integrative modelling of transport and fate of endosulfan ... · the present document reports part...

C S I R O L A N D a nd WAT E R

Integrative modelling of transport and fate of

endosulfan in the riverine environment

Part I: Use of observed riverine concentrations to identify

contributions of airborne and waterborne transport

Peter R. Briggs, Michael R. Raupach, Bruce Cooper and Monika Muschal

CSIRO Land and Water

Technical Report 14/98, June 1998

Integrative modelling of transport and fate of

endosulfan in the riverine environment

Part I: Use of observed riverine concentrations to identify

contributions of airborne and waterborne transport

Peter R. Briggs, Michael R. Raupach, Bruce Cooper and Monika Muschal

CSIRO Land and Water

Technical Report 14/98, June 1998

Copyright

© 2002 CSIRO Land and Water.To the extent permitted by law, all rights are reserved and no part of this publication covered bycopyright may be reproduced or copied in any form or by any means except with the writtenpermission of CSIRO Land and Water.

Important Disclaimer

To the extent permitted by law, CSIRO Land and Water (including its employees and consultants)excludes all liability to any person for any consequences, including but not limited to all losses,damages, costs, expenses and any other compensation, arising directly or indirectly from using thispublication (in part or in whole) and any information or material contained in it.

Integrative Modelling of Transport and Fate of Endosulfan in the RiverineEnvironment, Part 1: Use of Observed Riverine Concentrations to Identify

Contributions of Airborne and Waterborne Transport

P.R. Briggs1, M.R. Raupach1, B. Cooper2, M. Muschal2

1CSIRO Land and Water, Canberra, ACT2NSW Department of Land and Water Conservation, Sydney, NSW

Summary: To reduce endosulfan contamination in rivers and waterways, it is important toknow the relative significances of the possible airborne and waterborne transport pathwaysresponsible for moving endosulfan from farm to river. This work develops a method foridentifying the relative contributions of airborne and waterborne transport through model-based analysis of riverine endosulfan concentration data. The method is applied to 5 years ofobserved endosulfan concentrations and river flows in the Namoi River, gathered by the NSWDepartment of Land and Water Conservation under its Central and North West Regions WaterQuality Program (CNWRWQP). The river data are supplemented with concurrent dailyrainfall observations from 23 local Bureau of Meteorology stations.

The work has two stages. The first identifies concentration “signatures”, or typical patterns ofconcentration change following a transport event, for three airborne transport pathways (spraydrift, vapour transport and dust transport) and two waterborne pathways (runoff andsubsurface leaching). Properties of the concentration signatures are determined throughphysical reasoning, modelling and appeal to measurement. The most important properties arethe mix of α, β and sulphate species in the riverine endosulfan (airborne pathways carry α andβ almost exclusively, whereas waterborne pathways carry mainly but not only sulphate); andthe timing of concentration changes in the river following a single transport event (waterbornetransport can only occur after antecedent rainfall). Concentration signatures also show thatdust transport and subsurface leaching are of negligible magnitude.

The second stage is to devise and apply a method for associating observed riverineconcentrations with transport pathways. This is done by plotting each observed concentrationpeak on a plane in which the vertical axis is the fraction of endosulfan α and β in the peakconcentration, and the horizontal axis is a measure of antecedent rainfall. On this plane,concentration peaks fall mainly into two distinct regions: (a) events with high fractions ofendosulfan α and β and little antecedent rainfall, and (b) events with low α and β fractionsand high antecedent rainfall. Events in region (a) can be attributed primarily to airbornetransport, and those in region (b) primarily to waterborne transport. Some events occur in athird region (c) with low α and β fractions and low antecedent rainfall, arising from thecontinued presence of endosulfan sulphate with a long riverine residence time. Very fewevents are observed in the fourth region (high α and β fractions and high antecedent rainfall).Events in regions (a) and (b) are both of significant magnitude, with (a) being dominant atlocations close to cotton farms and less so at locations distant from farms. Events in region (c)are smaller. It is concluded that airborne transport pathways account for a significant fraction(around half) of the endosulfan concentrations in slow-flowing waterways close to cottonfarms.

2

1. Introduction

1.1 Background

The present document reports part of the results of LWRRDC Project CEM5, "IntegrativeModelling of Transport and Fate of Endosulfan in the Riverine Environment". This project isa component of the Research Program “Minimising the Impact of Pesticides on the RiverineEnvironment” (henceforth “the Pesticides Program”), jointly funded by the Land and WaterResources Research and Development Corporation (LWRRDC), the Cotton Research andDevelopment Corporation (CRDC) and the Murray-Darling Basin Commission (MDBC).

Work to date in the Pesticides Program has established that endosulfan (and other agriculturalchemicals) can be transported from farms into the riverine environment by several alternativepathways involving both airborne and waterborne transport. Possible pathways include:

1. Spray drift (the direct deposition of off-target spray droplets);

2. Vapour transport (in which endosulfan sprayed onto a crop volatilises into the air,disperses as a gas and is then deposited on water or other surfaces downwind);

3. Dust transport (the windblown transport of endosulfan-contaminated dust from farms);

4. Runoff or tailwater spillage (discharge of the endosulfan-contaminated overflow fromfarm drainage systems into waterways, because of inability to store all rainfall on-farm duringlarge rainfall events);

5. Subsurface leaching (the movement of endosulfan into waterways through the soil).

Of these, the first three pathways are airborne and the last two are waterborne. It is essential todetermine the relative significance of these transport pathways, as the management options forlimiting pesticide movement into waterways are quite different for different pathways.

Project CEM5 (the project within the Pesticides Program to which this report is acontribution) has the overall goal of determining the relative and absolute contributions of thevarious transport pathways to riverine endosulfan concentrations, through physical modellingand data interpretation. The specific aims of Project CEM5 as set out in the Project Scheduleare: (1) to develop and validate a simple predictive analysis for runoff discharge to river,including water, sediment and pesticide transport; (2) to clarify the relative contributions ofspray drift, vapour transport, dust transport and runoff; (3) to clarify the in-stream fate ofendosulfan by combining models for each transport pathway with a chemistry model and amodel for riverine dispersion; and (4) to attempt a closure of the riverine endosulfan budget,thus providing a validated assessment of the contributions from all significant pathways.

Project CEM5 extends modelling work already undertaken within the Pesticides Program onthe vapour transport pathway for endosulfan (Raupach, Ford and Briggs 1996, henceforthRFB96) and endosulfan transport by multiple pathways (Raupach and Briggs 1996, henceforthRB96). For earlier modelling work on the vapour transport pathway, see Raupach (1993a,b).

3

1.2 This report

To meet the overall goal and specific objectives of Project CEM5 as outlined above, we havefound it necessary to use two different, complementary approaches: (1) data interpretationwith model scenarios (interpretations of observed data on riverine concentrations with the aidof model-generated scenarios the riverine concentration patterns produced by differenttransport pathways); and (2) synthesis through realistic modelling (physical modelling ofendosulfan transport by all pathways under realistic scenarios for farm management,geography and meteorological conditions). This report describes work undertaken under thefirst approach. The outcomes of the second will be reported separately.

The aim of the work described here is to identify features of the riverine concentration datawhich indicate the relative significance of the various transport pathways. This is donethrough a statistical analysis and model-based interpretation of more than 5 years of observedendosulfan concentrations and river flows in the Namoi River, together with concurrent dailyrainfall observations from 23 stations in the Namoi Valley. The concentration and river flowdata were recorded by the NSW Department of Land and Water Conservation under itsCentral and North West Regions Water Quality Program (CNWRWQP).

This work proceeded in two main stages. The first (described in Section 2) was the applicationof modelling results to define concentration “signatures”, or typical patterns of concentrationchange following a transport event, for the various airborne and waterborne transport routes.These signatures characterise the mix of endosulfan species (α-endosulfan, β-endosulfan andendosulfan sulphate) occurring in the river as a result of a transport event through a particularpathway, and the timing of the peak and subsequent decay of the endosulfan concentrations.Thus, through both species mix and timing, associations can be drawn between observedriverine concentrations and transport pathways, at least on a probabilistic basis. The secondstage began with the assembly, description, quality control and preliminary analysis of data onendosulfan concentration, river discharges and daily rainfall from all available stations in theNamoi valley (described in Section 3). Then, we tested several statistical approaches foridentifying the transport pathways leading to observed peaks in riverine endosulfanconcentrations. These tests, described in Section 4, were done in four ways: by examininggross correlations between hydrological events (antecedent rainfall and river discharge) andendosulfan concentrations; by examining the relationship between concentration and thetiming of antecedent rainfall on an event-by-event basis; and by examining the species mix onan event-by-event basis; and by examining storm events.

2. Characteristics of Airborne and Waterborne Endosulfan Transport

2.1 Endosulfan species and transformations

As part of stages 1 and 2 of the “Cotton Resistance Management Strategy”, endosulfan issprayed to control Heliothis infestation on most cotton crops, typically from November toJanuary at intervals of 2 to 4 weeks. Endosulfan (C9H6O3Cl6S) exists as two isomers, α-endosulfan and β-endosulfan, which occur in the ratio 2:1 in the "technical endosulfan"applied to cotton crops in spray form. Once in the natural environment, endosulfan undergoesseveral chemical and physical transformations. First, both the α and β isomers oxidise toendosulfan sulphate in the presence of biotic material, over a time scale of several days. The

4

α, β and sulphate forms are all of comparable toxicity. Second, all three forms graduallyhydrolyse in water to endosulfan diol, which is much less toxic and can be regarded as a sinkfor endosulfan from the standpoint of toxicity. Third, endosulfan in aqueous solution isadsorbed onto and desorbed from colloidal particles in a rapid, effectively instantaneous two-way process. Fourth, endosulfan in aqueous solution is exchanged with the atmosphere, inanother two-way process with a time scale which depends on the water depth H; this timescale is short (hours) when H is around 0.1 m, and long (several days) when H is 1 m or more(RFB96). Figure 1 shows schematically the relationships between the three toxic endosulfanspecies (α, β and sulphate); the three phases in which endosulfan exists in the riverineenvironment (vapour, dissolved and particle-bound); and the four main transformationprocesses (oxidation, hydrolysis, adsorption-desorption between particles and water, and air-water exchange). A detailed endosulfan chemistry model incorporating all these species,phases and processes is described in RFB96 (Appendices 2 and 3) and RB96 (Appendices Aand B).

Ca

diol

CwCw Cw

Cp Cp Cp

Ca Ca

10hr 10hr

500hr

500hr

10hr

1hr1hr1hr

30hr 10 hr5

30hr

air

water

particle

α β sulphate

ExchangeOxidationHydrolysis

Figure 1: Schematic diagram of the physical and chemical interactions included in theendosulfan chemistry model, with approximate reaction rates expressed as time scales.

5

A consistent notation is vital, because of the need to keep track of the concentrations of threeendosulfan species in three different phases. For endosulfan species x (= α, β or f, where fdenotes the sulphate), the concentration in air as vapour is written as Ca

x, the concentration inwater in dissolved form as Cw

x, and the concentration in water in particle-bound form as Cpx

(all in kg-endosulfan m-3). The total concentration in air is written as Ca = Caα + Ca

β + Caf, and

likewise Cw = Cwα + Cw

β + Cwf (dissolved in water) and Cp = Cp

α + Cpβ + Cp

f (on particles inwater).

2.2 Concentration Signatures for Transport Pathways: Qualitative Properties

To achieve the aim of identifying features of the riverine concentration data which indicate therelative significance of the various transport pathways, it is necessary to identify a“concentration signature” for each pathway. Only through differences among these signaturescan an analysis of riverine concentrations distinguish between different pathways. Theimportant properties of the concentration signature for a particular pathway relate to thespecies mix of the transported endosulfan; the timing of concentration changes in the riverfollowing a single transport event, and the magnitude of the transport. Properties of theconcentration signatures can be identified with a combination of physical reasoning,modelling and appeal to measurement.

This section summarises six important qualitative properties of the concentration signatures,with brief justifications. Of these, properties 1 and 2 relate to species mix, 3 and 4 relate totiming, and 5 and 6 to relative magnitudes of pathways. Some of the justifications rely on amore detailed modelling analysis in which a typical concentration signature for each transportpathway is determined using a scenario, consisting of specified typical conditions for atransport event. To keep the main argument simple, this detailed scenario analysis is describedseparately in Section 2.3.

The six qualitative properties of the concentration signatures are:

1. Airborne pathways (spray drift, vapour transport, dust transport) carry endosulfan αand β but practically no sulphate.

This assertion can be made with very high confidence, on the following grounds. The activeingredient in spray drift is “technical endosulfan”, partitioned in the ratio 2:1:0 between α, βand sulphate (thus containing no sulphate). Vapour transport is much less effective forsulphate than for α (with β being intermediate) because of the relative saturation vapourpressures of the three species. The saturation vapour pressures are 1.9, 0.092 and 0.023 mPafor α, β and sulphate respectively, at 25 deg C (Hoechst 1993). Dust transport can carry allthree species but is much lower in magnitude than either spray drift or vapour transport, so thesulphate carried by this pathway is insignificant in practice.

2. Waterborne pathways can carry significant amounts of all three endosulfan species(α, β and sulphate), with a tendency for sulphate to predominate.

This property derives from observational evidence. Figure 2 shows flow discharges andendosulfan species concentrations recorded as part of the CNWRWQP during 9 storm eventsin four different waterways. It is likely that the endosulfan detected during these storm eventsis transported to the waterways by runoff, though this cannot be proved. The highestconcentrations (Cw > 1 µg L-1, in the Gwydir at Brageen) are observed on the rising limb of the

6

storm hydrograph, with sulphate making up 0.5 to 0.7 of the total. There are a few periodswhen the species mix is dominated by the sum of α and β, but these tend to be short duration.There is a tendency for sulphate to dominate when total concentrations are low.

9/12/93 10/12/93 11/12/93 12/12/930.0

0.1

0.2

0.3

0

1000

2000

3000

4000

21/1/95 22/1/95 23/1/95 24/1/95

End

osul

fan

Con

cent

ratio

n(µ

gl-1

)

0.00

0.05

0.10

0.15

Dis

char

ge(M

ld-1

)

0

5000

10000

15000

4/1/96 5/1/96 6/1/96 7/1/960.0

0.2

0.4

0.6

0

2500

5000

7500

10000

22/1/96 24/1/96 26/1/96 28/1/96 30/1/960.00

0.25

0.50

0.75

1.00

0

5000

10000

15000

20000

25000

Q

Q

Q

Q

tot

tot

tot

tot

α β

f

α

βf

α,β

f

f

α

β

Figure 2a: Storm event sampling for Coxs Creek at Boggabri, including discharge (Q), α and β,,sulphate (f), and total endosulfan. Sampling times for discharge and endosulfan are marked bysymbols on the discharge plot.

7Figure 2b: As above, for Mooki River at Ruvigne

Figure 2c: As above, for Namoi River at Tarriaro

9/2/92 10/2/92 11/2/920.0

0.1

0.2

1

2

3

4

β

f

totQ

20/1/95 21/1/95 22/1/95 23/1/95 24/1/95

End

osul

fan

Con

cent

ratio

n(µ

gl-1

)

0

1

2

3

4

5

Dis

char

ge(M

ld-1

)

0

2000

4000

6000

29/1/97 31/1/97 2/2/970.0

0.5

1.0

1.5

2.0

0

2000

4000

6000

8000

αβ

α

β

f

f

tot

tot

Q

Q

23/1/95 24/1/95 25/1/950.00

0.05

0.10

0

200

400

600

3/1/96 5/1/96 7/1/96 9/1/96 11/1/96

End

osul

fan

Con

cent

ratio

n(µ

gl-1

)

0.00

0.05

0.10

0.15

Dis

char

ge(M

ld-1

)

0

5000

10000

15000

20000

Q

tot

αβ

f

f,tot

Qα,β = 0

Figure 2d: As above, for Gwydir River at Brageen Crossing.

8

3. Endosulfan α and β remain in the river for only a few days, whereas sulphate remainsin the river for substantial periods (months).

This property has high confidence, based on the known behaviour of the removal mechanismsfor different endosulfan species. Once in the river, endosulfan is removed mainly byvolatilisation to the atmosphere and hydrolysis to endosulfan diol. As indicated in Figure 1and RB96, both processes are fairly rapid for α and β, with time scales of a few days at most.However, they are very much slower for sulphate, because sulphate is less volatile than α andβ, and also the hydrolysis of sulphate to diol is insignificant.

4. Endosulfan peaks caused by the runoff pathway occur after rainfall.

This important property is obvious from the fact that rainfall is required to produce runoff. Itimplies that a means of diagnosing the significance of the runoff pathway is to look for atemporal relationship between endosulfan peaks and antecedent rainfall.

5. Dust transport and subsurface leaching are negligible.

The assertion that dust transport is negligible can be made with high confidence, on the basisof measurements by Leys et al. (1998) and incorporation of these measurements into scenariosfor the transport pathways, as described in more detail in Section 2.3. Subsurface leaching isregarded as a negligible pathway because of the strong tendency of endosulfan to bind to soilparticles (Howard, 1991; Wauchope et al., 1992). Endosulfan α and β are also susceptible tooxidation by soil microorganisms (Howard, 1991; NRCC, 1975). On these grounds,subsurface leaching is not considered further here.

6. Spray drift and runoff are both potentially large contributors to riverineconcentrations.

This assertion is based on scenarios for typical transport events developed below (Section2.3). Although there is substantial variability among different transport events, such scenariosprovide guidance on the likely rankings of the relative transports through the differentpathways in typical circumstances.

2.3 Concentration Signatures for Transport Pathways: Scenario Analysis

Some of the justifications for the above qualitative properties of concentration signaturescome from analysis of a scenario for each transport pathway, consisting of specified typicalconditions for a transport event. With the aid of process-based models, we can estimate thetypical amount of endosulfan transported through each pathway under the specifiedconditions. This section describes the pathway scenarios, the process models used todetermine endosulfan transport, and the derived concentration signatures.

Spray drift: Current spraying practice usually uses Ultra-Low-Volume (ULV) techniques,producing droplets of around 80 µm diameter. The active ingredient (technical endosulfan,partitioned 2:1 between the α and β isomers) is mixed with an inert carrier to a mass fractionof about 0.25. The spray is applied at a typical dose rate Ddose of 0.72 kg-endosulfan ha-1 =0.72×10-4 kg-endosulfan m-2.

9

The input of endosulfan from spray drift to any downwind surface (including a river) isdetermined by the off-target spray deposition onto the surface (Ds

x kg m-2 of endosulfanspecies x). This can be expressed as a fraction fdrift of the intended deposition or dose rate overthe target area (Ddose

x kg-endosulfan m-2 for species x), so that Dsx = fdrift Ddose

x. The depositionfraction fdrift depends on the dispersive droplet motions only, and is therefore independent ofthe species x. It is determined by a model of spray dispersion and droplet deposition, in thiscase a settling-Gaussian-plume algorithm for spray dispersion and a settling-velocityalgorithm for particle deposition (RB96, Appendix C).

The spray drift scenario is based on model results shown in RB96, Figure C3, which assumethat a field of area 1 km × 1 km is sprayed with droplets having a Gaussian droplet sizespectrum with mode 80 µm and standard deviation of 20 µm. A wind of speed of 4 m s-1, inthermally neutral conditions, carries the drifting spray to a river at a distance X furtherdownwind from the leeward edge of the sprayed field. The model results (RB96, Figure C3)show that in these conditions the fractional deposition fdrift is 0.03 when X = 1 km, falling to0.008 when X = 3 km and 0.004 when X = 5 km. Our spray drift scenario assumes a total doserate Ddose of 0.72×10-4 kg-endosulfan m-2, partitioned 2:1:0 between α, β and sulphate, and afractional deposition fdrift of 0.01 (appropriate for X in the range 2 to 3 km). It follows that thespray deposition for each species, given by Ds

x = fdrift Ddosex, is Ds

α = 0.48×10-6 kg m-2, Dsβ =

0.24×10-6 kg m-2 and Dsf = 0.

Vapour transport: Vapour transport begins with the volatilisation of endosulfan from thecrop, a continuous process which eventually removes up to 70% of the total endosulfandeposited during a spray. This vapour is dispersed by wind and atmospheric turbulence, anddeposited on downwind surfaces (including waterways). The deposition process is driven bythe concentrations of each endosulfan species in the air just above the deposition point (in thiscase a waterway), which can remain at significant levels for at least a day or two after a crop issprayed, because the endosulfan on the crop continues to volatilise over that period of time.

The concentration Cax(t) in the air above the water surface is a function of time t since

spraying at t = 0. Direct measurements (Ahmad et al. 1995) and model results (RFB96,Raupach 1993a,b) show that, for a field-river distance X of a few kilometres or less, the initialtotal air concentration Ca(0) above the river is of order 1 µg m-3, decaying to zero after a fewdays. This typical behaviour can be captured by assuming that for any species x (α, β or f):

where Ta is a decay time. The vapour transport scenario is based on Equation (1) with a totalinitial concentration Ca(0) = Ca

α(0) + Caβ(0) of 1 µg m-3 and a time constant Ta of 2 days. The

total air concentration Ca(t) = Caα(t) + Ca

β(t) is partitioned in the ratio 2:1:0 between the α, βand sulphate species at all times t, as in technical endosulfan.

Dust transport: The dust transport pathway operates by the windblown movement ofendosulfan-bearing dust from a cotton farm into the riverine environment. Potentialmechanisms for on-farm dust generation include dust uplift during wind erosion events, upliftby vehicular traffic on unpaved roads, and uplift by agricultural operations. This dust is thendispersed and deposited downwind.

C t C t Tax

ax

a( ) ( ) exp= −0 b g (1)

10

The input of endosulfan to a river (or other downwind surface) from a single dust transportevent is determined by the dust deposition (Ddust kg-dust m-2) and the mass fraction ofendosulfan on the dust (rp

x kg-endosulfan per kg-dust). The endosulfan input for species x isthen Ddust rp

x kg-endosulfan m-2. Careful field measurements of airborne dust by John Leysand colleagues (Leys et al. 1998) have shown that the mass fraction for total endosulfan(rp = rp

α + rpβ + rp

f) is about 1×10-6 for dust uplifted by vehicle traffic from unpaved roads,and about 1.8×10-6 for dust uplifted from fields by cultivation. For dust uplifted by regional-scale wind erosion, the mass fraction would certainly be lower because of dust dilution. Noinformation about the species mix (α, β or sulphate) is available from this work, but earliermeasurements by V. Edge and colleagues (reported as a personal communication in RB96, p.9) give the ratio of α:β:f as 60:35:5, not far from the species mix of technical endosulfan. Themix for technical endosulfan is therefore assumed. Taking an average value for rp of 1.5×10-6

(dimensionless), we have rpα = 1.0×10-6 and rp

β = 0.5×10-6.

The regional dust deposition rate (at distances of more than a few hundred metres from acotton farm) was measured by Leys et al. (1998) to be around 2 g-dust m-2 month-1,incorporating dust from all sources. Higher dust deposition rates are observed close to sourceson-farm, but this short-range dust transport relocates endosulfan on-farm rather than moving itover sufficient distances to reach the riverine environment. To make an upper-limit estimateof the dust transport per event, we assume that all of monthly regional dust deposition occursin a single event in each month, giving a dust deposition for this event of Ddust =2×10-3 kg-dust m-2.

This scenario, based on the recent field measurements of Leys et al. (1998), implies anendosulfan input per dust transport event (Ddust rp

x) of 2×10-9 kg m-2 of α-endosulfan and1×10-9 kg m-2 of β-endosulfan. These estimates refine the initial upper-limit estimates used inRB96. They also indicate that the deposition of endosulfan by dust transport is nearly a factorof 1000 lower than the typical endosulfan deposition by spray drift (Ds

x), estimated above asDs

α = 1.44×10-6 kg m-2, Dsβ = 0.72×10-6 kg m-2 and Ds

f = 0. This confirms the finding of RB96that dust transport is not a significant pathway, and justifies property 5 in the previous section.

Runoff: Surface runoff is controlled on many Australian cotton farms by careful grading offields, furrow irrigation, and the recycling of runoff in water storages. Water storages andtailwater return channels improve water use efficiency, and also limit the flow ofcontaminated water into natural waterways. Concentrations of endosulfan in water storagesmay be as high as 50 µg L-1 (M. Silburn and D. Connelly, personal communication). Reviewsof farm management practices in the Macquarie and Namoi Valleys (O'Brien 1995, 1996)show that the state of development of water management systems varies substantially amongthe cotton growing areas of NSW. Occasionally, runoff from large rainfall events exceeds thecapacity of on-farm storage, allowing contaminated overflows to reach local rivers eitherdirectly or indirectly via floodways. From interviews with 85 cotton farmers in the NamoiValley, O'Brien (1996) estimated that 31% of Upper Namoi growers and 3% of Lower Namoigrowers cannot contain 25mm of rain on-farm.

The input of endosulfan species x to a river from runoff is determined by the endosulfanconcentrations in the runoff water, and the runoff discharge into the river. This runoff isassumed to enter the river at a single, well-defined point. The effect on the riverine endosulfanconcentrations can be specified in terms of a dilution factor fdil, defined by

11

where Q(runoff), Q(upstream) and Q(downstream) are the discharges (in m3 s-1) of the runoff and of theriver upstream and downstream of the runoff entry point. A mass balance shows that thedownstream concentrations of dissolved and particle-bound endosulfan in the river are madeup of weighted contributions from the upstream river and the runoff:

The runoff scenario is prescribed by assigning values to the quantities on the right hand sidethese equations. The upstream river is assumed to contain no endosulfan. The dilution factorfdil is arbitrarily assumed to be 0.05, a choice which determines only the magnitude of theriverine concentrations in the runoff scenario, not the species mix or the timing of theconcentration decay after the runoff event. The dissolved concentrations Cw

x(runoff) are assumed

to be Cwα = 1 µg L-1, Cw

β = 1 µg L-1, Cwf = 8 µg L-1, so that most of the endosulfan in the

runoff water is in sulphate form. The particle-borne concentrations Cpx(runoff) are assumed to be

in chemical equilibrium with the dissolved concentrations, with a waterborne particle densityof 0.1 kg m-3, resulting in only a small contribution of particle-borne endosulfan to the overallendosulfan in the runoff (that is, Cp

x << Cwx).

The most important choice in this scenario is that of the ratio of (α+β) to sulphateconcentration in the runoff water, which determines the initial species mix of the riverineendosulfan. The above scenario corresponds with a choice for this ratio of 0.2. This can bejustified on two grounds: first, it is broadly consistent with the measurements in Figure 2.Second, it is broadly consistent with the sources of endosulfan in farm runoff water, whichmust come from one of three major stores: endosulfan on plants immediately after spraying,endosulfan in the soil, and endosulfan in water storages. For all three, the residence time ofendosulfan α and β is of the order of a few days at most, but the residence time of endosulfansulphate is much longer, because of the combined effects of volatilisation, oxidation andhydrolysis. A significant contribution of endosulfan α and β in runoff water is thereforeexpected only when rainfall occurs within a few days of spraying. Since spray events onaverage are 2 to 4 weeks apart, the probability of this occurring is around 0.2, so this figureprovides a first estimate of the average fraction of endosulfan α and β in the runoff water.

Predicted scenarios: Figure 3 shows the predicted time course of the riverine endosulfanconcentrations resulting from a single transport event through each of the spray drift, vapourtransport, dust transport and runoff pathways. Following Section 2.2, subsurface leaching isregarded as negligible. Endosulfan chemistry is treated with a detailed model incorporating thespecies, phases and processes mentioned in Section 2.1 (RFB96, Appendices 2 and 3; RB96,Appendices A and B). The calculations yield the concentrations in a column of river water, ofarbitrary horizontal size, which moves with the flow. This water column is assumed to be wellmixed vertically and to stay intact as it moves down the river. Sediment-water exchanges are

f Q Q Q Q Qdil runoff downstream downstream runoff upstream= = +( ) ( ) ( ) ( ) ( ), (2)

C f C f C

C f C f C

w downstreamx

dil w runoffx

dil w upstreamx

p downstreamx

dil p runoffx

dil p upstreamx

( ) ( ) ( )

( ) ( ) ( )

= + −

= + −

1

1

b gb g (3)

12

treated as if the sediment is all in suspension. The calculations use the following commonassumptions: the river contains a suspended particulate concentration of 0.05 kg m-3, typicalof inland New South Wales rivers. The river water depth is H = 2 m. The transport eventoccurs at time t = 0, prior to which the river water is assumed to be free of endosulfan, so thatall initial riverine concentrations are zero: Cw

x(0) = 0, Cpx(0) = 0.

For vapour transport, the peak total concentration occurs at about t = 2 days, as a result ofuptake from the air into the water body at early times when Ca is high, and removal ofendosulfan from the water by hydrolysis and by revolatilisation back to the air at later timeswhen Ca is low.

For spray drift, initial concentrations are quite high and are entirely in α and β form, but thesedecay rapidly because of hydrolysis and volatilisation back into the endosulfan-free air abovethe water.

For dust transport, the initial endosulfan load in the water is entirely in particle-bound form,having arrived on dust particles. This moves quickly into dissolved form over several hours,

Figure 3: Evolution of endosulfan species concentrations f fw w w p p p, , , , ,C C C C C Cα β α β for individual events

in the vapour transport, spray drift, dust transport and runoff pathways. Assumptions: medium scenarios,water depth H = 2.0 m, suspended particulate concentration = 0.05 kg m-3..

Time (days)

0.01 0.1 1 10 100 1000

0.00.20.40.6

0.000

0.001

0.002

0.00.10.20.3

Con

cent

ratio

nin

Wat

er(µ

gL-1

) 0.00

0.01

0.02

α waterβ watersulphate waterα particleβ particlesulphate particletotal endosulfan

Vapour

Spray

Dust

Runoff

13

following which the α and β components decay through hydrolysis and volatilisation but thesulphate decays much more slowly. However, the total concentration is very low as notedabove.

For runoff, the initial species mix in the river is the same as in the runoff water because of theassumption of dilution of the runoff by a clean upstream flow. The sulphate component,assumed to be 0.8 of the endosulfan in the runoff on grounds argued above, persists for a longtime in the river water while the α and β components decay over a few days throughhydrolysis and volatilisation.

These scenarios confirm the six qualitative properties of concentration signatures listed inSection 2.2. In magnitude, the spray drift and runoff scenarios produce roughly equal riverineconcentrations, though with quite different species mixes and subsequent timings forendosulfan decay. The vapour transport scenario produces small riverine concentrations aboutan order of magnitude less than spray drift and runoff, and the dust transport scenarioproduces riverine concentrations which are an order of magnitude smaller still and arenegligible in practice.

Readers familiar with RB96 will notice some changes to the outcomes of scenarios given inRB96 Figure 5 compared with the present Figure 3. These changes reflect new informationand understanding about the various transport processes. Three changes have been made: first,concentrations in the dust scenario are lower in the present work to accord with the fieldresults of Leys et al. (1998). This further reinforces the conclusion of RB96 that dust transportis negligible. Second, the present runoff scenario uses somewhat higher concentrations with adifferent mix of species (namely, a greater proportion of sulphate). This is based on thearguments justifying the runoff scenario given above, and is also consistent with findings ofthe analyses described later. Third, the time scale for the oxidation of α and β to sulphate inwater has been lengthened from 100 to 500 hours, following advice from Professor IvanKennedy (personal communication) that this is not likely to be a significant process in water.The consequence is that α and β are available to be revolatilised (and lost to the system) for alonger period before oxidising to the less volatile sulphate, so that less sulphate is seen in thevapour and spray scenarios at large times after the transport event (over 10 days). We note thatthere is still considerable uncertainty about the appropriate time scales for both the oxidationand hydrolysis pathways in Figure 1 (cf. Brooks, 1998), which can have a substantial effect onthe residence times and magnitudes of the various species.

3. Data

3.1 Concentration and River Discharge

Since September 1991, the NSW Department of Land and Water Conservation (DLWC) hascollected and analysed endosulfan concentrations in surface waters at up to 35 “water qualitystations” in the Macquarie, Namoi, Gwydir, Darling, and Border River basins as part of theCentral and North West Regions Water Quality Program (CNWRWQP). This study usesendosulfan concentration data from nine sites in the Namoi basin: five on the main channel ofthe Peel/Namoi River, and four on tributary creeks (Table 1, Figure 4).

14

Routine sampling of endosulfan and discharge data was conducted weekly during the peakspraying season (December to February), fortnightly within a two-month shoulder period(November and March), and monthly during the off-season (Cooper, 1996). As shown inTable 1, only four of the nine water quality stations were sampled during all growing seasonsfrom 1991-92. The endosulfan measurements were from grab samples collected at 0.25 mdepth, or at mid-depth when water depth was less than 0.5 m. Care was taken to minimisechanges to the sample in transportation, and the samples were tested ideally within 24 hoursand generally within 48 hours at the DLWC Water Environmental Laboratory (Cooper, 1996).River discharges coincident with the endosulfan sampling were recorded at all stations exceptPeel River at Bective Reserve. March 1997 is the time of the most recent data considered here,but the CNWRWQP monitoring program is ongoing.

We recognise but do not deal explicitly with the difficulty of endosulfan measurement errors,treating the measurements as unbiased.

At several CNWRWQP water quality stations, routine measurements were supplemented bymultiple samples taken on the same day during several storm events. In these cases the peaktotal endosulfan value per day was included in the analysis of routine samples. The only suchstation among the Namoi Valley stations used here for primary analysis (Table 1) is CoxsCreek at Boggabri. Sub-diurnal aspects of the storm event data are treated separately, usingmeasurements not only from this station but also from other stations not involved in the rest ofthe analysis.

Measurements During Growing Season

Station Name Ident Lat Long 91/92 92/93 93/94 94/95 95/96 96/97

Namoi River at Bugilbone 419021 -30.27 148.82 17 19 13 17 15 13

Pian Creek at Rossmore 419064 -30.09 149.07 17 19 13 11 14 13

Namoi River at Weeta Weir 419068 -30.29 149.34 13

Gunidgera Creek D/S Regulator 419061 -30.20 149.43 12

Narrabri Creek at Narrabri 419003 -30.33 149.78 14

Coxs Creek at Boggabri (Warragrah) 419032 -30.78 149.99 3 3 15 7

Namoi River at Gunnedah 419001 -30.97 150.25 17 19 14 14 14 7

Peel River at Bective Reserve 41910115 -30.97 150.73 17 8

Peel River at Paradise Weir 419024 -31.10 150.94 16 17 13 9 13 9

Table 1: List of water quality stations considered in the study, including station number, location,and the number of endosulfan measurements taken during each growing season. Italicised stationsare on the main channel of the Namoi and Peel Rivers. After an initial assessment, Peel Riverstations were not used due to the lack of endosulfan activity (up-valley from the main cottongrowing areas). For Coxs Creek at Boggabri, multiple measurements on the same day are countedas one measurement.

15

148.5 149.0 149.5 150.0 150.5 151.0 151.5

-31.0

-30.5

-30.0

Wee Waa

Gunnedah

Boggabri

Narrabri

Tamworth

Endosulfan MeasurementsRainfall StationsRoadsDrainage NetworkBuilt Areas

200 400 600 800 1000 1200 1400

Elevation (m)

Figure 4a: Location of endosulfan and discharge measurement stations, Bureau of Meteorologyprecipitation stations, relief, infrastructure and drainage network.

148.5 149.0 149.5 150.0 150.5 151.0 151.5

-31.0

-30.5

-30.0

213

4

5

7

6

8

910

14

3

1211

1

1516

17

18 1920

2122

23

Namoi Riverat Bugilbone

Pian Creekat Rossmore

Namoi RiverD/S Weeta Weir

Gunidgera CreekD/S Regulator

Narrabri Creekat Narrabri

Cox's Creek atBoggabri (Warragrah)

Namoi Riverat Gunnedah Peel River at

Bective Reserve

Peel River atParadise Weir

Endosulfan and DischargeMeasurements

Endosulfan Only

Bureau of Meteorology Rainfall Stations

1.Burren Junction (Waterford) 52066 9. Narrabri (Narrabri West P.O.) 53030 17. Carroll (The Ranch) 550552. Pilliga (Pilliga Post Office) 52023 10. Narrabri Bowling Club 54120 18. Somerton (Kallaroo) 551383. Burren Junction P.O. 52001 11. Boggabri (Be-Bara) 55268 19. Somerton (Glen Burn) 551404. Rowena (Myall Plains) 52021 12. Boggabri (Wilga Park) 55273 20. Somerton (Bective Estate) 550035. Pilliga (Coolabah Downs) 52074 13. Turrawan (Wallah) 55058 21. Tamworth (Inverness) 552796. Wee Waa (Pendennis) 53034 14. Boggabri (Milchengowrie) 55034 22. Warral (Hillsia) 551587. Wee Waa Post Office 53044 15. Gunnedah (Gunnedah Pool) 55023 23. Tamworth (Bahreenah) 551668. Narrabri (Mollee) 53026 16. Gunnedah (Gunnedah Scs) 55024

Figure 4b: Name key for discharge/endosulfan and precipitation stations in figure 4a.

16

▲

▲

▲

▲

▲

▲

▲▲

▲

▲

Flo

od

Irrig

ated

Cotto

n

Dry

land

Cotto

n

En

dosu

lfan

Meas

ure

men

ts

Mai

nC

hann

el(N

am

oi-P

eel)

▲Trib

uta

ryC

reek

s

50

km

Figure 4c: Location of cotton farms and endosulfan measuring stations in the Namoi River Catchment.The map is modified from Peasley (1996) to remove non-cotton areas by colour masking. Cotton farmlocations were determined by air photo interpretation and limited site inspection, and will vary from yearto year. Used with permission of DLWC.

17

3.2 Rainfall

Daily rainfall totals provide different information and a more continuous record of hydrologicevents than periodic river discharges. Using Metaccess software, we compiled daily rainfallrecords for 23 Bureau of Meteorology (BoM) rainfall stations in the Namoi Valley, for theperiod 1 June 1991 to 28 February 1997. No data for March 1997 were available throughMetaccess. The rainfall station locations are shown in Figure 4a, with a site index in Figure4b, and the locations of cotton growing areas in 4c (after Peasley, 1996). Rainfall stationswere selected for proximity to the nine water quality stations in Table 1, and also for theircompleteness of record, especially continuity through weekends. For each water qualitystation, a record of mean local daily rainfall was constructed as the average of each day's totalfor the three closest rainfall stations. Periods of missing rainfall data were covered byinterpolation using data from nearby stations. In several cases, the same rainfall station wasrequired to create the local rainfall totals for more than one water quality station.

Rain events resulting in floods may occur over several days, and overflows from waterstorages may be the result of several rain events occurring in a short period of time. For thesereasons it is important to consider the relationship between endosulfan concentrations andantecedent rainfall, that is, the total rainfall occurring over a specified number of days prior toand including the day of the endosulfan measurement. For each endosulfan measurement ateach water quality station, one-day to thirty-day antecedent rainfall totals were calculated fromthe records of mean local daily rainfall.

3.3 Sampling Considerations

As suggested by Cooper (1996) the data were divided into "in-season" (November-March) and"off-season" (April-October) because of the known strong seasonality of endosulfanconcentrations which coincides with the aerial spraying season. This seasonality is shown inFigure 5, where ensemble monthly averages of endosulfan concentration and local rainfall arepresented for the period June 1991 to February 1997 for two water quality stations (Pian Creekat Rossmore and Namoi River at Bugilbone). Although there is a strong apparent correlationbetween rainfall and endosulfan concentration, this shows only that rainfall and endosulfanuse have similar seasonal cycles. These results emphasise the need to treat in-season and off-season data separately in any analysis of the relationship between hydrological events andendosulfan concentrations.

The water quality sampling schedule adopted by DLWC balances a requirement forcontinuous environmental monitoring against the non-trivial cost and workload of collectionand analysis. Unfortunately, weekly sampling presents some difficulties in analysis becauseof the potential for aliasing, which arises when temporally varying quantities such as risingand falling endosulfan concentrations are sampled insufficiently frequently (formally, lessfrequently than twice per period of the highest-frequency wave contributing to the variancespectrum). If endosulfan concentrations have periodicities associated with flood hydrographsor aerial spraying, which can occur fortnightly or more frequently, it may be difficult toidentify a relationship unambiguously from weekly samples. The sub-diurnal sampling of anumber of storm events is potentially useful for relating endosulfan concentrations to runoff,but does not provide any information about airborne transport. With a weekly samplingstrategy, near-peak concentrations resulting from airborne transport are only likely to be

18

sampled occasionally. This tendency is made more severe by the short-term nature of theconcentration signatures from airborne transport (Sections 2.2 and 2.3).

4. Results and Discussion

Results are presented in five parts. The first (Section 4.1) sets the scene by describing thespatial and temporal patterns of rainfall in the Namoi Valley. In the second (Section 4.2),possible links between endosulfan concentration, discharge and antecedent rainfall areexplored through scatter plots; these do not produce any useable relationships. To find analternative approach, we exploit the properties of the concentration signatures (Section 2.2)which lead us to examine event timing and species mix. Section 4.3 focuses on the timing ofendosulfan concentration peaks relative to antecedent rainfall, and Section 4.4 introduces

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5 6 7 8 9 10 11 12

Month

Ave

rag

eM

on

thly

Rai

nfa

ll(m

m)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Ave

rag

eE

nd

osu

lfan

Co

nce

ntr

atio

n( µ

gl-1

)b

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5 6 7 8 9 10 11 12

Month

Ave

rag

eM

on

thly

Rai

nfa

ll(m

m)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Ave

rag

eE

nd

osu

lfan

Co

nce

ntr

atio

n

( µg

l-1)

Mean Rainfall

Mean Sulfate

Mean Total Endo

a

Figure 5: Ensemble monthly averages of measurements of sulphate and totalendosulfan (06/91 to 02/97) compared with local average monthly rainfall for the sameperiod, for a) Pian Creek at Rossmore and b) Namoi River at Bugilbone.

19

species mix as an additional parameter. Section 4.5 discusses the results of storm eventsampling.

4.1 Rainfall in the Namoi Valley

Rainfall in the Namoi Valley is strongly seasonal, with the majority falling during the summergrowing season. Figure 6a shows the mean regional monthly totals (averaged over the 23BoM rainfall stations) for June 1991 to February 1997, with historical perspective provided bythe median monthly totals for three stations with long rainfall records. In Figure 6b, regionalaverage rainfall totals are binned by season and expressed as the mean monthly rainfall duringeach period. Crosses indicate the total for the peak month of the period. High year-to-yearvariability is apparent in both figures, with the effect of the 1994 drought being particularlyevident.

Figure 7 shows the spatial pattern of rainfall in the (a) growing season and (b) off-season inthe Namoi Valley, including the locations of the water quality stations from Figure 4. Theseareal estimates are interpolated by kriging using monthly totals from 188 BoM stations withinand outside the plot area, drawn from the source data of Cogley and Briggs (1995). The period1950-79 was used (a common standard for climatic normals), with all stations having at least20 years of data in that period. Comparing Figure 7 with the relief in Figure 4a, it is clear thattopography exerts a strong control over the pattern of rainfall in both the growing season andoff-season. The correlation (Pearson's r) of mean annual rainfall with station elevation in theregion is 0.58. On the valley floor, a gradient of about 100 mm of mean annual rainfall existsbetween the downstream and upstream endosulfan measurement stations, a distance ofroughly 225 km. The local maximum in the top centre of both plots is from a single elevatedstation, Barraba (Mount Lindsay) in the Nandewar Range, with a mean annual rainfall of1006 mm.

Month

06/9107/9108/9109/9110/9111/9112/9101/9202/9203/9204/9205/9206/9207/9208/9209/9210/9211/9212/9201/9302/9303/9304/9305/9306/9307/9308/9309/9310/9311/9312/9301/9402/9403/9404/9405/9406/9407/9408/9409/9410/9411/9412/9401/9502/9503/9504/9505/9506/9507/9508/9509/9510/9511/9512/9501/9602/9603/9604/9605/9606/9607/9608/9609/9610/9611/9612/9601/9702/97

To

talM

on

thly

Rai

nfa

ll(m

m)

0

20

40

60

80

100

120

140

160Namoi Monthly Total(23 Station Average)

Growing SeasonOff Season

Narrabri West P.O.Gunnedah PoolTamworth Airport

Historical Median Rainfall

Figure 6a: Monthly rainfall in the Namoi Valley between Bugilbone and Tamworth from June 1991 toFebruary 1997. Bars are the regional monthly totals averaged from 23 stations: black during the cottongrowing season (November to March), grey during the off-season (April to October). For historicalperspective, line plots show the median monthly rainfall for three stations in the region:Narrabri West Post Office (65 years of record), Gunnedah Pool (115 years), andTamworth Airport (111 years).

20

Figure 6b: Comparison of rainfall in and out of cotton growing season in the Namoi Valley. Black bars are themonthly average rainfall during the growing season (November to March). Grey bars are the monthly averagerainfall during the off-season (April to October). The crosses are the maximum monthly total in each period. Allmonthly averages are weighted by month length. All statistics are based on daily rainfall averaged over 23 stationsin the Namoi Valley between Bugilbone and Tamworth. The 1991 off-season values are for June to October only.The 1996-97 on-season values are for November to February only.

Figure 7: Mean long-term arealrainfall totals (mm) for the NamoiValley during cotton growing season(a. Nov-Mar) and out of season (b.Apr-Oct). Estimates were kriged(spherical variogram) fromaccumulated mean monthly totals of188 rainfall stations in and around theregion with at least 20 years of databetween 1950-79. Mean annual totalsare the sum of both plots. Diamondsshow the locations ofendosulfan/discharge measuringstations.

Season

91 Off91-92 On

92 Off92-93 On

93 Off93-94 On

94 Off94-95 On

95 Off95-96 On

96 Off96-97 On

Rai

nfa

ll(m

m)

0

20

40

60

80

100

120

140

160

180

148.5 149.0 149.5 150.0 150.5 151.0 151.5

-31.0

-30.5

-30.0

148.5 149.0 149.5 150.0 150.5 151.0 151.5

-31.0

-30.5

-30.0a

b

21

4.2 Concentration-Discharge and Concentration-Rainfall Correlations

For each of the nine water quality stations used here, all measurements of endosulfan areshown on 6-year time series plots in Figures 8a to 8i. Endosulfan readings from April toOctober are uniformly low (< 0.05 µg L-1). Pian Creek at Rossmore shows somewhat morevariability in off-season endosulfan concentrations, with a peak value of 0.12 µg L-1 on28/9/93 followed by 0.08 µg L-1 on 12/10/93. Because they occur near the end of the off-season, these are surprising results and may require an alternative explanation. Cooper (1996)has suggested that the onset of spring irrigation flows may remobilise contaminatedsediments, but sediment sampling in the Macquarie Valley has shown no evidence ofendosulfan residues at this time of year (Cooper 1996).

The Peel River stations at Paradise Weir and Bective Reserve show uniformly low endosulfanreadings (mostly zero) throughout the year because of their distance from cotton growingareas.

The magnitude of endosulfan peaks over time is clearly related to the area of cotton plantedupstream from the measuring site (Figure 4c), possibly scaled as a fraction of the totalupstream catchment area. Coxs Creek and Pian Creek show higher peaks compared to NamoiRiver stations. This may be due to the combination of higher cotton-to-catchment-area ratios(a waterborne transport factor) and closer proximity to cotton farms (an airborne transportfactor). The Coxs Creek dataset is biased toward sampling of storm events, which may resultin higher peaks than otherwise, but Coxs Creek also drains a large area of dryland cotton forwhich there is little on-farm storage of contaminated runoff. Narrabri Creek at Narrabri isnotable for relatively low endosulfan levels. As shown in Figure 4c, there is relatively littlearea planted to cotton upstream. Narrabri Creek may also experience greater clean runofffrom enhanced precipitation in the upland areas of the Nandewar Range (Figure 4a).

We now use the data in Figure 8 to examine gross relationships between (a) endosulfanconcentration and discharge, and (b) concentration and antecedent rainfall.

Namoi River at Bugilbone

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Jun 91 Dec 91 Jun 92 Dec 92 Jun 93 Dec 93 Jun 94 Dec 94 Jun 95 Dec 95 Jun 96 Dec 96 Jun 97

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

AlphaBetaSulfateTotal

a

Figure 8: Complete time series of endosulfan measurements by species for 9 stations in the Namoi Valley.Plots (a-i) are in order of increasing distance up-valley. Note the varying scale of concentration axes.

22

Figure 8 continued.

Pian Creek at Rossmore

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


b

Namoi River D/S Weeta Weir

0

0.02

0.04

0.06

0.08

0.1

0.12


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


c

Gunidgera Creek D/S Regulator

0

0.02

0.04

0.06

0.08

0.1

0.12


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


d

23

Figure 8 continued.

Narrabri Creek at Narrabri

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


e

Coxs Creek at Boggabri (Warragrah)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


f

Namoi River at Gunnedah

0

0.02

0.04

0.06

0.08

0.1

0.12


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


g

24

Figure 8 continued.

Concentration and Discharge: In Figure 9, in-season total endosulfan concentrations areplotted against coincidentally measured river discharges (on a log axis) for the seven stationsfurthest down the valley (excluding the Peel River stations). All plots show considerablescatter with no strong evidence of a positive relationship. At best it can be said that there aregenerally more points in the upper right quadrant of the plots than the upper left, but there arealso many occurrences of relatively high discharges with relatively little endosulfan present(for instance Namoi at Gunnedah). For Coxs Creek at Boggabri the plot includes endosulfanmeasurements during a number of storm events. Only the peak endosulfan per day and thecorresponding discharge measurement are plotted here. A handful of endosulfan concentrationmeasurements were recorded at several sites during periods of zero discharge (not plotted).They were generally low and do not affect the overall picture. The highest values during zerodischarge were 0.09, 0.10, and 0.08 µg L-1, recorded at Coxs Creek at Boggabri in successiveweeks during January 1997.

Peel River at Paradise Weir

0

0.002

0.004

0.006

0.008

0.01

0.012


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


h

Peel River at Bective Reserve

0

0.002

0.004

0.006

0.008

0.01

0.012


En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)


i

25

.

Namoi River D/S Weeta Weir

0

0.02

0.04

0.06

0.08

0.1

0.12

0.01 0.1 1 10 100 1000 10000 100000

Discharge (megalitres/day)

To

talE

nd

osu

lfan

(mg

l-1)

c

Namoi River at Bugilbone

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.01 0.1 1 10 100 1000 10000 100000


To

talE

nd

osu

lfan

(mg

l-1)

a Pian Creek at Rossmore

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0.01 0.1 1 10 100 1000 10000


To

talE

nd

osu

lfan

(mg

l-1)

b

Gunidgera Creek D/S Regulator

0

0.02

0.04

0.06

0.08

0.1

0.12

0.01 0.1 1 10 100 1000 10000


To

talE

nd

osu

lfan

(mg

l-1)

d

Narrabri Creek at Narrabri

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.01 0.1 1 10 100 1000 10000 100000


To

talE

nd

osu

lfan

(mg

l-1)

e

Namoi River at Gunnedah

0

0.02

0.04

0.06

0.08

0.1

0.12

0.01 0.1 1 10 100 1000 10000


To

talE

nd

osu

lfan

(mg

l-1)

g

Coxs Creek at Boggabri

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.01 0.1 1 10 100 1000 10000 100000


To

talE

nd

osu

lfan

(mg

l-1)

f

◆ Routine

▲ Storm

Figure 9: In-season (Nov-Mar) total endosulfan concentration versus coincident log-discharge for 7stations in the Namoi Valley (Peel River stations excluded). Plots (a-g) are in order of increasingdistance up-valley. Note the varying axes scales. Storm events for Coxs Creek are the peak endosulfanmeasurement for the day. Erratum: Y-axis concentrations should be µg l-1, not mg l-1

26

Concentration and Antecedent Rainfall: In Figure 10a-g plots of in-season total andsulphate endosulfan concentration versus accumulated antecedent rainfall are shown for theseven down-valley water quality stations (excluding the Peel river stations). For each station,pairs of endosulfan measurements are plotted against accumulated local rainfall (the mean ofthe nearest three rainfall stations) during the previous 1, 3, 7, 10, 20 and 30 days, where 1refers to the rainfall on the same day as the measurement.

Short period antecedent rainfall totals (1, 3 days) are clearly uncorrelated with endosulfanconcentration at all sites. A poor correlation with one-day rainfall is not surprising in a systemwhere lags are expected and threshold rain events leading to water storage overflow mayrequire several days of rain. However, the three-day result is a little more surprising. Toexplore this, the highest 120 values for the local 3-day antecedent rainfall (in-season) wereidentified for the water quality stations at Pian Creek at Rossmore and Namoi River atBugilbone. These 120 values arose from 25 discrete rain events (in which rainfall occurred onconsecutive days). Of these 25 rain events, less than a quarter had endosulfan concentrationmeasurements taken on days associated with peak 3-day antecedent rainfall. This illustratesthe difficulty of finding correlations based on phenomena with short time scales whenprocesses are sampled infrequently compared with those time scales.

Extending the antecedent rainfall period provides more variation, but no evidence of a robustrainfall-endosulfan relationship. Although some of the plots for 10, 20, or 30 day antecedentrainfall might suggest a relationship on cursory visual inspection, this impression relies on twoor three sulphate/total endosulfan pairs per plot which represent the same two or three basin-wide events at each measuring station. Rank correlation coefficients (Helsel and Hirsch 1992)were generated for a number of these plots. A few statistically significant correlationsemerged, but there was very little spatial or temporal consistency.

In summary, no useful correlations can be found in these data between endosulfanconcentration and either discharge or local antecedent rainfall over any period from 1 to 30days.

27

1-day

0 10 20 30 40 50 600.0

0.1

0.2

0.3

0.4

a) Namoi River at Bugilbone

7-day

0 25 50 75 100 125 150 175 200

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.0

0.1

0.2

0.3

0.4

10-day

0 25 50 75 100 125 150 175 200

20-day

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

30-day

Local Antecedent Rainfall (mm)

0 50 100 150 200 250 300

3-day

0 20 40 60 80 100 120

Total EndosulfanEndosulfan Sulphate

Figure 10a: Namoi River at Bugilbone. Total endosulfan and endosulfan sulphate versus antecedentrainfall for periods of 1, 3, 7, 10, 20, and 30 days prior to each endosulfan measurement. Each pairof total and sulphate measurements are aligned on the rainfall axis. Where they overlap, the samplewas all sulphate. The scale of the rainfall axis increases with increasing length of the antecedentperiod. The scale of the endosulfan axis varies between stations (a-g).

28

1-day

0 5 10 15 20 250.0

0.5

1.0

1.5

b) Pian Creek at Rossmore

7-day

0 25 50 75 100 125 150 175 200

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.0

0.5

1.0

1.5 10-day

0 25 50 75 100 125 150 175 200

20-day

0 50 100 150 200 2500.0

0.5

1.0

1.5 30-day


0 50 100 150 200 250

3-day

0 20 40 60 80 100 120


1-day

0 5 10 15 20 25 30 350.00

0.02

0.04

0.06

0.08

0.10

c) Namoi River D/S Weeta Weir

7-day

0 25 50 75 100 125 150 175 200

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.00

0.02

0.04

0.06

0.08

0.10 10-day

0 25 50 75 100 125 150 175 200

20-day

0 25 50 75 100 125 150 175 2000.00

0.02

0.04

0.06

0.08

0.10 30-day


0 50 100 150 200 250

3-day

0 25 50 75 100 125 150


Figure 10b and c: As above,for Pian Creek at Rossmore andNamoi River D/S Weeta Weir.

29

1-day

0 5 10 15 20 25 30 350.00

0.02

0.04

0.06

0.08

0.10

d) Gunidgera Creek D/S Regulator

7-day

0 25 50 75 100 125 150 175

En

do

sulf

anC

on

cen

trat

ion

(µg

l-1)

0.00

0.02

0.04

0.06

0.08

0.10 10-day

0 25 50 75 100 125 150 175

20-day

0 25 50 75 100 125 150 175 2000.00

0.02

0.04

0.06

0.08

0.10 30-day


0 50 100 150 200 250

3-day

0 25 50 75 100 125 150


1-day

0 5 10 15 200.00

0.02

0.04

0.06

e) Narrabri Creek at Narrabri

7-day

0 50 100 150 200

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.00

0.02

0.04

0.06 10-day

0 50 100 150 200 250

20-day

0 50 100 150 200 2500.00

0.02

0.04

0.06 30-day


0 50 100 150 200 250 300 350

3-day

0 20 40 60 80


Figure 10d and e: As above,for Gunidgera Creek D/SRegulator and Narrabri Creekat Narrabri.

30

1-day

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

f) Coxs Creek at Boggabri

7-day

0 20 40 60 80 100

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.0

0.2

0.4

0.6

0.8

1.0

10-day

0 20 40 60 80 100

20-day

0 25 50 75 100 125 150 175 2000.0

0.2

0.4

0.6

0.8

1.0

30-day


0 25 50 75 100 125 150 175 200

3-day

0 20 40 60 80 100


1-day

0 5 10 15 20 250.00

0.02

0.04

0.06

0.08

0.10

g) Namoi River at Gunnedah

7-day

0 20 40 60 80 100

En

do

sulf

anC

on

cen

trat

ion

( µg

l-1)

0.00

0.02

0.04

0.06

0.08

0.10 10-day

0 20 40 60 80 100 120

20-day

0 25 50 75 100 125 1500.00

0.02

0.04

0.06

0.08

0.10 30-day


0 25 50 75 100 125 150 175 200

3-day

0 10 20 30 40 50 60


Figure 10f and g: As above, forCoxs Creek at Boggabri andNamoi River at Gunnedah.. Forsub-diurnal storm event data,only the peak endosulfanmeasurement per day is plotted.

31

4.3 Temporal Relationships Between Concentration Peaks and Antecedent Rainfall

We next examine, on an event-by-event basis, the temporal relationship betweenconcentration peaks (as far as they can be discerned from weekly sampling) and the dailyrecords of antecedent rainfall. This is motivated by the obvious fact that endosulfan peakscaused by runoff occur after rain (property 4 of the concentration signatures in Section 2.2).

For this analysis we used the water quality stations with the longest data records: Namoi Riverat Bugilbone and Pian Creek at Rossmore. For the in-season (November to March) periods,we identified each occurrence of a local temporal peak in total endosulfan concentration,defined as an occasion when Cw(ti) > max[Cw(ti+1), Cw(t i-1)] (where ti-1, ti and ti+1 aresuccessive water quality sampling times). In Figure 11, the local daily rainfall record aroundeach concentration peak is plotted for the period from ti-1 to ti+1 (that is, from the measurementbefore the peak time until the measurement after the peak), with the origin set at the observedpeak time (ti) in most cases and at the midpoint between identical measurements when a peakwas sustained over two observations. Typically the plots cover a period from 6 to 14 dayseither side of this peak time. For both the Namoi River at Bugilbone (Figure 11a) and PianCreek at Rossmore (Figure 11b), these plots show that on average, there is a considerablyhigher probability of rain activity before peaks in endosulfan concentration than after peaks.

If there was no relationship between rainfall and endosulfan concentration, the expecteddistribution of rain events would be relatively even on either side. Therefore Figure 11demonstrates that at least some endosulfan peaks are temporally correlated with antecedentrainfall, and hence with the runoff pathway.

The plots do not show a consistent time lag between the peak rainfall event and the peakendosulfan measurement, which is to be expected for two reasons: runoff from rainfall eventshas a variable travel time to the river depending on rainfall intensity, rainfall distribution andantecedent soil moisture; and with a (typically) weekly sampling interval for endosulfanconcentrations, the timing of concentration peaks is very poorly resolved.

The antecedent rain events have a variety of magnitudes, some being characterised by intenserain over a one to three day period. The dates of eight of the endosulfan peaks are shared byboth stations, as noted in the Figure 11 legend, demonstrating that peak endosulfanconcentrations can occur simultaneously over significant distances.

32

For comparison, plots of the rainfall before and after in-season local troughs in endosulfanconcentration (occasions when Cw(ti) < min[Cw(ti+1), Cw(t i-1)]) are shown in Figures 12a and12b. There are fewer minima plotted because some of the downward trends extend into theoff-season and are not included. The endosulfan trough plots for both stations show less rainactivity overall than the peak plots, and a tendency for concentration troughs to occur beforesubstantial rain events. This relationship is a reciprocal result to the relationship in Figure 11between concentration peaks and antecedent rain. Figure 12 is a reciprocal result to Figure 11:if rain tends to precede concentration peaks, it is also likely to follow concentration troughs.Figure 12 implies no causality (that is, concentration troughs do not bring rain), in contrastwith Figure 11 which we interpret as suggesting a causal relationship between at least someconcentration peaks and antecedent rainfall.

Figure 11: Local daily rainfall before and after endosulfan peaks during the growing season (Nov-Mar) for (a) Namoi at Bugilbone and (b) Pian Creek at Rossmore. Each line shows the daily rainfallrecord from one measurement before to one measurement after the occurrence of a local maximumendosulfan reading. The time between endosulfan measurements was generally 7-14 days. Legendentries are in chronological order of the date of the peak. Boldfaced dates are peak events occurringat both stations. Italicised dates are the midpoint between peaks sustained for two measurements.Plot colours do not correspond between the two graphs.

Days Before or After Endosulfan Maximum

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Loca

lDai

lyR

ainf

all(

mm

)

0

20

40

60

80

100

12017-Dec-917-Jan-9217-Feb-923-Nov-9224-Nov-9215-Dec-925-Jan-9325-Jan-9314-Dec-935-Jan-9416-Feb-9430-Jan-9514-Mar-9519-Dec-9515-Jan-9612-Feb-9630-Dec-9611-Feb-97



Days Before or After Endosulfan Maximum

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Loca

lDai

lyR

ainf

all(

mm

)

0

20

40

60

80

100

120 19-Nov-913-Dec-9117-Dec-917-Jan-923-Feb-9231-Mar-923-Nov-927-Dec-9222-Dec-925-Jan-9325-Jan-9314-Dec-938-Mar-9414-Nov-946-Feb-9519-Dec-959-Jan-9626-Jan-9612-Feb-9625-Nov-9623-Dec-964-Feb-97

33

4.4 Species mix

Figure 11 shows that some endosulfan peaks are preceded by large rainfall events, but this isnot always the case. Overlap obscures a number of plot lines in Figure 11 that show very littlerainfall activity prior to the endosulfan concentration maximum. While this figure shows thatrainfall is implicated in many endosulfan peaks, it does not provide any indication of thecauses of peaks occurring without significant antecedent rainfall.

A further indicator which can distinguish between airborne and waterborne transport is thefraction of endosulfan α and β in the total concentration:

f C C C C Cw w w w wαβα β α β= + + +c h c hf (4)

Figure 12: Local daily rainfall before and after endosulfan troughs during the growing season (Nov-Mar) for (a) Namoi at Bugilbone and (b) Pian Creek at Rossmore. Each line shows the daily rainfallrecord from one measurement before to one measurement after the occurrence of a local minimumendosulfan reading. The time between endosulfan measurements was generally 7-14 days. Legendentries are in chronological order of the date of the peak. Italicised dates are the midpoint betweentroughs sustained for two measurements. Plot colours do not correspond between the two graphs.

Days Before or After Endosulfan Minimum

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Loca

lDai

lyR

ainf

all(

mm

)

0

20

40

60

80

100

120

30-Dec-9128-Jan-9211-Nov-927-Dec-9222-Dec-9220-Jan-9320-Dec-935-Feb-9416-Jan-957-Mar-952-Jan-9630-Jan-9620-Jan-97


Days Before or After Endosulfan Minimum

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Loca

lDai

lyR

ainf

all(

mm

)

0

20

40

60

80

100

12025-Nov-919-Dec-9130-Dec-9113-Jan-9218-Mar-9211-Nov-9215-Dec-9229-Dec-9220-Jan-938-Nov-938-Feb-9412-Dec-942-Jan-9615-Jan-965-Feb-969-Dec-9624-Jan-97


34

In Section 2.2, it was argued that fαβ is close to 1 for endosulfan deposited in waterways byairborne transport (property 1 of the concentration signatures), but is much lower on averagefor waterborne endosulfan (property 2). In Section 2.3, we argued that fαβ is typically around0.2 in runoff water, though with significant variability. However, fαβ needs to be interpretedwith care, because of the long residence time of sulphate in a waterway relative to α and β(property 3 of the concentration signatures). Sulphate can be present in a water body as aresult of transport events occurring for a long time before measurement (up to months),whereas α and β are likely to be removed relatively quickly (within days).

In Figure 13, we plot the events represented by the endosulfan concentration peaks in Figure12, on a plane in which the vertical axis is the α+β fraction fαβ and the horizontal axis is ameasure of antecedent rainfall. In Figure 13a, this measure is the rainfall on the rainiest day; inFigure 13b, it is the average antecedent rainfall over the interval (typically 7 or 14 days)between the event time ti and the time of the previous measurement at time ti−1. The purposeof these plots is to show simultaneously those aspects of the concentration signaturerepresented by species mix (properties 1 and 2 from Section 2.2, whereby high fαβ isassociated with airborne transport) and those aspects represented by antecedent rainfall(property 4, whereby significant antecedent rainfall is a prerequisite for waterborne transport).Figures 13a and 13b both show that, for the data from Namoi River at Bugilbone and PianCreek at Rossmore:

• High values of fαβ (exceeding 0.2) in endosulfan peaks are always associated with dryantecedent conditions over the previous 7-14 days (less than 30 mm peak daily rainfall or5 mm average daily rainfall).

• Endosulfan peaks occurring after large storms or prolonged rain invariably have fαβ lessthan 0.2, and are hence at least 80% sulphate.

Figure 13 can therefore be divided into four regions: (a) high fαβ (> 0.2) and low antecedentrainfall over the previous 7-14 days (average daily rainfall < 5 mm or peak daily rainfall< 30 mm); (b) low fαβ and high antecedent rainfall; (c) low fαβ and low antecedent rainfall; and(d) high fαβ and high antecedent rainfall. These regions are indicated on Figure 13b. All thedata occur in regions (a), (b) and (c), with region (d) being vacant.

Concentration peaks lying in region (a) are likely to be due to airborne transport, both becauseof the high fαβ and the low antecedent rainfall which implies an absence of runoff. For pointson the left hand side of region (a) with zero or very low antecedent rainfall, waterbornetransport can be excluded with near-certainty. The presence of some sulphate in these peaks(which could not arrive by direct airborne transport) is understandable because of the longresidence time of sulphate in the river, leading to sulphate from “old” transport events beingpresent as a longer-term background.

Events in region (b) are likely to be associated with waterborne transport, because of the highantecedent rainfall. For those events in region (b) with very small or zero fαβ, airbornetransport can be excluded with near-certainty, provided that the oxidation of α and β tosulphate in waterways is a very slow process as supposed in Figure 1. If there is significantconversion of α and β to sulphate by this pathway, then sulphate in the river could be due toairborne transport followed by oxidation.

35

Events in region (c) are of indeterminate origin. These events do not show clear signatures oftransport through either pathway, and are probably associated with the long-term (seasonal)background of “old” sulphate with long residence time scales. Binding of endosulfan toparticles, leading to retention in sediments, is likely to contribute to this background.

The absence of points from region (d) supports the conclusion in Section 2.3 that theendosulfan carried by waterborne transport is mainly in sulphate form, with fαβ < 0.2, andreinforces the concept that transport events fall into two distinct populations represented byregions (a) and (b) in Figure 13, with region (c) being an overlap region.

It is noteworthy that Pian Creek at Rossmore has a higher proportion of events in region (a),which we associate with a high probability of airborne transport, than the Namoi River atBugilbone. This is likely to reflect a closer proximity of Rossmore station to cotton farms andconsequently a greater potential for airborne transport, particularly by spray drift.

Maximum Local Daily RainfallPrior to Endosulfan Peak (mm)

0 20 40 60 80 100 120

α+β

as%

ofT

otal

End

osul

fan

0

20

40

60

80

100

Average Local Daily RainfallPrior to Endosulfan Peak (mm)

0 5 10 15 20 25 30

α+β

as%

ofT

otal

End

osul

fan

0

20

40

60

80

100

Namoi River at BugilbonePian Creek at Rossmore

a

c b

d

a

b

Figure 13: For the peaks inFigure 11, α+β species as apercentage of the totalendosulfan are plotted againsttwo measures of antecedentrainfall: (a) the rainfall on therainiest day; (b) the averageantecedent rainfall over theinterval (typically 7 or 14 days).In (b), regions suggestingprobable transport mechanismsare shown in grey: (a) mostlikely airborne transport; (b)most likely waterbornetransport; (c) indeterminate;(d) rare due to dominance ofsulphate in waterbornetransport.

36

Figure 13 enables a statement to be made about the likely pathway leading to a riverineendosulfan peak, but it does not indicate the magnitude of the concentration and therefore thesignificance of the pathway. Therefore, we replot Figures 13a and 13b in three dimensionsusing total endosulfan concentration as the z axis, producing Figure 14. Here, Figures 13a and13b have become the “floors” of Figures 14a and 14b, with total endosulfan concentrationbeing represented by the height of each point above the floor.

Figure 14 shows that there is a broad similarity in the magnitude of the largest endosulfanpeaks in region (a) (probably airborne) and region (b) (probably waterborne). Peaks in region(c) tend to be smaller, as do peaks in region (b) with fαβ = 0.

0.0

0.5

1.0

1.5

020

4060

80100

120

020

4060

80

Tot

alE

ndos

ulfa

n(µ

gl-1

)

Peak Rainfall (mm)

% α+β Endosulfan

● Namoi at Bugilbone● Pian Creek at Rossmore

0.0

0.5

1.0

1.5

05

1015

2025

30

020

4060

80

Tot

alE

ndos

ulfa

n(µ

gl-1

)

Average Rainfall (mm)

% α+β Endosulfan


Figure 14a: For the peaks inFigure 11, α+β species as apercentage of total endosulfanplotted against rainfall on therainiest day in the antecedentinterval (typically 7 or 14 days).This is a replot of Figure 13awith the addition of a z-axisshowing the amount of totalendosulfan.

Figure 14b: For the peaks inFigure 11, α+β species as apercentage of total endosulfanplotted against the averagerainfall in the antecedentinterval (typically 7 or 14 days).This is a replot of Figure 13bwith the addition of a z-axisshowing the amount of totalendosulfan.

37

Figure 15 is a variation on Figure 14, with total endosulfan on the z axis given as a waterbornemass flux of endosulfan (in kg-endosulfan s-1 or grams of endosulfan per day, equal to theproduct of concentration and discharge). While the concentrations in Figure 14 are broadlysimilar between regions (a) and (b), Figure 15 shows that three events in region (b) involve thetransport of large volumes of endosulfan. These are events where a relatively highconcentration of endosulfan was recorded during periods of high river flow. For these threeevents, waterborne transport is almost certainly the dominant pathway from farm to river.Although the mass fluxes are expressed in grams per day, it should be noted that they areinstantaneous fluxes at the time of measurement.

0

1000

2000

3000

4000

5000

020

4060

80100

120

020

4060

80

Tot

alE

ndos

ulfa

nF

lux

(gd-1

)

Peak Rainfall (mm)

% α+β Endosulfan

0

1000

2000

3000

4000

5000

05

1015

2025

30

020

4060

80

Tot

alE

ndos

ulfa

nF

lux

(gd-1

)


% α+β Endosulfan


Figure 15b: For the peaks inFigure 11, α+β species as apercentage of total endosulfanplotted against the averagerainfall in the antecedentinterval (typically 7 or 14 days).This is a replot of Figure 14busing the mass flux of totalendosulfan as the z-axis.

Figure 15a: For the peaks inFigure 11, α+β species as apercentage of total endosulfanplotted against rainfall on therainiest day in the antecedentinterval (typically 7 or 14 days).This is a replot of Figure 14ausing the mass flux of totalendosulfan as the z-axis..

38

We conclude this section by mentioning three caveats. First, this analysis has been carried outfor just two Namoi Valley water quality stations with long endosulfan concentration records(over 5 years). We have extended the analysis to other Namoi Valley water quality stations(Table 1) producing the results in Figures 16, 17, and 18. These are broadly consistent withthe conclusions above, although three points now fall in region (d). These turn out to be frompeaks recorded at three different water quality stations on the same day (9/12/96) associatedwith a large basin-wide rain event. We interpret these points as a caution that the conclusionsabove are probabilistic rather than absolute.

Second, this analysis has been restricted to endosulfan peaks. This has been done to focus onidentifiable transport events causing a rise in riverine concentrations. The effect of includingall concentration data (not only peaks) is to introduce many more points into region (c) andinto region (b) close to the fαβ = 0 axis, as well as somewhat more scatter into the overallpicture. Because the additional points are not peak events, they have generally low totalconcentrations (that is, they are close to the floor in Figures 14 or 17) and do not alter thegeneral conclusions about the relative magnitudes of airborne and waterborne transport.

Third, we have not included endosulfan data in storm event sampling in this analysis, becausethese data are deliberately biased to select waterborne transport events. Restriction to theroutine monitoring data is intended to produce an unbiased sample of airborne and waterbornetransport events.

39

Maximum Local Daily RainfallPrior to Endosulfan Peak (mm)

0 20 40 60 80 100 120

α+β

as%

ofT

otal

End

osul

fan

0

20

40

60

80

100

Average Local Daily RainfallPrior to Endosulfan Peak (mm)

0 5 10 15 20 25 30

α+β

as%

ofT

otal

End

osul

fan

0

20

40

60

80

100

Namoi River at BugilbonePian Creek at RossmoreNamoi River D/S Weeta WeirGunidgera Creek D/S RegulatorNarrabri Creek at NarrabriCoxs Creek at BoggabriNamoi River at Gunnedah

Figure 16: α+β species as a percentage of the total endosulfan for 7 down-valley stations plottedagainst two measures of antecedent rainfall: (a) the rainfall on the rainiest day; (b) the averageantecedent rainfall over the interval (typically 7 or 14 days). The three boxed outliers weremeasured at different stations on the same day (9/12/96).

40

0.0

0.5

1.0

1.5

020

4060

80100

120

020

4060

80

Tot

alE

ndos

ulfa

n(µ

gl-1

)

Peak Rainfall (mm)

% α+β Endosulfan

0.0

0.5

1.0

1.5

05

1015

2025

30

020

4060

80

Tot

alE

ndos

ulfa

n(µ

gl-1

)


% α+β Endosulfan

● Namoi at Bugilbone● Pian Creek at Rossmore● Namoi River D/S Weeta Weir● Gunidgera Creek D/S Regulator● Narrabri Creek at Narrabri● Coxs Creek at Boggabri● Namoi River at Gunnedah

Figure 17b: α+β species as apercentage of total endosulfan plottedagainst average rainfall in theantecedent interval (typically 7 or 14days). This is a replot of Figure 16bwith the addition of a z-axis showingthe amount of total endosulfan.

Figure 17a: α+β species as apercentage of total endosulfan plottedagainst rainfall on the rainiest day inthe antecedent interval (typically 7 or14 days). This is a replot of Figure16a with the addition of a z-axisshowing the amount of totalendosulfan.

41

0

1000

2000

3000

4000

5000

020

4060

80100

120

020

4060

80

Tot

alE

ndos

ulfa

nF

lux

(gd-1

)

Peak Rainfall (mm)

% α+β Endosulfan

0

1000

2000

3000

4000

5000

05

1015

2025

30

020

4060

80

Tot

alE

ndos

ulfa

nF

lux

(gd-1

)


% α+β Endosulfan

● Namoi at Bugilbone● Pian Creek at Rossmore● Namoi River D/S Weeta Weir● Gunidgera Creek D/S Regulator● Narrabri Creek at Narrabri● Coxs Creek at Boggabri● Namoi River at Gunnedah

Figure 18a: α+β species as apercentage of total endosulfanplotted against rainfall on therainiest day in the antecedentinterval (typically 7 or 14 days).This is a replot of Figure 17ausing the mass flux of totalendosulfan as the z-axis.

Figure 18b: α+β species as apercentage of total endosulfanplotted against average rainfallin the antecedent interval(typically 7 or 14 days). This is areplot of Figure 17b using themass flux of total endosulfan asthe z-axis.

42

4.5 Storm Event Sampling

Endosulfan concentrations and river discharges have been measured as part of theCNWRWQP at daily or sub-diurnal intervals during a number of storms in the Namoi andGwydir valleys. Time series plots from these storms, already presented in Figure 2, are nowdiscussed in a little more detail. As pointed out by Cooper (1996), vastly different endosulfanresponses can accompany local storm events, implying different transport mechanisms anddynamics both spatially and temporally. For Coxs Creek at Boggabri (Figure 2a), rising levelsof endosulfan with high α+β fractions following the flood peak on 9/12/93 and prior to theevent on 26/1/96 suggest that airborne transport may be involved. The falling limb of thehydrograph coincides with falling concentrations during the events at Coxs Creek at Boggabrion 21/1/95 and at Mooki River at Ruvigne (Figure 2b) on 3/1/96 to 5/1/96, both of whichinvolve small amounts of endosulfan, most or all of which is sulphate. Mobilisation ofcontaminated sediment may be involved in these cases. A similar pattern with larger fractionsof α+β is observed during the 24/1/95 Mooki River event (Figure 2b), while the 4/1/96 CoxsCreek event (Figure 2a) involves large total endosulfan amounts and a sudden drop and thenrise with large α+β fractions throughout. In contrast the falling limb of the Namoi River atTarriaro event (Figure 2c) coincides with a sudden and sustained rise in high sulphateendosulfan, perhaps indicating water storage overflow. These results emphasise thecomplicated nature of the endosulfan-hydrology relationship.

5. Concluding Summary

In this report we have shown that useful information about the influence of hydrology onriverine endosulfan and the importance of airborne and waterborne transport can be derivedfrom riverine concentration data, provided these are supplemented with local daily rainfallrecords. We have considered only the Namoi Valley, but the analysis could usefully be appliedto endosulfan measurements from other areas. We have used “concentration signatures”, ortypical patterns of concentration change following a transport event through specificpathways, to identify features of the concentration data which can be used as indicators oftransport pathways. The main results are:

1. Concentration signatures have been identified for the spray drift, vapour transport, dusttransport, and runoff pathways. The following six properties of these signatures have beenderived from physical reasoning, modelling and appeal to measurement: (1) airborne pathways(spray drift, vapour transport, dust transport) carry endosulfan α and β but practically nosulphate; (2) waterborne pathways can carry significant amounts of all three endosulfanspecies (α, β and sulphate), with a tendency for sulphate to predominate; (3) endosulfan α andβ remain in the river for only a few days, whereas sulphate remains in the river for substantialperiods (months); (4) endosulfan peaks caused by the runoff pathway occur after rainfall; (5)dust transport and subsurface leaching are negligible; and (6) spray drift and runoff are bothpotentially large contributors to riverine concentrations. Of these, properties 1 and 2 relate tospecies mix, 3 and 4 relate to timing, and 5 and 6 to relative magnitudes of pathways. Some ofthe justifications, particularly those relating to magnitude, rely on the determination of typicalconcentration signatures for each transport pathway using a scenario consisting of specifiedtypical conditions for a transport event.

43

2. Gross correlations between concentration and discharge or antecedent rainfall, basedon scatter plots, led to no relationships which could be used to attribute pathways to observedendosulfan concentrations.

3. The timing relationship between concentration peaks and rainfall was investigated byplotting the local daily rainfall record around a large number of concentration peaks. Thisshowed that there is a higher probability of rainfall before a peak than after, demonstrating acausal relationship between at least some concentration peaks and antecedent rainfall.

4. To incorporate species mix as an indicator of transport pathway, each peakconcentration observation was plotted on a plane in which the vertical axis is the fraction fαβ

of endosulfan α and β in the total concentration, and the horizontal axis is a measure ofantecedent rainfall. On this plane, observed concentration peaks fall into two distinct regions:(a) events with high fractions of endosulfan α and β and little antecedent rainfall, and (b)events with low α and β fractions and high antecedent rainfall. Using the characteristicproperties of the concentration signatures, events in region (a) are attributed primarily toairborne transport, and those in region (b) primarily to waterborne transport (noting that theseattributions are probabilistic rather than absolute). Some events occur in a third region (c) withlow α and β fractions and low antecedent rainfall, arising from the continued presence ofendosulfan sulphate with a long riverine residence time. Very few peak concentration eventsare observed in the fourth region (high α and β fractions and high antecedent rainfall).

5. Events in regions (a) and (b) are of significant magnitude in total concentration, with(a) being dominant at locations close to cotton farms. Events in region (c) have smaller totalconcentrations. The conclusion is that airborne transport pathways therefore dominateendosulfan movement close to cotton farms.

The attribution of transport pathways to specific events or endosulfan concentration peakswould be made substantially more precise if the concentration monitoring data weresupplemented with certain ancillary data, not available for this report. In decreasing order ofimportance they are the location and timing of aerial spraying; irrigation scheduling and on-farm water storage capacities; annual locations of cotton plantings; and river discharge time-series. Even single values integrated over the catchment area of each measuring station wouldgreatly assist the kind of analysis given here.

Finally, we note that the results presented here have significant implications for themanagement of endosulfan in the riverine environment. These will be discussed in Part II ofthis report.

Acknowledgments: This work was funded by the Land and Water Resources Research andDevelopment Corporation (LWRRDC), the Cotton Research and Development Corporation(CRDC) and the Murray-Darling Basin Commission (MDBC) as part of the “Minimising theImpact of Pesticides on the Riverine Environment” Research Program. The support of thesebodies is gratefully acknowledged. We are also appreciative of discussions with colleagues inthe Program, particularly Vic Edge, Ivan Kennedy, John Leys and Bruce Peasley.

44

6. References

Ahmad, N., Edge, V. and Rohas, P. (1995). Aerial transport of endosulfan. Proc. Workshop onPesticides and the Riverine Environment, Sydney, August 1995 (Land and Water ResourcesResearch and Development Corporation).

Brooks, A. (1998). The Effects of Endosulfan on Aquatic Macroinvertebrate Communities inArtificial Ponds. Report to LWRRDC, Project NDW10, Aquatic Ecosystem Section, WaterQuality Services, NSW Department of Land and Water Conservation.

Cogley, J.G. and Briggs, P.R. (1995). Global Precip Climatology & Topography, 1-Degree.Ancillary dataset DS768.0, Data Support Section, Scientific Computing Division, NationalCenter for Atmospheric Research, Boulder, CO.

Cooper, B. (1996). Central and north west regions water quality program: 1995/96 report onpesticides monitoring. NSW Department of Land and Water Conservation, Technical ServicesDirectorate, Parramatta, NSW.

Helsel, D.R. and Hirsch, R.M. (1992). Statistical Methods in Water Resources. Elsevier, NewYork.

Hoechst (1993). HOE 002671, Endosulfan - volatility and photochemical oxidativedegradation in the atmosphere. Document A50940.

Howard, P.H. (1991). Endosulfan, in Handbook of Environmental Fate and Exposure Data forOrganic Chemicals, Vol. 3, P.H. Howard (ed). Lewis Publ., Chelsea, Michigan.

Leys, J.F., Larney, F.J., Müeller, J.F., Raupach, M.R., McTainsh, G.H., and Lynch, A.W.(1998). Anthropogenic dust and endosulfan emission on a cotton farm in northern New SouthWales, Australia. Submitted to Science of the Total Environment.

National Research Council Canada (1975). Endosulfan: Its Effects on Environmental Quality.Report 11, NRC Associate Committee on Scientific Criteria for Environmental Quality,Subcommittee on Pesticides and Related Compounds, Ottawa.

O'Brien, J.M. (1995). Central and North West Regions Water Quality Program. 1994/95Macquarie cotton industry benchmarking. NSW DLWC, Document TS 95.029.

O'Brien, J.M. (1996). Central and North West Regions Water Quality Program. 1995/96Namoi cotton industry audit and review. NSW DLWC.

Peasley, B.A. (1996). Namoi River catchment area: irrigation and riparian vegetation,1:500,000 scale map, NSW DLWC.

Raupach, M.R. (1993a). Pesticide transport to a nature reserve. Technical Report No. 55,CSIRO Centre for Environmental Mechanics.

Raupach, M.R. (1993b). Pesticide transport to a nature reserve: second study. TechnicalReport No. 61, CSIRO Centre for Environmental Mechanics.

45

Raupach, M.R., Ford, P.W. and Briggs, P. (1996). Modelling the aerial transport of endosulfanto rivers. Part I: the vapour transport pathway. Technical Report No. 113, CSIRO Centre forEnvironmental Mechanics.

Raupach, M.R. and Briggs, P. (1996). Modelling the aerial transport of endosulfan to rivers.Part II: transport by multiple pathways. Technical Report No. 121, CSIRO Centre forEnvironmental Mechanics.

Wauchope, R. D., Buttler, T. M., Hornsby, A. G., Augustijn-Beckers, P. M., and Burt, J. P.(1992). The SCS/ARS/CES pesticide properties database for environmental decision making.Reviews of Environmental Contamination and Toxicology. 123:1-155.

integrative modelling of transport and fate of endosulfan ... · the present document reports part...

Documents