quantifying the redistribution of soils and sediments ... · quantifying the redistribution of...

CSI ROAUST RALIA

CSIRO LAND and WATER

Quantifying the redistribution of soils andsediments within a post-harvested forestcoupe near Bombala, New South Wales,

AustraliaEdited by Dr P.J. Wallbrink with contributions by (in alphabetical order)

Dr J.M. Olley, Mr B.P. Roddy, Dr P.J. Wallbrink

CSIRO Land and Water Technical report no. 7/97, August 1997

Quantifying the redistribution of soils and

sediments within a post-harvested forest

coupe near Bombala, New South Wales,

Australia

Research report produced by CSIRO, Land and Water,

on behalf of the NSW Environment Protection Authority

Keywords: soil erosion, sediments, forest harvesting, sediment budgets, tracers, runoff manage-

ment, fallout radionuclides, water quality, Caesium-137, Lead-210, filter strips, log landings, snig

tracks, erosion mitigation, hardwood forests

Wallbrink, P.J., Roddy, B.P. and Olley, J.M., 1997. Quantifying the redistribution of soils and

sediments within a post-harvested forest coupe near Bombala, New South Wales, Australia,

CSIRO Land and Water Technical Report series 7/97, August 1997.

Authors contact address: CSIRO Land and Water, P.O. Box 1666, ACT 2601, Australia

Quantifying the redistribution of soil and sediment within a post-harvest coupe - Page 2

Executive summary

This study measures soil erosion and redistribution after timber harvesting, using two radionuclide

tracers. This was undertaken in a small drainage basin within compartment 1708, Bondi State

Forest, near Bombala, SE NSW. The study area was divided into several distinct elements; log

landings, snig tacks (and access roads), general harvest area (GHA), cross banks (and windrows),

and the filter strip of native vegetation left adjacent to the major streamline. The two tracers used

for this study were 137Cs and 210Pbex. The results from these were integrated into budgets describing

the movment of soil within and between the various landscape elements.

The 137Cs budget showed that no net loss of soil material had occurred from within the study area,

within uncertainties, 109 + 14 %. Conversely, the 210Pbex budget showed a total retention of 78 +

12 %. The deficit of 210Pbex compared to that of 137Cs was best explained by a combination of

analytical and sampling uncertainties, losses of 210Pbex associated with combustion and/or transport

of litter and organic mater from the site, and some small loss of surface soil (to a depth of 2 mm).

However, no evidence of surface derived topsoil material was found in sediments currently being

transported from the site.

Both tracer budgets revealed a net loss of soil from the snig tracks and log landings. This was

quantified to be 28 + 13 mm and 48 + 29 mm from these land forms respectively. This is equiva-

lent to annual losses of 70 + 33 t ha-1 yr-1 for the snig tracks and 120 + 70 t ha-1 yr-1 for the log

landings, given the ~six year gap between harvesting and the time of measurement. Up to 20 + 10

% of this loss could be directly attributable to the creation of the cross banks by bulldozer blading.

The greater loss of material from the log landings is not surprising given the greater pressure from

vehicular traffic and machinery in these locations. Material eroded from the log landings was

accounted for within the cross banks, GHA, and buffer strip. Similarly the soil material eroded

from the snig tracks was accounted for within the cross banks, GHA, and the buffer strip. The

GHA was found to be a net trap of soil and sediment from the 137Cs budget, although the 210Pbexbudget suggests that this may be offset by a loss of organics and litter.

The filter strip had a 60 % increase in 137Cs areal concentrations and negligible increase in 210Pex.

The increase in 137Cs may suggest a preferential trapping of fine grained material in preference to

coarser grains. Presumably the coarser material being deposited upslope of the filter strip. The

fine grained material may have been sourced from either the snig tracks, log landings or the GHA.

The total activity of 137Cs estimated to be removed from the snig tracks was about 5.9 MBq. Of

this amount ~20 + 10 % was incorporated within the cross banks, 32 + 4 % within the filter strip,

and the remainder 48 + 7 % within the GHA. This suggests that the current system of spreading

flow from high compaction areas (such as the snig tracks and log landings) onto the low compac-

tion area of the GHA, was effective at retaining soil and sediment within the study area. In this

regard however it is clear that careful placement of the cross banks and the maintainenance of high

soil infiltrability and surface roughness within the GHA and filter strip areas remains critical.


Acknowledgments

This report was funded by NWSEPA. Mr Matthew Rake of CSIRO Land and Water assisted the

authors in the collection of field samples. Initial development of the research was assisted by Dr

Andrew Murray, of the Riso laboratory Denmark. The research was conducted in Compartment

1708 of the Bondi State Forest under Special Purposes Permit for Research no 04991 authorised by

State Forests of NSW Research Division. The authors are particularly grateful to Mr Peter Fogarty,

DCLM and Dr Peter Hairsine, CRCCH, for their constructive reviews of this report.


Table of Contents

225.5 Calculation of total activity of 137Cs and 210Pbex in the study area . . . . . . . . . . . . . . . . . . .

215.4 Areal tracer concentrations of the different landscape elements. . . . . . . . . . . . . . . . . . . .

205.3 Depth distributions of 137Cs and 210Pbex from profiles 1 and 2 in the reference area . . . .

195.2 Surface areas of the different landscape elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185.1 Particle size separation experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185. RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

174.3.2 Gamma Spectrometry methods for analysis of low level radioactivity . . . . . . . . . . . .

164.3.1 Preparation of soil samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

164.3 Laboratory techniques for radionuclide analysis of soil samples . . . . . . . . . . . . . . . . . . . .

164.2.5 Sampling and processing of soil material for particle size experiment . . . . . . . . . . .

164.2.4 Radioactivity in the filter strip resulting from natural erosion . . . . . . . . . . . . . . . . .

154.2.3 Determining reference area activities; soil depth distributions of 137Cs & 210Pbex .

154.2.2 Radioactivity of 137Cs and 210Pbex within the landscape elements of the study area .

144.2.1 Calculating the surface area of the different landscape elements . . . . . . . . . . . . . . . .

144.2 Experimental methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

144.1 Overall experimental design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

144. EXPERIMENTAL DESIGN AND RESEARCH METHODS . . . . . . . . . . . . . . . . . . . . . . . . .

133.2 Logging treatment for compartment 1708. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123.1 Physical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123. THE STUDY AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

112.4 General comments on the application of fallout radionuclide methods . . . . . . . . . . . . . .

112.3.2 Applications of Lead-210 to geomorphic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102.3.1 Production of Lead-210 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102.3 Fallout Lead-210 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102.2.4 Sediment budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92.2.3 Quantifying soil loss using depletion of 137Cs amount . . . . . . . . . . . . . . . . . . . . . . . . .

92.2.2 Applications of 137Cs to geomorphic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92.2.1 Production of Caesium-137 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92.2 Fallout Caesium-137 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92.1 Choice of study method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92. A REVIEW OF THE METHODS USED IN THIS STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71.2 This report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71.1 Preamble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. .71. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


. 40APPENDIX A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 359. BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

348.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

338.1 This report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

338. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

317. CALCULATING SOIL LOSS FROM THE SNIG TRACKS AND LOG LANDINGS . . . .

286.4 Formally accounting for the differences in retention of 137Cs and 210Pbex . . . . . . . . . . . . .

286.3 Differences in the retention of 137Cs and 210Pbex in the study site . . . . . . . . . . . . . . . . . . . .

276.2 Constructing soil and sediment budgets based on 210Pbex . . . . . . . . . . . . . . . . . . . . . . . . . .

256.1 Constructing soil and sediment budgets based on 137Cs . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

6. SEDIMENT BUDGETS DESCRIBING THE MOVEMENT OF SOIL AND

SEDIMENT WITHIN THE STUDY AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


1. INTRODUCTION

1.1 Preamble

The timber industry is an important sector of the Australian domestic economy. In 1993, it had a

turnover of approximately $10 billion dollars and employed ~60,000 people (ABARE, 1993). A

considerable amount of timber industry activities occur in regional Australia, making it an impor-

tant contributor to the economies of these areas (State Forests of NSW, 1994a). This is particularly

true of the Eden region on the south coast of NSW. Logging has occurred in this area since 1850.

In 1970, the Forestry Commission of NSW issued a license to Harris-Daishowa to export pulp

wood, and consequently a chip mill and loading facility were built near the township (Harris-

Daishowa, 1986). The timber for the chip mill is sourced from a geographic region known as the

Eden State Forests Region Management Area (EMA). This was established in 1975 for the better

management of timber resources in this region (Forestry Commission of NSW, 1975).

Timber is harvested from the EMA using a practice known as integrated logging. This allows the

harvesting of both sawlogs and pulpwood, where the harvesting of sawlogs alone would be insuffi-

cient to offset the infrastructure costs necessary for establishing a forestry enterprise.

Timber harvesting has the potential to increase soil erosion above natural levels. The potential

source areas for soil movement come from highly trafficked areas, such as snig tracks, log

landings, compartment roads and other disturbed surfaces. The rate of erosion from snig tracks in

the EMA has been measured at between 2.3 and 11.1 t ha-1 using a rainfall simulator emulating

storm based rainfall (Croke et al., 1997), whereas production from unsealed forest roads has been

measured at 50-90 t ha-1, from runoff trapping experiments (Grayson et al., 1993). This is similar

to northern hemisphere studies which have estimated rates of 1-5 m3 ha-1 yr-1 for abandoned roads

(Megahan and Kidd, 1972) and up to 100-150 m3 ha-1 yr-1 for roads during use (Reid and Dunne,

1984).

Soil erosion has implications for the long term sustainability of the industry. Natural rates of soil

generation in Australia are thought to be low, if not negligible (Edwards, 1987). Thus it is doubt-

ful whether any net loss of the soil (and nutrients) from forest systems can be tolerated above

background rates. Nonetheless, many off site impacts have been identified including; declining

water quality due to elevated sediment and nutrient levels, changes to light availability and

temperature, and adverse impacts on stream chemistry, biota and ecology (Campbell and Doeg,

1989).

There has also been a change in the social valuing of forest resources in terms of its important role

in water quality, wildlife habitats, recreation, and the tourist industry (Cocks et al, 1980; Cocks,

1992). Consequently, management of forests is now perceived as a major issue within many

sectors of the community.

1.2 This report

The primary aims of this report are to i) quantify the net soil loss that occurs from an enclosed

basin within a coupe after logging in a dry sclerophyll forest, ii) quantify loss from, and redistribu-

tion between, various landscape elements within the basin, and finally iii) investigate the efficacy


of cross banks and buffer strips in mitigating post harvest soil erosion and delivery of sediment to

channels.

Section 2 contains a review of the tracer methods used in this study. The study area is described in

Section 3. The experimental design and the field and laboratory methods are given in Section 4.

The results are presented in Section 5. A soil and sediment budget for the study area is constructed

in Section 6. Section 7 presents an analysis of soil loss measured from the snig tracks and log

landings of the study area, and Section 8 discusses the implications of this research and suggests

areas of future work.


2. A REVIEW OF THE METHODS USED IN THIS STUDY

2.1 Choice of study method

In this study the fallout nuclides 137Cs and 210Pbex are used to estimate soil erosion from, and

within, a logged coupe. The following sections briefly describe each of these nuclides, their use in

erosion studies, and as the basis for constructing soil and sediment budgets.

2.2 Fallout Cesium-137

2.2.1 Production of Cesium-137

Cesium is an alkali metal with a chemistry similar to that of sodium, potassium and other elements

of Group I in the Periodic Table. Caesium-137 has a half life of ~30 years and is found in the

environment as a consequence of above ground atmospheric nuclear detonations that occurred

between 1945 and 1974 (Wise, 1980; Longmore, 1982; Ritchie and McHenry, 1990). The nuclide

was injected into the stratosphere by the nuclear blast and eventually re-enters the troposphere

where it is returned to the earths surface, principally by wet precipitation (Environmental Radioac-

tive Special Topic Steering Committee, 1993). There is about an order of magnitude higher fallout

in the northern hemisphere, primarily because more detonations occurred in the northern

hemisphere (Ritchie and McHenry, 1990) and stratospheric exchange between the hemispheres is

generally weak. Overall fallout of this nuclide is strongly related to local patterns and rates of

precipitation (Davis, 1963; Longmore, 1982). Upon reaching the soil surface, 137Cs strongly binds

to soil particles, particularly the fine clay fraction (Lomenick and Tamura, 1965; Rogowski and

Tamura, 1965, 1970; Squire and Middleton, 1966; McHenry et al., 1973). 137Cs not only complexes

strongly with the soil particles, but appears to occur in an almost non exchangeable form.

Lomenick and Tamura (1965) reported that they removed less than 1 % of 137Cs from sediment by

a variety of salt, base, oxidizing, weak acid , and water solutions. In undisturbed soils 137Cs is

generally found with a maximum concentration slightly below the soil surface from where it tails

off to detection limits at below ~200-250 mm depth (Peart and Walling, 1986; Walling and

Woodward, 1992; Wallbrink and Murray, 1993; Zhang et al., 1994, Basher et al., 1995).

2.2.2 Applications of 137Cs to Geomorphic Studies

Measurements of 137Cs have been used to i) determine the age of sediment horizons (Day and

Campbell, 1986; Longmore, 1982), ii) quantify rates of sediment accumulation (eg. Robbins and

Eddington, 1975; Ritchie et al., 1975; Sharma et al., 1987; Walling and Bradley, 1987; Bishop et

al., 1991; Walling et al., 1992) iii) fingerprint sediments (Burch et al., 1988; Wallbrink and

Murray, 1993; Walling et al., 1993), and most importantly iv) investigate erosional and deposi-

tional processes (Longmore et al., 1983; 1986). An outline of its use to both directly quantify soil

loss and describe its redistribution are outlined below.

2.2.3 Quantifying soil loss using depletion of 137Cs amount

Most methods using 137Cs for measuring erosion and deposition, rely on establishing a

background or reference amount of 137Cs. The reference amount of 137Cs in the soil is derived

from fallout processes alone, with no loss or gain due to soil erosion or deposition. The reference

value itself is established by sampling within a stable area in which no erosion or deposition has


occurred (McCallan et al., 1980; Day and Campbell, 1986). In this way the residual soil store of137Cs after erosion, is compared to the amount expected to be present prior to disturbance, and thus

used to calibrate loss (Walling and Quine, 1990). All values below the reference level represent

net erosion, and areas above represent net deposition (Longmore et al., 1983).

The conversion of the differences (between the reference value and those measured elsewhere) into

quantitative amounts of soil loss can be undertaken by i) regression relationships derived from

erosion plot data (e.g., Campbell et al., 1986; Elliott et al., 1990; Sutherland, 1991), ii) various

theoretical models including the proportional model (e.g., Kachanoski and de Jong, 1984), grave-

metric method (e.g., Brown et al., 1981), mass balance models (e.g., Fredricks and Perrens, 1988;

Brunskill et al., 1984) and iii) profile distribution and/or direct calibration methods ( Zhang et al.,

1990; Gillieson et al., 1996). Most regression relationships and models are designed for use on

cultivated fields. The latter approach (iii) however is suitable for application to the study area. In

essence it is based on a detailed depth description of 137Cs in the undisturbed reference area. The

percentage of 137Cs contained within each soil layer (by depth), compared to the total inventory

amount, is then calculated. By comparing the percentage depletion of 137Cs in disturbed areas, with

the detailed depth profile, depth of soil loss can be determined.

2.2.4 Sediment Budgets

Sediment budgets based on tracers such as 137Cs can also provide a powerful framework for soil

erosion studies. For example, Ritchie et al. (1974) first constructed a coarse budget using 137Cs for

a forest, grass, and grass/crop watershed. Since then it has also been applied to a variety of other

geomorphic locations (Walling et al., 1986; Owens et al., 1997). In Australia, Loughran et al.

(1992) used the nuclide as the basis for developing a sediment budget within a small drainage

basin, including actively cultivated vineyard slopes. It has also been used in other Mediterranean

climates (see Quine et al., 1994). Soil and sediment budgets based on measurements of 137Cs also

require a reference value for 137Cs. The areas of the different landscape units within the study area

are also determined. The mean radionuclide activity of 137Cs (in Becquerels) of each landscape

element is then calculated by analysis of representative soil cores taken within them. These

amounts are then summed together to calculate the total amount of each tracer contained within the

study area. This value is then compared with the amount expected to be present in its prior undis-

turbed condition (from analysis of the reference area) to reveal any net loss from the system.

Differences in radionuclide amounts between the landscape elements can then also be used to

reveal the flow of material within and between them. A similar approach can be undertaken using

fallout 210Pb.

2.3 Fallout Lead-210

2.3.1 Production of Lead-210

Lead is located in the Group IVA of the Periodic Table. Of the four radioactive isotopes of Lead,210Pb is the only one to have a half life over 12 hours (Doe, 1969). 210Pb (half life 22 years) is part

of the 238U decay series and is present in all soils and rocks. It has two components, the first is

formed from decay of 226Ra through a number of short lived gases (the longest of which is 222Rn,

(Smith, (1982)) to form in-situ or supported 210Pb in the soil. The second component is formed

when some of the 222Rn escapes into the atmosphere where it also decays to 210Pb (Turekian et al.,

1977). This second component then returns to the soil surface by rain scavenging (Koide et al.,


1972; Smith, 1982) where it then attaches to soil and organic particles to form an excess or

unsupported amount. This second component (210Pbex) forms the basis of the measurements used in

this study.

Robbins and Edgington (1975) found that 210Pbex tends to remain in fixed association with the

solid phase after deposition. Unsupported 210Pbex also appears to be preferentially associated by

mass, with smaller size grains in soils. For example, in grain size distribution measurements at 3

depths Chanton et al. (1983), found that the specific activity of 210Pbex in clay was 3.2 times that of

silt and 24.7 times that of sand. A similar phenomenon was observed by Wallbrink (1996). In

undisturbed soils the concentration of 210Pbex also declines exponentially with depth.

2.3.2 Applications of Lead-210 to geomorphic studies

Because 210Pbex binds irreversibly with soil particles, is generally immobile in sediments, and has a

constant input, it is an ideal tracer for dating the age of sediments in lakes, estuaries and coastal

marine sediments (Koide et al., 1972; Robbins and Edgington, 1975; Goldberg et al., 1977). This

tracer is frequently used for dating sediment layers deposited up to around 100-150 years before

present (Oldfield and Appleby, 1984).

210Pbex can also be used to infer rates and patterns of sediment accumulation in a similar manner to137Cs, although most applications of this type are recent ( He and Walling, 1996). 210Pbex can also be

combined with 137Cs and 7Be, as well as other physical and chemical characteristics to fingerprint

the source of sediment in drainage systems (Wasson et al., 1987; Wallbrink and Murray, 1993;

Collins et al., 1997).

2.4 General comments on the application of fallout radionuclide methods

Many authors have reported the measurement of fallout radionuclides as useful tools to character-

ise geomorphic processes (e.g., Walling and Quine, 1990; Wise, 1980; Longmore et al., 1983;

McCallan et al., 1980). However, while these methods are widely applied, their application to

quantifying soil erosion rates is still complex. Quine (1995) states that there is no generally

accepted calibration procedure for converting 137Cs depletion amounts to net soil loss, due to

factors such as variations in 137Cs concentrations at the soil surface during atmospheric fallout,

seasonal variation of erosion and fallout, and variation in depth and extent of erosion. Because137Cs is preferentially associated with clay particulates, results may be in error where selective

sorting occurs in either deposition or erosion areas (Cooper et al., 1987; Govers et al., 1996). Any

calibration of control sites is generally only valid for areas studied at the same time and using

identical land management strategies (Quine, 1995).

Another factor that must be considered when using fallout tracers is that the initial distribution of

these nuclides may be non-uniform. Sutherland (1994) and Wallbrink et al. (1994) provide data

showing that the variability of 137Cs may be as large as 40 %. Because of this spatial heterogeneity,

more than one sample needs to be taken to ensure that a true estimate of the mean value is

obtained. Consequently, for the work presented in this report a minimum of 30 replicate soil cores

were taken from any given area in which an estimate of the mean value was required.


3. THE STUDY AREA

3.1 Physical characteristics

The study area is a small basin (approximately 12 ha) within compartment 1708 in the

Bondi State Forest, approximately 24 km south of Bombala in the south east of NSW (see

Figure 3.1). The area lies within the Bega Batholith, which consists of three major igneous

rock types; granite-ademellite, ademellite and quartz diorite-granodiorite (Reinson, 1976),

of which the granite-ademellite is predominant at the study site. The typical soil type in this

area is a light yellow coloured, uniformly coarse textured and weakly structured Orthic

Tenosol, (Uc 5.22, Northcote, 1971). The particle size distribution within these soils is 67

% sand, 16 % silt and 17 % clay by weight.

Figure 3.1. Site location map showing Compartment 1708, (as the thick solid line), and

the study area within it (as light hatched area).

The study site is 650 m above sea level and has a mean annual rainfall of ~1000 mm. The

rainfall patterns are highly variable both monthly and annualy. The lowest rainfall periods

tend to be from midwinter to early spring, and the highest rainfall periods from midsummer

to early winter (State Forests of NSW, 1994a). The dominant species of trees across the

study site is Silvertop Ash (Eucalyptus Sieberi) with associated species of Grey Gum (E.


Logged coupe

Compartment 1708

Studyarea

Referenceslope

Referencebuffer strip

Referenceprofile 1

Referenceprofile 2

0 1 km

BondiCreek

Way

Forest

Coolanguburra

37o06'

149o20'

New

South

Wales

Studyarea

cypellocarpa) and Brown Barrel (E. fastigata). Middle story regrowth vegetation consists

mainly of smaller eucalypts, acacias and Musk Daisy Bush (Olearia argophylla) (Croke et

al., 1997). An ephemeral creek approximately 300 m long forms the major drainage feature

of the study area. In its present form it is reasonably well incised, with vertical banks of

about one metre height at its headwaters. At the basin outlet the channel banks are about 4

m high, with inclination of ~450. There was a history of some prior grazing in this area

from surrounding pastoral landholders, and earlier selective timber extraction. A major

wildfire in this compartment was documented in the 1982/83 season. Wildfires are known

to occur in this region and are documented within the Eden Management Area EIS (Harris

Daishowa, 1986, State Forests NSW, 1994a).

3.2 Logging treatment for compartment 1708

Logging commenced in compartment 1708 in May 1990 and was completed by October

1990. The trees within this compartment were cut by chainsaw and the branches and

crowns left for later burning. The logs were dragged (or snigged) by bulldozers along snig

tracks to the log dumps where they were stripped of bark, and loaded onto semi trailers for

transport out of the coupe via the compartment road (State Forests of NSW, 1994a). The

compartment, and our study area within it, were harvested under the Standard Erosion

Mitigation Guidelines (SEMGL) which, among other conditions, prescribed that all catch-

ments greater than 40 ha are filtered by a zone of vegetation of 20 m (State Forests of NSW,

1994a). Trees were harvested from this area, but only if they could be felled and snigged

without machinery entering the exclusion zone.

After harvesting, the remaining crowns, leaves, and slash were burnt to reduce the fuel load

available for wildfire. In this way, it is aimed that logging slash is reduced by about 35 %

and fine fuel by about 80 % (State Forests of NSW, 1994c). Burning also facilitates regen-

eration of trees by releasing nutrients and initiating seed release. Some of the log dumps in

Compartment 1708, and the study area, were planted with Eucalyptus nitens, to promote

revegetation of these areas. The log dumps were also ripped to a depth of about 40 cm after

harvesting to facilitate regeneration of native tree species.

Post harvesting treatments to reduce sediment loss is principally the construction of cross

banks on the snig tracks. The cross banks are typically spaced between 20 m and 30 m,

depending on the slope. They are designed to reduce water velocity on the tracks, and divert

water and sediment from them onto the GHA. The cross banks are generally constructed by

blading material off the snig track to form a mound of material at right angles to the

predominant water flow on the track. (note: In this report, the term cross banks also

includes the mounds of material, sometimes called windrows that form alongside the snig

tracks as part of the bulldozer blading process). Water and sediment from the log landings

were also retained by cross banks, with flow being diverted into the GHA.

For the purposes of this investigation we have defined five distinct landscape elements

within the study area, these being; snig tracks, log landings, GHA, cross banks, and the filter

strip. The aim of this report is to estimate soil loss from these landscape units, and redistri-

bution between them.


4. EXPERIMENTAL DESIGN AND RESEARCH METHODS

4.1 Overall experimental design

Measurement of the soil redistributed between the various landscape elements of the study

site was undertaken using tracer based (137Cs and 210Pbex) soil and sediment budgets. This

approach is based on determining the total amount of each tracer contained within the

overall study area, and also in each of the individual landscape units within it, both before

and after harvesting. In an enclosed basin, such as the study area, the sum of the tracer

amounts contained within each of the landscape units should sum to the overall amount

calculated to be present if no net losses have occurred. Any losses from this system

however, are revealed as a deficit of tracer in the total budget amount. Analysis of the tracer

values contained within individual landscape elements will reveal where this loss is likely to

have occurred.

In order to construct a sediment budget using fallout nuclides we need to determine:

1) The relative surface areas of each landscape element.

2) The activities of 137Cs and 210Pbex within each of these elements.

3) A reference activity (for 137Cs and 210Pbex) from an undisturbed coupe, similiar in nature to

the logged study area.

Because natural background erosion has occurred within the study area over the last 30

years (the period since fallout of 137Cs), we also need to determine:

4) the amount of downslope movement of 137Cs and 210Pbex (and thus soil) which is not due

to forest harvesting.

In order to compare tracer budgets of 137Cs and 210Pbex with one another, it is also important

to confirm that they are tracing similar components of the soil matrix. Therefore we also

investigate:

5) the dependence of 137Cs and 210Pbex concentration on particle size and organic matter

content.

4.2 Experimental methodology

4.2.1 Calculating the surface area of the different landscape elements

The surface areas of the different landscape units within the study area were determined by

digitising an areal photo (blown up from 1:25000) using ARC/INFO. Different coverages

were created for the snig tracks, log landings, and the buffer strip. The area of the GHA was

obtained by subtracting the sum of the areas of the log landings, snig tracks and filter strip

areas from the total study area.

The digitising of areal photos involves errors due to optical distortion and relief displace-

ment (Curran, 1985; Avery, 1968). There is also some digitised image stretching which

occurs when the goe-referencing points (or tick marks) identified on the digitised areal

photo coverage are aligned with the same tick marks identified in the ARC/INFO contour


coverage. (The georeferencing points are selected because they are readily identifiable

points both on the aerial photo and in the digitised contour coverage. An example is a

stream junction or cross road). The 5 tick marks used for this study site formed a box

around the logged coupe with sides of approximately 1000 m. The final image was stretched

about 49 m when overlayed onto the contour map. This represents an absolute error of

about 5 % for the entire image. This is used as an upper estimate of the error associated

with determining the area of each of the landscape units.

4.2.2 Radioactivity of 137Cs and 210Pbex within the landscape elements of the study area

The soil samples used to determine the tracer amount of the landscape units were obtained

by hand augering. A metal ring was hammered into the ground and three augers (of known

depth and surface area) were used to extract the soil. Samples were collected at depth incre-

ments of 0-2 cm, 2-5 cm, and 5-25 cm; surface area was 0.00785 m2 for the 0-2 cm and 2-5

cm, and 0.00581 m2 for the 5-25 cm depths.

Sampling was undertaken in a grid pattern within each landscape element to ensure spatial

representativness of the tracer estimates. Where grid sampling was not possible (i.e., the

snig tracks), they were sampled linearly. Thirty individual cores were taken from within

each landscape unit, except for the filter strip at the base of the logged slope in which 45

cores were taken. These were mixed together in groups of 15 to reduce demand on analyti-

cal facilities, the samples from the other areas were mixed together in groups of 10. The 0-2

cm layer was mixed separately, as were the 2-5 cm, and 5-25 cm depths.

In this study, the sediment trapped behind the cross banks adjacent to the log landing was

included in the sampling of the log landing itself. Material retained behind cross banks on

the snig tracks was included in the sampling of the snig tracks. The sediment tongues that

developed from the cross banks and trespassed into the GHA, were included in the sampling

of the GHA.

4.2.3 Determining reference area activities and soil depth distributions of 137Cs and210Pbex

The reference area activities of 137Cs and 210Pbex were measured in an adjacent unlogged,

undisturbed native forest of similar slope and aspect to the study area site (see Figure 3.1)

by the same process of hand augering described in 4.2.2 above. Thirty individual samples

were taken from within the reference slope. A further 30 samples were taken from within a

strip of forest of equivalent width to the study area filter strip at the base of this slope.

Measuring soil loss using a direct calibration approach requires a detailed reference curve to

be measured. This was undertaken by excavating soil profiles at two sites which were

selected within the adjacent undisturbed forest area, approximately 100 m and 250 m

respectively from the coupe boundary. Detailed depth penetration information at these sites

was obtained by cutting thin layers of soil sequentially down through the soil with a known

surface area (0.16 m2). One mm layers were taken down to two mm depth. Two mm layers

were then taken to 24 mm depth; 10 mm layers from 24 mm to 104 mm; 20 mm layers from

104 mm down to 204 mm, and 50 mm layers from 204 mm down to the final depth of 304

mm. This was undertaken using apparatus described in Wallbrink and Murray (1996).


4.2.4 Radioactivity in the filter strips resulting from natural erosion

The amount of radioactivity contained within the study area filter strip comprises radioactiv-

ity contained on i) in-situ preharvesting soil particles, ii) soil particles deposited as a

function of natural pre-harvesting erosion processes, and iii) soil particles trapped within the

filter as a result of post-harvesting disturbance.

An estimate of the amount of radioactivity, in Bq m-2, from (ii) above can be obtained from

the amount by which the filter strip in the reference area was elevated above the reference

slope. This can be calculated using Equation 4.2.4 below;

Equation 4.2.4: Natural slope erosion (Bq m-2) = Reference filter (Bq m-2) - Reference Slope (Bq m-2)

This value (for both tracers) was then subtracted from the study area filter strip areal

concentration value.

The cross banks were also carefully considered. The approximate volume of soil contained

within the cross banks was calculated by multiplying their average width, depth and height

dimensions by the total number of them in the study area. A similiar approach was adopted

for the snig track windrows adjacent to the snig tracks. The methodology and assumptions

used for these analyses is outlined in Appendix A.

4.2.5. Sampling and processing of soil material for particle size experiment

Separate soil samples for the particle size analysis were obtained from the reference slope

using a similar auger method to described in section 4.2.2. Thirty six cores (to 2 cm depth)

were taken and then mixed together, the total mass from these was ~5.5 kg. This sample

was weighed, dried and weighed again. A 300 g sub sample was taken from the dry mass for

casting. The remainder was then slaked in water and then mechanically agitated in sieves

with apertures of; 500 µm, 250 µm, 125 µm, and 63 µm. Each sample was then washedagain by hand in the appropriate sieve until only clear water emerged from the base of the

sieves. The

4500 C for 48 hours to determine the Loss on Ignition (LOI). The samples (except organics)

were then ground in a rock mill to a fine powder.

This powder was then cast in a polyester resin matrix in either a cup geometry (~250 g),

disk (~30 g), or stick (~10 g), depending on the sample size. The samples were then

stored for 28 days (6 half lives of 222Rn) to ensure equilibrium with the 222Rn parent, 226Ra.

This enables concentrations of 226Ra to be determined with a higher sensitivity from the222Rn daughters (Murray et al., 1987).

4.3.2 Gamma spectrometry methods for analysis of low level radioactivity

Both 137Cs and 210Pb are gamma emitters, and as such are readily detectable by routine, high

resolution gamma spectrometry techniques (Hamada and Kruger, 1965). 137Cs emits gamma

rays at 662 KeV while 210Pb emits at 42.52 KeV. Analysis for these nuclides at the CSIRO

laboratories follows the methods described in Murray et al. (1987). The detectors used for

the experimental work in this study are n-type closed ended co-axials. These were

calibrated for Uranium (238U) series radionuclides using Canadian Centre for Mineral

Energy Technology (CANMET) uranium ore BL-5, and a standardized 137Cs solution

(Amersham International). The detectors were calibrated for 226Ra by using a standardised

solution from ANSTO. Independent checks on the calibrations were undertaken by partici-

pating in International Atomic Energy Association (IAEA) intercomparisons.


5. RESULTS

5.1 Particle size separation experiment

The particle size experiment was undertaken to determine the relative amounts of the two

tracers contained within the different particle size fractions of the Yellow Granite soil in the

study site. The results of this work are given in Table 5.1 below.

Table 5.1. Results of the particle size experiment determining the relative amounts of

the two tracers contained within each soil separate and the organic fraction.

Note: Uncertainties equivalent to one standard error are given as subscripts

in the least significant figure.

1002.71002.4226.014.039.41.35006Total

30.70.714.90.469.31.45.90.01403120503Organics

11.21.77.90.425.33.93.10.31321.60.11910> 500

8.01.18.20.918.22.73.20.42033.60.4911250-500

6.60.85.60.614.91.92.20.23044.30.4508125-250

14.61.210.71.133.12.74.20.4806101540663-125

3.60.24.10.18.10.61.60.155411114740-63

4.20.35.10.29.60.72.00.177616112420-40

5.40.27.40.312.20.52.90.185420114310-20

10.40.423.00.723.50.99.10.19033512622-10

5.30.213.10.411.90.45.10.1130557190< 2

210Pbex% of

Total

Activity

137Cs

% of

Total

Activity

Total210PbexActivity

(Bq)

Total137Cs

Activity

(Bq)

210PbexActivity

(Bq/kg)

137Cs

Activity

(Bq/kg)

Dry

Weight

(g)

Size (µm)

The sum of the activity of the tracers contained within each particle size separate (in

Becquerels) was equivalent to the total amount estimated to be present by simply multiply-

ing the average concentration (in Bq kg-1) of the tracers in the bulk soil (plus those of three

replicates in the reference area) by the total bulk soil mass. The 210Pbex total concentration

results varied by only 1.4 % for 210Pbex and by 7.8 % for 137Cs.

The distribution of the percentage of 137Cs and 210Pbex contained within the different particle

size separates is shown graphically in Figure 5.1. This diagram suggests that there is a

systematic trend for 137Cs to be retained with smaller size particles than 210Pbex. For

example, there is approximately twice the amount of 137Cs contained with the


Figure 5.1. Comparison of the amount of Becquerels of each nuclide, for each separate, as a percentage of the total amount in the original sample.

Nonetheless, this information shows that there is no gross dissimilarity in the adsorptionbehaviour of 137Cs and 210Pbex to mineral soil material, and that a comparison of budgetsbased on these two independent tracers is valid. However, their differences with respect tothe organic component are shown to have a potentially significant impact on the interpreta-tion of the tracer based budgets in Section 6.3 below.

5.2 Surface areas of the different landscape elements

The different areas of the landscape units within the study area (calculated using theARC/INFO method described in Section 4.2.1) are given in Table 5.2.1. below.

Table 5.2.1. Landscape element proportions determined by ARC/INFO analysis of (1:25000) digitised aerial photograph. Note: Uncertainties calculated as per discussion in section 4.2.1.

Landscape Unit Area (m2) Percent ofstudy Area

Snig Track 222001100* 17Log Landing 3200200 3Filter Strip 6900300 6GHA 917004600 74Total 1240004700 100

Note: Snig track area caluclated by multiplying total sniglength by survey width of 4.5 m.

137Cs and 210PbexActivity Percentagesby Size Fraction.

Size Fraction.

% A

CT

IVIT

Y

0

5

10

15

20

25

30

137C s % 210Pb ex %

< 2

2-10

10-2

0

20-4

0

40-6

3

63-1

25

125-

250

250-

500

> 5

00

Org

anic

s

The amount of disturbance from the snig tracks and log landings (~20 %) is typical of

compartment 1708 as a whole, but is higher than for other compartments in the EMA. For

example, Matthews and Croke (1997) measured 14, 15 and 16 % disturbance for three

compartments subject to similiar logging operations, while 15 % has been quoted as a

typical figure for disturbance in coupes elsewhere (State Forests, 1994a).

5.3 Depth distributions of 137Cs and 210Pbex from profiles 1 and 2 in the reference area

The measured depth distributions of 137Cs and 210Pbex from the depth profiles 1 and 2 in the

reference area are given in Figure 5.3 below. As expected from the literature (Section 2.2

and 2.3) maximum concentrations of 137Cs occur slightly below the soil surface, and at the

soil surface for 210Pbex. Note different concentration scales used for the two tracers.

Depth

(mm)

0

50

100

137Cs Profile 1, (Bq kg -1)

0 4 8 12 16

0

50

100

137Cs Profile 2 (Bq kg-1)

0 4 8 12 16

Depth

(mm)

0

50

100

210Pbex

Profile 1, (Bq kg-1)

0 50 100 150 200

Depth

(mm)

0

50

100

210Pbex

Profile 2, (Bq kg-1)

0 50 100 150 200

Figure 5.3. Measured depth distributions of 137Cs and 210Pbex from detailed depth profiles

1 and 2 within the reference area adjacent to Compartment 1708, near

Bombala.


5.4 Areal tracer concentrations of the different landscape elements

The areal concentrations of the two tracers (in Bq m-2) within each of the landscape units of

the study area as well as the reference profiles, reference slope, and reference filter strip are

given in Table 5.4.1 below.

Table 5.4.1. Areal concentrations of 137Cs and 210Pbex (derived from bulked cores) for

landscape elements within the study area, and adjacent reference slope near

Bombala, NSW. Note: Uncertainties equivalent to one standard error are

given as subscripts in the least significant figures for bulked cores. Uncertain

ties for means are derived from ANOVA analysis of entire data set.

Note:* Value represents the sum of 32 individual depth layer measurements

10

10

10

30

13709010501201560901330130

57040500406604058090

Reference Filter Strip

Mean

10

10

10

30

1200110840120850150960130

49030450404307046090

Reference Slope

Mean

1*

2

1380901300130

4503042090

Reference Profile 2*

Mean

1*12207039030Reference Profile 1*

10

10

10

30

300160250140130170230130

3004021080508019090

Snig Tracks

Mean

10

10

10

30

108090760150710150850130

58040610504804056090

General Harvest Area

(GHA)

Mean

10

10

10

30

019010019015019080130

40130220502006115090

Log Landing

Mean

15

15

15

45

1140210155017013302001340130

720601030708306079090

Filter Strip

Mean

Number of cores

within sample

210Pbex(Bq m-2)

137Cs

(Bq m-2)

Landscape Unit

It should be noted that when the areal concentration values from within the study area filter

strip are used in soil and sediment budgets in Section 5, they are reduced (by 123 Bq m-2 and

364 Bq m-2 for 137Cs and 210Pbex respectively, as per Equation 4.2.4). This is to account for

natural soil movement on the slope since 137Cs fallout ceased in ~1968. It should also be

noted that the cross banks do not appear in Table 5.4.1. This is because they have been

created from material bladed off the snig tracks and log landings. Consequently, the 137Cs

and 210Pbex within them is distributed throughout the entire soil matrix and not just contained

within the top 20 cm or so of the soil. Thus, the total activity of 137Cs and 210Pbex (in

Becquerels) within the cross banks is calculated by multiplying their average concentration


(in Bq kg-1) by the total mass of soil (in kg) contained within them. The total mass has been

calculated from measurement of their average widths, heights, and lengths in the study area

(see Appendix A). The total amount of 137Cs and 210Pbex estimated to be contained within

the cross banks is given in Table 5.4.2 below.

Table 5.4.2. Total activity of 137Cs and 210Pbex contained within the crossbanks of the

study area in compartment 1708. Note: Uncertainties equivalent to one

standard error are given as subscripts.

2.77.43.9810.98678270339141500452.1822.61210Pbex

1.210.61.780.89678270339141500452.1822.61137Cs

Total

MBq

Average

Activity

(Bq kg-1)

Total kgBulk

Density

(kg/m3)

Total

volume (m3)

Tracer

5.5 Calculation of total activity of 137Cs and 210Pbex in the study area and the

landscape elements contained within it

The total activity of each of the tracers (in Becquerels) contained within the various

elements of the study area, and adjacent reference areas, is required to complete a sediment

budget (see section 4.1). This is obtained by multiplying the surface area of each of the

landscape units (in square metres, see Table 5.2.1) by their measured areal concentrations

(in Bq m-2, see Table 5.4.1).

For example, the study area is 123900 m2 (12.4 ha). The reference activity for 137Cs and210Pbex is 460 ± 90 and 960 ± 130 Bq m-2 respectively. Consequently, the total activity of137Cs and 210Pbex contained within the study area (including filter strip) prior to logging is

calculated to be 57 ± 11 MBq for 137Cs, and 120 ± 16 MBq for 210Pbex. Similiar calculationscan be undertaken for each of the landscape units, and the results for 137Cs are given in

Table 5.5.1. Note that the total activity contained within the cross banks is derived from

Table 5.4.2.

Table 5.5.1. Post harvesting activity of 137Cs contained within the landscape elements of

the study area. Note: Uncertainties are derived from the square root of the

summation in quadrature of the surface area errors (of Table 5.2.1) and

measurement errors (of Table 5.4.1.).

10914.561.98.3100Total

211.20.6Cross Banks

73.44.11.91909022200110017Snig Tracks

10.10.50.31509032001603Log Landings

9.15.10.67409069003006Filter Strip

9014518.15609091700460074GHA

% of

Total

Total Post

Logging Activity

(MBq)

Post Logging

Areal Activity

(Bq m-2)

Area

(m2)

% of Sub

Catchment

Area

Landscape Unit


The total activity of 210Pbex contained within each of the landscsape elements of the study

area can be calculated in a similiar manner as for 137Cs above (see Table 5.5.2).

Table 5.5.2. Post harvesting activity of 210Pbex contained within the landscape elements of

the study area. Note: Uncertainties are derived from the square root of the

summation in quadrature of the surface area errors (of Table 5.2.1) and

measurement errors (of Table 5.4.1.).

77.812.492.714.1100Total

2.36.32.77.4Cross Banks

4.22.55.032.822712622200110017Snig Tracks

0.20.40.270.48412632001603Log Landings

5.60.86.720.8698012669003006Filter Strip

65.410.477.9711.585012691700460074GHA

% of

Total

Total Post

Logging Activity

(MBq)

Post Logging

Areal Activity

(Bq m-2)

Area

(m2)

% of Sub

Catchment

Area

Landscape Unit

A summary of the change in activity as a percentage of the total activity within the study

area for each landscape element before and after harvesting, is presented in Table 5.5.3

below.

Table 5.5.3. Summary of percentage change as a function of total study area activity in

the landscape elements for 137Cs and 210Pbex. (Note: This incorporates adjust

ment to 137Cs and 210Pbex total activities as per equation 4.2.4).

+ 2.05.5+ 2.01Cross Banks

- 14.08.4- 10.04.6Snig Tracks

- 2.04- 2.00.2Log Landings

0.0.0.8+ 4.00.5Filter Strip

- 9.01.5+ 18.02.6GHA

210Pbex Change

(%)

137Cs Change

(%)

Landscape Unit

These results show that the greatest loss of both 137Cs and 210Pbex is from the snig tracks.

The log landings also show a deficit of both these tracers. The greatest gain for 137Cs is

clearly within the GHA, although the same is not true for 210Pbex. The filter strip has gained137Cs although there was no change in 210Pbex activity, given uncertainties.

The same pattern is revealed when the absolute changes in pre and post harvesting tracer

activity amount for each landscape unit are compared in Table 5.5.4 below. The snig tracks

and log landings have lost more than 50 % of their initial store of both 137Cs and 210Pbex.

(The greater loss of 210Pbex than137Cs is presumably because more 210Pbex is contained in the

soil surface than 137Cs). The GHA shows a 20 % increase in 137Cs but a deficit of 210Pbex.

The initial store of 137Cs in the filter strip increased by 66 %, but gained only fractionally

more 210Pbex. This difference may be partly due to the preferential deposition of fine grained


material in the filter strip, the fines containing more 137Cs than 210Pbex by mass (see Figure

5.1). This also implies that the coarser grained material has been deposited upslope,

presumeably within the GHA.

Table 5.5.4. Absolute changes in total activity of 137Cs and 210Pbex within each of the

landscape elements, before and after harvesting. (Note: Total 137Cs and 210Pbexactivities have been adjusted as per equation 4.2.4).

26.76.6665.13.1Filter Strip

-1277.988.22150.941.9GHA

-910.33.1-670.51.5Log Landings

-76521.3-594.210.1Snig Tracks

Percent

change

(%)

210Pbexamount

after

(MBq)

210Pbexamount

before

(MBq)

Percent

change

(%)

137Cs

amount

after

(MBq)

137Cs

amount

before

(MBq)

Landscape element

In the next section these differences in total activity are used as the basis for constructing a

soil and sediment budget for the study area. Some explanations for the relative differences

in recoveries of the two tracers (from Table 5.5.3 and 5.5.4) are also discussed.


6. SEDIMENT BUDGETS DESCRIBING THE MOVEMENT OF

SOIL AND SEDIMENT WITHIN THE STUDY AREA

6.1 Constructing soil and sediment budgets based on 137Cs

The total activity of 137Cs and 210Pbex (in Becquerels) contained in the post harvest landscape

elements form the basis of a budget describing the movement of soil and sediment within

the study area. The data (of Table 5.4.1, 5.4.2 and 5.5.1) have been expressed graphically in

Figure 6.1 for 137Cs to describe the flow of tracers (and thus sediment and soil) within and

between the various landscape elements of the study area.

+163

Generalharvest

area

Snig

track

-21

-105

Log landing

INPUT

100%

+21

+31

Cross banks

Buffer strip

109% (+14)

Caesium-137

(1%)

(7%)

(2%)

(90%)

(9%)

Figure 6.1: Sediment budget ~6 years after harvesting showing areas of erosion and

deposition in the landscape elements of the study area, based on measure

ments of fallout 137Cs. Note: Values within arrows represent percentage

change of total study area activity within each landscape unit. Values in

parentheses represent the percent amount of total study area post harvest

activitycontained within each landscape unit. Uncertainties are the sum of

measurement errors of the tracers plus those associated with scanning the

airphoto image into ARC/INFO format.


Figure 6.1 suggests that within uncertainties, 109 ± 14 %, the 137Cs budget balances. This isconsistent with no net loss of material occurring from the study area. However, soil and

sediment redistribution has clearly occurred within it. Losses, as a percentage of total 137Cs

budget, have occurred from the snig tracks, -10 ± 4.6 % and log landings, -2 ± 0.2 %. Therehas been a net accumulation of material within the GHA, +18 ± 2.6 %, the filter strip, +4 ±0.5 % and the cross banks, +2 ± 0.2 %. The material contained within the crossbanks ispresumably primarily sourced form the snigtracks alone, due to their method of construc-

tion. The additional soil material within the GHA is probably derived from a combination

of the snigtracks, log landings and crossbanks. The material contained within the filter

strips is potentially sourced from the snigtracks, log landings and crossbanks as well as from

the GHA. However the proportional amounts by which these areas are contributing

sediment is unknown.

The redistribution of tracers (and thus soil) shown in Figure 6.1 is consistent with the

experimental work of Croke et al. (1997). These authors demonstrate that the GHA is a net

trap of soil and sediment material generated from snig tracks and cross banks. The 137Cs

budget also suggests that the filter strip is efficiently trapping soil material. For example,

the total net amount of 137Cs retained within the filter increased from 460 Bq m-2 to 790 + 90

Bq m-2 in the ~six years post logging. This represents a ~60 % increase in 137Cs concentra-

tions within this element (see Table 5.5.4; Note, this includes the subtraction of 123 + 90 Bq

m-2 from equation 4.2.4. See section 4.2.4). Clearly the filter strip has effectively retained

soil and sediment derived from the upslope area.

The total amount of activity of 137Cs removed from the snig tracks was ~5.9 + 0.6 MBq. Of

this amount ~20 + 10 % could be accounted for within the cross banks (given the assump-

tions in Appendix A), and a further 32 + 4 % within the filter strip at the base of the slope.

The remainder ~48 + 7 % was presumably retained within the GHA. Due to their method of

sampling (section 4.2.2) the cross banks contained no fluvially deposited soil material, and

so the ~20 % of material within them presumably results from the mechanical movement of

soil that occurred during their initial construction. Nonethless, these 137Cs results suggests

that the runoff management system in the study area has effectively dispersed flow, and thus

sediments, from the compacted snig track areas onto regions of higher infilitrability and

surface roughness within the GHA and filter strips. The key components of this system are

the effectiveness of the cross banks to intercept and disperse material from the source

areas, and the maintenance of well drained and heterogeneous soil conditons in the deposi-

tion GHA and filter areas. In this context however, it is possible that smaller cross banks

may have been equally effective at trapping and dispersing inflowing material in the study

area. This would have the dual advantage of decreasing the amount of soil removed from

the snig tracks to construct them, and also reduce the propensity for the region immediately

upslope of the cross bank to develop into an erosion source itself. (The latter process was

observed where bulldozer blades had breached the dispersable b horizon of the soil. In

these instances small active rills and gullies to 30 cm had formed upslope of the cross bank).

It is clear however that the entrainment of particles from snig tracks and their subsequent

trapping and deposition as wedges behind cross banks, within sediment tongues or dispersed

over the GHA, is a complex, and dynamic, process. A better understanding of these

processes requires a more formal separation, and high resolution sampling of the snig

tracks, cross banks, cross bank sediment wedges, and GHA sediment tongues as landscape

elements in their own right.


6.2 Constructing soil and sediment budgets based on 210Pbex

A similiar budget has been constructed for 210Pbex, see Figure 6.2. This budget agrees well

with the 137Cs results in identifying the log landings (- 2 ± 4 %), and the snig tracks (- 14 ±8.4 %) as areas of soil loss. Similarly to the 137Cs results, the cross banks also contain about

2 ± 5.5 % of the total 210Pbex activity. However, the 210Pbexbudget is different from the 137Csin that indicates soil loss from the GHA (- 9 ± 1.5 %) and negligible change in the amountof tracer contained within the buffer strip. The total amount of 210Pbex retained within the

study area is 78 + 12 % of the amount calculated to be present from analysis of the reference

cores.

-92

Generalharvest

areaSnig

track

-24

-148

Log landing

INPUT

100%

+26

01

Cross banks

Buffer strip

78% (+12)

LOSS

22%

Lead-210

(

6.3 Differences in the retention of 137Cs and 210Pbex within the study site

The difference between the total amounts 137Cs and 210Pbex calculated to be retained within

the study area after harvesting is ~20 ± 12 %, ie. a greater loss of 210Pbex than 137Cs hasoccurred. Some reasons for this potential difference are given below:

a) The total amounts of 137Cs (109 ± 14 %) and 210Pbex (78 ± 12 %) retained within the studyarea are reported with standard errors. The difference between them can be almost

accounted for at the two sigma level, assuming that the errors of the two budgets are

independent.

b) It is possible that the tracers are not binding equally to soil particles of different size. The

results of the soil fractionation experiment (Section 5.1.) shows that 137Cs has a greater

affinity with fine clays (< 2 µm), and that more 210Pbex is proportionally found on the coars-est fraction (> 500 µm). Thus, to account for the difference in the budgets by this logicrequires a large mass of coarse material to be exported off the site, in conjunction with a

preferential retention of fine clays. However, this is counterintuitive to the generally

accepted view that fine grains are selectively eroded and transported from slopes under

normal erosion conditions, and as such is discounted from further consideration.

c) These two tracers have different depth penetration characteristics. The maximum activity

of 210Pbex is found at the soil surface, whilst that of137Cs is found at depth (see Figure 5.3).

In this way, any loss of surface soil removes proportionally more 210Pbex than137Cs.

d) A greater amount of total 210Pbex activity is held in the surface organic/litter layer than137Cs. Thus, post harvest burning of the litter and slash in the study area, may have preferen-

tially removed more of the 210Pbex than137Cs, by processes such as volatilization, transport in

post harvest runoff waters and wind blowing of the ash load.

Consequently, the differences in the total activity of these two tracers retained in the study

area can be accounted for by a combination of i) statistical error, ii) preferential association

and subsequent loss of 210Pbex with organic material, and iii) loss of surface soil and thus

proportionally greater loss of 210Pbex than137Cs due to their contrasting depth penetration

characteristics.

6.4 Formally accounting for the differences in retention of 137Cs and 210Pbex

An understanding of the degree to which each of these processes contribute to the difference

can be obtained by considering some relevant information. For example, from Section 3.2 it

is known that the study area was burnt after logging operations ceased. The ignition

temperature of organics can be as low as 350o C (Luke and McArther, 1978). A light fuel

burn in the field (such as in a fuel reduction situation) can reach temperatures of 510o C and

windrow fires up to 780o C at the surface (Floyd, 1966). Indeed, experiments in both light

fuel burns and heavy fuel burns (such as in windrows or bark heaps) by Luke and McArthur,

(1978), suggest that the temperature at the soil surface can easily reach 3500 C in these situa-

tions. In the latter case, the soil temperature at 50 mm depth was still elevated to 1300 C

after a period of nine hours after combustion of surface materials was complete. Thus it is


possible that combustion of both litter at the surface and organic matter contained in the top

few millimetres of the soils, may account for some of the loss of 210Pbex. A further compo-

nent of ash is presumably lost as a function of post harvest rainfall.

The actual amount of 210Pbex available to be lost through this process can be estimated from

the available data. For example in the two detailed reference profiles approximately 11 + 1

% of total 210Pbex was retained within the litter layer compared to only 1 + 0.1 % for137Cs.

The particle size experiment confirmed that approximately two and half times the amount of210Pbex was retained within the organic component of that soil compared to

137Cs (see Table

5.1; Figure 5.1). In this way, if the litter layer and the organics to a depth of 8 mm, were

effectively combusted, then this would account for 20 + 1 % of the total 210Pbex inventory,

conversely the calculated loss of 137Cs from this process would be only 3 + 1 %.

However, it is unlikely that total combustion of organics to a depth of 8 mm occurred

across the entire study area. A more likely scenario is that the loss of 210Pbex included a

component attached to soil particulates. In this case the loss of the surface litter layer, plus

the associated soil and organics to a depth of only 2 mm, would account for 16 + 6 % loss of

the total amount of 210Pbex. In this scenario, the total amount of137Cs would be negligibly

reduced, and remains within uncertainties of 100 % ie., 99 + 5 %.

A loss of soil particulates to this depth is equivalent to an average net soil loss of ~3.3 m3

ha-1 yr-1 in the six years between harvesting and our experimental work. It would also repre-

sent the net removal of some 250 cubic metres of topsoil material from the study basin over

this time. If this were true, then some of this topsoil eroded material should be evident

within the streamline draining the study area. The activity (Bq kg-1) of 137Cs and 210Pbex on

these sediments should be very high. Deposits of surface eroded material should also be

found in the next order streamline at the base of the catchment. To address this question,

five samples of deposited sediment were taken along the length of the drainage line starting

from its outlet at the base of the study slope up to its headwaters. Additional samples were

taken in the stream channels above and below the confluence with the study area drainage

line. Each of these samples represented the amalgamation of ten or more subsamples taken

along a length of ~20 m. The results of these analyses are given in Table 6.4 below.

Table 6.4. Measurement of 137Cs and 210Pbex tracer concentrations on deposited sediment

samples within the drainage line of the study area, and from channels both

upstream and downstream of its confluence with the main channel.

0.0 0.1-3.5 2.1Downstream channel 2

0.1 0.3-1.9 2.5Downstream channel 1

0.3 0.20.0 2.41Upstream channel

10.1 0.370.5 3.4Drainage line 5, headwaters

0.1 0.2-1.8 2.4Drainage line 4

0.8 0.25.2 3.3Drainage line 3

0.1 0.4-3.3 3.3Drainage line 2

0.4 0.22.05 2.2Drainage line 1, base of slope

137Cs Activity (Bq kg-1)210Pbex Activity (Bq kg-1)Sample number


The concentrations of 137Cs and 210Pbex in the study area drainage line are generally very low,

(and not distinguishable from zero). However in one case, Drainage line # 5, the tracer

concentrations are consistent with its derivation from topsoil sources. This sample was

taken at the uppermost part of the drainage line, where in some cases the terminal cross

banks of snig tracks were observed to within 2-3 m of the streamline. Nonetheless, the

majority of sediment material below this point was clearly not derived from erosion of soil

from the surface of the catchment. The most likely source of this material was from the

numerous small sections of bank collapse, animal burrowings and channel pop-outs lining

the banks of this feature. (Note: the term pop-out refers to features that remain in the

channel bank when ovoid sections of soil material up to 50 cm diameter, 10 cm depth, are

released into the channel as a function of either pore pressure or bank instablility).

The dominance of the drainage channel itself as a source of post harvesting sediment is

consistent with the results of Olley et al. (1996). It is also consistent with the very low

tracer concentrations of sediment material found within the channel of the upstream catch-

ment (that was not harvested). Similarly, no evidence of surface eroded material was found

in the samples taken from the channel downstream of the study area.

A large mass of sediment was found where the drainage line exited the 12.4 ha basin of the

study area. This was light grey in colour and coarse grained in texture. It was difficult to

ascertain the age of this material, although some small trees and profuse vegetation were

evident growing within it. A further series of samples were taken from a bench deposit

formed from this material. The concentrations of 137Cs and 210Pbex were also very low,

suggesting that the material was also probably of subsoil origin.

It is concluded, on the basis of the 137Cs budget that the majority of material detached from

various landscape elements within the study site as a result of harvesting, was retained

within the catchment boundary itself. The difference between the 137Cs budget and that of210Pbex is best explained by a combination of analytical and sampling uncertainties, loss of210Pbex through combustion of litter and organic material and export of ash, and some small

loss of topsoil. However, samples of sediments within the drainage lines of the study area,

did not confirm that material currently being transported from this site was of topsoil origin.


7. CALCULATING SOIL LOSS FROM THE SNIG TRACKS AND

LOG LANDINGS.

Both the 137Cs and 210Pbex budgets show a net loss of material from the snig tracks and log

landings. The amount of soil removed from these can be calculated by comparing Caesium

activity retained on these features, with its known initial distribution in the soil. In Figure

7.1 a polynomial curve has been fitted to the average shape of the areal concentration values

of the two detailed reference depth profiles of 137Cs (given in Figure 5.3). This curve is then

normalised to the average areal concentration value of the reference area (460 Bq m-2). In

this way retention of 100 percent of 137Cs activity represents no soil loss at all, whereas

depletions from this can be directly converted to depth of soil loss (assuming that the distri-

bution of 137Cs in the reference areas was present in the snig track and log landing soils prior

to harvesting). The soil depth loss amounts can then be converted to a total net loss in

tonnes per hectare using the measured bulk density of these soils (1.5 g cm3).

Depletion of137

Cs reference amount (%)

0 20 40 60 80 100

Depth

ofsoilloss(m

m)

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

Soilloss(tha

-1)

0

500

1000

1500

2000

2500

Log landing

Snig Tracks

Figure 7.1 Distribution of 137Cs areal concentration with depth in the reference slope

adjacent to compartment 1708, derived from the average of reference

profiles 1 and 2.

The percentage amounts by which the snig tracks and log landings were depleted were 49 %

and 67 % respectively, equivalent to net soil depth losses of 28 + 13 mm and 48 + 29 mm

respectively. These calculate to annual losses of 70 + 33 t ha-1 yr-1 for the snig tracks and

120 + 70 t ha-1 yr-1 for the log landings, using the ~six year gap between harvesting and

measurement of their radionuclide content. However, it should be noted that the


radionuclide based values are a measure of total loss of soil material from these snig and log

landing features. Consequently, they include losses due to physical processes such as:

mechanical removal of soil by bulldozer blading, export on truck wheels, tyres and chassis,

as well as losses due to wind and water erosion. These values of loss can also be compared

to those of 4 - 5.8 t ha-1 (Croke et al., 1997) derived from rainfall simulator experiments on

snig tracks within the study area. The difference between the radionuclide estimates of soil

loss, and those of Croke et al. (1997) presumably represent a combination of differential

production rates from the snig tracks over time, different post harvest rainfall and weather

patterns, and the mechanical losses outlined above.


8. CONCLUSIONS

8.1 This report

This report aimed to i) quantify the total net soil loss that occurs from a basin within a

coupe after logging in a dry sclerophyll forest, ii) quantify loss from, and redistribution

between, various landscape elements within the coupe, and finally iii) investigate the

efficacy of cross banks and filter strips to mitigate post harvest soil erosion.

Total net loss from within the study area was determined by constructing tracer based soil

and sediment budgets. The 137Cs budget showed no net loss of soil material from within the

study area (within uncertainties). Conversely, there was a deficit 22 + 12 % of 210Pbexcompared with that expected to be present. This difference was best explained by a combi-

nation of analytical and sampling uncertainties, loss of 210Pbex associated with transport

and/or combustion of litter and organic mater from the site, and some small loss of surface

soil (to a depth of 2 mm). However in this latter case some 250 cubic metres of surface

material should have been transported from the study area basin. No evidence for this was

found in sediments currently being transported from the site.

Both tracer budgets showed a similiar loss of material from the snig tracks and log landings.

The 137Cs budget showed a gain within the GHA, conversely the 210Pbex budget showed a

loss for the same area. As this is proportionally the largest region of the study area, it is

presumably where the greatest effect of the combustion of litter and organics would occur.

This is also a heterogeneous part of the landscape, and it is possible that an understanding of

soil movement processes within this region would benefit from more investigation of this

area.

The net depth loss of material from the snig tracks and log landings was quantified to be 28

+ 13 mm and 48 +29 mm respectively. This is equivalent to losses of 70 + 33 t ha-1 yr-1 for

the snig tracks and 120 + 70 t ha-1 yr-1 for the log landings, using the ~six year gap between

harvesting and measurement of their radionculide content. Up to 20 + 10 % of the losses

from the snig tracks is shown to result from physical removal of soil material to create the

cross banks. The greater proportional loss of material from the log landings than the snig

tracks is also not surprising given the pressure from additional vehicular traffic and machin-

ery in these locations. The tracer based budgets suggest that material eroded from the log

landings is contained within either the cross banks, GHA, or filter strip. Similarly the

material from the snig tracks is accounted for within the cross banks, GHA and filter. The

GHA was found to be a net trap of soil and sediment from the 137Cs budget although the210Pbex budget suggests that this is balanced against a loss of organics and litter.

The filter strip showed a ~60 % increase in levels of 137Cs and negligible increase in 210Pbex.

The increase in 137Cs concentrations with respect to that of 210Pbex may suggest a preferential

trapping of fine grained material in preference to coarser grains. This material was sourced

from either the snig tracks, log landings or the GHA. The total amount of 137Cs activity

estimated to be removed from the snig tracks was ~ 5.9 MBq. Of this amount approxi-

mately 20 + 10 % could be accounted for within the cross banks, 32 + 4 % within the filter

strip and the remainder, 48 + 7 % within the GHA. On this basis the GHA plays a funda-

mental role in the trapping of material generated from the snig tracks. It also suggests that


the runoff management system of dispersing flow from the highly compacted snig tracks, by

cross banks, into the less compacted (and larger area) GHA and filter strips has effectively

retained soil and sediment mobilised as a result of harvesting at this site. It is probable that

the same effect could have been achieved using cross banks of smaller size.

8.2 Future work

The assessment of soil erosion and redistribution within the study site, is partially dependent

on a careful assessment of the uncertainties associated with the laboratory analysis of 137Cs

and 210Pbex, and their initial distribution in the landscape. Consequently, the application of

the tracer based budgets in this report would benefit from a reduction in the errors attribut-

able to both these factors. Analytical errors can be minimised by extended periods of count-

ing by gamma spectrometry equipment, however the heterogeneity of these tracers in the

field is more complex. An attempt was made to address this issue by taking many individ-

ual core samples, although it is probable that estimates of the uncertainties on average

values may well benefit from a more thorough field sampling program. Areas that would

benefit from such an approach would be the characterisation of 210Pbex in the reference slope

and the GHA.

A more complete understanding of the relationship between erosion of sediment from snig

tracks, and subsequent deposition in wedges upslope of cross banks, and tongues in the

GHA, would be obtained by a more detailed sampling and treatment of these areas as

separate elements within their own right.

The deficit of total 210Pbex with respect to that of137Cs is also intriguing. Clearly our under-

standing of the fate of nutrients and associated elements within forest organic material

would benefit from a more systematic investigation of the relationship between depth of

litter layer, the retention of 210Pbex, organic content of forest soils and the amounts by which

these materials are lost from catchments as a function of postharvest burning and erosion

processes.

It is also possible that the small mass of material potentially eroded (250 cubic metres) from

this catchment occurred within a short space of time after logging ceased. This possibility

could be addressed by a programme monitoring the particle size and tracer composition of

sediments leaving harvested catchments immediately prior to, and post harvesting.


9. BIBLIOGRAPHY

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