release of dissolved organic carbon from upland peat

HYDROLOGICAL PROCESSESHydrol. Process. 16, 3487–3504 (2002)Published online 13 November 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1111

Release of dissolved organic carbon from upland peat

F. Worrall,1* T. P. Burt,2 R. Y. Jaeban,2 J. Warburton2 and R. Shedden3

1 Department of Geological Sciences, Science Laboratories, South Road, Durham DH1 3LE, UK2 Department of Geography, Science Laboratories, South Road, Durham DH1 3LE, UK

3 Northumbrian Water Ltd, Abbey Road Industrial Estate, Pity Me, Durham DH1 5EZ, UK

Abstract:

This study examines the release of dissolved organic carbon (DOC) from upland peat during the period of the autumnflushing. Hydroclimatic conditions were monitored in conjunction with measurements of absorbance and the E4/E6ratio of the stream draining an 11Ð4 km2 upland peat catchment in northern England. During two months of monitoringthe effects of 67 separate rainfall events were examined showing that:

1. The peat behaves hydrologically as if it were a two end-member system consisting of old, interevent, and new,event, water. Runoff is initiated by percolation excess of new water at the acrotelm–catotelm interface.

2. The discharge of dissolved organic matter behaves like a three end-member system with the between-event waterbeing low in DOC and storm events being characterized by two types of water. Initial runoff being characterizedby new water rich in DOC that gives way to new water depleted in DOC. This transition can be ascribed to therunoff progressing from throughflow within the acrotelm progressing to saturation-excess overland flow.

3. Depletion of DOC during storm events is accompanied by a change in the character of the DOC as the E4/E6 ratiochanges. This suggests that the decrease in DOC during events is the result of exhaustion of reserves rather thanchanges in the flowpaths being utilized by runoff.

4. The amount of carbon released in any event is critically dependent upon the time between events during whichoxidation processes generate a reservoir of available carbon. Production of available carbon in the catchment is ashigh as 4Ð5 g C per day per m3 of peat, suggesting a turnover rate of peat of the order of 42 years. Copyright 2002 John Wiley & Sons, Ltd.

KEY WORDS DOC; water colour; peat

INTRODUCTION

Dissolved organic carbon (DOC) is a major component of river water: it contributes to the transport of metalsand organic micropollutants (Worrall et al., 1997); acts as an energy source (Hynes, 1983); affects lightpenetration (Schindler et al., 1996); plays a role in pH buffering (Kerekes et al., 1986); controls the partitionof components between the water and sediment (Worrall et al., 1997); is a source of nutrients (Qualls et al.,1991); represents a major issue in the treatment of water (Naden and McDonald, 1989); and contributes tothe global carbon cycle (Meybeck, 1993).

It has been shown that the major source of DOC to rivers is the most carbon-rich soils within a catchment(Urban et al., 1989; Hope et al., 1997a,b; Aitkenhead et al., 1999; Elder et al., 2000). The release of DOChas been the focus of several studies: most of which have focused upon forested catchments. Mcknight et al.(1993) and Hornberger et al. (1994) both showed that DOC concentration in the stream of a forested mountaincatchment was closely linked to flushing of the surface organic-rich horizons of the catchment’s soils. Boyeret al. (1997) showed that the catchment as a whole integrated the flushing response of each of the identifiable

* Correspondence to: Dr F. Worrall, Department of Geological Sciences, Science Laboratories, South Road, Durham DH1 3LE, UK.E-mail: [email protected]

Received 20 August 2001Copyright 2002 John Wiley & Sons, Ltd. Accepted 11 March 2002

3488 F. WORRALL ET AL.

source areas within a catchment. However, forested catchments would be expected to behave very differentlyfrom peat-covered, unforested uplands. The presence of mineral horizons in forests soils can adsorb and retardDOC movement; thus DOC variation in forest streams is related to the depth in the soil profile (Zechs et al.,1994, Borken et al., 1999).

For peat systems the discharge of dissolved organic matter to surface streams is less likely to be controlledby adsorption to mineral soils. Grieve (1990a) found a positive correlation between discharge and DOC, butchanges of DOC during runoff events were small compared with seasonal changes, except in late summer.The marked seasonality in DOC from peat has been noted by a number of authors (e.g. Naden and McDonald,1989) and can be ascribed to the low water tables, low runoff yet high turnover of carbon during the summermonths, the labile organic matter produced being flushed out in the early autumn period. This annual cycleis extenuated by drought conditions, and DOC concentrations in streams draining peat have been observedto rise after drought (Mitchell and McDonald, 1992, Vogt and Muniz, 1997). This study sets out to performa detailed analysis of DOC discharge events in an upland stream during the early autumn period as a meansof understanding the flowpaths and processes controlling release of dissolved carbon to surface waters.

STUDY SITE

Moor House National Nature Reserve (NNR) is situated in the North Pennine upland region, to the southof the summit of Cross Fell (National grid reference NY 7 56 326, Figure 1). The Moor House NNR is aterrestrial and freshwater site which is part of the UK Environmental Change Network (ECN). The ECNcollect various hydrological data from the Trout Beck catchment that lies within the Moor House NNR;this catchment is one of the primary headwater tributaries of the River Tees and shares a watershed withheadwaters of the River Tyne. The rivers Tees and Tyne are the major rivers of the north-east of England andare dominated by processes in the North Pennines, including retaining high DOC concentrations throughouttheir length. The Trout Beck catchment lies largely above 450 m OD, with the highest point being thesummit of Cross Fell at 893 m OD. The underlying geology is a succession of Carboniferous limestones,sands and shales with intrusions of the doleritic whin sill (Johnson and Dunham, 1963). This solid geologyis covered by glacial till, the poor drainage of which has contributed to the development of blanket peat.Blanket peat covers 90% of Trout Beck catchment (Evans et al., 1999), some of which is either naturallyeroded, or damaged by the practice of gripping (artificial drainage channels). The vegetation of the reserve isdominated by Eriophorum sp. (cotton grass), Calluna vulgaris (heather) and Sphagnum sp. (moss). The meanannual temperature (1992–2000) is 5Ð8 °C; air frosts are recorded on over 100 days in a year. Mean annualprecipitation (1953–2000) is 1953 mm, with snow being a significant proportion of precipitation—annualaverage snow cover at 500 m is 55 days (Archer and Stewart, 1995). Any rainfall in the upper catchmentproduces a rapid runoff response: studies at Moor House have shown that the lag between peak rainfallintensity and peak flow can be as little as 30 min (Burt et al., 1998). The catchment area above the TroutBeck gauging site is 11Ð4 km2.

METHODS

Monitoring of dissolved organic carbon

In order to understand the release of DOC from blanket peat, detailed observations were made of DOCconcentrations in relation to hydrological and hydrochemical parameters. A Rock and Taylor pump samplerwas put in place on the Trout Beck, just upstream of the gauging station (Figure 1). The sampler originallywas set to collect a sample every hour, for 48 h, i.e. until all 48 bottles were full. The full bottles werecollected and taken to the laboratory, and the sampler was restocked with empty bottles and reset to takeanother set of samples. After the first set of samples had been analysed, the plan was revised so that the pumpsampler was set to fill a sample bottle every two hours, as this was deemed to be sufficiently frequent to

Copyright 2002 John Wiley & Sons, Ltd. Hydrol. Process. 16, 3487–3504 (2002)

DISSOLVED ORGANIC CARBON FROM PEAT 3489

Figure 1. The location of the Moor House study site

quantify temporal changes in DOC over changing flow conditions. Sampling at Moor House continued for aperiod of two months, 7 September 1999 to 2 November 1999 with minor breaks during times of equipmentfailure. This sampling period was planned to coincide with the maximum in the annual cycle of DOC releasefrom upland peat in the UK (Naden and McDonald, 1989). Every effort was made to coincide visits to thesampling site with the end of the sampling cycle, such that the equipment could be kept running continuously,thus producing DOC data that would cover as many storm hydrographs as possible.

Once the samples had been collected from Moor House they were analysed as quickly as possibleowing to the unstable nature of aquatic humic substances. Where possible the samples were processedon the same day that they were brought back to the laboratory; if not analysis was carried out thefollowing day. When delays in analysis were unavoidable, the samples were kept refrigerated at 4 °C and inthe dark.

Samples were filtered using a 0Ð45 µm membrane filter, before being put in a 1-cm cuvette and run througha UV-VIS spectrophotometer (Camspec m302, Camspec Ltd, Cambridge, UK). Previous studies have used arange of absorbances to measure DOC concentrations, here absorbance at 400 nm was used (Thurman, 1985),and also at 465 nm and 665 nm. Absorbance measurements at 465 and 665 nm are used to calculate theE4/E6 ratio, taken as indicative of the humification of the DOC (Thurman, 1985).

Environment Change Network monitoring data

The Moor House site, as part of the ECN monitoring network, is instrumented with an automatic weatherstation, and a river flow gauge. The water sampler used in this study was sited at this river gauge. From theautomatic weather station hourly summaries of rainfall were taken to give duration, intensity and total volume



of all rainfall events. River stage and gypsum block data were also analysed to give hourly summaries ofdischarge and soil moisture content.

The outflow at Trout Beck is monitored by a multiparameter water quality monitoring probe (GreenspanTechnology Smart Sonde) for pH, conductivity and dissolved oxygen on a quasicontinuous basis—pHand conductivity were used for the purposes of this study. Conductivity measurements were normalizedautomatically to 25 °C with a tolerance of š0Ð5%. The pH measurement was compensated automatically fortemperature changes and has a tolerance of 0Ð2 pH. In addition, the ECN monitoring programme samples arange of water quality parameters in the Trout Beck stream on a weekly basis. For the purpose of this studyrecords of DOC and absorbance at 436 nm from this ECN sampling programme were utilised in order tofacilitate calibration of absorbance results.

Full details of sampling protocols for all of the parameters monitored as part of the ECN study are reportedin Sykes and Lane (1996).

Calibration experiment

As part of the ECN, Trout Beck is monitored for both its DOC and its absorbance at 436 nm on a weeklybasis. To compare results from this study with those results a calibration experiment was performed. On 20June 2000 all major tributaries of the Tees and Tyne were sampled from Moor House to the tidal limit ofeach river. The date was chosen as it followed 5 days of dry weather and so baseflow conditions could besaid to exist for both rivers over their entire length. Both rivers were chosen because their headwater sourcesare adjacent to each other at the Moor House site. The rivers were sampled along their lengths to enable arange of DOC concentrations to be sampled. In all 39 sites were sampled. Each sample was treated as aboveand analysed for pH, conductivity and absorbance at 400, 436, 445 and 665 nm following the methods givenabove and compared with the ECN samples to provide a calibration that links absorbance measures of DOCto actual DOC concentrations.

Event-based statistics

In order to analyse the Moor House data, the record was considered as a series of events for which multipledescriptors could be calculated to create a multivariate database of parameters describing each event. Thisdatabase enables underlying controls on and groupings of events to be examined. Such an approach has beensuccessful in determining controls on runoff events in agricultural catchments (Heppell et al., 2002). Rainfallwas considered the driver for both discharge and DOC release. Rainfall events were defined as any rainfallseparated by at least 1 h of no rainfall from previous rainfall, with the minimum amount of rainfall measurablebeing 0Ð2 mm. For each rainfall event the duration, intensity, total volume and the antecedent soil moisturecontent were recorded. For each rainfall event the corresponding flow period was examined to assess whetheran increase in flow had occurred; if so, an event was recorded. For each flow event identified the antecedentflow and the peak discharge were recorded. Equally, for each rainfall event identified the absorbance recordwas also examined for any increase. If an event in the absorbance record occurred the pre-event absorbanceand the peak absorbance were recorded. In addition, for each absorbance event the time to the peak and thetime since the last absorbance event were measured. This meant that for a rainfall event up to nine descriptorswere calculated.

In order to understand what conditions trigger a response in either the flow or the absorbance, logisticregression was used. Logistic regression is the most appropriate technique for predicting a binary outcome(e.g. runoff event versus non-event) from continuous explanatory variables (e.g. rainfall intensity). This methodtransforms from a probability scale (0, 1) to the scale of continuous variables (1, �1). The transformationused is the logit transform, y D log��/�1 � �� where � is the probability of an event. The transformedparameter y can then be related linearly to the chosen explanatory variables, in this case the event descriptors.This regression method does not use a least-squares fitting method, but rather uses maximum likelihoodestimation. In addition logistic regression allows the significance of parameters included in the model to be



assessed, thus aiding the mechanistic interpretation of the model. Linear discriminant analysis may be used toclassify a binary response to one of two groups given a set of explanatory variables. However, discriminantanalysis does not offer a probability scale and predictions may fall outside acceptable regions. In the sameway, neural networks do not offer a probabilistic scale of classification and thus the facility to interpret resultsrelative to each other is lost.

RESULTS

Complete monitoring records were collected for four periods: 7–13 September; 22 September to 4 October;13–25 October; and 29 October to 11 November (Table I).

Calibration experiment

Results from the ECN monitoring show a clear linear trend between absorbance at 436 nm and the measuredDOC (Figure 2). Equally, taking results from the calibration experiment the relationship between absorbanceat 400 and 436 nm is clearly linear with an almost perfect correlation, but with absorbance at 436 nm alwaysbeing lower than that at 400 nm (Figure 3). Therefore, a significant linear relationship between measuredDOC and absorbance at 400 nm can be established (Figure 4)

DOC D 110 Abs400 C 0Ð75 r2 D 0Ð80 �1�

where DOC is the dissolved organic carbon content in mg C/L, and Abs400 is absorbance at 400 nm.

Absorbance measurements

The range in absorbance over the period of the study shows that the release of DOC ranges from 7 mg/Lto 29Ð5 mg/L. Over the study period a range of styles of response in the absorbance record can be observed(Figure 5).

1. Increases in the flow correspond to increases in the levels of absorbance, for example, the runoff eventthat starts on the 27 September corresponds with a 50% increase in absorbance, with the absorbancelevels declining at a very gentle rate subsequent to the event. Examining the flow–absorbance relationshipfor this runoff event alone shows that levels of absorbance indicate anticlockwise hysteresis (Figure 6).Anticlockwise hysteresis is indicative of the component of interest lagging behind the flow, but the shapeof this hysteresis loop shows the very rapid changes in absorbance on the rising limb of the event notobserved on the recession limb.

2. In contrast the runoff event of the 29 September shows that decreases in absorbance levels coincide withthe peak in the runoff—the reverse of that observed for the event of 27 September. During the eventof 29 September a peak in absorbance precedes the peak in the runoff. Examining the flow–absorbancerelationship for this event shows a clockwise hysteresis (Figure 7), confirming the observation that in thisevent absorbance precedes flow, but as with the event of 27 September the change in DOC concentrationson the rising limb is rapid compared with that of the change on the recession limb.

Table I. Periods of sampling and the number of events in each period

Sampling period Numberof days

Number ofrainfall events

Number offlow events

7–13 September 6 3 322 September to 4 October 13 42 2829 October to 11 November 14 22 14



0

0.02

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0.08

0.1

0.12

0.14

0.16

0 5 10 15 20 25 30

DOC (mg C/l)

Ab

s (4

36 n

m)

Figure 2. Comparison of absorbance at 436 nm with measured DOC concentrations at the Moor House site

The events recorded during the monitoring programme can be divided between these two styles—clockwiseand anticlockwise events: 11 events out of all the identifiable events were of the anticlockwise type (someof these are compound events, e.g. 23 September) and three were of the clockwise type. The latter aredistinguished from the former not only in the style of the absorbance response but also in the amount ofrunoff in the event; of the three clockwise events the smallest has a peak flow of 3Ð97 m3/s (11 November),whereas the largest of the anticlockwise event is 3Ð3 m3/s. Therefore, the scale of the event appears crucial indefining the absorbance response. The event of 3 October is difficult to define as either of the types delineatedabove as it appears to be overprinted by an event on 2 October with an anticlockwise-type response. Grieve(1990b) observed no differences between runoff events in terms of their DOC response.

E4/E6 measurements

Changes in the E4/E6 ratio reflect changes in the type of organic matter being mobilized. Comparing thesame events as above again highlights a contrast between the types of event (Figure 8). For the event of 23September there is only a slight decrease in the E4/E6 ratio, whereas for the event of 29 September thereis more marked decline; such a marked decline also is observed for the event of 1 October. Anticlockwiseevents tend to cause little or no change in the E4/E6 ratio, indicating no change in the nature of the DOC overthese events and suggesting that the water in these events is sampling a homogeneous reservoir of availablecarbon. In contrast, the larger clockwise events seem to indicate a fractionation, with only a component of theavailable carbon being mobilized. There is a possible exception to this pattern with the event of 21 October,where increases in E4/E6 ratio are observed at the start of a series of small events. The event of 21 Octoberand the subsequent monitored events behave like an anticlockwise event. However, the 21 October event



y = 0.6094x - 0.0011R2 = 0.9974

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Ab

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Figure 3. Comparison of absorbance at 436 nm with absorbance at 400 nm for samples taken throughout the Tees catchment

follows a dry spell, which may be an explanation for the nature of the DOC mobilized at the start of thisevent. The three events that follow the 21 October event of the succeeding three days cause no change in theE4/E6 ratio.

pH and conductivity measurements

These two parameters show no real contrast between the types of event proposed above. The time-seriesfor the measured conductivity across the events examined above (Figure 9) shows that conductivity decreasesover all recognizable events, with the decrease in conductivity being proportional to the maximum dischargeof the event. The relationship between flow and conductivity confirms that, firstly, events follow a singletrend (Figure 10), i.e. no separate trends or groupings are observed in this plot, but secondly that there ismixing of two types of water. High conductivity waters exist at the lowest flows and represent ‘old water’,i.e. water that has had time to evolve its solute composition. The conductivity of this ‘old water’ componentvaries: at approximately constant low flow there exists a range of conductivity values; this is because at lowflows waters have had differing lengths of time to evolve in the peat matrix. In contrast, the low conductivitywaters occurring at high flows represent an unreacted or ‘new water’, and perhaps can be thought of as thewaters most akin to rainwater. Unlike the ‘old water’ end-member that showed varying conductivity at anapproximately constant flow, the ‘new water’ end-member shows a smaller range of conductivity at a rangeof flows, approximately 20 µS/cm for high-flow samples (>1 m3/s) as opposed to 100 µS/cm for low-flowsamples (<1 m3/s). This variation results from the fact that the source of the ‘new water’, i.e. rainwater, hasan approximately constant conductivity over the period of observation but that once this new water comesto dominate the flow, any size of flow is possible. The region between the flows dominated by either ‘old’



y = 1790.7x + 11.821R2 = 0.8357

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DO

C (

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Figure 4. Comparison of absorbance at 400 nm with DOC concentrations at the Moor House site

or ‘new’ water is not a straight but rather it is a curve. This conservative mixing of the two end-memberscan be explained by the fact that they may represent distinct chemical compositions and not variations in asingle ion. For example, it is easy to envisage that rainwater is dominated by Na and Cl ions but that the peatmatrix is dominated by different ions, e.g. Ca and HCO3

�. A conservative mixing of these waters dominatedby different ions would give a conservative mixing with respect to any one element, but there is a not a linearrelationship between concentrations of such species and their conductivity.

This behaviour is replicated for pH, for which a similar mixing line is observed (Figure 11). The mixing linefor pH is conservative given the logarithmic nature of pH—low pHs constituting the high-flow end-member.The existence of such relationships means that the mixing relationships may be inverted to give the proportionof old and new water during the period of monitoring. When the proportion of new water is compared withthe absorbance measure of DOC (Figure 12) the data can be bound by an approximate triangle suggestingthree end-members controlling the DOC levels. These three water bodies represent:

1. A new water end-member low in absorbance (point A) that can be taken as the end-member closest torainwater.

2. A composition that has the lowest proportion of new water in the measured samples and also has a lowabsorbance (point B). This end-member can attributed to old groundwater in the acrotelm.

3. An end-member with high absorbance and composed of new water (point C). This must be event waterthat is sampling a supply of available carbon, but this event water is a mixture of new water and old waterflushed out during rainstorms.



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Figure 5. Absorbance record compared with flow over the period of study: 22 September to 4 October 1999

These identified end-members add detail to the two types of event highlighted above. This plot suggests thatthe difference between the two events is not necessarily as distinct as suggested above but rather that mostevents consist in some part of all three end-members.

Event statistics

In all 67 separate rainfall events were recorded during the study period. Of the 67 rainfall events, 37 werealso flow events and 46 were absorbance events. In 21 cases there was an absorbance event but no flow eventassociated with a rainfall event; in all but three of these cases the total rainfall volume was less than 0Ð4 mmand the rain lasted no more than an hour. These events where absorbance goes up but apparently not flowoccur within hours of a very large event and although they prolong the recession limb of any event theydo not contribute enough water to create a separate peak in the flow record. They do, however, contributeto new water flow and so can create an increase in the absorbance record, for example the rainfall event of0400 h, 1 October (Figure 5). Equally, there are 12 events where a flow event occurs without any change inthe absorbance record.

Given the corrections made above, the logistic regression to separate runoff events from non-events gave

loge��/�1 � �� D 0Ð504Fp C 2Ð254T � 0Ð824 �2�

where � is the probability of a runoff event, Fp is the river discharge prior to the start of rainfall and T is thetotal volume of rainfall falling in the rain event (mm). Only variables with a significant fit (at least at the 95%level) or that significantly improved the classification of runoff event versus non-event were included. Theequation correctly classifies 88% of the events. The relationship can be plotted against isoprobability lines of



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Ab

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Figure 6. Anticlockwise hysteresis for the runoff event on 27 September

an event occurring (Figure 13). The figure and the significance level of the Fp variable suggests that the mostimportant variable controlling runoff was the total amount of rainfall; in this study non-events were recordedonly for rainstorms less than T D 1 mm. The pre-event flow (Fp) can be taken as a proxy for the position ofthe water table prior to the rainfall. The importance of the water table in the generation of runoff confirmsthe result of Evans et al. (1999) that runoff mechanisms are controlled by saturation-excess overland flowor by subsurface stormflow within the acrotelm rather than by infiltration-excess overland flow. It should benoted that the shallow subsurface stormflow generated in the upper layer of the peat, the acrotelm, has beentermed ‘percolation-excess’ by Evans et al. (1999) and by Holden (2000) in that it represents an excess ofwater over that which percolates down into the lower peat, the catotelm. Percolation excess means that thewater table rises into the more permeable acrotelm and lateral subsurface flow increases substantially.

The logistic regression for absorbance events is

loge��/�1 � �� D 0Ð756Fp C 1Ð3502�DI/T� � 2Ð1134 �3�

where � is the probability of an event in the absorbance record, Fp is the river discharge prior to the start ofrainfall, D is the duration of the rainfall event (h), I is the peak intensity of the rainfall event (mm/h) and T isthe total volume of rainfall falling in the rain event (mm). This equation correctly classifies 76% of the events.The DI/T parameter provides a slightly better classification than using duration (D), peak intensity (I) andtotal volume (T) separately. This parameter, used previously by Heppell et al. (2002), has several advantages.First, it enables the relationship to be easily visualized (Figure 14). Second, this parameter does have physicalinterpretation, representing the ratio of the peak intensity to the average intensity of the rainstorm, i.e. thepeakedness of any rainstorm. Thirdly, the parameter as given is dimensionless. Unlike the relationship forflow events, the relative importance of the variables in this equation is more equal.



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Figure 7. Clockwise hysteresis for the runoff event on 29 September

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Figure 8. E4/E6 series compared with flow over the period of study: 22 September to 4 October 1999



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Figure 9. Conductivity series compared with flow for the period 22 September to 4 October 1999

To understand the concentrations of DOC, as measured from absorbance measurements, in the Trout Beckdischarge, the height of the peak and the difference between the peak and the pre-event levels in the absorbancerecord were predicted using stepwise multiple regression; only those variables and constants that are significantat the 95% level are included. All records were converted to DOC concentration to aid interpretation. Theresult for the peak DOC concentration is

DOCpeak D 0Ð895 DOCpre C 0Ð19t r2 D 0Ð89, n D 38 �4�

where DOCpeak is the DOC concentration on the peak of the event (mg C/L); DOCpre is the DOCconcentration immediately prior to the start of rainfall and t is the time between events in the absorbancerecord.

Interpretation of Equation (4) suggests that DOC movement is effectively a flushing mechanism, where thepeak DOC is made up of the pre-event DOC plus that which has developed between events. The coefficientof t (time between events) for Equation (4) suggests a zero-order production rate of available carbon betweenevents: this rate being between 4Ð5 and 4Ð6 mg C/L�1 per day. Examining individual events this productionrate is up to 1022 kg C/day, which equates to 9 g/day per m2 of catchment area. Assuming an average depthof peat in the catchment is 2 m (Holden et al. 2001), then the production rate is 4Ð5 g/day per m3 of peat.Taking figures from Mitchell (1991) for peat from Scar House, Yorkshire, suggests a volume production rateof 3Ð4 g/day per m3 of peat.



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0.01 0.1 1 10 100

Flow (m3/s)

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Figure 10. Conductivity versus flow for the entire period of the study

DISCUSSION

Is it possible to establish a model of DOC discharge from this upland peat? There are several lines of evidence.Firstly, the separation of the hydrograph at this site is simple, i.e. only two end-members need to be invokedto describe the hydrograph. These two end-members are a rainwater (or ‘new water’,) and an ‘old water’ thatcan be associated with water stored in the peat matrix in the catotelm. During an event new water enters thesystem and comes to dominate the discharge, suppressing the conductivity and the lowering the pH of thestream: this is true for all the observed runoff events. It should be noted that new water dominates all runoffevents: this is not true in many other settings, where old water has been shown to dominate. For example, inforested catchments the water dominating runoff events is typically groundwater stored from previous eventsor accumulated between events that has an evolved composition indicative or interaction with components ofthe soil or rock in which it was stored (Peters et al., 1995). The dominance of old water in settings such asforests or catchments with deep mineral soils has been ascribed to two mechanisms, either the dominance ofvertical flow paths to the soil–rock interface, or the presence of old water at shallow depths in riparian areasthat is forced out by a head of new water developing upslope during rainstorms. The lack of such old waterdominance in this setting suggests that both mechanisms can be discounted here, i.e. that vertical flowpathsare not important and that riparian zones do not dominate the nature of the stream discharge.

The absorbance response is decoupled from the hydrology and is controlled by a different set of stores.Firstly, there is a catotelm water that represents the most evolved or old water but that has been largelyexhausted of its available carbon. Secondly, there is a new water that has little absorbance; this new watershould not be thought of as rainwater but as the nearest equivalent. The third component is water thatacquired additional available carbon from the acrotelm but is still substantially new water. Combining these



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Figure 11. pH versus flow for the entire period of the study

three water types with the observation of the two types of hysteresis and the information from Equation (1)makes it possible to describe what happens during a runoff event.

Prior to any event, during baseflow conditions, water is slowly draining from the catotelm; this water haslargely exhausted its supply of available carbon with the supply coming from background turnover in thecatotelm or from storage of water coming from the acrotelm during or between events. As a rainstorm startsthe rainwater enters the peat and causes the water table to rise into the acrotelm. The acrotelm, dry prior to theevent, has been the site of oxidation, turnover of carbon and thus production of available carbon. As the watertable rises this available carbon is acquired and moved out of the system. The water entering the stream atthis stage is the water that is flushing the acrotelm, but it is new water. This represents ‘percolation-excess’,i.e. runoff generated in the acrotelm at the catotelm–acrotelm interface. As an event progresses evidencesuggests that runoff is now of new water with low absorbance; this can be generated by two possibilities.If the whole profile saturates, overland flow occurs or macropores become viable that allow the transportof new water directly to the stream. Alternatively, the acrotelm becomes exhausted of carbon as the watertable rises through it. Transport in macropores or as overland flow would occur with minimal interactionwith the available carbon-rich pore-water and so the absorbance levels in the stream begin to decrease as thecharacter of the water becomes increasingly like rainwater (new water). This also would explain the hysteresisobserved. The second possibility is that as the water table rises through the catotelm the supply of availablecarbon in this region is rapidly exhausted and so absorbance levels drop as the supply becomes exhausted.Either mechanism could explain the decrease in stream levels of DOC, the hysteresis and the dominance ofnew water. In a forest in Japan, Sakamoto et al. (1999) showed that clockwise and anticlockwise hysteresisoccurred for DOC movement, but in that case it was related to temperature control on the biological productionof available carbon.



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Figure 12. Comparison of the proportion of new water to absorbance

Do these two mechanisms explain the change in E4/E6 ratio observed across different styles of event? Overthe anticlockwise events no change in DOC composition is observed, but a decrease in E4/E6 is observedfor clockwise events. If the decrease in absorbance seen during larger events is caused by flow throughmacropores, or by overland flow, then to explain any observed fractionation would mean that this occurred inthe exchange between the fluid-filled peat matrix and the quickflow. Alternatively, if the observed decreasein absorbance during large events is the result of an exhaustion effect, then the fractionation would resultfrom the fact that some forms of the carbon are more mobile than others. This latter mechanism appearsmore physically realistic and thus exhaustion effects also must be contributing to the decrease in DOC levelsduring large events.

It might now be hypothesized that the occurrence of any exhaustion effect is not related only to the size ofthe runoff event, i.e. the amount of flow through the soil, but also to the time between events. As time after anevent increases the oxidation of carbon continues, so adding to the pool of carbon available. The longest dryperiod experienced during this study led to the highest E4/E6 ratios observed. Changes in the characteristicsof carbon released across an event are of concern if the change in characteristics also reflects a change in, forexample, the trihalomethane (THM) formation potential of the carbon being released—THM is a carcinogen,the concentration of which is limited in domestic water supply (Rook, 1974). However, it also must be truethat such changes in character must transfer through the catchment.

The events monitored in this study were only for one part of the year, judged a priori to be the re-wettingperiod of the upland peat. During such a re-wetting period it would be expected that the amount of carbonbeing released would be at an annual maximum as the water table rises through the acrotelm (Evans et al.,1998) and that carbon oxidized over the summer period is flushed out. However, how increased levels overthis period of re-wetting translate across the rest of the year is not possible to judge from this record.



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Figure 13. Probability of a runoff event occurring. Isoprobability lines of an event occurring

CONCLUSIONS

This detailed investigation of the release of DOC to a first-order stream from upland peat has shown that:

1. The release of DOC from peat is event-based, i.e. the highest levels are associated with runoff events.2. Release of DOC is partly decoupled from the hydrological behaviour of the peat.3. The peat behaves hydrologically a is if it were a two end-member system consisting of old, interevent

water and new, event water. During events new water dominates.4. The discharge of organic matter behaves like a three end-member system, with the between-event water

being low in DOC and events being characterized by two types of water, the initial stage event beingcharacterized by new water rich in DOC that gives way to new water depleted in DOC as an eventprogresses, which can be ascribed to the runoff generation being initially by percolation excess progressingto saturation-excess.

5. Evidence from the E4/E6 ratio suggests that exhaustion of reserves of available carbon during events isaccompanied by fractionation in the type of carbon being mobilized.

6. Production of available carbon in the catchment is as high as 4Ð5 g C/day per m3 of peat, suggesting aturnover rate of peat of the order of 42 years.

The length of the study means that it is not possible to statistically test this model nor to understand thecontext of these results within the annual pattern of peat hydrology.



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Figure 14. Probability of an absorbance event occurring. Isoprobability lines of an event occurring

ACKNOWLEDGEMENTS

The authors are grateful to Northumbrian Water Ltd for their financial support for this project. TheEnvironmental Change Network (ECN) for provision of additional data.

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