Land management as a factor controlling dissolved organic carbon release from upland peat soils 2: Changes in DOC productivity over four decades

Increasing DOC concentrations in surface waters have been observed across parts of Europe and North America over the past few decades. Most proposed explanations for these widespread trends invoke climate change or reductions in sulphate deposition. However, these factors do not seem apposite to explain either the fine-scale (within kilometres) or regional-scale spatial variation in DOC concentrations observed across the UK.We have reconstructed DOC concentrations and land use for one North Pennine and five South Pennine catchments (UK), located in three discrete areas, over the last four decades. Rainfall, temperature and sulphate deposition data, where available, were also collated and the potential influence of these factors on surface water DOC concentrations was assessed.Four of the six catchments examined showed highly significant (p < 0.001) increases (53-92%) in humic coloured DOC (hDOC) concentrations in drainage waters over the period 1990-2005. Changes in temperature and sulphate deposition may explain 20-30% of this trend in these four catchments. However, the rapid expansion of new moorland burn on blanket peat can explain a far greater degree (> 80%) of the change in hDOC. Far smaller increases in hDOC (10-18%) were identified for the two remaining catchments. These two sites experienced similar changes in sulphur deposition and temperature to those that had seen largest increases in DOC, but contained little or no moorland burn management on blanket peat.This study shows that regional-scale factors undoubtedly underlie some of the recent observed increases in drainage humic coloured DOC. However, changes in land management, in this case the extensive use of fire management on blanket peat, are a far more important driver of increased hDOC release from upland catchments in some parts of the UK. It suggests that the recent rapid increase in the use of burning on blanket peat moorland has implications for ecosystem services and carbon budgets.     

Land management as a factor controlling dissolved organic carbon release from upland peat soils 1: Spatial variation in DOC productivity

The importance of soil storage in global carbon cycling is well recognised and factors leading to increased losses from this pool may act as a positive feedback mechanism in global warming. Upland peat soils are usually assumed to serve as carbon sinks, there is however increasing evidence of carbon loss from upland peat soils, and DOC concentrations in UK rivers have increased markedly over the past three decades. A number of drivers for increasing DOC release from peat soils have been proposed although many of these would not explain fine-scale variations in DOC release observed in many catchments.We examined the effect of land use and management on DOC production in upland peat catchments at two spatial scales within the UK. DOC concentration was measured in streams draining 50 small-scale catchments (< 3 km²) in three discrete regions of the south Pennines and one area in the North Yorkshire Moors. Annual mean DOC concentration was also derived from water colour data recorded at water treatment works for seven larger scale catchments (1.5-20 km²) in the south Pennines. Soil type and land use/management in all catchments were characterised from NSRI digital soil data and ortho-corrected colour aerial imagery.Of the factors assessed, representing all combinations of soil type and land use together with catchment slope and area, the proportion of exposed peat surface resulting from new heather burning was consistently identified as the most significant predictor of variation in DOC concentration. This relationship held across all blanket peat catchments and scales.We propose that management activities are driving changes in edaphic conditions in upland peat to those more favourable for aerobic microbial activity and thus enhance peat decomposition leading to increased losses of carbon from these environments.     

Link between DOC in near surface peat and stream water in an upland catchment

Hydrologic transport of dissolved organic carbon (DOC) from peat soils may differ to organo-mineral soils in how they responded to changes in flow, because of differences in soil profile and hydrology. In well-drained organo-mineral soils, low flow is through the lower mineral layer where DOC is absorbed and high flow is through the upper organic layer where DOC is produced. DOC concentrations in streams draining organo-mineral soils typically increase with flow. In saturated peat soils, both high and low flows are through an organic layer where DOC is produced. Therefore, DOC in stream water draining peat may not increase in response to changes in flow as there is no switch in flow path between a mineral and organic layer. To verify this, we conducted a high-resolution monitoring study of soil and stream water at an upland peat catchment in northern England. Our data showed a strong positive correlation between DOC concentrations at -1 and -5 cm depth and stream water, and weaker correlations between concentrations at -20 to -50 cm depth and stream water. Although near surface organic material appears to be the key source of stream water DOC in both peat and organo-mineral soils, we observed a negative correlation between stream flow and DOC concentrations instead of a positive correlation as DOC released from organic layers during low and high flow was diluted by rainfall. The differences in DOC transport processes between peat and organo-mineral soils have different implications for our understanding of long-term changes in DOC exports. While increased rainfall may cause an increase in DOC flux from peat due to an increase in water volume, it may cause a decrease in concentrations. This response is contrary to expected changes in DOC exports from organo-mineral soils, where increase rainfall is likely to result in an increase in flux and concentration.     

Spatial variation in concentrations of dissolved nitrogen species in an upland blanket peat catchment

The concentration of nitrogen (N), particularly as nitrate (NO3-N), in upland streams, lakes and rivers is frequently used as a diagnostic of the vulnerability of upland ecosystems to increased atmospheric nitrogen deposition and N saturation. The N content of running waters, however, is generally assessed on the basis of sampling at a limited number of points in space and time within the catchment under investigation. The current study was conducted at Trout Beck, an 11.5 km2 blanket peat-dominated catchment in the North Pennine uplands of the UK. Results from sampling at 33 sites within this catchment demonstrated that the concentrations of all dissolved N species were highly variable, even over short distances. Statistical relationships between the concentrations of NO3-N and dissolved organic nitrogen (DON) and percentage catchment cover of Calluna/Eriophorum and Eriophorum vegetation were found. However, it was also noted that in catchments containing limestone outcrops, NO3-N concentration was much higher than in catchments where runoff was sourced directly from the blanket peat surface. It is possible that NH4-N and DON leached from the blanket peat are mineralised and nitrified, providing a source for the NO3-N found in the river channels. Overall, the current study suggests that interpretations of N-saturation based on river water chemistry measurements at a single point must be treated cautiously, and that the influence of catchment-scale physical factors, such as vegetation and geology cover on the concentration of dissolved N species in upland river waters should not be ignored.     

Manganese and land-use in upland catchments in Scotland

Manganese (Mn) in surface waters is a micronutrient, but elevated concentrations are toxic to fish and impair drinking water quality. In Scotland, undesirable Mn concentrations (> 0.05 mg l(-1)) occur predominantly in upland freshwaters because the acidic pH and organic nature of catchment soils favour Mn mobilisation. The relationship between upland land-use in Scotland and Mn concentrations in surface waters is reviewed. Conifer afforestation is associated with enhanced Mn in runoff. Mn is leached from conifer foliage and litter, and mature conifers enhance acid deposition and loss of Mn from acidified catchment soils. After harvesting, increased soil pools of water-soluble Mn and elevated Mn concentrations in runoff have been observed. Liming, fertiliser addition, drainage ditch construction and ploughing to improve upland pastures, and muirburn on grouse moors may also increase Mn concentrations in runoff, but the evidence is less clear-cut. The extent to which land-use influences Mn concentrations in upland catchments in Scotland is modified by catchment hydrology and soil type. Catchment geology, instream processes and standing water stratification are probably lesser influences on Mn concentrations in surface waters of upland catchments in Scotland. The location of land-use in upland catchments, especially in the riparian zone, is critical in determining its effect on Mn in runoff. Climate change is expected to increase Mn concentrations in runoff from upland catchments in Scotland because of predicted changes in soil hydrology.     

The nitrogen composition of streams in upland Scotland: some regional and seasonal differences

The nitrogen (N) composition of streams draining four upland regions of Scotland was compared in samples collected monthly between April 1997 and April 1998. Stream samples were analysed for total N (TN), particulate N (PN), nitrate (NO3), ammonium (NH4), dissolved organic N (DON) and dissolved organic carbon (DOC). Concentrations of TN were small, generally less than 1 mg l(-1) , dominated by dissolved forms of N, and varied significantly between upland regions. Nitrate accounted for most of the variability in TN; largest concentrations were observed in the Southern Uplands and smallest concentrations were observed in the Highlands. Nitrate concentrations were positively correlated with the percentage cover of improved grasslands and brown forest soils and negatively correlated with the percentage cover of peat. Concentrations of DON also varied between regions, but to a lesser extent than those of NO3. Largest concentrations occurred in SW Scotland and smallest concentrations in the Cairngorms. Although a significant positive correlation between DON and DOC was observed, stream water DON content was not related to the percentage cover of peat in the catchment, as was the case for DOC. The average DOC:DON ratio was narrower for streams in the Southern Uplands than for those in the Cairngorms and Highlands. Nitrate and DON displayed contrasting seasonal trends; NO3 concentrations were larger in the winter while DON concentrations were larger in the summer. Only a small proportion, < 8% and < 7%, of TN was PN and NH4, respectively, the majority of N was present as either NO3 or DON. Nitrate was the dominant fraction (58-65%) in all regions except the Highlands where DON accounted for 57% of TN. However, the relative importance of the DON component increased in the summer in all regions. This study has demonstrated that the DON fraction is an important component of the total N transported by streams from upland catchments in Scotland. Thus, assessments of anthropogenic impacts on N losses from upland ecosystems need to consider not only the dissolved inorganic species but also DON.     

Temporal variations in dissolved organic carbon concentrations in upland and lowland lakes in North Wales

Over a period of 18 months, the dissolved organic carbon (DOC) concentration of a series of four lakes in North Wales was measured monthly. The lake catchment profiles consisted of an upland thin peat/soil (Llyn Cwellyn), an upland thin peat/soil associated with an adjacent area of small bog (Llyn Teyrn), an upland blanket bog (Llyn Conwy), and large lowland fen and fertile agricultural area (Llyn Cefni). The results examine the indirect effect of temperature and precipitation on the DOC concentrations found in the lakes fed by the catchments. The lowest DOC of the four sites was observed for Llyn Teyrn, varying from 1.2 to 3.30 mg/L, and with the highest being recorded for Llyn Cefni (5.45–10.83 mg/L). Temperature and rainfall data were both collected. Correlations with the DOC exhibited significant relationships with temperature for three of the sampled lakes Cwellyn (r 0.490), Teyrn (r 0.640) and Cefni (r 0.472). Recomputation versus 30‐ and 60‐day temperature lag times improved the correlation coefficients. The data showed weak and insignificant correlations for DOC versus rainfall for the three lakes, but the upland lake, Llyn Conwy, with its blanket bog catchment, did not demonstrate any statistical correlation with temperature, although it did show a significant correlation for DOC versus rainfall (r 0.553, P < 0.05). Over the sampling period, although tentative relationships were found among temperature, rainfall and DOC levels, an indirect association tempered by site hydrology is suggested.     

Spatial and temporal variability in the relationship between water colour and dissolved organic carbon in blanket peat pore waters

The transfer of carbon from terrestrial peat to the fluvial environment forms an important component of the peatland carbon cycle, and has major implications for water quality. Dissolved organic carbon (DOC) is generally considered the largest constituent of aquatic carbon and tends to be the most intensively monitored, particularly in peatland catchments. However, many long-term records for DOC are based on proxy studies that use water colour as a surrogate. This paper tests the robustness of using spectrophotometric techniques to monitor water colour, based on absorbance from a single wavelength at 400 nm, as a surrogate for true DOC determination. The general ability of spectrophotometric analysis to measure low DOC concentrations depends on the calibration used; thus, the minimum mass of DOC detectable varies considerably and in this study was found to be as high as 10.32 mg C L^− 1. While there is often a significant correlation between water colour and DOC, it was found that the use of single or even “pooled” regressions to predict DOC concentrations could result in miscalculations of more than 50%. Further, the water colour-DOC relationship in blanket peat pore waters was found to vary significantly between peat layers, land management treatments and through time. Thus, studies using long-term water colour records as a proxy for long-term DOC concentrations in peatlands must be treated with a certain degree of caution, especially in cases where changes may have taken place to DOC production, such as those caused by land management change, during the course of investigation.     

Ditch blocking, water chemistry and organic carbon flux: evidence that blanket bog restoration reduces erosion and fluvial carbon loss

The potential for restoration of peatlands to deliver benefits beyond habitat restoration is poorly understood. There may be impacts on discharge water quality, peat erosion, flow rates and flood risk, and nutrient fluxes. This study aimed to assess the impact of drain blocking, as a form of peatland restoration, on an upland blanket bog, by measuring water chemistry and colour, and loss of both dissolved (DOC) and particulate organic carbon (POC). The restoration work was designed to permit the collection of a robust experimental dataset over a landscape scale, with data covering up to 3 years pre-restoration and up to 3 years post-restoration. An information theoretic approach to data analyses provided evidence of a recovery of water chemistry towards more ‘natural’ conditions, and showed strong declines in the production of water colour. Drain blocking led to increases in the E4:E6 ratio, and declines in specific absorbance, suggesting that DOC released from blocked drains consisted of lighter, less humic and less decomposed carbon. Whilst concentrations of DOC showed slight increases in drains and streams after blocking, instantaneous yields of both DOC and POC declined markedly in streams over the first year post-restoration. Attempts were made to estimate total annual fluvial organic carbon fluxes for the study site, and although errors around these estimates remain considerable, there is strong evidence of a large reduction in aquatic organic carbon flux from the peatland following drain-blocking. Potential mechanisms for the observed changes in water chemistry and organic carbon release are discussed, and we highlight the need for more detailed information, from more sites, to better understand the full impacts of peatland restoration on carbon storage and release.     

Is in-stream processing an important control on spatial changes in carbon fluxes in headwater catchments?

Data on small-scale spatial variations in instantaneous fluxes and concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and free carbon dioxide (CO2) are presented for a small acidic headwater stream in NE Scotland. Chloride is used as a conservative element to estimate additional, diffuse inputs of water into the main stem of the stream, other than those from tributaries. Downstream changes in instantaneous carbon fluxes were calculated and then used to estimate losses and gains of carbon within the stream system. Dissolved organic carbon concentrations in the stream ranged from 1.19-6.06 mg l(-1) at its source to a maximum of 10.0-25.3 mg l(-1) as the stream passed through deep peats; DOC concentrations then declined in the lower part of the catchment. DIC concentrations were initially low, increased to 1.5-3.0 mg l(-1) and then decreased to 0.1-1.65 mg l(-1) at the lowest site. Free CO2 concentrations increased from 0.35 mg l(-1) at the stream source to 3.30 mg l(-1) as the stream passed through the peat dominated area. Continually high inputs of CO2-rich water (> 6.0 mg l(-1)) from tributaries maintained these high concentrations in the main stem, until approximately 1.74 km downstream, when there was a rapid decline in concentration. Significant changes in DOC, DIC and CO2 fluxes occur over a distance of 2.7 km downstream from the stream source to the catchment outlet. Between 5.64-41.5 mg C s(-1) as DOC and 2.52-16.2 mg C s(-1) as DIC are removed from the water column. Between 6.81 and 19.0 mg C s(-1) as CO2 is lost along the stream length as progressive equilibration with the atmosphere occurs. We estimate that 11.6-17.6% of the total DOC flux is removed from streamwater by in-stream processes. Dissolved inorganic carbon (HCO3- and free CO2) losses are in excess of nine times its measured flux at the outlet of the catchment. These results suggest that in-stream processing of DOC and DIC and outgassing of CO2 are important controls on the spatial variability of carbon fluxes within headwater streams in upland catchments dominated by organic-rich soils.     

Carbon budget for a British upland peat catchment

This study describes the analysis of fluvial carbon flux from an upland peat catchment in the North Pennines. Dissolved organic carbon (DOC), pH, alkalinity and calcium were measured in weekly samples, with particulate organic carbon (POC) measured from the suspended sediment load from the stream outlet of an 11.4-km(2) catchment. For calendar year 1999, regular monitoring of the catchment was supplemented with detailed quasi-continuous measurements of flow and stream temperature, and DOC for the months September through November. The measurements were used to calculate the annual flux of dissolved CO(2), dissolved inorganic carbon, DOC and POC from the catchment and were combined with CO(2) and CH(4) gaseous exchanges calculated from previously published values and the observations of water table height within the peat. The study catchment represents a net sink of 15.4+/-11.9 gC/m(2)/yr. Carbon flows calculated for the study catchment are combined with values in the literature, using a Monte Carlo method, to estimate the carbon budget for British upland peat. For all British upland peat the calculation suggests a net carbon sink of between 0.15 and 0.29 MtC/yr. This is the first study to include a comprehensive study of the fluvial export of carbon within carbon budgets and shows the size of the peat carbon sink to be smaller than previous estimates, although sensitivity analysis shows that the primary productivity rather than fluvial carbon flux is a more important element in estimating the carbon budget in this regard.     

Buffering of recovery from acidification by organic acids

In the United Kingdom, as in other regions of Europe and North America, recent decreases in surface water sulphate concentrations, due to reduced sulphur emissions, have coincided with marked increases in dissolved organic carbon (DOC) concentrations. Since many of the compounds comprising DOC are acidic, the resulting increases in organic acidity may have the potential to offset the benefits of a decrease in mineral (sulphate) acidity. To test this, we used a triprotic model of organic acid dissociation to estimate the proportional organic acid buffering of reduced mineral acidity as measured in the 22 lakes and streams monitored by the UK Acid Waters Monitoring Network. For an average non-marine sulphate decrease of 30 mueq l(-1) over 15 years from 1988-2003, we estimate that around 28% was counterbalanced by rising strong organic acids, 20% by rising alkalinity (partly attributable to an increase in weak organic acids), 11% by falling inorganic aluminium and 41% by falling non-marine base cations. The situation is complicated by a concurrent decrease in marine ion concentrations, and the impact this may have had on both DOC and acidity, but results clearly demonstrate that organic acid increases have substantially limited the amount of recovery from acidification (in terms of rising alkalinity and falling aluminium) that have resulted from reducing sulphur emissions. The consistency and magnitude of sulphate and organic acid changes are consistent with a causal link between the two, possibly due to the effects of changing acidity, ionic strength and aluminium concentrations on organic matter solubility. If this is the case, then organic acids can be considered effective but partial buffers to acidity change in organic soils, and this mechanism needs to be considered in assessing and modelling recovery from acidification, and in defining realistic reference conditions. However, large spatial variations in the relative magnitude of organic acid and sulphate changes, notably for low-deposition sites in northwestern areas where organic acid increases apparently exceed non-marine sulphate decreases, suggest that additional factors, such as changes in sea-salt deposition and climatic factors, may be required to explain the full magnitude of DOC increases in UK surface waters.     

Can climate change explain increases in DOC flux from upland peat catchments?

Long-term increases in DOC concentration in rivers draining areas of upland peat are a ubiquitous phenomenon in the UK. Several hypotheses have been proposed to explain these increases, but one compelling explanation is the observed long-term increase in temperature in UK uplands causing increases in peat decomposition rates, and increasing the depth of oxidation as evaporation increases depth to the water table. The study constructed an empirical model for water table depth and decomposition rate calibrated against observations from the Environmental Change Network monitoring site at Moor House in the North Pennines, UK. The study shows: (i) Depth of the water table has not changed significantly over a 30-year period, reflecting the fact that blanket peat is well buffered against climate change. (ii) Increases in temperature are responsible for a 12% increase in DOC production while an approximate 78% increase in DOC production has been observed. (iii) Overall DOC production is predicted to rise by 6% but observation suggests increases on the scale of 97%. (iv) The model inadequately represents changes in production and supply of DOC during periods of severe drought. The study shows that temperature change alone is insufficient to explain observed increases in DOC production. Alternative explanations for large increases in DOC production could include changes in land management, but an enzymic latch mechanism, i.e. derepression of anaerobic degradation, causing increased decomposition rates in response to severe drought is preferred.     

Stream water hydrochemistry as an indicator of carbon flow paths in Finnish peatland catchments during a spring snowmelt event

Extreme hydrological events are known to contribute significantly to total annual carbon export, the largest of which in Arctic and boreal catchments is spring snowmelt. Whilst previous work has quantified the export of carbon during snowmelt, the source of the carbon remains unclear. Here we use cation hydrochemistry to trace the primary flowpaths which govern the export of carbon during the snowmelt period; specifically we aim to examine the importance of snowpack meltwater to catchment carbon export. The study was carried out in two forested peatland (drained and undrained) catchments in Eastern Finland. Both catchments were characterised by base-poor stream water chemistry, with cation concentrations generally decreasing in response to increasing discharge. Streamflow during the snowmelt period was best described as a mixture of three sources: pre-event water, snowpack meltwater and a third dilute component we attribute to the upper snow layer which was chemically similar to recent precipitation. Over the study period, pre-event water contributed 32% and 43% of the total stream runoff in Välipuro (undrained) and Suopuro (drained), respectively. The results also suggest a greater near-surface throughflow component in Suopuro, the drained catchment, prior to snowmelt. CO₂ and DOC concentrations correlated positively with cation concentrations in both catchments indicating a common, peat/groundwater flowpath. CH₄ concentrations were significantly higher in the drained catchment and appeared to be transported in near-surface throughflow. Meltwater from the snowpack represented an important source of stream water CO₂ in both catchments, contributing up to 49% of total downstream CO₂ export during the study period. We conclude that the snowpack represents a potentially important, and often overlooked, transient carbon store in boreal snow-covered catchments.     

Drain blocking: an effective treatment for reducing dissolved organic carbon loss and water discolouration in a drained peatland

Peatlands are an important terrestrial carbon store. However, heightened levels of degradation in response to environmental change have resulted in an increased loss of dissolved organic carbon (DOC) and an associated rise in the level of discolouration in catchment waters. A significant threat to peatland sustainability has been the installation of artificial drainage ditches. However, recent restoration schemes have pursued drain blocking as a possible strategy for reducing degradation, fluvial carbon loss and water discolouration. This paper investigates the effect of open cut drainage and the impact of drain blocking on DOC and colour dynamics in blanket peat soil-water solutions. Three treatments (intact peat, drained peat and drain-blocked peat) were monitored in an upland blanket peat catchment in the UK. DOC and colour values were significantly higher on the drained slopes compared with those of the intact peat, which in turn had greater DOC and colour values than the drain-blocked slopes. Consequently, drain blocking is shown to be a highly successful technique in reducing both the DOC concentration and level of discolouration in soil waters, even to values lower than those observed for the intact site, which suggests a process of store exhaustion and flushing may operate. The colour per carbon unit (C/C) ratio was significantly higher at the drain-blocked site than either the intact or the drained treatments, while the E4/E6 ratio (fulvic acid/humic acid) was significantly lower at the blocked site compared to the two other treatments. The high C/C and low E4/E6 ratios indicate that drain blocking also modifies the composition of DOC, such that darker-coloured humic substances become more dominant compared to the intact site. This implies disturbance to DOC production and/or transportation processes operating within the peat.     

Trends in stream water quality in Environmental Change Network upland catchments: the first 5 years

Hydrochemical data from catchment streams at three Environmental Change Network (ECN) upland catchments in north-east Scotland, south-east Scotland and northern England have been subjected to statistical analysis to separate the components due to long-term trend, season and flow. The relative variances due to each are presented for pH, Ca, SO4-S, NO3-N, DOC and Cl. Long-term trends have been identified with increases in concentration for NO3-N and DOC, decreases in SO4-S concentrations, along with short-term seasonal fluctuations. Application of this trend technique has allowed the detection of changes and contributed to an improved understanding of catchment behaviour.     

Interaction of nitrogen deposition and land use on soil and water quality in Scotland: issues of spatial variability and scale

Over large areas of the Scottish uplands anthropogenic sulfur (S) deposition is declining in response to stringent national and European controls on S emissions. At the same time, however, the relative contribution of nitrogenous (N) compounds to the total anthropogenic deposition loading has increased. To investigate the significance of N deposition on the potential acidification of surface waters, national, regional, and catchment databases were developed to assess the relationships between N deposition, soil C/N ratios, land use and surface water NO3 concentrations. National classification schemes for land use and soils were used as only limited empirical data are available at such large spatial scales. Data were screened to eliminate areas where N inputs are dominated by non-atmospheric sources. From these screened datasets, it was apparent that areas with the highest risk of N leaching were situated predominantly in the upland areas of south-west and west Scotland (areas with low soil C/N ratios). At the regional scale, surface-water NO3 concentration in afforested catchments was negatively correlated with soil C/N ratios below 20. This relationship was not evident in moorland catchments, where NO3 leaching was strongly related to N deposition and the loch/catchment ratio, rather than the soil C/N ratio. Temporal trends of regional water quality highlighted as increasing loch NO3 concentrations between 1988 and 1996-1997, presumably reflecting an increase in N deposition, enhanced leaching losses from the terrestrial component of the catchment, or altered in-lake processes. The hydrochemical records for two catchments in NE Scotland (Lochnagar and Allt a Mharcaidh) highlight the importance of within catchment process in controlling the nitrogen response observed in surface waters. The potential mechanisms through which vegetation and soils may modify incoming deposition are discussed.     

Source characterisation and loads of metals and pesticides in urban wet weather discharges

Contaminants in urban wet weather discharges originate from a number of sources such as materials from wet and dry atmospheric deposition, wastewaters, urban surface erosion, traffic-related activities, in-sewer deposits, etc. In the current study, four contributions (rainwater, dry atmospheric deposition, dry weather discharge and catchment surface + possible erosion of in-sewer deposits) to the total concentrations of priority substances have been assessed at the outlet of two urban catchments (one residential catchment with a combined system and one industrial area with a separate stormwater system) for 12 storm events (six for each catchment). Mass balances were calculated for seven metals and four pesticides, as well as for total suspended solids and chemical oxygen demand. The respective contributions of dry and wet atmospheric deposition, wastewater and catchment surface differ for each pollutant type, corresponding to different land use, activities, environments and sewer systems. For most of the pollutants, the catchment surface appears to be the main contribution, with significant storm event variability, excepted for atrazine in one catchment.     

The influence of summer seasonal extremes on dissolved organic carbon export from a boreal peatland catchment: evidence from one dry and one wet growing season

A small peatland catchment in eastern Finland was monitored for runoff and export of dissolved organic carbon. To exemplify the response of a peatland under years of different meteorological conditions, data from one very dry and one wet growing season are presented. Runoff was 194 and 387 mm, and DOC export was 4.2 and 11.3 gC m^− 2 in the dry and wet year respectively. Analysis of the hydrographs showed that in both years most runoff and DOC export was generated during peak flow events, with low baseflow in the intermediate periods. Runoff response to rain events was strongest under a high water table in the spring and autumn, with a summer runoff minimum. The effect of drought on runoff occurred only after mid June when the effects of snowmelt had dissipated. Snowmelt therefore dominated DOC export in the dry year (61%) but contributed much less (29%) in the wet year. The relationship between runoff and the water table was highly similar in both years. Any variation that was observed herein was potentially related to a superficial subsidence of the peat surface under drier summer conditions and to raised spring water levels, causing restricted drainage due to flooding. The observed variation of DOC concentrations in both years generally had a limited impact on DOC export. During peak runoff events, concentrations dropped proportionally to the magnitude of the events. This decrease in available DOC is argued to be caused by a dominance of leaching over supply and production and by the specific hydraulic characteristics of the peatland surface. DOC concentrations were some 25% higher under drought conditions. These elevated concentrations were dispelled by spring snowmelt of the next year, demonstrating that a winter snowpack in boreal regions may be instrumental in limiting the longer term hydrological and biochemical effects of a drought.     

Laboratory study of impacts of concrete fragment sizes on wetland water chemistry

This study investigated the effects of urban concrete materials, of different particle sizes, immersed in water through a laboratory-based experiment. Water was sourced from a high conservation-value wetland (Blue Mountains upland swamp). Prior to the experiment, wetland water was dilute (32.5 μS/cm), acidic (pH 5.3) and had detectable major ion concentrations of only sodium and chloride. Water was exposed to three treatments of different concrete fragment sizes (whole, crushed and fine). All treatments increased conductivity and pH and also resulted in modified ionic composition where calcium, potassium, hydroxide, carbonate and sulphate were recorded at much high levels. The extent and speed of water chemistry changes was linked to the particle size of the concrete. The results of this study support the hypothesis that concrete can be an environmentally hazardous material influencing water quality in urban catchments.