Characterizing Pb Mobilization from Upland Soils to Streams Using

Dec 2, 2009 - Upland Soils to Streams Using. 206Pb/207Pb Isotopic Ratios. JULIAN J. C. DAWSON,*, †. DOERTHE TETZLAFF,† ANNE-MARIE CAREY,‡...
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Environ. Sci. Technol. 2010, 44, 243–249

Characterizing Pb Mobilization from Upland Soils to Streams Using 206 Pb/207Pb Isotopic Ratios J U L I A N J . C . D A W S O N , * ,† DOERTHE TETZLAFF,† ANNE-MARIE CAREY,‡ A N D R E A R A A B , ‡,§ C H R I S S O U L S B Y , † KENNETH KILLHAM,‡ AND ANDREW A. MEHARG‡ Northern Rivers Institute, School of Geosciences, University of Aberdeen, St. Mary’s Building, Elphinstone Road, Aberdeen AB24 3UF, U.K., Institute of Biological and Environmental Sciences, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen AB24 3UU, U.K., and Department of Chemistry, University of Aberdeen, Meston Building, Meston Walk, Aberdeen AB24 3UE, U.K.

Received September 2, 2009. Revised manuscript received November 21, 2009. Accepted November 23, 2009.

Anthropogenically deposited lead (Pb) binds efficiently to soil organic matter, which can be mobilized through hydrologically mediated mechanisms, with implications for ecological and potable quality of receiving waters. Lead isotopic (206Pb/207Pb) ratios change down peat profiles as a consequence of longterm temporal variation in depositional sources, each with distinctive isotopic signatures. This study characterizes differential Pb transport mechanisms from deposition to streams at two small catchments with contrasting soil types in upland Wales, U.K., by determining Pb concentrations and 206Pb/207Pb ratios from soil core profiles, interstitial pore waters, and stream water. Hydrological characteristics of soils are instrumental in determining the location in soil profiles of exported Pb and hence concentration and 206Pb/207Pb ratios in surface waters. The highest Pb concentrations from near-surface soils are mobilized, concomitant with high dissolved organic carbon (DOC) exports, from hydrologically responsive peat soils with preferential shallow subsurface flows, leading to increased Pb concentrations in stream water and isotopic signatures more closely resembling recently deposited Pb. In more minerogenic soils, percolation of water allows Pb, bound to DOC, to be retained in mineral horizons and combined with other groundwater sources, resulting in Pb being transported from throughout the profile with a more geogenic isotopic signature. This study shows that 206Pb/207Pb ratios can enhance our understanding oftheprovenancesandtransportmechanismsofPbandpotentially organic matter within upland soils.

1. Introduction Fluvial losses of soil organic matter cause discolorization of waters and are important for calculating terrestrial carbon budgets (1). Moreover, potentially toxic elements (PTEs) binding to dissolved organic carbon (DOC) or particulate * Corresponding author phone: +44 (0)1224 273728; fax: +44 (0)1224 272331; e-mail: [email protected]. † Northern Rivers Institute, School of Geosciences. ‡ Institute of Biological and Environmental Sciences. § Department of Chemistry. 10.1021/es902664d

 2010 American Chemical Society

Published on Web 12/02/2009

material will also be exported (2-5). Mobilization of organically bound PTEs can impact quality of surface waters, with implications for riverine biodiversity and economic consequences for potable waters (1, 2, 4). Furthermore, floodplain or riverbed deposition of terrestrially derived suspended particulate matter may also lead to contamination of such habitats (4). Understanding how soil biogeochemical and hydrological processes interact (6) to control fluvial transport of PTEs is important for underpinning sustainable management strategies. Trends in atmospherically deposited lead (Pb) have been relatively consistent in ombrotrophic peat profiles across Western Europe (7). In the U.K., anthropogenic Pb deposition in peats increased from pre-Industrial Revolution levels to maximal concentrations (occurring ca. 1860 in the Southern Pennines of N. England and 1940s to late 1960s across Scotland) as metal-ore mining, industrial usage, leaded petroleum (1930s to 1990s), and coal combustion (up to late 1960s) increased. Elevated Pb concentrations were still apparent in the late 1980s, but have declined steadily in recent years as sources of anthropogenic Pb declined (4, 7-9). Lead isotopic ratios (e.g., 206Pb/207Pb) vary down ombrotrophic peat profiles as a consequence of long-term temporal variation in depositional sources, each with distinctive isotopic signatures (10-12). Lead from Australian ore, commonly used for petroleum additives in the U.K., has been characterized by low 206Pb/207Pb ratios (∼1.04); Pb ratios in coal were ∼1.16-1.18 (8, 13). Preindustrial soil-deposited Pb generally had higher ratios of ∼1.17-1.18 and geogenicderived 206Pb/207Pb ratios maybe even higher; crustal and bedrock ratios can be >1.20 (11, 14-17). Atmospherically deposited Pb is stored within ombrotrophic peats and tends not to undergo “postdepositional mobility” through profiles (4, 18), providing an accurate record of deposition. However, studies in more mineral-rich soils have suggested that vertical migration of a proportion of deposited Pb occurs, binding to organic matter, clay minerals, or oxides in lower horizons or potentially redistributed by soil invertebrates, such as enchytraeid worms (11, 15, 17, 19). Other studies have indicated translocation of Pb in soil to groundwater and surface water (10). In upland environments, the majority of Pb deposited binds efficiently to organic matter in soils and subsequently mobilized through hydrologically mediated DOC or particulate export (3, 4, 20, 21). Strong positive correlations with Pb have supported the importance of DOC concentrations in its transport (2, 22). The variability in the Pb content on suspended particulates during hydrological events has been explained by differences in organic matter content of the suspended material and erosion processes (4, 20, 23). To understand mechanisms controlling mobilization of Pb in upland catchments, mechanisms that describe movement of water through soils are required as the export of DOC and particulate material from soils is linked to discharge in response to precipitation events (24-27). In many U.K. upland catchments, flow path partitioning and mean transit times are dependent on topography and soils with contrasting hydrological responsiveness (28-30). Responsive soils, including hill peats, tend to be poorly drained, to remain close to saturation, and to be dominated by rapid runoff mechanisms, such as overland and preferential subsurface flows (30). If a catchment is wetting up during periods of higher discharges, increasing DOC concentrations, substantial soil organic matter translocation, and erosion occur (1, 26) and with it removal of soilbound Pb (23). Upland areas also contain less responsive soils, characterized by increased percolation and recharge to deeper VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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soil horizons and fractured bedrock; at times of lower precipitation, loss of connectivity between parts of the soil profile and drainage network occurs (27, 31, 32). The distribution and temporal connectivity of soils in relation to the drainage system can be a major influence on controlling DOC inputs to surface waters (27, 33). Recently, 206Pb/207Pb ratios have been used to assess mobility of atmospherically deposited lead in a variety of environmental matrixes, e.g., soil profiles (15, 34), landfill leachates to groundwater (14), and precipitation and surface waters in upland catchments (35, 36). It is hypothesized that different soil types with their inherent hydrological (28) and chemical properties will differentially affect the provenance of the soil-bound Pb that is transported to surface waters. The mobilization of Pb from varying depths within the soil profile will ultimately influence the composition of the 206 Pb/207Pb ratios in stream water. Thus, by determining Pb concentrations and 206Pb/207Pb ratios from bulk soil profiles, interstitial pore waters, dissolved (bound to DOC) and particulate phases of surface waters, we aimed to trace and characterize transport mechanisms of Pb from deposition and local geogenic origins to fluvial export within two small proximal moorland (organic-rich nonforested upland) catchments with contrasting soils.

2. Methods 2.1. Study Area. The two north-facing study sites (site A ) 0.36 km2; site B ) 0.41 km2) are situated in subcatchments of the Eunant Fawr, an upland catchment that drains in an easterly direction to the Lake Vyrnwy reservoir at Llanwddyn in Powys, Wales, U.K. A detailed description of the wider Lake Vyrnwy catchment area is given in the Supporting Information. Site A contains soils that have shallow organic horizons with a deeper gley minerogenic component and deep peat profiles; site B is wholly peat-dominated, characteristic of other upland moorland areas in Wales with peat depths of >1 m present (26). Both study sites with relative locations of soil cores, soil pore water, and stream water sampling are shown in Figure S1 of the Supporting Information. 2.2. Sample Collection, Preparation, and Analysis. Field sampling was conducted at the two sites on three separate occasions during the summer of 2008 (July 10-11, August 13-14, and September 11-12). Soil core profiles, pore water, and stream water (dissolved and particulate) samples were collected as detailed in the Supporting Information. Bulk soil and particulate samples were microwave-digested with a combination of concentrated nitric acid and hydrogen peroxide prior to chemical analysis. 2.2.1. Chemical Analysis. All aqueous samples (including digest extracts) were diluted accordingly for Pb, 206Pb/207Pb ratios, and other trace elements (Ba, Cu, Mn, Sr, and Zn) for analysis by an ICP-MS (Agilent 7500 series) that had been calibrated with appropriate standards and an internal standard of 10 µg L-1 Rh (12). An aliquot of each filtrate from pore and stream water samples was also used to determine DOC concentrations using ultraviolet oxidation and subsequent detection by infrared gas analysis (LABTOC, Pollution and Process Monitoring). 2.2.2. Statistical Analysis. General linear modeling was performed using Mintab v.15 on ranked data to assess significant differences between sites and temporal variations within sites for all soil, pore water, and stream water data. Pearson correlation coefficients were derived to assess relationships between DOC, pH, and all trace elements described, both within sites and across the total data set.

3. Results 3.1. Hydrological and Chemical Characteristics of Study Sites. The two spatially close sites produced contrasting bulk soil characteristics (Table S1) in terms of their ranges of 244

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percentage of carbon (site A, 10.6-53.7%; site B, 38.7-55.9%) and percentage of nitrogen (site A, 0.59-3.51%; site B, 1.30-2.30%). Intersite hydrochemistry was also significantly different (p > 0.05). However, hydrological and chemical data provided no significant temporal differences within each site in soil cores, soil pore waters, or stream water. During all three visits, wet antecedent conditions prevailed: samples were obtained during a wetting-up period in July; in August and September, on descending limbs of the hydrograph (Figure S2). Specific discharge on sampling days was relatively consistent (ca. 30-55 L s-1 km-2). Site A had significantly (p < 0.05) lower soil C/N ratios compared to site B as a consequence of lower carbon and higher nitrogen contents in their respective cores. Most cores revealed an increase in C/N ratio with depth (Table S1, Figure S3). Furthermore, site B and two cores at site A (west of the stream) were peat soils >70 cm with a maximum depth of 101 cm. However, the shortest core at site A (core I, 55 cm) was obtained from an area covered by humic/stagnogley soils. This core was characterized by shallow organic matter to 10 cm overlaying gleyed mineral soils with fragmented parent material present at the deepest section of the core. The lack of highly organic soils in this profile was confirmed with its lowest percentage of carbon. Although only three cores were sampled within each site, the integration of differing soil characteristics was apparent in their respective stream water analyses. In stream water, DOC concentrations were significantly lower (p < 0.001) with concomitantly higher pH values at site A than those recorded at site B. Moreover, at both sites, a significant decrease in DOC occurred from the first sampling site to the lowest downstream site (site A: 18.4 ( 2.67 to 7.62 ( 0.70 mg L-1; site B: 26.4 ( 3.98 to 17.4 ( 1.98 mg L-1; Figure S4a). The pH data produced an opposite trend with increasing values downstream (Figure S4b). The maximum pH was observed at site A (5.74) and minimum pH at site B (4.02). 3.2. Pb Concentrations and 206Pb/207Pb Isotope Ratios in Soil Cores. Lead concentrations in bulk soil were similar at both sites near the surface (top 10 cm) with concentrations between 150 and 270 mg of Pb kg-1 (Figure 1a). The maximum value at site A of 269 mg of Pb kg-1 was at the 2-4 cm depth; at site B, the maximum value was at 4-6 cm depth (214 mg of Pb kg-1). However, there was a sharper decline in the Pb concentrations at site A, which had reduced to 30 cm. At site A, 206Pb/207Pb ratios were lowest in the top 4 cm (1.135-1.138) from the surface, rising to 1.174 by -30 cm. The highest 206Pb/207Pb ratio was reached at 40 cm depth (1.189). Ratios subsequently varied considerably from this point and the three separate cores ranged from 1.116 to 1.177. At site B, surface (top 4 cm) 206Pb/207Pb ratios were lower (1.104-1.111) and consistent between the three separate cores. This steadily increased to values up to 1.165 at 20-40 cm depth from which a decline at depth in all 3 separate cores occurred but with large variability (values ranged from 1.054 to 1.162). 3.3. Pb Concentrations and 206Pb/207Pb Ratios in Soil Pore Waters. Lead concentrations of interstitial pore waters analyzed at three different depths in the soil profile exhibited no particular profile trend (Figure 1c). However, Pb concentrations at site A ranged from 0.04 to 0.70 µg L-1, which was substantially lower than concentrations in pore waters

FIGURE 1. Lead concentrations and 206Pb/207Pb ratios for soil (a/b) and pore waters (c/d) at the two study sites. Bulk soil samples were obtained from three individual soil cores (site A ) 9 site B ) 0). Soil pore water solution samples were obtained on three separate occasions at 10, 30, and 70 cm depths from three soil pits (site A ) b; site B ) O). from site B (0.45-2.46 µg L-1). Generally, highest concentrations were observed in the -10 cm extract, in proximity to the highest Pb concentrations in near-surface bulk soils. The 206 Pb/207Pb ratios (Figure 1d) produced a more variable signal at site A at all pore water extraction depths with deviation from its surrounding bulk soil after -10 cm. Site B, by -70 cm, also showed considerable variation and deviation from bulk soil of similar depth. 3.4. Pb Concentrations and 206Pb/207Pb Ratios in Stream Water. The changes in downstream Pb concentrations in both dissolved and particulate fractions are shown in Figure 2a. At site A, Pb concentrations were highest in the particulate phase with concentrations of 3.15 ( 0.74 µg L-1 and dissolved concentrations 0.05) differences were observed in DOC concentrations between sampling depths or dates at either study site. However, site A (13.24 ( 8.09 mg L-1) produced significantly (p < 0.001) lower pore water DOC concentrations compared to site B (26.54 ( 7.80 mg L-1). At individual study sites, the

DOC in pore waters was correlated with only Pb (site A: p ) 0.003; site B: p ) 0.046, Figure 3a); across both sites, DOC was correlated with both Pb (p < 0.001) and Cu (p ) 0.001). No other trace metals showed any significant relationship with DOC (Figure S5). In stream water, significant relationships were observed between DOC and Pb at both study sites (Figure 3b). However, Cu produced only a positive relationship with DOC at site A. Across both sites, DOC concentrations were inversely proportional to Mn and Sr (Figure S6). The pH, which was inversely related to Pb and DOC concentrations (p < 0.001), also showed positive relationships (p < 0.05) with Mn, Zn, Sr, and Ba concentrations across sites.

4. Discussion 4.1. Pb Concentrations and 206Pb/207Pb Ratios in Soil, Soil Pore Water, and Stream Water. The characteristic soil depth profiles of Pb concentrations (Figure 1a) and 206Pb/207Pb ratios (Figure 1b) at the two study sites are in general agreement with deposition patterns in other parts of the U.K. that are distant from main industrial sources. In an upland peat catchment in NE Scotland, near-surface Pb concentrations ranged between 238 and 489 mg kg-1 (9); upper and lower moorland peat profiles in Central Scotland had contaminated profiles with ∼260 mg kg-1 of Pb (11). Contamination in the Southern Pennines, N. England, is higher (maximum ∼1600 mg of Pb kg-1) because of the proximity to larger areas of anthropogenic production but retain similar patterns of deposition (4, 5). Maximal Pb concentrations in this study (∼270 mg kg-1) tend to have been recorded very close to the VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Lead concentrations (a) and 206Pb/207Pb ratios (b) for stream water at the two study sites. Five contiguous downstream water samples were obtained on three separate occasions and separated into solution (site A ) b; site B ) O) and particulate fractions (site A ) 1; site B ) 4).

FIGURE 3. DOC relationships with Pb in (a) pore water and (b) stream water from each of the two study sites (site A ) b; site B ) O). Mean concentrations and bidirectional standard errors relate to samples obtained on three separate occasions at the five contiguous downstream sites. surface of the soil profile when compared to other peat profiles studied (mainly occur at a 5-10 cm depth) (4, 5, 19). At site A in particular, there is a lack of reduction in depositional Pb load at the surface in contrast to previously studied ombrotrophic peats elsewhere in the U.K., as atmospheric Pb deposition has reduced in recent years (4, 5, 7, 9, 19). Thus, sloping terrain with areas not classified as peat and potentially lower peat accretion may account for some differences in Pb characteristics; other factors may include aquatic erosional losses or redeposition downslope of top soil in recent times. The lower 206Pb/207Pb ratios close to the surface, followed by a rise (to >1.15) between 20 and 40 cm depth and then a general decline, is similar to peat cores and long-term uninhabited areas of St. Kilda (Gleann Mo´r) in the Western Isles of Scotland (12). At least part of the elevated Pb concentrations is due to Industrial Revolution deposition as detailed by the 206Pb/207Pb ratios of 10 cm (Figure 1b,d), suggesting percolation of Pb via pore waters (17) and possible interaction with other water sources at greater depths. This does not appear to occur at site B at -10 and -30 cm, although some migration of Pb with different signatures by lower depths (-70 cm) has also occurred from 206

higher up the profile, upslope groundwater infiltration or upwelling of deeper groundwater. Although the deposition of total Pb concentrations is similar at both site A and site B, the 206Pb/207Pb ratios in the stream water (both dissolved and in particulate) at site A were less depleted than site B (Figure 2b), suggesting different export mechanisms of Pb to the stream. At site A, the significantly lower DOC concentrations and higher pH (Figure S4) suggest an increased influence of more minerogenic horizons (26, 37) and possibly deeper groundwater contributions (31, 38) containing lower Pb concentrations. This results in reduced Pb concentrations in the dissolved phase of surface waters (Figure 2a). Moreover, as water percolates through the soil profile or downslope, DOC is removed from soil solution by retention on mineral surfaces, reducing DOC export and preventing export of Pb bound to the retained DOC. In highly organic soils, such as those prevalent at site B, only small reductions in pore water DOC concentrations occur with depth due to a lack of available mineral binding capacity (1). The decreasing trend of DOC concentrations downstream (correlated with the complexed Pb) is common in moorland streams (33, 39), as a reduction in the contributory influence of upstream organic soils occurs. This is also inferred by pH, where increasing pH suggests (i) a decrease in organic acids as DOC decreases and (ii) increased minerogenic and deeper groundwater contributions to discharge containing increased base cation concentrations. This is substantiated by higher stream water concentrations of Ba and Sr (known geogenic markers) at site A compared to site B (Figure S6), which also increase downstream. 4.2. Differential Transport Mechanisms of Pb at the Two Study Sites. Hydrologically, the more minerogenic soil present at site A is characteristic of model C (mineral or humic gley soils) in the hydrology of soil types (HOST) response model descriptors (28). These soils that undergo seasonal saturation allow some percolation of water contributing to groundwater recharge and surface waters (28-30). This is corroborated with 206Pb/207Pb ratio data with a more predominantly geogenic signature in the stream water (Figure 2b), suggesting that a mixture of Pb sources throughout the profile, not just the subsurface with their highest concentrations of Pb (depleted 206Pb/207Pb signature), have contributed to the Pb in surface waters. Moreover, an apparent increased erosion and reduced structural integrity in parts of the soil system of site A subcatchment may account for comparatively higher losses of Pb from its profile on particulate matter. It has been noted in physically eroding peatlands, with or without mineral soils, that the transport of Pb can be significant on peat particles (23). At site B, a more anthropogenically influenced, depleted 206 Pb/207Pb ratio is apparent in surface waters (Figure 2b), more similar to soils of 10-20 cm depth and pore waters between 1.14 and 1.16, measured at -10 cm. These shallow subsurface soils also contained maximal concentrations of Pb, associated with highest DOC and Pb concentrations in the soluble phase. This is in agreement with a previous study at an upland peatland site in NE Scotland, where during high flows 30-40% of the Pb exported was coupled with DOC and a 206Pb/207Pb signature (∼1.14) representative of shallow subsurface flow (36). Hydrologically, the soils at site B represent more typically responsive soils, with lateral flow common through shallow subsurface soil horizons (30). Such responsive soils are characteristic of peaty upland sites in the U.K. and when hydrologically connected to surface waters during higher flow conditions strongly influence its chemistry (27, 30). The dominance of soil-derived contributions of Pb at discharges above baseflow encountered during this sampling regime indicates the dominance of shallow sub-

surface runoff generation processes with increased Pb concentrations to surface waters at site B. 4.3. Trace Elements and DOC Relationships. The strong binding ability and co-transport of Pb with DOC was apparent in both pore waters and stream water at both study sites (Figure 3). Relationships between other trace elements and DOC were only sometimes present in stream water and not in pore waters, indicating the greater utility of Pb as a potential tracer of organic matter transport as DOC or on particulate matter from soils to surface waters compared with other PTEs. Stream water, by integrating flows from the soil profile and groundwater inputs at the catchment scale, allows relationships between DOC and other PTEs, such as Cu and Mn, to be assessed. It is acknowledged that the behavior of PTEs in soils is dictated by a range of physicochemical parameters that govern trace element partitioning between solid and solution phases and eventual mobilization to surface waters (40-42). Although soluble Cu is known to complex preferentially with DOC rather than soil organic matter (43), pH is also a major factor in controlling solubility of certain PTEs, such as Zn, as its binding to organic matter tends to be weaker (2). The Ba and Sr concentrations produced an inverse relationship with soil-derived DOC concentrations as both these trace elements are indicative of a more geogenic character, as noted previously. 4.4. Characterizing Pb Mobilization. Differences in Pb transport mechanisms at two spatially close upland sites with contrasting soils were derived from three sampling periods when similar discharge levels had occurred. However, stream discharge and its hydrochemistry can be highly variable (24, 38). Within each site, the dynamics of Pb mobilization will vary with hydrological fluctuations. Peatlands can exhibit “flashy hydrological regimes” and increased flows have a major impact on the export of dissolved Pb (21), as more near-surface soil horizons containing the highest concentrations of Pb are hydrologically connected to surface waters. During drier conditions, these shallow subsurface soil-derived sources will reduce with concomitant changes in stream water Pb concentrations and 206Pb/207Pb signatures. Total Pb concentrations in surface waters during high flow events reported by Rothwell et al. (5) sometimes exceeded the EU standard of 10 µg L-1 (mean ) 15.7 µg L-1; Rothwell personal communication). Their results from a severely eroded peatland in the Southern Pennines, N. England, suggest considerable quantities of Pb were mobilized during storm events and were much higher than the study presented here, where a combined dissolved and particulate export of ∼4 ug L-1 has been attained. However, concentrations of Pb in Southern Pennines soils were significantly higher (∼1600 mg kg-1); thus, the proportion of Pb transported in stream water to that deposited in the soil is actually higher in the current study. This suggests that, in these Welsh soils, historically deposited Pb is more easily transported to surface waters. This might be as a consequence of soil type and differential solid-solution binding affinity of Pb, DOC export variability, rainfall-runoff characteristics, or total catchment wetness. Moreover, differences in seasonal or hydrological conditions are likely to impact Pb transport mechanisms, potentially increasing the Pb load to receiving waters under increased discharges. With the limited amount of data available to date, further sampling at a range of stream water discharges from base to storm flow at both sites, coupled with annual Pb export budgets, would be required to determine temporality of catchment-specific Pb mobilization. It is important to understand processes controlling spatial heterogeneity of hydrologically mediated carbon and PTE transport within upland source areas contributing to drinking water. This study has indicated that 206Pb/207Pb ratios, allied with the strong binding affinity of Pb for organic matter, can VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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enhance our understanding of the provenances and transport mechanisms of Pb, and potentially organic matter, in upland catchments.

Acknowledgments The authors thank UKPopNet (under the auspices of NERC) for funding, Severn-Trent Water as landowners and The Royal Society for Protection of Birds (RSPB) for permission to access the Lake Vyrnwy Nature Reserve, and Jared Wilson (LIFE Blanket Bog Project) and Mike Walker (RSPB) for facilitating field work. We would also like to acknowledge Debbie Coldwell and Matt Walker at the University of York (UKPopNet) for administration and guidance on-site. Finally, we acknowledge the British Atmospheric Data Centre (http:// www.badc.nerc.ac.uk/home) for temperature and precipitation data.

Supporting Information Available Full methods; Table S1, % N and % C in soil cores; Figure S1, locations for soil cores, soil pits, and stream water samples at the two study sites; Figure S2, specific discharge covering the sampling period; Figure S3, C/N ratio of soil cores; Figure S4, downstream DOC and pH; Figure S5, DOC relationships with trace elements in soil pore water; Figure S6, DOC relationships with trace elements in stream water. This material is available free of charge via the Internet at http:// pubs.acs.org.

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