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1.
Acid‐neutralizing capacity (ANC) is an important index for streamwater acidification caused by external factors (i.e. chronic acid deposition) and internal factors such as soil acidification due to nitrification. In this study, the influence of forest clear‐cutting and subsequent regrowth on internal acidification was investigated in central Japan, where stream pH (near 7·0) and ANC (above 0·1 meq L?1) are high. pH, the concentrations of major cations (Na+, K+, Mg2+ and Ca2+), major anions (NO3?, Cl? and SO42?) and dissolved silica (Si), and ANC were measured in 33 watersheds of various stand ages, during 2002 to 2004. Only NO3? concentration decreased with stand age, whereas pH, ANC, and concentrations of the sum of base cations (BC) and Si were negatively correlated with the minimum elevation of the watershed. The correlation between the BC/Si ratio and minimum elevation suggested that factors contributing to acid neutralization changed at 1100 m above sea level. In watersheds at lower elevations (?1100 m), the relatively high contribution of soil water with longer soil contact times should result in higher ANC, and cation exchange reactions should be the dominant process for acid neutralization due to deposition of colluvial soils on the lower slope. In contrast, in higher‐elevation watersheds (≥1100 m), weathered residual soils are thin and the small contribution of deeper groundwater results in lower ANC. These results suggest that the local acid sensitivity is determined by the hydrological and geomorphologic factors generated by steep topography. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

2.
In the decades since dramatic reductions in acid emissions and subsequent deposition in North America and Europe, the vast majority of research on aquatic chemical recovery has focused on trends in acid–base concentrations during baseflow conditions. Missing from such assessments is consideration of higher flow periods, when flow paths and chemical concentrations change and episodic acidification may occur. Stream samples collected weekly and bi‐hourly during high‐flow storm events from 1992 to 2015 at three watersheds representing a gradient in response to acid deposition in the south‐eastern United States were used to evaluate temporal trends in acid anions, base cations, acid neutralizing capacity (ANC), and pH for each of five distinct flow exceedance regimes, as well as trends in concentration–discharge (C–Q) relationships. For the most well‐buffered watershed (mean ANC = 220 μeq L?1), ANC and pH increased at a similar rate for baseflow and storm flow conditions. Decreases in sulfate controlled ANC trends at higher flows, whereas smaller sulfate decreases in combination with base cation increases resulted in similar ANC improvements for low‐flow periods. For the most acid sensitive watershed (mean ANC = 10 μeq L?1), no trends in ANC or pH were observed for the lowest flows (>90% flow exceedance), whereas reductions in sulfate resulted in significant increases in ANC during higher flow conditions. At all sites, greater rates of sulfate decline during high‐flow, as compared with low‐flow, conditions are likely a result of a reduced capacity of near surface soils to adsorb and retain sulfur in these non‐glaciated watersheds. Overall, consistent increases in pH (~0.01–0.02 pH units year?1) during higher flow conditions (<10% flow exceedance) in contrast to the variable trends observed during lower flows (>50% flow exceedance) illustrate that episodic acidification is recovering at an equal or greater rate than chronic acidification in these watersheds.  相似文献   

3.
The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO3?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO3?, SO42?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO3? (4.4 ± 2.9 mg N/L), SO42? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO3?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd.  相似文献   

4.
Quantifying the effects of anthropogenic processes on groundwater in arid regions can be complicated by thick unsaturated zones with long transit times. Human activities can alter water and nutrient fluxes, but their impact on groundwater is not always clear. This study of basins in the Trans‐Pecos region of Texas links anthropogenic land use and vegetation change with alterations to unsaturated zone fluxes and regional increases in basin groundwater NO3? concentrations. Median increases in groundwater NO3? (by 0.7–0.9 mg‐N/l over periods ranging from 10 to 50+ years) occurred despite low precipitation (220–360 mm/year), high potential evapotranspiration (~1570 mm/year), and thick unsaturated zones (10–150+ m). Recent model simulations indicate net infiltration and groundwater recharge can occur beneath Trans‐Pecos basin floors, and may have increased due to irrigation and vegetation change. These processes were investigated further with chemical and isotopic data from groundwater and unsaturated zone cores. Some unsaturated zone solute profiles indicate flushing of natural salt accumulations has occurred. Results are consistent with human‐influenced flushing of naturally accumulated unsaturated zone nitrogen as an important source of NO3? to the groundwater. Regional mass balance calculations indicate the mass of natural unsaturated zone NO3? (122–910 kg‐N/ha) was sufficient to cause the observed groundwater NO3? increases, especially if augmented locally with the addition of fertilizer N. Groundwater NO3? trends can be explained by small volumes of high NO3? modern recharge mixed with larger volumes of older groundwater in wells. This study illustrates the importance of combining long‐term monitoring and targeted process studies to improve understanding of human impacts on recharge and nutrient cycling in arid regions, which are vulnerable to the effects of climate change and increasing human reliance on dryland ecosystems.  相似文献   

5.
Stable isotopic (δDVSMOW and δ18OVSMOW) and geochemical signatures were employed to constrain the geochemical evolution and sources of groundwater recharge in the arid Shule River Basin, Northwestern China, where extensive groundwater extraction occurs for agricultural and domestic supply. Springs in the mountain front of the Qilian Mountains, the Yumen‐Tashi groundwater (YTG), and the Guazhou groundwater (GZG) were Ca‐HCO3, Ca‐Mg‐HCO3‐SO4 and Na‐Mg‐SO4‐Cl type waters, respectively. Total dissolved solids (TDS) and major ion (Mg2+, Na+, Ca2+, K+, SO42?, Cl? and NO3?) concentrations of groundwater gradually increase from the mountain front to the lower reaches of the Guazhou Basin. Geochemical evolution in groundwater was possibly due to a combination of mineral dissolution, mixing processes and evapotranspiration along groundwater flow paths. The isotopic and geochemical variations in melt water, springs, river water, YTG and GZG, together with the end‐member mixing analysis (EMMA) indicate that the springs in the mountain front mainly originate from precipitation, the infiltration of melt water and river in the upper reaches; the lateral groundwater from the mountain front and river water in the middle reaches are probably effective recharge sources for the YTG, while contribution of precipitation to YTG is extremely limited; the GZG is mainly recharged by lateral groundwater flow from the Yumen‐Tashi Basin and irrigation return flow. The general characteristics of groundwater in the Shule River Basin have been initially identified, and the results should facilitate integrated management of groundwater and surface water resources in the study area. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

6.
In the Youngcheon Diversion Tunnel area, South Korea, 46 samples of tunnel seepage water (TSW) and borehole groundwater were collected from areas with sedimentary rocks (mainly sandstone and shale) and were examined for hydrogeochemical characteristics. The measured SO4 concentrations range widely from 7·7 to 942·0 mg/l, and exceed the Korean Drinking Water Standard (200 mg/l) in about half the samples. The TDS (total dissolved solid) content generally is high (171–1461 mg/l) from more shale‐rich formations and also reflects varying degrees of water–rock interaction. The water is classified into three groups: Ca? SO4 type (61% of the samples collected), Ca? SO4? HCO3 type (15%) and Ca? HCO3 type (24%). The Ca? HCO3 type water (mean concentrations=369 mg/l Ca, 148 mg/l HCO3 and 23 mg/l SO4) reflected the simple reaction between CO2‐recharged water and calcite, whereas the more SO4‐rich nature of Ca? SO4 type water (mean concentrations=153 mg/l Ca, 66 mg/l HCO3 and 416 mg/l SO4) reflected the oxidation of pyrite in sedimentary rocks and fracture zones. Pyrite oxidation resulted in precipitation of amorphous iron hydroxide locally within the tunnel as well as in high concentrations of Ca (mean 153 mg/l) and Na (mean 49 mg/l) for TSW, and is associated with calcite dissolution resulting in pH buffering. The pyrite oxidation required for the formation of Ca? SO4 type water was enhanced by the diffusion of oxygenated air through the fractures related to the tunnel's construction. The subsequent outgassing of CO2 into the tunnel resulted in precipitation of iron‐bearing carbonate. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

7.
Wetlands often form the transition zone between upland soils and watershed streams, however, stream–wetland interactions and hydrobiogeochemical processes are poorly understood. We measured changes in stream nitrogen (N) through one riparian wetland and one beaver meadow in the Archer Creek watershed in the Adirondack Mountains of New York State, USA from 1 March to 31 July 1996. In the riparian wetland we also measured changes in groundwater N. Groundwater N changed significantly from tension lysimeters at the edge of the peatland to piezometer nests within the peatland. Mean N concentrations at the peatland perimeter were 1·5, 0·5 and 18·6 µmol L?1 for NH4+, NO3? and DON (dissolved organic nitrogen), respectively, whereas peatland groundwater N concentration was 56·9, 1·5 and 31·6 µmol L?1 for NH4+, NO3? and DON, respectively. The mean concentrations of stream water N species at the inlet to the wetlands were 1·5, 10·1 and 16·9 µmol L?1 for NH4+, NO3? and DON, respectively and 1·6, 28·1 and 8·4 µmol L?1 at the wetland outlet. Although groundwater total dissolved N (TDN) concentrations changed more than stream water TDN through the wetlands, hydrological cross‐sections for the peatland showed that wetland groundwater contributed minimally to stream flow during the study period. Therefore, surface water N chemistry was affected more by in‐stream N transformations than by groundwater N transformations because the in‐stream changes, although small, affected a much greater volume of water. Stream water N input–output budgets indicated that the riparian peatland retained 0·16 mol N ha?1 day?1 of total dissolved N and the beaver meadow retained 0·26 mol N ha?1 day?1 during the study period. Nitrate dominated surface water TDN flux from the wetlands during the spring whereas DON dominated during the summer. This study demonstrates that although groundwater N changed significantly in the riparian peatland, those changes were not reflected in the stream. Consequently, although in‐stream changes of N concentrations were less marked than those in groundwater, they had a greater effect on stream water chemistry—because wetland groundwater contributed minimally to stream flow. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

8.
To improve quantitative understanding of mixed‐land‐use impacts on nutrient yields, a nested‐scale experimental watershed study design (n = 5) was applied in a 303(d), clean water act impaired urbanizing watershed of the lower Missouri River Basin, USA. From 2010 to 2013, water samples (n = 858 sample days per site) were analysed for total inorganic nitrogen (TIN‐N), nitrite (NO2–N) nitrate (NO3–N), ammonia (NH3–N), and total phosphorus (TP‐P). Annual, seasonal, and monthly flow‐weighted concentrations (FWCs) and nutrient yields were estimated. Mean nutrient concentrations were highest where agricultural land use comprised 58% of the drainage area (NH3 = 0.111 mg/l; NO2 = 0.045 mg/l; NO3 = 0.684 mg/l, TIN = 0.840 mg/l; TP = 0.127 mg/l). Average TP‐P increased by 15% with 20% increased urban land use area. Highly variable annual precipitation was observed during the study with highest nutrient yields during 2010 (record setting wet year) and lowest nutrient yields during 2012 (extreme drought year). Annual TIN‐N and TP‐P yields exceeded 10.3 and 2.04 kg ha?1 yr?1 from the agricultural dominated headwaters. Mean annual NH3–N, NO2–N, NO3–N, TIN‐N, and TP‐P yields were 0.742, 0.400, 4.24, 5.38, and 0.979 kg ha?1 yr?1, respectively near the watershed outlet. Precipitation accounted for the majority of the explained variance in nutrient yields (R2 values from 0.68 to 0.85). Nutrient yields were also dependent on annual precipitation of the preceding year (R2 values from 0.87 to 0.91) thus enforcing the great complexity of variable mixed‐land‐use mediated source‐sink nutrient yield relationships. Study results better inform land managers and best management practices designed to mitigate nutrient pollution issues in mixed‐land‐use freshwater ecosystems. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

9.
Concerns related to climate change have resulted in an increasing interest in the importance of hydrological events such as droughts in affecting biogeochemical responses of watersheds. The effects of an unusually dry summer in 2002 had a marked impact on the biogeochemistry of three watersheds in the north‐eastern USA. Chemical, isotopic and hydrological responses with particular emphasis on S dynamics were evaluated for Archer Creek (New York), Sleepers River (Vermont) and Cone Pond (New Hampshire) watersheds. From 1 August to 14 September 2002, all three watersheds had very low precipitation (48 to 69 mm) resulting in either very low or no discharge (mean 0·015, 0·15 and 0·000 mm day?1 for Archer Creek, Sleepers River and Cone Pond, respectively). From 15 September to 31 October 2002, there was a substantial increase in precipitation totals (212, 246 and 198 mm, respectively) with increased discharge. Archer Creek was characterized by a large range of SO42? concentrations (152 to 389 µeq L?1, mean = 273 µeq L?1) and also exhibited the greatest range in δ34S values of SO42? (?1·4 to 8·8 ‰ ). Sleepers River's SO42? concentrations ranged from 136 to 243 µeq L?1 (mean = 167 µeq L?1) and δ34S values of SO42? ranged from 4·0 to 9·0 ‰ . Cone Pond's SO42? concentrations (126–187 µeq L?1, mean = 154 µeq L?1) and δ34S values (2·4 to 4·3 ‰ ) had the smallest ranges of the three watersheds. The range and mean of δ18O‐SO42? values for Archer Creek and Cone Pond were similar (3·0 to 8·9 ‰ , mean = 4·5 ‰ ; 3·9 to 6·3 ‰ , mean = 4·9 ‰ ; respectively) while δ18O‐SO42? values for Sleepers River covered a larger range with a lower mean (1·2 to 10·0 ‰ , mean = 2·5). The difference in Sleepers River chemical and isotopic responses was attributed to weathering reactions contributing SO42?. For Archer Creek wetland areas containing previously reduced S compounds that were reoxidized to SO42? probably provided a substantial source of S. Cone Pond had limited internal S sources and less chemical or isotopic response to storms. Differences among the three watersheds in S biogeochemical responses during these storm events were attributed to differences in S mineral weathering contributions, hydrological pathways and landscape features. Further evaluations of differences and similarities in biogeochemical and hydrological responses among watersheds are needed to predict the impacts of climate change. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
Abstract

A field experiment was conducted on a sloping grassland soil in southwest England to investigate the downslope transport of nitrogen in soil water following the application of cattle manure, slurry and inorganic fertilizer. Transport of nitrogen (N) species was monitored on hydrologically isolated plots. Manure (50 t ha?1), slurry (50 m3 ha?1) and fertilizer (250 kg N ha?1) were applied in February/March 1992. Subsurface water movement, by both matrix and preferential flow, was the dominant flow route during the experiment. Subsurface and surface nutrient flow pathways were monitored by analysing soil water and surface runoff for NO3-N, NH4-N and total N. Subsurface flow chemistry was dominated by NO3-N, with concentrations usually between 2 and 5 mg NO3 ?N dm?3. Differences between fertilizer and manure treatments and the untreated control were not significant. Significantly elevated NO3-N concentrations were observed in soil water in the buffer zone, indicating the importance of a buffer zone at least 10 m wide between manure spreading zones and an adjacent water course.  相似文献   

11.
Field surveys on atmospheric deposition and stream water chemistry were conducted in an evergreen forest in northeastern Thailand characterized by a tropical savanna climate with distinct dry and wet seasons. Atmospheric deposition of ion constituents by throughfall and stemflow was shown to increase in the beginning and end of the wet season, reflecting the precipitation pattern. The pH and electrical conductivity of stream water increased with alkalinity and base cation concentrations due to mineralization of organic matter by the first rain and retention of anions in soil during the start of the wet season. After initial alkalinization, the pH and alkalinity declined rapidly with the highest SO42? concentration displayed in the middle towards the end of the wet season. The magnitude of peaks in SO42? concentration (13.5–60.6 μmolc/L) reflects deposition during the first 2 months of the wet season (March and April) in respective years (60.8–170 molc/ha). Release of SO42? with H+, which is retained in soil during the early wet season, may cause acidification later in the season. The deposition and concentration of SO42? declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO42? and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO42? concentration/deposition and increased precipitation in the middle–late wet season. The retention‐release cycle of SO42? largely contributed to both seasonal and interannual variations in stream water chemistry in the tropical savanna climate studied.  相似文献   

12.
A two‐dimensional variable‐density groundwater flow and transport model was developed to provide a conceptual understanding of past and future conditions of nitrate (NO3) transport and estimate groundwater nitrate flux to the Gulf of Mexico. Simulation results show that contaminant discharge to the coast decreases as the extent of saltwater intrusion increases. Other natural and/or artificial surface waters such as navigation channels may serve as major sinks for contaminant loading and act to alter expected transport pathways discharging contaminants to other areas. Concentrations of NO3 in the saturated zone were estimated to range between 30 and 160 mg?L?1 as NO3. Relatively high hydraulic vertical gradients and mixing likely play a significant role in the transport processes, enhancing dilution and contaminant migration to depth. Residence times of NO3 in the deeper aquifers vary from 100 (locally) to about 300 years through the investigated aquifer system. NO3 mass fluxes from the shallow aquifers (0 to 5.7 × 104 mg?m?2?day?1) were primarily directed towards the navigation channel, which intersects and captures a portion of the shallow groundwater flow/discharge. Direct NO3 discharge to the sea (i.e. Gulf of Mexico) from the shallow aquifer was very low (0 to 9.0 × 101 mg · m?2?day?1) compared with discharge from the deeper aquifer system (0 to 8.2 × 103 mg?m?2?day?1). Both model‐calibrated and radiocarbon tracer‐determined contaminant flux estimates reveal similar discharge trends, validating the use of the model for density‐dependent flow conditions. The modelling approach shows promise to evaluate contaminant and nutrient loading for similar coastal regions worldwide. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

13.
The relationship between solute concentrations and discharge can inform an integrated understanding of hydrological and biogeochemical processes at watershed scales. Recent work from multiple catchments has shown that there is typically little variation in concentration relative to large variations in discharge. This pattern has been described as chemostatic behavior. Pond Branch, a forested headwater catchment in Maryland, has been monitored for stream nitrate (NO3?) concentrations at weekly intervals for 14 years. In the growing season and autumn of 2011 a high‐frequency optical NO3? sensor was used to supplement the long‐term weekly data. In this watershed, long‐term weekly data show that NO3? concentrations decrease with increasing discharge whereas 6 months of 15‐minute sensor observed concentrations reveal a more chemostatic behavior. High‐frequency NO3? concentrations from the sensor collected during different storm events reveal variable concentration–discharge patterns highlighting the importance of high resolution data and ecohydrological drivers in controlling solute export for biologically reactive solutes such as NO3?.  相似文献   

14.
Long-term ecosystem studies are valuable for understanding integrated ecosystem response to global changes in atmospheric deposition and climate. We examined trends for a 35-year period (1982/83–2017/18) in concentrations of a range of solutes in precipitation and stream water from nine headwater catchments spanning elevation and surficial geology gradients at the Turkey Lakes watershed (TLW) in northeastern Ontario, Canada. Average annual water year (WY, October to September) concentrations in precipitation significantly declined over the period for sulphate (SO42−), nitrate (NO3−) and chloride (Cl−), while calcium (Ca2+) and potassium (K+) concentrations increased, resulting in a significant pH increase from 4.2 to 5.7. Trends in stream chemistry through time are generally consistent with expectations associated with acidification recovery. Concentration of many stream water solutes (SO42−, Cl−, calcium [Ca2+], magnesium [Mg2+] and NH4+ generally decreased, while others (silica [SiO2] and dissolved organic carbon [DOC]) generally increased. Increases were also observed for alkalinity (six of nine catchments), acid neutralizing capacity ([ANC]; six of nine catchments) and pH (eight of nine catchments), while conductivity declined (six of nine catchments). Variability in trends among catchments are associated with differences in surficial geology and wetland cover. While absolute solute concentrations were generally lower at bedrock dominated high-elevation catchments compared to till dominated lower elevation catchments, the rate of change of concentration was often greater for high elevation catchments. This study confirms continued, but non-linear stream chemistry recovery from acidification, particularly at the less buffered high and moderate elevation sites. The heterogeneity of responses among catchments highlights our incomplete understanding of the relative importance of different mechanisms influencing stream chemistry and the consequences for downstream ecosystems.  相似文献   

15.
Monitoring the effects of acidic deposition on aquatic ecosystems in the Northeastern US has generally required regular measurements of stream buffering chemistry (i.e. acid‐neutralizing capacity (ANC) and calcium Ca2+), which can be expensive and time consuming. The goal of this paper was to develop a simple method for predicting baseflow buffering chemistry based on the hydrogeomorphic properties of ten nested watersheds in the Neversink River basin (2·0–176·0 km2), an acid‐sensitive basin in the Catskill Mountains, New York State. The tributaries and main reach watersheds have strongly contrasting mean baseflow ANC values and Ca2+ concentrations, despite rather homogeneous vegetation, bedrock geology, and soils. A stepwise regression was applied to relate 13 hydrogeomorphic properties to the mean baseflow ANC values and Ca2+ concentrations. The regression analysis showed that watersheds with lower ANC values had a higher mean ratio of ‘quickflow’ runoff to precipitation during 20 non‐snowmelt runoff events (referred to as mean runoff ratio). The mean runoff ratio could explain at least 80% of the variability in mean baseflow ANC values and Ca2+ concentrations among the ten watersheds. Greater mean runoff ratios also correlated with steeper slopes and greater drainage densities, thus allowing the prediction of baseflow ANC values (r2 = 0·75) and Ca2+ concentrations (r2 = 0·77) with widely available spatial data alone. These results indicate that hydrogeomorphic properties can predict a watershed's sensitivity to acid deposition in regions where the spatial sources of stream buffering chemistry from the bedrock mineralogy and soils are fairly uniform. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

16.
Mass balance studies in forested catchments in the northeastern USA show that S losses via streamwater SO42? exceed measured atmospheric S inputs. Possible sources of the excess S loss include underestimated dry deposition, mineralization of organic S in soils, desorption of soil sulphate, oxidation of recently formed sulphides and mineral weathering. Evaluating the relative contribution of these sources and processes to SO42? export is important to our understanding of S cycling as well as to policy makers in their evaluation of the efficacy of S emission controls. In order to evaluate the potential for mineral weathering contributions to SO42? export, we measured concentration and isotopic composition (δ34S and δ18O) of SO42? in stream water, and concentration and δ34S values of four S fractions in bedrock and soil parent material in catchments of varying geological composition. Geological substrates with low S concentrations were represented by catchments underlain by quartzite and granite, whereas geological substrates with high S concentrations were represented by catchments underlain by sulphidic slate, schist and metavolcanic rocks. Catchments with S‐poor bedrock had stream‐water SO42? concentrations <100 µeq L?1 and isotopic values consistent with those of atmospheric SO42? that had been cycled through the organic soil pool. Catchments with S‐rich bedrock had stream‐water SO42? concentrations ranging from 56 to 229 µeq L?1. Isotopic values deviated from those of SO42? in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral δ34S values precluded the isotopic detection of a weathering contribution. These results, along with evidence suggesting formation of secondary sulphate minerals in bedrock weathering rinds, indicate that mineral weathering may be an important source of S in the surface waters of some forested catchments in the northeastern USA. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

17.
The southwestern Adirondack region of New York receives among the highest rates of atmospheric nitrogen (N) deposition in the USA. Atmospheric N deposition to sensitive ecosystems, like the Adirondacks, may increase the acidification of soils through losses of exchangeable nutrient cations, and the acidification of surface waters associated with enhanced mobility of nitrate (NO3?). However, watershed attributes, including surficial terrestrial characteristics, in‐lake processing, and geological settings, have been found to complicate the relationships between atmospheric N deposition and N drainage losses. We studied two lake‐watersheds in the southwestern Adirondacks, Grass Pond and Constable Pond, which are located in close proximity (～26 km) and receive similarly high N deposition, but have contrasting watershed attributes (e.g. wetland area, geological settings). Since the difference in the influence of N deposition was minimal, we were able to examine both within‐ and between‐watershed influences of land cover, the contribution of glacial till groundwater inputs, and in‐lake processes on surface water chemistry with particular emphasis on N solutes and dissolved organic carbon (DOC). Monthly samples at seven inlets and one outlet of each lake were collected from May to October in 1999 and 2000. The concentrations of NO3? were high at the Grass Pond inlets, especially at two inlets, and NO3? was the major N solute at the Grass Pond inlets. The concentrations of likely weathering products (i.e. dissolved Si, Ca2+, Mg2+, Na+) as well as acid neutralizing capacity and pH values, were also particularly high at those two Grass Pond inlets, suggesting a large contribution of groundwater inputs. Dissolved organic N (DON) was the major N solute at the Constable Pond inlets. The higher concentrations of DON and DOC at the Constable Pond inlets were attributed to a large wetland area in the watershed. The DOC/DON ratios were also higher at the Constable Pond inlets, possibly due to a larger proportion of coniferous forest area. Although DON and DOC were strongly related, the stronger relationship of the proportion of wetland area with DOC suggests that additional factors regulate DON. The aggregated representation of watershed physical features (i.e. elevation, watershed area, mean topographic index, hypsometric‐analysis index) was not clearly related to the lake N and DOC chemistry. Despite distinctive differences in inlet N chemistry, NO3? and DON concentrations at the outlets of the two lakes were similar. The lower DOC/DON ratios at the lake outlets and at the inlets having upstream ponds suggest the importance of N processing and organic N sources within the lakes. Although an inverse relationship between NO3? and DOC/DON has been suggested to be indicative of a N deposition gradient, the existence of this relationship for sites that receive similar atmospheric N deposition suggest that the relationship between NO3? and the DOC/DON ratio is derived from environmental and physical factors. Our results suggest that, despite similar wet N deposition at the two watershed sites, N solutes entering lakes were strongly affected by hydrology associated with groundwater contribution and the presence of wetlands, whereas N solutes leaving lakes were strongly influenced by in‐lake processing. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

18.
This paper examines the impact of contrasting antecedent soil moisture conditions on the hydrochemical response, here the changes in dissolved nitrogen (NO3?, NH4+ and dissolved organic nitrogen (DON)) and dissolved organic carbon (DOC) concentrations, of a first‐order stream during hydrological events. The study was performed in the Hermine, a 5 ha forested watershed of the Canadian Shield. It focused on a series of eight precipitation events (spring, summer and fall) sampled every 2 or 3 h and showing contrasted antecedent moisture conditions. The partition of the eight events between two groups (dry or wet) of antecedent moisture conditions was conducted using a principal component analysis (PCA). The partition was controlled (first axis explained 86% of the variability) by the antecedent streamflow, the streamflow to precipitation ratio Q/P and by the antecedent groundwater depth. The mean H+, NO3?, NH4+, total dissolved nitrogen and DOC concentrations and electrical conductivity values in the stream were significantly higher following dry antecedent conditions than after wetter conditions had prevailed in the Hermine, although the temporal variability was high (17 to 138%). At the event scale, a significantly higher proportion of the changes in DON, NO3?, and DOC concentrations in the stream was explained by temporal variations in discharge compared with the seasonal and annual scales. Two of the key hydrochemical features of the dry events were the synchronous changes in DOC and flow and the frequent negative relationships between discharge and NO3?. The DON concentrations were much less responsive than DOC to changes in discharge, whereas NH was not in phase with streamflow. During wet events, the synchronicity between streamflow and DON or NO3? was higher than during dry events and discharge and NO3? were generally positively linked. Based on these observations, the hydrological behaviour of the Hermine is conceptually compatible with a two‐component model of shallow (DON and DOC rich; variable NO3?) and deep (DON and DOC poor; variable NO3?) subsurface flow. The high NO3? and DOC levels measured at the early stages of dry events reflected the contribution from NO3?‐rich groundwaters. The contribution of rapid surface flow on water‐repellent soil materials located close to the stream channel is hypothesized to explain the DOC levels. An understanding of the complex interactions between antecedent soil moisture conditions, the presence of soil nutrients available for leaching and the dynamics of soil water flow paths during storms is essential to explain the fluxes of dissolved nitrogen and carbon in streams of forested watersheds. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
Subsurface brines with high nitrate (NO3?) concentration are common in desert environments as atmospheric nitrogen is concentrated by the evaporation of precipitation and little nitrogen uptake. However, in addition to having an elevated mean concentration of ～525 mg/L (as N), NO3? in the coastal sabkhas of Abu Dhabi is enriched in 15N (mean δ15N ～17‰), which is an enigma. A NO3? solute mass balance analysis of the sabkha aquifer system suggests that more than 90% of the nitrogen is from local atmospheric deposition and the remainder from ascending brine. In contrast, isotopic mass balances based on Δ17O, δ15N, and δ18O data suggest approximately 80 to 90% of the NO3? could be from ascending brine. As the sabkha has essentially no soil, no vegetation, and no anthropogenic land or water use, we propose to resolve this apparent contradiction with a density‐driven free‐convection transport model. In this conceptual model, the density of rain is increased by solution of surface salts, transporting near‐surface oxygenated NO3? bearing water downward where it encounters reducing conditions and mixes with oxygen‐free ascending geologic brines. In this environment, NO3? is partially reduced to nitrogen gas (N2), thus enriching the remaining NO3? in heavy isotopes. The isotopically fractionated NO3? and nitrogen gas return to the near‐surface oxidizing environment on the upward displacement leg of the free‐convection cycle, where the nitrogen gas is released to the atmosphere and new NO3? is added to the system from atmospheric deposition. This recharge/recycling process has operated over many cycles in the 8000‐year history of the shallow aquifer, progressively concentrating and isotopically fractionating the NO3?.  相似文献   

20.
The temporal variability in nitrogen (N) transport in the Corbeira agroforestry catchment (NW Spain) was analysed from October 2004 to September 2008. Nitrate (NO3–N) and total Kjeldahl nitrogen (TKN) loads and concentrations were determined at various timescales (annual, seasonal and event). The results revealed a strong intra‐annual and inter‐annual variability in N transport influenced by weather patterns and consequently by the hydrological regime. Mean annual export of total N in the catchment was 5.5 kg ha?1 year?1, with NO3–N being the dominant form. Runoff events comprised 10% of the study period but contributed 40 and 61% of the total NO3–N and TKN loads, respectively. The NO3–N and TKN concentrations were higher during runoff events than under baseflow conditions, pointing to diffuse sources of N. The mobilization of TKN during runoff events was attributed to surface runoff, while NO3–N might be related to subsurface and groundwater flow. Runoff events were characterized by high variability in N loads and concentrations. Higher variability was observed in N loads than in N concentrations, indicating that event magnitude plays an important role in N transport in this catchment; event magnitude explained approximately 96% of the NO3–N load. However, a combination of variables related to runoff event intensity (rainfall, discharge increase and kinetic energy) explained only 66% of the TKN load. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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