Stream SO42 Research Articles

Widespread chemical dilution of streams continues as long-term effects of acidic deposition slowly reverse

Studies of recovery from acidic deposition have focused on reversal of acidification and its associated effects, but as recovery proceeds slowly, chemical dilution of surface waters is emerging as a key factor in the recovery process that has significant chemical and biological implications. This investigation uses long-term chemical records from 130 streams in the Adirondack region of New York, USA, to evaluate the role of ongoing decreases in conductance, an index of dilution, in the recovery of these streams. Stream chemistry data spanning up to 40 years (1980s–2022) showed that acid-neutralizing capacity has increased in 92% of randomly selected streams, but that harmful levels of acidification still occur in 37% of these streams. Conductance and Ca2+ concentrations decreased in 79% of streams, and SO42− concentrations in streams continued to show strong decreases but remained several times higher than concentrations in precipitation. These changes were ongoing through 2022 even though acidic deposition levels were approaching those estimated for pre-industrialization. Further dilution is continuing through ongoing decreases in stream SO42−. Nevertheless, Ca2+ continued to be leached from soils by SO42−, organic acids and NO3−, limiting the replenishment of available soil Ca2+, a prerequisite to stem further dilution of stream water.

Lithological and glacial controls on sulfide weathering and the associated CO2 budgets in the Tibetan Plateau: New constraints from small catchments

The Tibetan Plateau, a region that hosts low to middle-latitude glaciers and a high density of metasedimentary rocks, is a hotspot of continental weathering and carbon cycling. However, quantitative estimations of sulfide oxidation and how it affects CO2 budgets from the plateau weathering are lacking. Here we use chemical and isotopic datasets from the studied small catchments in the Tibetan Plateau to estimate that 69.1% to 77.0% of stream SO42– originates from sulfide oxidation. Thus, more than 80% of the CO2 consumed by silicate weathering is counterbalanced by transient CO2 release from H2SO4-driven carbonate dissolution in the study area. In addition, the sulfide oxidation rate for glacial catchments is 5 times that of nonglacial catchments. The CO2 release rate from acid generated by sulfide oxidation in metasedimentary catchments is approximately 3 times that of granitic catchments. We propose that sulfide oxidation has greatly modified the CO2 budget of chemical weathering of the Tibetan Plateau. This study reveals new constraints on the CO2 budget in the Tibetan Plateau weathering, and will provide important insights into the carbon cycle during the plateau uplifting in Cenozoic.

Comparison of retention and output of sulfur in limestone soil and yellow soil and their responses to acid deposition in a small karst catchment of Guizhou Province, Southwest China.

Investigating the responses of retention and output of sulfur (S) is significant to understand the impact of atmospheric S deposition on the S cycling in soils and its environmental effects in the karst catchments of Southwest China. This study analyzed the contents and δ34S values of different S forms (total S, carbon-bonded S, ester-bonded SO42-, SO42-, and total reduced inorganic sulfur [TRIS]), the δ34S values of stream SO42-, the δ13C values of soil organic carbon, and sulfate-reducing bacteria (SRB) quantity in limestone soil and yellow soil profiles in a typical small karst catchment of Southwest China. The results showed that under the same acid deposition level, the limestone soil and yellow soil profiles are significantly different from the distribution of contents and δ34S values of different S forms and the number of SRB. At the same time, more than 70% of the SO42- in the stream water draining the sampling slopes came from soils at different depths in limestone soil and yellow soil profiles. These results indicate the different response of retention and output of S in the limestone soil and yellow soil to S deposition input. The organic S formation and dissimilatory SO42- reduction (DSR) to form TRIS are S retention processes that exist in both limestone soil and yellow soil profiles. There are processes of transport and accumulation of SO42- at the bottom layer in yellow soil profile; therefore, retaining S as absorbed SO42- is also a main S retention process in yellow soil. At present, the output of SO42- through stream water mainly comes from the deposited SO42- which undergoes DSR reaction driven by SRB, not from organic S mineralization and desorption of adsorbed SO42- in the limestone soil and yellow soil profiles. However, organic S is the main S form in limestone soil and yellow soil. After the annual S deposition flux is significantly reduced, organic S mineralization in limestone soil and yellow soil profiles may release a large amount of SO42- into the surface water.

Exploring the trend of stream sulfate concentrations as U.S. power plants shift from coal to shale gas

Since the early 2000s, an increasing number of power plants in the U.S. have switched from burning coal to burning gas and thus have released less SO2 emissions into the atmosphere. We investigated whether stream chemistry (i.e., SO42−) also benefits from this transition. Using publicly available data from Pennsylvania (PA), a U.S. state with heavy usage of coal as fuel, we found that the impact of SO2 emissions on stream SO42− can be observed as far as 63 km from power plants. We developed a novel model that incorporates an emission-control technology trend for coal-fired power plants to quantify potentially avoided SO2 emissions and stream SO42− as power plants switched from coal to gas. The results show that, if 30% of the electricity generated by coal in PA in 2017 had been replaced by that from natural gas, a total of 20.3 thousand tons of SO2 emissions could have been avoided and stream SO42− concentrations could have decreased as much as 10.4%. Extrapolating the model to other states in the U.S., we found that as much as 46.1 thousand tons of SO2 emissions per state could have been avoided for a similar 30% coal-to-gas switch, with potential amelioration of water quality near power plants. The emission-control technology trend model provides a valuable tool for policy makers to assess the benefits of coal-to-gas shifts on water quality improvements as well as the effectiveness of emission control technologies.

Effects of climate and atmospheric deposition on a boreal lake chemistry: A synthesis of 36 years of monitoring data

Reduction in SO42− and NO3− atmospheric deposition in the past decades has improved surface water quality in several catchments but recent studies suggest an increasing influence of climate and dissolved organic carbon (DOC). Here, we report on long-term trends in climate variables, strong acid anions and base cations concentrations in precipitation and at the lake outlet (stream) of a boreal catchment in Québec, Canada, and assess the combined effects of these trends on stream chemistry. Annual SO42− and NO3− depositions respectively decreased by ~85% (from 23 to ~3 kg ha−1) and ~70% (from 18 to ~5 kg ha−1 yr−1) from 1981 to 2016. As a response, stream SO42− and Ca2+ concentrations decreased by 50% (from 3.9 to 1.9 mg L−1) and ~35% (from 2.4 to 1.5 mg L−1), respectively. Stream NO3− concentration decreased by ~89% (from 0.6 to 0.07 mg L−1) mainly due to the decline in NO3− deposition and possibly to increased vegetation N uptake. Unexpectedly, stream alkalinity decreased, likely due to the decline in Ca2+ concentration and to an increase in DOC concentration. Variations in stream pH and Na+ concentrations were best explained by climatic changes than by changes in acid deposition, likely reflecting the effect of climate change on chemical weathering in the region. In addition, the average daily temperature between May and September had a strong influence on stream Ca2+ concentration in the last two decades (negative relationship), suggesting an increasing vegetation nutrient uptake caused by improved growth conditions. Overall, decreased acidic deposition resulted in a general recovery of surface water although the parallel increase in DOC concentration prevented from an increase in water alkalinity. Our data also indicate an increasing influence of climate on water chemistry at the study site, probably mediated by increasing weathering rate and vegetation nutrient uptake.

Boreal forest riparian zones regulate stream sulfate and dissolved organic carbon

In boreal forest catchments, solute transfer to streams is controlled by hydrological and biogeochemical processes occurring in the riparian zone (RZ). However, RZs are spatially heterogeneous and information about solute chemistry is typically limited. This is problematic when making inferences about stream chemistry. Hypothetically, the strength of links between riparian and stream chemistry is time-scale dependent. Using a ten-year (2003−2012) dataset from a northern Swedish catchment, we evaluated the suitability of RZ data to infer stream dynamics at different time scales. We focus on the role of the RZ versus upslope soils in controlling sulfate (SO42−) and dissolved organic carbon (DOC). A priori, declines in acid deposition and redox-mediated SO42− pulses control sulfur (S) fluxes and pool dynamics, which in turn affect dissolved organic carbon (DOC). We found that the catchment is currently a net source of S, presumably due to release of the S pool accumulated during the acidification period. In both, RZ and stream, SO42− concentrations are declining over time, whereas DOC is increasing. No temporal trends in SO42− and DOC were observed in upslope mineral soils. SO42− explained the variation of DOC in stream and RZ, but not in upslope mineral soil. Moreover, as SO42− decreased with time, temporal variability of DOC increased. These observations indicate that: (1) SO42− is still an important driver of DOC trends in boreal catchments and (2) RZ processes control stream SO42− and subsequently DOC independently of upslope soils. These phenomena are likely occurring in many regions recovering from acidification. Because water flows through a heterogeneous mosaic of RZs before entering the stream, upscaling information from limited RZ data to the catchment level is problematic at short-time scales. However, for long-term trends and annual dynamics, the same data can provide reasonable representations of riparian processes and support meaningful inferences about stream chemistry.

The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory

Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO2 during weathering. We evaluated the potential of gray shale to take up CO2 from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ13CDIC) along water flow paths in a well-characterized critical zone observatory (Susquehanna Shale Hills catchment). In this catchment, chemical weathering in shallow soils is dominated by clay transformation as no carbonates are present, and soil pore waters are characterized by low DIC and pH. In shallow soil porewaters, the DIC, dominated by dissolved CO2, is in chemical and isotopic equilibrium with CO2 in the soil atmosphere where pCO2 varies seasonally to as high as 40 times that of the atmosphere. The degradation of ancient organic matter is negligible in contributing to soil CO2. The chemistry of groundwater varies along different flowpaths as soil pore water recharges to the water table and then dissolves ankerite or secondary calcite under the valley floor. Weathering of carbonate leads to much higher concentrations of DIC (∼2500μmol/L) and divalent cations (Ca2+ and Mg2+) in groundwaters than soil waters. The depth to the ankerite weathering front is hypothesized to be roughly coincident with the water table but it varies due to heterogeneities in the protolith composition. Groundwater chemistry therefore shows different saturation indices with respect to ankerite depending upon location along the valley. The δ13CDIC values of these groundwaters document mixing between the ankerite and soil CO2. The major element concentrations, DIC, and δ13CDIC in the first-order stream incising the valley of the catchment are derived from groundwater and soil waters in proportions that vary both spatially and temporally. The CO2 degassed slightly in the stream but little evidence of C isotopic equilibration with the atmosphere is observed, due to the short length of the stream and short contact time with air.The ankerite reaction front also lies close to the pyrite dissolution front. Pyrite oxidation in bedrock likely released sulfuric acid and played a minor role in the ankerite dissolution, shifting groundwater δ13CDIC slightly above the expected mixing values. At the catchment scale, the stream SO42− is also dominantly derived from wet deposition, as stream has δ34SSO4 values around 3‰, well within the range of acid deposition.A mass balance calculation shows that silicate and ankerite dissolution of the Rose Hill shale at Shale Hills consumes CO2 at a rate of ∼44 and ∼42–48molm−2ky−1 respectively, while degradation of ancient organic matter releases CO2 at a rate of ∼1.3molm−2ky−1. Silicate dissolution at the shallow soils is facilitated by low pH and high soil pCO2. As ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration, CO2 consumption by shale weathering is thus limited by initiation of rock disintegration (e.g., fractures) and exposure of fresh surface area to infiltrating CO2- and O2-rich water.

Long-Term Annual and Seasonal Patterns of Acidic Deposition and Stream Water Quality in a Great Smoky Mountains High-Elevation Watershed

The recovery potential of stream acidification from years of acidic deposition is dependent on biogeochemical processes and varies among different acid-sensitive regions. Studies that investigate long-term trends and seasonal variability of stream chemistry in the context of atmospheric deposition and watershed setting provide crucial assessments on governing biogeochemical processes. In this study, water chemistries were investigated in Noland Divide watershed (NDW), a high-elevation watershed in the Great Smoky Mountains National Park (GRSM) of the southern Appalachian region. Monitoring data from 1991 to 2007 for deposition and stream water chemistries were statistically analyzed for long-term trends and seasonal patterns by using Seasonal Kendall Tau tests. Precipitation declined over this study period, where throughfall (TF) declined significantly by 5.76 cm year−1. Precipitation patterns play a key role in the fate and transport of acid pollutants. On a monthly volume-weighted basis, pH of TF and wet deposition, and stream water did not significantly change over time remaining around 4.3, 4.7, and 5.8, respectively. Per NDW area, TF SO42- flux declined 356.16 eq year−1 and SO42- concentrations did not change significantly over time. Stream SO42- remained about 30 μeq L−1 exhibiting no long-term trends or seasonal patterns. SO42- retention was generally greater during drier months. TF monthly volume-weighted NH4+ and NO3- concentrations significantly increased by 0.80 μeq L−1 year−1 and 1.24 μeq L−1 year−1, respectively. TF NH4+ fluxes increased by 95.76 eq year−1. Most of NH4+ was retained in the watershed, and NO3- retention was much lower than NH4+. Stream monthly volume-weighted NO3- concentrations and fluxes significantly declined by 0.56 μeq L−1 year−1 and 139.56 eq year−1, respectively. Overall, in NDW, inorganic nitrogen was exported before 1999 and retained since then, presumably from forest regrowth after Frazer fir die-off in the 1970s from balsam wooly adelgid infestation. Stream export of NO3- was greater during winter than summer months. During the period from 1999 to 2007, stream base cations did not exhibit significant changes, apparently regulated by soil supply. Statistical models predicting stream pH, ANC, SO42-, and NO3- concentrations were largely correlated with stream discharge and number of dry days between precipitation events and SO42- deposition. Dependent on precipitation, governing biogeochemical processes in NDW appear to be SO42- adsorption, nitrification, and NO3- forest uptake. This study provided essential information to aid the GRSM management for developing predictive models of the future water quality and potential impacts from climate change.

Processes affecting the response of sulfate concentrations to clearcutting in a northern hardwood forest, Catskill Mountains, New York, U.S.A.

The effects of disturbance on the biogeochemical processes that affect the sulfur (S) cycle in forested ecosystems are important, but have been studied in only a few locations. In this investigation, the mechanisms that caused large decreases in stream SO42− concentrations after clearcutting a small forested catchment in the Catskill Mountains of southeastern New York in 1997 were identified through an examination of pH and SO42− concentrations in soil solutions, bulk deposition of SO42− in throughfall collectors, adsorbed SO42− concentrations in buried soil bags, and spatial variations in SO42− concentrations in shallow groundwater. The load of SO42−–S in stream water during the first 2 years after clearcutting was about 2 kg ha−1 year−1 less than the background value of 8–10 kg ha−1 year−1. The 10 and 19% decrease in net throughfall flux of SO42−–S during the 2nd and 3rd year after the clearcut, respectively, reflects reduced dry deposition of S after removal of the canopy, but this decrease accounts for 0 and 43%, respectively, of the decrease in SO42− load in streamflow for these 2 years. The pH of B-horizon soil water decreased from 4.5 to 4.0 within 8 months after the clearcut, and SO42− concentrations decreased from 45 µmol L−1 to less than 20 µmol L−1 during this time. A strong correlation between SO42− concentrations and pH values (r2 = 0.71, p < 0.01) in B-horizon soil water during the post-harvest period (1997–1999) reflects increased SO42− adsorption in response to soil acidification. Sulfate concentrations in groundwater from 21 spatially distributed wells were inversely related to a topographic index that served as a surrogate for soil wetness; thus, providing additional evidence that SO42− adsorption was the dominant cause of the decreased SO42− concentrations in the stream after clearcutting. These results are consistent with those from a 1985 whole-tree harvest at the Hubbard Brook Experimental Forest in New Hampshire in which increased SO42− adsorption resulting from decreased soil pH was the primary cause of decreased SO42− concentrations in stream water.

A MODEL TO SIMULATE THE RESPONSE OF A NORTHERN HARDWOOD FOREST ECOSYSTEM TO CHANGES IN S DEPOSITION

Watershed studies across the northeastern United States have shown that stream losses of SO42− exceed atmospheric sulfur (S) deposition. Understanding the processes responsible for this additional source of S is critical to quantifying ecosystem response to ongoing and potential future controls on SO2 emission. An integrated biogeochemical model, PnET-BGC, was used to investigate inputs and dynamics of S in a northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. The changes in soil S pools and stream-water SO42− were simulated to assess the response to both atmospheric S deposition and forest clear-cutting disturbances. Model simulation using the measured dry-to-bulk deposition ratio of 0.21 resulted in an underprediction of soil S pools and stream-water SO42− concentrations in the biogeochemical reference watershed (Watershed 6). However, the depiction of biotic processes (e.g., plant uptake, mineralization) in the model reduced the discrepancy in stream SO42− concentration between measured and model predicted value by ∼50% compared to a previous modeling effort that only considered abiotic processes. Long-term simulations (∼150 yr) indicated that elevated anthropogenic S deposition has increased stream SO42− concentrations and enhanced the incorporation of S in adsorbed SO42− and organic S soil pools. Following the implementation of the 1970 and 1990 Amendments to the Clean Air Act, decreases in S deposition resulted in the net release of S from soil pools, including soil organic S. Model simulation of forest clear-cutting of Watershed 5 at the HBEF showed that NO3− leaching and associated acidification following this disturbance increased adsorption of SO42− to soil. Compared to the reference watershed, stream-water SO42− concentrations were slightly higher, and soil adsorbed SO42− pools were substantially higher in the clear-cut watershed 4–5 yr after disturbance. Simulation of stable S isotopes showed that fractionation associated with the mineralization of soil organic S might explain the depletion in 34S observed between throughfall and stream water in the reference watershed. There is a need for further research on: (1) the rates of dry S deposition and (2) the rate of weathering of various minerals and the isotopic composition of these minerals in order to fully assess the discrepancy (i.e., greater export of S than can be accounted for by atmospheric deposition) in watershed S mass balances. The results of forecasts of the future response to anticipated decreases in S deposition are highly dependent on the nature of this additional source of S to forest watersheds. However, the large size and relatively long turnover time of soil organic S pools compared to adsorbed SO42− pools suggest that a model depicting only abiotic processes will not be suitable for predicting the long-term recovery of stream water from acidification by atmospheric deposition in northern forests.

Sources of stream sulphate in headwater catchments in Otter Creek Wilderness, West Virginia, USA

AbstractUpland forested catchments in the Appalachian Plateau region receive among the greatest rates of atmospheric sulphur (S) deposition in the eastern USA, although coal mines and S‐bearing minerals in bedrock may also contribute to stream acidity in this region. Watershed mass balance and stable S isotopic values (δ34S) of sulphate (SO42−) were used to assess the contributions to stream SO42− from atmospheric and lithogenic sources at Yellow Creek (YC), a headwater catchment on the Appalachian Plateau in West Virginia. Oxygen isotopic values (δ18O) of water were used to study catchment hydrology. Stream output of SO42− was c. 60% of atmospheric S deposition during a relatively dry year, whereas atmospheric S input was nearly balanced by stream output during a year with above normal amounts of precipitation. The temporal patterns and values of δ34S were similar between bulk precipitation and stream water at two upper elevation sites. At the lowest elevation site, stream δ34S values were similar to bulk precipitation values during the dormant season but were slightly lower than precipitation during the low‐flow summer, probably as the result of a greater proportion of stream water being derived from deep hydrological flowpaths that have contacted S‐bearing minerals with low δ34S values in coal seams. Stream δ34S values at YC were significantly higher than at Coal Run, a catchment containing abandoned coal prospects and having a greater amount of S‐bearing minerals than YC. Results suggested that lithogenic S is a relatively minor source and that atmospheric deposition is the principal source of stream SO42−, and thus stream acidity, at YC. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Changes in soil sulfur constituents in a forested watershed 8 years after whole-tree harvesting

Soil S constituents were evaluated before and after the whole-tree harvesting of Watershed 5 (W5) at the Hubbard Brook Experimental Forest, New Hampshire. Soil solution and stream water concentrations of SO42-, NO3-, and H+ were compared between W5 and W6 (reference watershed). Whole-tree harvesting increased phosphate-extractable SO42- (PSO4) in the E horizon, from 2 mg S·kg-1 soil in pre-harvest to 9 and 10 mg S·kg-1 soil 3 and 8 years post-harvest, respectively. Harvesting increased PSO4 in the Bh horizon from 11 mg S·kg-1 soil prior to harvesting to 20 and 25 mg S·kg-1 soil 3 and 8 years after harvesting, respectively. Temporal patterns in soil chemistry were also reflected in stream SO42-, NO3-, and H+ concentrations. Eight years after harvesting, PSO4 concentrations in the mineral soil increased with elevation. This elevational pattern was likely due to the higher concentrations of SO42- and H+ in soil solutions that enhanced SO42- adsorption at the higher elevations. The high H+ concentrations were attributed to enhanced nitrification and differences in vegetation at upper elevations. The importance of these factors were discussed with respect to the effects of forest harvesting and changes in atmospheric S deposition.

Changes in soil sulfur constituents in a forested watershed 8 years after whole-tree harvesting

Soil S constituents were evaluated before and after the whole-tree harvesting of Watershed 5 (W5) at the Hubbard Brook Experimental Forest, New Hampshire. Soil solution and stream water concentrations of SO42-, NO3-, and H+ were compared between W5 and W6 (reference watershed). Whole-tree harvesting increased phosphate-extractable SO42- (PSO4) in the E horizon, from 2 mg S·kg-1 soil in pre-harvest to 9 and 10 mg S·kg-1 soil 3 and 8 years post-harvest, respectively. Harvesting increased PSO4 in the Bh horizon from 11 mg S·kg-1 soil prior to harvesting to 20 and 25 mg S·kg-1 soil 3 and 8 years after harvesting, respectively. Temporal patterns in soil chemistry were also reflected in stream SO42-, NO3-, and H+ concentrations. Eight years after harvesting, PSO4 concentrations in the mineral soil increased with elevation. This elevational pattern was likely due to the higher concentrations of SO42- and H+ in soil solutions that enhanced SO42- adsorption at the higher elevations. The high H+ concentrations were attributed to enhanced nitrification and differences in vegetation at upper elevations. The importance of these factors were discussed with respect to the effects of forest harvesting and changes in atmospheric S deposition.

Inter-regional comparison of patterns and trends in surface water acidification across the United States

Temporal trends in acid-base chemistry are reported for surface waters in 6 regions of the United States. The lakes and streams are low ANC, dilute systems, selected to represent acid-sensitive aquatic resources in the 6 regions. The predominant trends observed were decreases in lake and stream SO42− concentrations in sites east of the Mississippi River, and increases in NO3− in the Adirondack lake and Catskill stream sites (both located in eastern New York State). Correlations of trend results from all sites with other factors indicated that trends in precipitation volume were highly correlated with the observed trend patterns. From the surface water trend results, three distinct clusters were identified that corresponded to three trend patterns: ‘dilution,’ ‘recovery,’ and ‘acidification.’ These were distributed across the LTM regions, with no particular geographic patterns.

Stable sulfur isotopes of sulfate in precipitation and stream solutions in a northern hardwood watershed

Stable S isotopes of SO42‐ in precipitation, throughfall, and stream water solutions in a northern hardwood watershed (Bear Brook Watershed, Maine) were examined to determine sources of stream SO42‐ and to identify watershed processes that may affect atmospherically deposited SO42‐ prior to reaching the streams. Similarity among temporal patterns in δ34S of precipitation, throughfall, stream SO42‐ (range: +3.4 to +9.4‰), and of stream SO42‐ (5–10%) indicate that the δ34S values of stream SO42‐ are controlled principally by atmospheric deposition. Lack of correlation between δ34S of stream SO42‐ and stream , elevational position, and stream discharge supports the interpretation that fractionation of stable S isotopes within the watershed appears to have, at most, a minor influence on δ34S of stream SO42‐.