Stream Water Concentrations Research Articles

Differential response of multiple stream ecosystem processes to basin- and reach-scale drivers

Stream ecosystems are inherently dependent on their surroundings and, thus, highly vulnerable to anthropogenic impacts, which alter both their structure and functioning. Anchored in biologically-mediated processes, the response of stream ecosystem functioning to environmental conditions exhibits intricate patterns, reflecting both natural dynamics and human-induced changes. Our study aimed at determining the natural and anthropogenic drivers influencing multiple stream ecosystems processes (nutrient uptake, biomass accrual, decomposition, and ecosystem metabolism) at a regional scale. By examining 38 natural and anthropogenic variables across 63 stream reaches in Gipuzkoa (northern Iberian Peninsula), we used structural equation modeling to unravel the cascading effect of basin- and reach-scale drivers onto ecosystem process. The results reveal significant variability in ecosystem processes, with contrasting spatial patterns, suggesting that studied processes respond differently to environmental factors. Urban land-use emerged as a primary basin-scale driver, whereas reach-scale variables reflected both natural and anthropogenic influence. Nutrient uptake rates were primarily driven by nutrient concentrations in stream water, but models for biomass accrual, decomposition, and ecosystem metabolism exhibited more complex cause-effect relationships. Our findings highlight the impact of urban areas on multiple ecosystem processes and services, disproportionate when considering their small land cover. The present study emphasizes the convenience of measuring multiple ecosystem functions simultaneously to get a comprehensive diagnosis of the functional status of rivers.

Quantifying the relative importance of agricultural land use as a predictor of catchment nitrogen and phosphorus concentrations

Agriculture is a major source of nitrogen (N) and phosphorus (P) in freshwater ecosystems, and different management strategies exist to reduce farmland nutrient losses and thus mitigate freshwater eutrophication. The importance of agricultural sources of N and P as drivers of water quality is known to vary spatially, but quantification of the relative importance of the nutrient sources shaping this variability remains challenging, especially with reference to inputs from waste water treatment works. Addressing this knowledge gap is key for targeting management strategies to where they are likely to have the greatest effect. To advance our understanding in this area, this study assesses the impact of population density as a driver of the relative importance of agricultural land use for predicting mean Total Oxidised Nitrogen (TON) and Reactive Phosphorus (RP) concentrations in rivers in England, using two different data-driven, statistical approaches: a generalised linear model and random forest. Our results show that agricultural N and P sources dominate in catchments with low population density, where stream water concentrations are lower and waste water treatment works are numerous, but smaller in terms of the population equivalent served. Agricultural N and P sources are not important predictors of N and P in catchments with high population density, where contributions from waste water treatment works dominate. These results require cautious interpretation, as model validation outcomes show that high TON and RP concentrations are consistently underpredicted. Altogether, our results suggest that the relative contribution of agricultural sources may be overestimated in densely populated catchments, relative to point sources from waste water treatment works, and that management strategies to reduce the contribution of agriculture to N and P in rivers may be better targeted towards catchments with lower population density, as this is where agricultural land use is the primary source of N and P.

Long-term Trends of Dissolved Organic Carbon Dynamics in a Subtropical Forest Responding to Environmental Changes.

Dissolved organic carbon (DOC) dynamics are critical to carbon cycling in forest ecosystems and sensitive to global change. Our study, spanning from 2001 to 2020 in a headwater catchment in subtropical China, analyzed DOC and water chemistry of throughfall, litter leachate, soil waters at various depths, and streamwater. We focused on DOC transport through hydrological pathways and assessed the long-term trends in DOC dynamics amidst environmental and climatic changes. Our results showed that the annual DOC deposition via throughfall and stream outflow was 14.2 ± 2.2 and 1.87 ± 0.83 g C m-2 year-1, respectively. Notably, there was a long-term declining trend in DOC deposition via throughfall (-0.195 mg C L-1 year-1), attributed to reduced organic carbon emissions from clean air actions. Conversely, DOC concentrations in soil waters and stream waters showed increasing trends, primarily due to mitigated acid deposition. Moreover, elevated temperature and precipitation could partly explain the long-term rise in DOC leaching. These trends in DOC dynamics have significant implications for the stability of carbon sink in terrestrial, aquatic, and even oceanic ecosystems at regional scales.

Sulfate runoff processes during rainfall events in a small forested catchment on the sea of Japan side recovering from acidification under climate change

AbstractChanges in rainfall patterns due to climate change may accelerate the runoff of sulfate (SO42−), which is anthropogenically emitted and deposited as an air pollutant, cycled in forest ecosystems, and partly accumulated in forest soils. A forested catchment on the Sea of Japan side in central Japan is significantly affected by transboundary air pollution from the Asian continent due to northwesterly seasonal winds in winter. In this study, intensive 24‐h observations were conducted every hour eight times from 2019 to 2020 to clarify changes in stream water quality and runoff processes during rainfall events. The pH, electrical conductivity, and SO42− concentration in stream water decreased with increasing hourly average discharge rate (L sec−1). The SO42− concentration was negatively correlated with discharge rate. Hydrograph separations using the water isotopic parameter (deuterium excess, d‐excess = δ2H – 8 × δ18O) showed that most of the stream flow during the rain events was derived from pre‐storm water. A significant negative correlation between the d‐excess and stream water discharge was found for all six events where the water isotope analysis was applied. However, the S isotope ratio (δ34S) in stream water was not correlated with discharge rate during rainfall events and was obviously different (>1.5‰) from rainwater δ34S in the same month. This suggests that rainwater SO42− during rainfall events did not directly flow to the stream but was retained in the forest ecosystem. The isotopically well homogenized internal SO42− appeared to be mainly released into the stream during rainfall events. Future climate change may further accelerate SO42− runoff from forest catchments and disrupt material cycles in the ecosystem if warming causes more intense rainfall.

Stream chemical response is mediated by hydrologic connectivity and fire severity in a Pacific Northwest forest

AbstractLarge‐scale wildfires are becoming increasingly common in the wet forests of the Pacific Northwest (USA), with predicted increases in fire prevalence under future climate scenarios. Wildfires can alter streamflow response to precipitation and mobilize water quality constituents, which pose a risk to aquatic ecosystems and downstream drinking water treatment. Research often focuses on the impacts of high‐severity wildfires, with stream biogeochemical responses to low‐ and mixed‐severity fires often understudied, particularly during seasonal shifts in hydrologic connectivity between hillslopes and streams. We studied the impacts of the 2020 Holiday Farm Fire at the HJ Andrews Experimental Forest where rare pre‐fire stream discharge and chemistry data allowed us to evaluate the influence of mixed‐severity fire on stream water quantity and quality. Our research design focused on two well‐studied watersheds with low and low‐moderate burn severity where we examined long‐term data (pre‐ and post‐fire), and instantaneous grab samples collected during four rain events occurring immediately following wildfire and a prolonged dry summer. We analysed the impact of these rain events, which represent the transition from low‐to‐high hydrologic connectivity of the subsurface to the stream, on stream discharge and chemistry behaviour. Long‐term data revealed total annual flows and mean flows remained fairly consistent post‐fire, while small increases in baseflow were observed in the low‐moderately burned watershed. Stream water concentrations of nitrate, phosphate and sulfate significantly increased following fire, with variance in concentration increasing with fire severity. Our end member mixing models suggested that during rain events, the watershed with low‐moderate severity fire had greater streamflow inputs from soil water and groundwater during times of low connectivity compared to the watershed with low severity fire. Finally, differences in fire severity impacts on concentration‐discharge relationships of biogenic solutes were most expressed under low catchment connectivity conditions. Our study provides insights into post‐wildfire impacts to stream water quality, with the goal of informing future research on stream chemistry responses to low, moderate and mixed severity wildfire.

Trade-offs between nitrogen and phosphorus removal with floodplain remediation in agricultural streams

To improve water quality and reduce instream erosion, floodplain remediation along agricultural streams can provide multiple ecosystem services through biogeochemical and fluvial processes. During floodplain inundation, longer water residence time and periodic anoxic conditions can lead to increased nitrogen (N) removal through denitrification but also mobilization of phosphorus (P), impeding overall water quality improvements. To investigate the capacity for N and P processing in remediated streams, we measured potential denitrification and nitrous oxide production and yields together with potential P desorption and P fractions in floodplain and stream sediments in ten catchments in Sweden. Sediment P desorption was measured as equilibrium P concentration, using P isotherm incubations. Denitrification rates were measured with the acetylene inhibition method. Sediment nutrient process rates were combined with hydrochemical monitoring along remediated streams and their paired upstream control reaches of trapezoidal shape to determine the impact of floodplains on water quality. The correlation between floodplain denitrification rates and P desorption (r = 0.53, p = 0.02) revealed a trade-off between soluble reactive P (SRP) and nitrate removal, driven by stream water connectivity to floodplains. Nitrous oxide production was not affected by differences in P processing, but nitrous oxide yields decreased with higher denitrification and P desorption. The release of SRP from floodplains (0.03 ± 0.41 mg P kg−1 day−1) was significantly lower than from trapezoidal stream banks (0.38 ± 0.37 mg P kg−1 day−1), predicted by long-term SRP concentrations in stream water and floodplain inundation frequency. The overall impact of SRP release from floodplains on stream SRP concentrations in remediated reaches was limited. However, the remediated reaches showing increased stream SRP concentrations were also frequently inundated and had higher labile P content and coarse soil texture in floodplain sediments. To fully realize the potential for water quality improvements with constructed floodplains in agricultural streams, the promotion of denitrification through increased inundation should be balanced against the risk of P release from sediments, particularly in streams with high SRP inputs.

Multi-year time series of daily solute and isotope measurements from three Swiss pre-Alpine catchments

Time series analyses of solute concentrations in streamwater and precipitation are powerful tools for unraveling the interplay of hydrological and biogeochemical processes at the catchment scale. While such datasets are available for many sites around the world, they often lack the necessary temporal resolution or are limited in the number of solutes they encompass. Here we present a multi-year dataset encompassing daily records of major ions and a range of trace metals in both streamwater and precipitation in three catchments in the northern Swiss Pre-Alps. These time series capture the temporal variability observed in solute concentrations in response to storm events, snow melt, and dry summer conditions. This dataset additionally includes stable water isotope data as an extension of a publicly available isotope dataset collected concurrently at the same locations, and together these data can provide insights into a range of ecohydrological processes and enable a suite of analyses into hydrologic and biogeochemical catchment functioning.

Assessing impacts of cemeteries on water quality in an urban headwater watershed with mixed human‐built infrastructure

AbstractCemeteries are understudied integral components to urban watersheds, which provide ecosystem services but can also export nutrients, trace elements, and other contaminants to nearby water bodies. In this study, we focus on Meadowbrook Creek, an urban headwater stream in Syracuse, New York (USA), which has shown significant nitrate contributions from a local cemetery. We collected biweekly surface water samples over the course of 1 year from 2022 to 2023 for analysis of major and trace elemental concentrations including Na, Ca, Mg, K, F, Cl, sulfate, and nitrate. Here, we aim to assess the impact of various human infrastructures on urban stream water quality with a particular focus on the cemetery and nitrate. A comparison between the new dataset in this study and previously reported water chemistry data in Meadowbrook in 2012 suggests a decade‐long impact of road salting and the cemetery on water quality particularly with respect to Na, Cl, and nitrate. Sulfate, Mg, Ca, and K are likely mainly geogenic. Stable nitrogen isotope data, the usage of concrete or steel vaults in the cemetery in the past 50 years, and the lack of correlation between nitrate and fluoride concentrations in stream water argue against burial decay products being a major source of nitrate to the stream. Instead, other nitrate sources that exist in the cemetery such as, fertilizer, decaying plant material, and wastewater, are more viable dominant nitrate sources. In addition, nitrate loading calculations indicate that the groundwater‐connected reach, including the cemetery, acts as an annual net sink for nitrate despite the seasonally varying sink‐source patterns.

Exploring the Complex Effects of Wildfire on Stream Water Chemistry: Insights From Concentration‐Discharge Relationships

AbstractWildfires are a worldwide disturbance with unclear implications for stream water quality. We examined stream water chemistry responses immediately (<1 month) following a wildfire by measuring over 40 constituents in four gauged coastal watersheds that burned at low to moderate severity. Three of the four watersheds also had pre‐fire concentration‐discharge data for 14 constituents: suspended sediment (SSfine), dissolved organic and inorganic carbon (DOC, DIC), specific UV absorbance (SUVA), major ions (Ca2+, K+, Mg2+, Na+, Cl−, , , F−), and select trace elements (total dissolved Mn, Fe). In all watersheds, post‐fire stream water concentrations of SSfine, DOC, Ca2+, Cl−, and changed when compared to pre‐fire data. Post‐fire changes in , K+, Na+, Mg2+, DIC, SUVA, and total dissolved Fe were also found for at least two of the three streams. For constituents with detectable responses to wildfire, post‐fire changes in the slopes of concentration‐discharge relationships commonly resulted in stronger enrichment trends or weaker dilution trends, suggesting that new contributing sources were surficial or near the surface. However, a few geogenic solutes, Ca2+, Mg2+, and DIC, displayed stronger dilution trends at nearly all sites post‐fire. Moreover, fire‐induced constituent concentration changes were highly discharge and site‐dependent. These similarities and differences in across‐site stream water chemistry responses to wildfire emphasize the need for a deeper understanding of landscape‐scale changes to solute sources and pathways. Our findings also highlight the importance of being explicit about reference points for both stream discharge and pre‐fire stream water chemistry in post‐fire assessment of concentration changes.

Effects of climate and acidic deposition on interannual variations of stream water chemistry in forested watersheds in the Shimanto River Basin, southern Japan

AbstractAlthough the amount of acidic deposition has recently decreased in Japan, it has still deteriorated some forest ecosystems during the past several decades. Moreover, recent climate changes can affect stream water chemistry. We investigated the temporal trend and effects of climate conditions on stream water chemistry for more than 20 years in two areas (Yusuhara and Taisho) in the Shmanto River Basin, southern Japan, where the effects of acidic deposition are considered to be modest. Stream water samples were collected monthly from three forest watersheds selected at each site. The annual means of the stream chemistry were predicted by multiple regression analysis. The ammonium, nitrate, and sulfate concentrations in the bulk precipitation have decreased at Yusuhara, and the sodium, magnesium, calcium, chloride, nitrate, and sulfate concentrations in the stream water have decreased in both areas. The nitrate and sulfate concentrations apparently responded to the decreasing input of acidic deposition. The sunlight hours were positively related with the potassium, magnesium, calcium, nitrate, sulfate, and bicarbonate concentrations in stream water. The results suggest that long sunlight hours boost the photosynthetic activities, thus promoting soil respiration and decomposition of soil organic matter. Moreover, a higher carbonic acid concentration in the soil solution promotes cation weathering and carbonic acid dissociation to bicarbonate. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previousinputs of acidic deposition enhanced the rate of rock weathering.

Hydrological and Biogeochemical Controls of the Seasonality of Particulate and Dissolved Nitrogen Exports During Rainfall Events From a Forested Watershed in Monsoon Asia, Japan

AbstractFew studies have investigated the effects of hydrological conditions such as rainfall magnitude and soil nitrogen dynamics on nitrogen export from forests during rainfall events. In this study, the seasonal export of particulate and dissolved nitrogen during 24 rainfall events (range: 3.0–417 mm) in a temperate monsoon forest watershed in Japan was measured along with the seasonality of the soil nitrate () pool size and the nitrification rate. The increase in nitrogen export with increasing streamflow was larger for particulate nitrogen than for and dissolved organic nitrogen, but was always the most abundant nitrogen component exported, even during extreme rainfall. For all rainfall events in summer, measured exports were higher than the averaged exports, regardless of rainfall magnitude or preceding rainfall. Nitrification activity in the soil was high in summer because of the high temperature and wet conditions of the soil. Nitrification also occurred in the upper slope in summer, whereas during other seasons it is hindered by dry conditions. These results indicate that nitrogen dynamics in the soil also affect export during rainfall. Comparisons with previous studies suggested that the effects of soil nitrogen dynamics on concentrations in stream water may be stronger during rainfall than during base flow conditions. They also showed that regions with wet summers due to rainfall are more sensitive to export, compared with other regions, and that increased summer rainfall can have a particularly large impact on nitrogen export from forest watersheds.

Sulfidic schist release of As, Cu, and Pb in laboratory experiments and across eleven watersheds in central Massachusetts, USA.

Sulfidic schists are important rock formations due to their trace metal and metalloid (TMM) content and carry the potential for pyrite and pyrrhotite to hydrate and oxidize leading to acid-enhanced chemical weathering. The objectives of this study were to compare TMMs in sulfidic schists to other co-occurring bedrock, evaluate conditions that optimize TMM rock weathering, and examine streamwater TMMs in relation to bedrock lithology and human development in eleven streams across central Massachusetts. Sulfidic schists samples had the highest As (72 ± 46mgkg-1), Cu (63 ± 21mgkg-1), and Pb (63 ± 33mgkg-1) concentrations. Electron Probe Microanalysis (EPMA) images show As and Pb were widely distributed across silicate and sulfide minerals in both the mica schist and sulfidic schists, not exclusively hosted in sulfide minerals as hypothesized. Batch reactors had TMM dissolution rates an order of magnitude higher for sulfidic schists than granite and mica schists. Furthermore, TMM dissolution was greatest under pH 9 than pH 7 or pH 5 and dissolution rates were two times greater under anoxic conditions compared to oxic conditions. Streamwater concentrations of As (0.01 to 10.3μg L-1), Cu (0.2 to 206μg L-1), and Pb (0.001 to 8.3μg L-1) were below Massachusetts Surface Water Quality Standards. Across the eleven watersheds, % sulfidic schists were positively correlated with mean streamwater S and Cu concentrations and area-normalized annual export. Streamwater As and Pb concentrations were significantly correlated with %Developed land and %Mica schist, which strongly covaried. Our study confirmed the elevated abundance of TMMs in sulfidic schists but laboratory experiments suggest the precipitation of amorphous Fe oxyhydroxide phases decreased dissolved TMMs during oxic weathering. Future work will need to incorporate groundwater and stable isotope systems to separate anthropogenic and geogenic analyses.

Detection of heavy metal contamination in Batlama Stream (Turkiye) and the potential toxicity profile

In this study, heavy metal pollution in Batlama stream flowing into the Black Sea from Giresun (Turkiye) province and the toxicity induced by this pollution were investigated by Allium test. Heavy metal concentrations in stream water were analyzed by using ICP-MS. Germination percentage, weight gain, root length, micronucleus (MN), mitotic index (MI), chromosomal abnormalities (CAs), proline, chlorophyll, malondialdehyde (MDA), antioxidant enzyme activities were used as indicators of physiological, cytogenetic and biochemical toxicity. In addition, Comet assay was performed for detecting DNA fragmentation. Anatomical changes caused by heavy metals in the root meristem cells were observed under the microscope. A. cepa bulbs are divided into two groups as control and treatment. The bulbs in the control group were germinated with tap water and the bulbs in the treatment group were germinated with stream water. As a result, heavy metals such as Al, Ti and Co and radioactive heavy metals such as Rb, Sr, Sb and Ba were detected in the stream water above the acceptable parametric values. Heavy metals in the water caused a decrease in germination, root elongation, weight gain, MI and chlorophyll values, and an increase in MDA, proline, SOD, CAT, MN and CAs values. Comet assays indicated the presence of severe DNA damage. In addition, heavy metals in stream water caused different types of CAs and anatomical damage in root meristem cells. As a result, it was determined that there is intense heavy metal pollution in the stream water and this pollution promotes multi-dimensional toxicity in A. cepa, which is an indicator organism. For this reason, the first priority should be to prevent pollution of water resources in order to prevent heavy metal-induced toxicity in water.

Assessment of Present-Day Heavy Metals Pollution and Factors Controlling Surface Water Chemistry of Three Western Siberian Sphagnum-Dominated Raised Bogs

This study investigated the heavy metal concentrations in bog and stream water compared to present-day atmospheric deposition, and concentrations in peat and vegetation within three typical raised bogs in Western Siberia located in urban area, close to oil and gas facilities and in the natural background area. Our data showed that elevated heavy metals deposition occurs not only near industrial centres but also in remote areas, which is a sign of regional atmospheric deposition of heavy metals associated with long-range transport and wildfires. Present-day atmospheric depositions of heavy metals are not always consistent with their contents in waters, and the content of Zn, Pb, Cu and Cd in waters is more correlated with their concentrations in vegetation and in the upper peat layer; this indicates a significant role of biological processes in heavy metal cycling. Temperature plays an important role in increasing the mobility and vegetation uptake of heavy metals. Heavy metals removal is largely determined by the size of the bog and its stage of development, which determines bog–river interaction. The seasonal catchment-scale budget indicated that 80–97% of Zn and Pb and 47–74% of Cu and Cd from atmospheric inputs remained within the catchments.

Linking Snow, Soil, and Stream During Snowmelt and Rain‐On‐Snow Events: Storage and Transport of Ions in an Acid‐Sensitive Alpine Catchment (Tatra Mountains, Poland)

AbstractThe purpose of the study was to examine the storage and transport of ions through snowpack, soils, and stream water in an acid‐sensitive alpine catchment (Tatra Mountains, Poland) during snowmelt and rain‐on‐snow events. Samples of snowpack layers, near‐surface soil horizons, and stream water were collected in the winter and snowmelt seasons of 2019. A laboratory experiment was conducted to determine the effect of temperature on the rate of soil nitrogen mineralization and nitrification. Our study has shown that snowpack is an important source of ions in the catchment. As the snow melts, the release of ions from snowpack occurs. A gradual and slow melting of snow starts even before the first snowmelt‐induced increase in stream discharge. ions eluted from available snowpack are temporarily stored in soil, which is shown by a large increase in the concentration of water‐soluble in the soil at that time. ions are washed out of soils and supplied to streams during the first snowmelt event. This is demonstrated by a large increase in the stream water concentration, termed an “ pulse.” The ion is a key acid anion responsible for the acidification of the studied stream during snowmelt season, as the pulse coincides with a decrease in bicarbonate alkalinity. Our field research and laboratory experiment have shown a minor role of mineralization and nitrification in production in soils in the winter and pulse formation in stream water during the early stages of the snowmelt season.

Vertical changes in sulfur isotopic ratio of water flowing through a forested catchment along the coast of the sea of Japan in central Japan–a buffer against seasonal transboundary air pollution

ABSTRACT Acidic substances, specifically sulfur (S) compounds, derived from atmospheric deposition play a major role in the acidification of forest ecosystems. This study conducted field surveys to clarify a buffering system against seasonal large S inputs in a forested catchment in central Japan that has historically suffered from transboundary air pollution. Results showed that atmospheric S fluxes significantly increased in winter due to north-westerly seasonal winds from the Asian continent; fluxes were 1.1 and 0.3 kmolc ha−1 in the cold and warm seasons, respectively, due to the large effects of sea salt and transboundary air pollution. Despite the large seasonality within atmospheric deposition, SO4 2– concentrations in stream water (SW) were found to be relatively stable throughout the year. Similarly, S isotopic ratios (δ34S) in rainwater showed clear seasonal variation, increasing to 12‰ in winter and decreasing to 2‰ in summer, whereas the δ34S value of SW was stable year-round at ~9‰. Flux-weighted mean δ34S values for rainfall (RF), throughfall, stemflow, and SW were similar, i.e. 8.5, 9.5, 9.0, and 9.0‰, respectively. Both the δ34S values and the SO4 2– concentrations in RF and soil solutions appear to converge at values of SW, suggesting that atmospheric deposition is a primary S origin in SW. The sulfur adsorption-desorption in soil appears to mainly buffer the large sulfur input and prevent sudden acidification, whereas a relatively small biological sulfur cycle was suggested by litterfall. Possible disturbances within this buffering system should be carefully monitored under a changing climate.

Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

AbstractThe oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that > 80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long‐term denudation rates, the observed OCpetro loss and the Re proxy are equivalent to a low OCpetro oxidation yield of < 1.7 × 10−2 tC km−2 yr−1. This is consistent with the low OCpetro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings.

Four decades of regional wet deposition, local bulk deposition, and stream-water chemistry show the influence of nearby land use on forested streams in Central Appalachia

Hydrologic monitoring began on two headwater streams (<1 km2) on the University of Kentucky's Robinson Forest in 1971. We evaluated stream-water (1974–2013) and bulk-deposition (wet + dust) (1984–2013) chemistry in the context of regional wet-deposition patterns that showed decreases in both sulfate and nitrate concentrations as well as proximal surface-mine expansion. Decadal time steps (1974–83, 1984–93, 1994–2003, 2004–2013) were used to quantify change. Comparison of the first two decades showed similarly decreased sulfate (minimum flow-adjusted annual-mean concentration of ≈13.5 mg/L in 1982 to 8.8 mg/L in 1992) and increased pH (6.6–6.8) in both streams, reflecting contemporaneous changes in both bulk and wet deposition. In contrast, concentrations of nitrate (0.14 to >0.25 mg/L) and base cations increased between these two decades, coinciding with expansion of surface mining between 1985 and 1995. In 2004, stream-water pH (6.7 in 2004), sulfate (9.2 mg/L), and nitrate (>0.11 mg/L) were similar to 1982, despite wet-deposition concentrations being lower. Base-cation concentrations were higher in the stream adjacent to ongoing surface mining relative to the stream situated near the middle of the experimental forest. However, pH decreased to approximately 5.7 by 2013 for both streams, which, combined with a shift in dominant cations from calcium to magnesium and potassium, indicates that the soil-buffering capacity of this landscape has been exceeded. Ratios of bulk deposition and stream-water concentrations indicate enrichment of sulfate (1.7–25.2) and cations (0.5–64.8), but not nitrogen (0.1–5.6), indicating that the Forest is not nitrogen saturated and that ongoing changes in water-quality are sulfate driven. When concentrations were adjusted to account for changes in streamflow (climate) over the 4 decades, external influences (land management/regulation) explained most change. The amount and direction of change differed among constituents, both between consecutive decades and between the first and last decades, reflecting the influence of localized surface mining even as regional wet deposition continued to improve due to the Clean Air Act. The implication is that localized stressors have the potential to out-pace the benefits of national environmental policies for communities that depend on local water-resources in similar environments.

Insights into the streamwater age in the headwater catchments covered by glaciers and permafrost, Central Tibetan Plateau

Improving our understanding of streamwater age knowledge is critical for revealing the complex hydrological processes in alpine cryosphere catchments. However, few studies on water age have been conducted in alpine cryosphere catchments due to the complicated and inclement environment. In this study, the Buqu catchment, a typical alpine catchment covered by glaciers and permafrost on the central Tibetan Plateau (TP), was selected as the study area. Using the sine-wave approach and a gamma model based on the seasonal cycle of stable isotopes in water, the young water fraction (Fyw) and mean transit time (MTT) of the Buqu catchment outlet and 23 sub-catchments was estimated to comprehensively reveal the potential driving mechanism of water age variability. The streamwater MTT for the entire catchment was 107 days, and 15.1 % of the streamwater was younger than 41 days on average. The estimated water age showed significant spatial heterogeneity with shorter water ages in high-elevation and glacier catchments and longer water ages in low-elevation and non-glacier catchments. Precipitation was the primary driver for spatial variations in water age, while the thickness of the permafrost active layer may function as an intermediate hub to drive water age variability. Mechanically, the thickness of the permafrost active layer controls the water ages by modifying the flow direction and length of water flow path. Spatially, this control mechanism is indirectly driven by the elevation gradient. The TDS concentration in streamwater is significantly related to water age, thus revealing a close link between water quality and hydrology. Our findings suggest that cryosphere retreats likely alter water age, thereby slowing water circulation rates and affecting water quality security under global warming. This study provides insights into the evolution of water ages, thereby deepening our understanding of the hydrological processes and guiding the protection of water resources in alpine headwater catchments.

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

AbstractThe compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1 while soil‐bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant.