Surface Water Chemistry Research Articles

Modelling inorganic nitrogen in runoff: Seasonal dynamics at four European catchments as simulated by the MAGIC model

Nitrogen (N) deposition is globally considered as a major threat to ecosystem functioning with important consequences for biodiversity, carbon sequestration and N retention. Lowered N retention as manifested by elevated concentrations of inorganic N in surface waters indicates ecosystem N saturation. Nitrate (NO3) concentrations in runoff from semi-natural catchments typically show an annual cycle, with low concentrations during the summer and high concentrations during the winter. Process-oriented catchment-scale biogeochemical models provide tools for simulation and testing changes in surface water and soil chemistry in response to changes in sulphur (S) and N deposition and climate. Here we examine the ability of MAGIC to simulate the observed monthly as well as the long-term trends over 10–35years of inorganic N concentrations in streamwaters from four monitored headwater catchments in Europe: Čertovo Lake in the Czech Republic, Afon Gwy at Plynlimon, UK, Storgama, Norway and G2 NITREX at Gårdsjön, Sweden. The balance between N inputs (mineralization+deposition) and microbial immobilization and plant uptake defined the seasonal pattern of NO3 leaching. N mineralization and N uptake were assumed to be governed by temperature, described by Q10 functions. Seasonality in NO3 concentration and fluxes were satisfactorily reproduced at three sites (R2 of predicted vs. modelled concentrations varied between 0.32 and 0.47 and for fluxes between 0.36 and 0.88). The model was less successful in reproducing the observed NO3 concentrations and fluxes at the experimental N addition site G2 NITREX (R2=0.01 and R2=0.19, respectively). In contrast to the three monitored sites, Gårdsjön is in a state of change from a N-limited to N-rich ecosystem due to 20years of experimental N addition. At Gårdsjön the measured NO3 seasonal pattern did not follow typical annual cycle for reasons which are not well understood, and thus not simulated by the model. CapsuleThe MAGIC model is able to simulate NO3 leaching on a monthly as well as an annual basis, and thus to reproduce the seasonal and short-term variations in N dynamics.

Comparison of surface water chemistry and weathering effects of two lake basins in the Changtang Nature Reserve, China

The geochemistry of natural waters in the Changtang Nature Reserve, northern Tibet, can help us understand the geology of catchments, and provide additional insight in surface processes that influence water chemistry such as rock weathering on the Qinghai–Tibet Plateau. However, severe natural conditions are responsible for a lack of scientific data for this area. This study represents the first investigation of the chemical composition of surface waters and weathering effects in two lake basins in the reserve (Lake Dogaicoring Qiangco and Lake Longwei Co). The results indicate that total dissolved solids (TDS) in the two lakes are significantly higher than in other gauged lakes on the Qinghai–Tibet Plateau, reaching 20–40g/L, and that TDS of the tectonic lake (Lake Dogaicoring Qiangco) is significantly higher than that of the barrier lake (Lake Longwei Co). Na+ and Cl− are the dominant ions in the lake waters as well as in the glacier-fed lake inflows, with chemical compositions mainly affected by halite weathering. In contrast, ion contents of inflowing rivers fed by nearby runoff are lower and concentrations of dominant ions are not significant. Evaporite, silicate, and carbonate weathering has relatively equal effects on these rivers. Due to their limited scope, small streams near the lakes are less affected by carbonate than by silicate weathering.

Concentration and distribution of major macro- and microelements in surface waters in the Altai

The macroelement (HCO3−, SO42−, Cl−, Ca2+, Mg2+, Na+ + K+, as well as NO3−, NO2−, and PO43−) and microelement (As, Cd, Cr, Cu, Co, Fe, Mn, Ni, Pb, Zn, and V) composition of natural waters in different physiographical provinces of the Altai have been studied and characterized. It has been found that the concentrations of dissolved forms of metals in the waters of the examined rivers generally do not exceed the MACwm, though in some cases, they exceed the world averages and depend on the features of water nourishment of rivers and the concentrations of mineral salts in water. A considerable portion of metals in Altai surface waters has been found to be transported as a component of suspension, though the specific concentration of metals in suspension decreases with increasing water turbidity. The effect of the structure and properties of the soil cover on the drainage basins of various physiographic provinces of the Altai on surface water chemistry is most distinct in the case of macrocomponents and typomorphic elements (Fe, Mn, Ca), as well as under anthropogenic impact on the watershed.

Scale-dependent groundwater contributions influence patterns of winter baseflow stream chemistry in boreal catchments

Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and biogeochemical responses to environmental change. Network-scale patterns of stream water reflect distinct hydrological processes among headwater units, as well as variable contributions from deeper groundwater stores, which may vary nonlinearly with drainage basin size. Here we explore the spatial variability of groundwater inputs to streams, and the corresponding implications for surface water chemistry, during winter baseflow in a boreal river network. The relative contribution of recent and older groundwater was determined using stable isotopes of water (δ18O) at 78 locations ranging from small headwaters (0.12 km2) to fourth-order streams (68 km2) in combination with 79 precipitation and 10 deep groundwater samples. Results from a two end-member mixing model indicate that deeper groundwater inputs increased nonlinearly with drainage area, ranging from ~20% in smaller headwater subcatchments to 70–80% for catchments with a 10.6 km2 area or larger. Increases in the groundwater contribution were positively correlated to network-scale patterns in surface stream pH and base cation concentrations and negatively correlated to dissolved organic carbon. These trends in chemical variables are consistent with the production of weathering products and the mineralization of organic matter along groundwater flow paths. Together, the use of stable isotopes and biogeochemical markers illustrate how variation in hydrologic routing and groundwater contributions shape network-scale patterns in stream chemistry as well as patchiness in the relative sensitivity of streams to environmental change and perturbation.

Hydrochemical characterisation of water quality in the Sarcheshmeh copper complex, SE Iran

Hydrochemical investigations were conducted to identify variations in surface water chemistry in different parts of the Sarcheshmeh copper complex. Twenty-three water samples from mine drainage, stream water, processing water of the concentrator plant and tailings dam water were collected to understand processes that control the elements’ mobility. Changes in trace metal concentrations and variation of major ions show two distinct water types: (a) neutral mine drainage (NMD) associated with the Sarcheshmeh mine waters containing lower trace metal, SO42− concentrations and neutral pH values, and (b) acid mine drainage (AMD) with waters from tailings impoundments and waste rock dumps corresponding to higher trace metals (Fe, Cu, Zn, Mo) and major anions (SO42− and Cl−) than the first type with typically acidic pH contents. As a result of Cu–Mo sulphides flotation process under alkaline conditions, high pH values, high Mo concentration and low contamination loads of Al, Mn, Fe, Cu and Cd were observed in reject waters from the concentrator plant. Sulphide oxidation (especially pyrite), dissolution–precipitation of the secondary minerals and discharge of the reject waters from concentrator plant were some of the most important factors that influence water quality at the tailings impoundments. The results of PHREEQC speciation and solubility modelling indicated supersaturation of the waters in the tailings impoundments with respect to hydronium jarosite, schwertmannite and gypsum.

Relación entre la composición química inorgánica del agua, la precipitación y la evaporación en la cuenca de Río Grande, Chone, Ecuador

(Recibido: 2015/01/19 - Aceptado: 2015/03/25)En la cuenca de Río Grande se construye la presa Propósito Múltiple Chone (PMCH) con una inversión aproximada de 66 millones de dólares que irrigará más de 7000 ha. Por las marcadas diferencias en las precipitaciones, éstas podrían perjudicar la calidad de las aguas, por consiguiente, el objetivo de este trabajo fue establecer la relación existente entre los iones y su ubicación en el gráfico: relación de los iones y mecanismos de procesos químicos de las aguas estableciendo su comportamiento y tendencia. El trabajo fue una investigación no experimental, transversal y descriptiva. Se tomaron 111 muestras simples en el conjunto de cauces que forman la cuenca de Río Grande, en los años 2013 y 2014, durante las épocas de estiaje y lluvia. Los parámetros medidos en las muestras fueron aniones y cationes. Los procesos que controlan la química del agua superficial de la zona de estudio en época de lluvia poseen una predisposición a la mineralización en equilibrio con las rocas, sin embargo, en el periodo de estiaje se observó la predisposición de las aguas hacia la zona donde predomina la evaporación con respecto a la precipitación, aumentando la composición química inorgánica de las aguas que en el tiempo podría ampliarse.

Assessment of surface water chemistry and algale biodiversity in the Bay of Mostaganem and the Cheliff estuary: North-western Algeria

Anthropogenic activities have led to water quality deterioration in many parts of the word, especially in Northwest Algeria. The current work investigated the spatiotemporal variations of water quality in the Cheliff River, samples for physico-chemical were performed at different periods from 2004 to 2007, the results chowed that nitrate (NO 3 - ) intake is very high especially in the month of February 2006 (26 mg/l) and February 2007 (37 mg/l), nitrite (NO 2 - ) values also exceed the standard for samples taken at the estuary (and the sea), ie 0.96 mg/l in the month of February 2006 and 0.98 mg/l in April 2007;the Ammonium (NH 4 + ) contributions are due to the River because the value recorded at the estuary (4.22 in February 2006) ;silicate (SiO 2 ) varies greatly depending on the River flow resulting from soil leaching SOUR to the estuary where we see the maximum values of 20.10 mg/l in the month of February 2007 and 19.1 mg/l in March 2005. The recorded values of elements phosphorus (PO 4 --- ) are high and very variable from 0.01 to 1.90 mg/l for the River, 0.01- 0.80 mg/l for the estuary and 0- 0.49 mg/l for the sea. The analyzed biological confirmed a total of 41 phytoplankton speciesand31 macroalgae species. So, Aquatic ecosystems are particulury vulnerable to environmental change and many are, at present, severely degraded.

Cryosphere: ice on Niwot Ridge and in the Green Lakes Valley, Colorado Front Range

Background: Subsurface ice preserved as ice lenses and within rock glaciers as well as glacial and lake ice provides sensitive indicators of climate change and serve as a late-season source of meltwater.Aims: We synthesise the results of geomorphological, geophysical and geochemical studies during the period of 1995–2014, building on a long history of earlier work focused on ice and permafrost studies on Niwot Ridge and the adjacent Green Lakes Valley (GLV), which is part of the Niwot Ridge Long Term Ecological Research Site.Methods: These studies are discussed in the context of how bodies of ice and rock glaciers reflect changing local climate. We review recent results from geophysical investigations (resistivity, seismic refraction and ground-penetrating radar) of the shallow subsurface, ongoing monitoring of the Arikaree Glacier, three rock glaciers and lake ice in the GLV, and interpretations of how subsurface ice melt regulates the flow and chemistry of alpine surface water after seasonal snowfields melt.Results and conclusions: Permafrost conditions reported from Niwot Ridge in the 1970s are generally absent today, but ice lenses form and melt seasonally. Ice is present permanently within the Green Lakes 5 rock glacier and at nearby favourable sites. The Arikaree Glacier has shown a marked decline in cumulative mass balance during the past 12 years after a 30-year period when net mass balance was ca. 0. Duration of seasonal lake ice increases with elevation in GLV, but duration has decreased at all seven lakes that have been monitored during the last three decades. This decrease has been most marked at the lowest elevation where it amounted to a reduction of about 1 d year−1 and least at Green Lake 5 where the loss has been at a rate of 0.5 d year−1. Surface temperature measurements from rock glaciers have not shown strong trends during the past 15 years. It has been suggested that almost all of the 2.5-mm year−1 increase in stream discharge from the upper GLV in September and October has been derived from melting of subsurface ice.

Assessment of Heavy Metal Contamination in Surface and Ground Water Resources around Udege Mbeki Mining District, North-Central Nigeria

The study assessed the surface water and groundwater chemistry with special concern to heavy metal contaminations/pollution in Udege Mbeki mining district. A total of eight mine ponds, eight hands – dog wells, three streams and one adit water samples were analysed for their heavy metals contents. The heavy metal concentrations ranged as follows: Cd (0.000-0.110 mg/L), Cr (0.003-0.146 mg/L), Co (0.110-1.260 mg/L), Cu (0.000-0.260 mg/L), Fe (0.015-8.398 mg/L), Pb (0.0001-1.853 mg/L) and Ni (0.001.112 mg/L). Concentrations of Cr, Cu and Ni are generally within the maximum permissible limit of the WHO (2006) guideline, whereas concentrations of Fe, Co, Pb and Cd are above the maximum permissible limit. The contamination/pollution level was estimated based on the geoaccumulation index (Ige0), enrichment factor (EF) and heavy metal pollution index (HPI). Sources of the heavy metals in the waters of the area are mainly anthropogenic. The heavy metal pollution index (HPI) calculated was 559.12, which is far above the critically pollution index value of 100 and therefore depict that the waters of the area are critically polluted with regards to heavy metals.

The Swedish monitoring of surface waters: 50 years of adaptive monitoring.

For more than 50 years, scientific insights from surface water monitoring have supported Swedish evidence-based environmental management. Efforts to understand and control eutrophication in the 1960s led to construction of wastewater treatment plants with phosphorus retention, while acid rain research in the 1970s contributed to international legislation curbing emissions. By the 1990s, long-time series were being used to infer climate effects on surface water chemistry and biology. Monitoring data play a key role in implementing the EU Water Framework Directive and other legislation and have been used to show beneficial effects of agricultural management on Baltic Sea eutrophication. The Swedish experience demonstrates that well-designed and financially supported surface water monitoring can be used to understand and manage a range of stressors and societal concerns. Using scientifically sound adaptive monitoring principles to balance continuity and change has ensured long-time series and the capability to address new questions over time.

CHEMICZNE WSKAŹNIKI JAKOŚCI WODY W ZLEWNI LASÓW PARCZEWSKICH

This paper presents the characteristics of the chemistry of surface and ground water in the bottom of the river valley reclaimed Ochoza. Drained grassland accounts for 20% of the total catchment area and are located on organic soils in the valley Tyśmienica classified to the Natura 2000 sites. Analysis of physico-chemical properties of water are to assess the effects of anthropogenic transformation and identify factors that influence water quality in the study area. Water samples were collected in the years 2011–2012 in several points. The walls were characterized by surface water stagnant in the trenches, in July, blueberry plantation. Characterized by the highest quality of surface water runoff river with the test

Hydrology and chemistry of surface water and ground water of Ryusendo region

Hydrology and chemistry of surface water and ground water of Ryusendo region

Environmental and ecological controls of coral community metabolism on Palmyra Atoll

Accurate predictions of how coral reefs may respond to global climate change hinge on understanding the natural variability to which these ecosystems are exposed and to which they contribute. We present high-resolution estimates of net community calcification (NCC) and net community production (NCP) from Palmyra Atoll, an uninhabited, near-pristine coral reef ecosystem in the central Pacific. In August–October 2012, we employed a combination of Lagrangian and Eulerian frameworks to establish high spatial (~2.5 km2) and temporal (hourly) resolution coral community metabolic estimates. Lagrangian drifts, all conducted during daylight hours, resulted in NCC estimates of −51 to 116 mmol C m−2 h−1, although most NCC estimates were in the range of 0–40 mmol C m−2 h−1. Lagrangian drift NCP estimates ranged from −7 to 67 mmol C m−2 h−1. In the Eulerian setup, we present carbonate system parameters (dissolved inorganic carbon, total alkalinity, pH, and pCO2) at sub-hourly resolution through several day–night cycles and provide hourly NCC and NCP rate estimates. We compared diel cycles of all four carbonate system parameters to the offshore surface water (0–50 m depth) and show large departures from offshore surface water chemistry. Hourly Eulerian estimates of NCC aggregated over the entire study ranged from 14 to 53 mmol C m−2 h−1, showed substantial variability during daylight hours, and exhibited a diel cycle with elevated NCC in the afternoons and depressed, but positive, NCC at night. The Eulerian NCP range was very high (−55 to 177 mmol C m−2 h−1) and exhibited strong variability during daylight hours. Principal components analysis revealed that NCC and NCP were most closely aligned with diel cycle forcing, whereas the NCC/NCP ratio was most closely aligned with reef community composition. Our analysis demonstrates that ecological community composition is the primary determinant of coral reef biogeochemistry on a near-pristine reef and that reef biogeochemistry is likely to be responsive to human behaviors that alter community composition.

Using integrated multivariate statistics to assess the hydrochemistry of surface water quality, Lake Taihu basin, China

Natural factors and anthropogenic activities both contribute dissolved chemical loads to lakes and streams. Mineral solubility, geomorphology of the drainage basin, source strengths and climate all contribute to concentrations and their variability. Urbanization and agriculture waste-water particularly lead to aquatic environmental degradation. Major contaminant sources and controls on water quality can be asssessed by analyzing the variability in proportions of major and minor solutes in water coupled to mutivariate statistical methods. The demand for freshwater needed for increasing crop production puulation and industrialization occurs almost everywhere in in China and these conflicting needs have led to widespread water contamination. Because of heavy nutrient loadings from all of these sources, Lake Taihu (eastern China) notably suffers periodic hyper-eutrophication and drinking water deterioration, which has led to shortages of freshwater for the City of Wuxi and other nearby cities. This lake, the third largest freshwater body in China, has historically beeen considered a cultural treasure of China, and has supported long-term fisheries. The is increasing pressure to remediate the present contamination which compromises both aquiculture and the prior economic base centered on tourism. However, remediation cannot be effectively done without first characterizing the broad nature of the non-point source pollution. To this end, we investigated the hydrochemical setting of Lake Taihu to determine how different land use types influence the variability of surface water chemistry in different water sources to the lake. We found that waters broadly show wide variability ranging from calcium-magnesium-bicarbonate hydrochemical facies type to mixed sodium-sulfate-chloride type. Principal components analysis produced three principal components that explained 78% of the variance in the water quality and reflect three major types of water chemistry. Agricultural land use is associated with greater concentrations of nutrients; urban areas with high concentrations of sodium, chloride, sulfate, fluoride and potassium; and natural weathering with calcium, magnesium and bicarbonate. Discriminant analysis and hierarchical cluster analysis produce complementary and similar results. Broadly speaking, future remediation to reduce nutrient loadings to the lake or industrial contamination can now be focused on specific land use practices, which are readily identifiable by using statistics in conjunction with GIS.

Integrating petrography, mineralogy and hydrochemistry to constrain the influence and distribution of groundwater contributions to baseflow in poorly productive aquifers: Insights from Gortinlieve catchment, Co. Donegal, NW Ireland

Identifying groundwater contributions to baseflow forms an essential part of surface water body characterisation. The Gortinlieve catchment (5km2) comprises a headwater stream network of the Carrigans River, itself a tributary of the River Foyle, NW Ireland. The bedrock comprises poorly productive metasediments that are characterised by fracture porosity. We present the findings of a multi-disciplinary study that integrates new hydrochemical and mineralogical investigations with existing hydraulic, geophysical and structural data to identify the scales of groundwater flow and the nature of groundwater/bedrock interaction (chemical denudation). At the catchment scale, the development of deep weathering profiles is controlled by NE-SW regional scale fracture zones associated with mountain building during the Grampian orogeny. In-situ chemical denudation of mineral phases is controlled by micro- to meso-scale fractures related to Alpine compression during Palaeocene to Oligocene times. The alteration of primary muscovite, chlorite (clinochlore) and albite along the surfaces of these small-scale fractures has resulted in the precipitation of illite, montmorillonite and illite-montmorillonite clay admixtures. The interconnected but discontinuous nature of these small-scale structures highlights the role of larger scale faults and fissures in the supply and transportation of weathering solutions to/from the sites of mineral weathering. The dissolution of primarily mineral phases releases the major ions Mg, Ca and HCO3 that are shown to subsequently form the chemical makeup of groundwaters. Borehole groundwater and stream baseflow hydrochemical data are used to constrain the depths of groundwater flow pathways influencing the chemistry of surface waters throughout the stream profile. The results show that it is predominantly the lower part of the catchment, which receives inputs from catchment/regional scale groundwater flow, that is found to contribute to the maintenance of annual baseflow levels. This study identifies the importance of deep groundwater in maintaining annual baseflow levels in poorly productive bedrock systems.

Initial effects of forest N, Ca, Mg and B large-scale fertilization on surface water chemistry and leaching from a catchment in central Sweden

Highly increased use of biomass production is placing great demands on Swedish forests. Several silvicultural measures can be implemented to increase forest production and fertilization being addressed in this paper is one. Forest companies are now increasingly applying fertilizer, with the main nutrient needed for high forest growth being nitrogen (N).This study investigated how commercial N fertilization (150kgN/ha, including also Ca, Mg and B) of the 45ha forest catchment Risfallet in central Sweden affected chemical composition and runoff export in stream water during one year after the fertilization. This well-defined and long-term monitored catchment proved very suitable for studies of water quality and nutrient losses. The fertilizer consisted of ammonium nitrate (50/50 ammonium and nitrate, respectively), plus calcium (Ca, 22kgha−1) and magnesium (Mg, 12kgha−1) to mitigate acidification and boron (B 1.1kgha−1) to compensate for decreased boron availability. The study was carried out according to the paired catchment method using a control area and a calibration period. Data from the after treatment period were compared with previous 25-year monitoring data for the catchment and also data for a similar 83ha control catchment, Gusseltjärn, in the same region. During the first year after treatment, the nitrate concentration in stream water increased from 0.05mgL−1 to 3.3mgL−1 on average. Other elements showing increased concentrations were ammonium (300%), B (3-fold), Mg (80%), Ca (60%), potassium (K) (50%) and sodium (Na) (40%). The pH decreased in the first half-year by 0.2 pH-units. One year after treatment, 14% of the N applied had leached from the catchment.

Surface water geochemical and isotopic variations in an area of accelerating Marcellus Shale gas development

Water samples were collected from 50 streams in an area of accelerating shale gas development in the eastern U.S.A. The geochemical/isotopic characteristics show no correlation with the five categories of Marcellus Shale production. The sub-watersheds with the greatest density of Marcellus Shale development have also undergone extensive coal mining. Hence, geochemical/isotopic compositions were used to understand sources of salinity and effects of coal mining and shale gas development in the area. The data indicates that while some streams appear to be impacted by mine drainage; none appear to have received sustained contribution from deep brines or produced waters associated with shale gas production. However, it is important to note that our interpretations are based on one time synoptic base flow sampling of a few sampling stations and hence do account potential intermittent changes in chemistry that may result from major/minor spills or specific mine discharges on the surface water chemistry.

In situ evaluation of model copper-cast iron canisters for spent nuclear fuel: a case of microbiologically influenced corrosion (MIC)

The Swedish method for disposal of spent nuclear fuel in a deep geological repository (KBS-3) relies on the stability of the granitic bed-rock and two engineered barriers: a copper-cast iron canister and highly compacted bentonite clay. In order to develop a better understanding of the internal corrosion processes that could take place if a leak were to occur in the outer copper canister, five miniaturised copper cast iron canisters were installed at a depth of 450 m at the Äspö Hard Rock Laboratory, in Sweden. The experiments differed in the density of the surrounding bentonite buffer, as well as in the number and position of leak points that were introduced in the copper shell. Several electrochemical techniques (e.g. AC impedance, linear polarisation resistance and electrochemical noise) were used to monitor the corrosion of different components of the experiment. Copper specimens were installed for post-test evaluation of the rate of general corrosion, localised corrosion and stress corrosion cracking (SCC). In addition, mechanical and environmental parameters, such as surface strain, hydrostatic pressure, redox potential, pH, water chemistry, dissolved gases, and microbial numbers, diversity, and activity were measured regularly. After five years of in situ exposure one of the canisters was retrieved and analysed to characterise and evaluate the corrosion processes that had occurred during the experiment. Extensive sulphide production by sulphate reducing bacteria led to rapid corrosion of iron, and the formation of iron sulphide deposits on the copper and iron electrodes disturbed the electrochemical measurements. This paper describes the various analyses that were carried out on the model canister and summarises the conclusions that can be drawn.

Biomass uptake and fire as controls on groundwater solute evolution on a southeast Australian granite: aboriginal land management hypothesis

Abstract. The chemical composition of groundwater and surface water is often considered to be dominated by water–rock interactions, particularly weathering; however, it has been increasingly realised that plant uptake can deplete groundwater and surface water of nutrient elements. Here we show, using geochemical mass balance techniques, that water–rock interactions do not control the hydrochemistry at our study site within a granite terrain in southwest Victoria, Australia. Instead the chemical species provided by rainfall are depleted by plant biomass uptake and exported, predominantly through fire. Regular landscape burning by Aboriginal land users is hypothesized to have caused the depletion of chemical species in groundwater for at least the past 20 000 yr by accelerating the export of elements that would otherwise have been stored within the local biomass. These findings are likely to be applicable to silicate terrains throughout southeast Australia, as well as similar lithological and climatic regions elsewhere in the globe, and contrast with studies of groundwater and surface water chemistry in higher rainfall areas of the Northern Hemisphere, where water–rock interactions are the dominant hydrochemical control.

Mobilization of selenium from the Mancos Shale and associated soils in the lower Uncompahgre River Basin, Colorado

This study investigates processes controlling mobilization of selenium in the lower part of the Uncompahgre River Basin in western Colorado. Selenium occurs naturally in the underlying Mancos Shale and is leached to groundwater and surface water by limited natural runoff, agricultural and domestic irrigation, and leakage from irrigation canals. Soil and sediment samples from the study area were tested using sequential extractions to identify the forms of selenium present in solid phases. Selenium speciation was characterized for nonirrigated and irrigated soils from an agricultural site and sediments from a wetland formed by a leaking canal. In nonirrigated areas, selenium was present in highly soluble sodium salts and gypsum. In irrigated soils, soluble forms of selenium were depleted and most selenium was associated with organic matter that was stable under near-surface weathering conditions. Laboratory leaching experiments and geochemical modeling confirm that selenium primarily is released to groundwater and surface water by dissolution of highly soluble selenium-bearing salts and gypsum present in soils and bedrock. Rates of selenium dissolution determined from column leachate experiments indicate that selenium is released most rapidly when water is applied to previously nonirrigated soils and sediment. High concentrations of extractable nitrate also were found in nonirrigated soils and bedrock that appear to be partially derived from weathered organic matter from the shale rather than from agricultural sources. Once selenium is mobilized, dissolved nitrate derived from natural sources appears to inhibit the reduction of dissolved selenium leading to elevated concentrations of selenium in groundwater. A conceptual model of selenium weathering is presented and used to explain seasonal variations in the surface-water chemistry of Loutzenhizer Arroyo, a major tributary contributor of selenium to the lower Uncompahgre River.