Groundwater Solute Concentrations Research Articles

Spatial and temporal dynamics of groundwater biogeochemistry in the deep subsurface of a high-energy beach

Intertidal sandy beach systems are considered complex biogeochemical reactors. At beach sites that are subject to high tidal and wave energy, seawater circulation can reach tens of meters deep into the subsurface and changing environmental conditions are assumed to lead to dynamic groundwater flow paths, saltwater-freshwater mixing zones, and a spatio-temporally variable groundwater biogeochemistry. Previous studies mainly focused on the upper meters of subterranean estuaries (STE), while the deep subsurface remained a black box. This study presents spatial (cross-shore) and temporal (~six-weekly, over 1.5 years) dynamics of the groundwater biogeochemistry that were observed down to 24 m below the ground surface (mbgs) of a sandy high-energy beach on Spiekeroog Island (Germany).In addition to redox conditions along a cross-shore transect ranging from oxic to Fe oxide reducing/slightly sulfidic, we found a previously unknown, distinct vertical redox zonation as well. Temporal variations of the biogeochemistry within low salinity groundwater at the most landward station close to the dune base were mainly driven by storm flood related seawater infiltration. Around the high water line, the extent of the upper saline plume (USP) varied over time. Furthermore, temporal dynamics of the O2 saturation at 6 mbgs indicated a seasonally shifting depth of the oxycline at this location. In the lower intertidal zone, groundwater solute concentrations displayed a temporally variable zone of deep freshwater discharge.Regarding the impact of the deep STE on the groundwater biogeochemistry of the discharge zone, our data revealed that nutrient, Mn, and Fe release along the deep flow paths through the USP towards the discharge zone was limited, likely due decreasing availability of labile organic matter and subsequent slowing down of metabolic processes with depth. High concentrations of metabolites in the upper ~2 mbgs of the discharge zone were, therefore, rather attributed to the incorporation of labile organic matter during continuous and storm flood related sediment relocation and/or the contribution of older waters, e.g., the subtidal saltwater wedge.

Hyperspectral imaging sediment core scanning tracks high-resolution Holocene variations in (an)oxygenic phototrophic communities at Lake Cadagno, Swiss Alps

Abstract. Pigments produced by anoxygenic phototrophic bacteria are valuable proxies of past anoxia in lacustrine and marine environments. Pigment measurement typically requires time-consuming and costly chemical extractions and chromatographic analyses, which limits the temporal resolution of paleoenvironmental reconstructions based on sedimentary pigments. Here, we evaluate the potential of in situ hyperspectral imaging (HSI) core scanning as a rapid, non-destructive method to document high-resolution changes in oxygenic and anoxygenic phototrophic communities at meromictic Lake Cadagno, Switzerland. Three distinct groups of pigments can be detected with the HSI method in the sediments of Lake Cadagno; each pigment group represents a different phototrophic community. Oxygenic phototrophs are indicated by total chloropigments (TChl; chlorophyll a, b, and derivatives). Two types of anoxygenic phototrophs were distinguished – purple sulfur bacteria (PSB), represented by bacteriochlorophyll a, and green sulfur bacteria (GSB), represented by bacteriochlorophyll c, d, and e. HSI pigment indices were validated by pigment measurements performed on extracted samples using spectrophotometer and high-performance liquid chromatography (HPLC). Bacteriochlorophylls were present throughout the past 10 kyr, confirming geochemical evidence of nearly continuous stratification and sulfidic conditions at Lake Cadagno. Major shifts in the anoxygenic phototropic communities are recorded at decadal to millennial scales. GSB and PSB communities coexisted from 10.2–8.8 kyr BP. Dominance of PSB over GSB from 8.8–3.4 kyr BP indicates strongly stratified conditions in the lake and strong light radiation at the chemocline. From 3.4–1.3 kyr BP, PSB were mostly absent, and GSB became dominant, implying lower light intensity at the chemocline due to a combination of factors including deforestation in the lake surroundings, increased flood frequency, cooler climatic conditions, and changes in groundwater solute concentrations. The high-resolution HSI data show that frequent flood events and mass movements disturbed the chemocline and the anoxygenic bacterial communities and that the PSB were particularly sensitive and slow to recover following these disturbance events. This study demonstrates for the first time that HSI can detect GSB-related pigments, making the method uniquely valuable as a rapid tool to study samples containing pigments of both oxygenic and anoxygenic phototrophs.

Numerical simulation of overbank hyporheic transport and biogeochemical reactions in a compound channel

AbstractTransverse hyporheic transport and biogeochemical reactions in a compound channel remain poorly understood. As floodplains are submerged, solutes in the surface water enter the hyporheic zone of the compound channel and react with the solutes upwelling below the floodplain. Aerobic respiration (AR) and denitrification (DN) processes in the hyporheic zone of the compound channel still need to be clarified. In this paper, a 3D hydrodynamic model coupled to a 2D groundwater flow and biogeochemical model was applied to investigate such processes. The model results were verified using laboratory experimental measurements. The effects of bank slope angle, ambient groundwater flow, and ambient groundwater solute concentration on aerobic and anaerobic biogeochemical processes were studied. The denitrification zone was found below the interface between the main channel and the floodplain. The lower bank slope angle favours nitrogen removal. Small losing groundwater discharges promote aerobic respiration and denitrification reactions. As the ambient groundwater oxygen concentration rises, O2 consumption increases while N removal decreases. As the NO3 concentration in ambient groundwater increases, NO3 consumption increases and proportionally NO3 consumption decreases. These findings not only provide a better understanding of biogeochemical reactions in a compound channel but can also be applied to river restoration to efficiently remove nitrate by modifying bank slope angles.

Numerical Modeling of Saltwater Intrusion in the Rmel-Oulad Ogbane Coastal Aquifer (Larache, Morocco) in the Climate Change and Sea-Level Rise Context (2040)

Many coastal aquifers have experienced seawater intrusion (SWI) into fresh groundwater aquifers. The principal causes of SWI include over-pumping and events such as climate change (CC) and rising sea levels. In northern Morocco, the Rmel-Oulad Ogbane coastal aquifer (ROOCA) supplies high-quality groundwater for drinking water and agriculture. This favorable situation has led to increased pumping, resulting in environmental challenges such as dropping water table and SWI. Furthermore, the climate has resulted in less recharge, with an estimated annual precipitation of 602 mm and an average temperature of 18.5 °C. The goal of this study is to determine how CC, over-pumping, and sea-level rise (SLR) affect SWI. Computational groundwater and solute transport models are used to simulate the spatial and temporal evolution of hydraulic heads and groundwater solute concentrations. The calibration is based on steady and transient groundwater levels from 1962 to 2040. SWI simulations show that the NW sector of the coastal area would be polluted, with the toe reaching 5.2 km inland with a significant salinity (15–25 g/L). To protect the fresh water in the reservoir from SWI, enhanced groundwater development and management approaches for this aquifer are required, such as artificial recharge from surface water.

Cycling of redox-sensitive trace metals in barrier island freshwater lenses

Freshwater lenses connect the terrestrial and marine realm via groundwater discharge at the edges of islands and serve as drinking water resources. We studied the redox-sensitive metals U, Mo, V, and Tl along the redox gradient of fresh groundwater lenses on Spiekeroog Island, northern Germany. Groundwater solute concentrations were linked to groundwater age and redox characteristics. We further quantified the contribution of precipitation, sea spray, and aquifer matrix to the groundwater metal concentrations and evaluated the sink and source function of the aquifer under oxic and reducing conditions.We found that biogeochemical processes altered the concentrations of the trace metals. In young, oxygen to nitrate reducing zones, the aquifer matrix represented the major metal source to the groundwater. For Tl, rain was an additional important (anthropogenic) source. Under manganese and iron oxide to sulfate reducing conditions, U and Tl were sensitive to redox dependent removal, whereas Mo and V were less affected by reductive precipitation/adsorption. In detail, 99% of dissolved Tl, 88% of U, 66% of Mo, and 44% of V were removed to the solid phase in comparison to values from less reducing zones. Large parts of the western freshwater lens on Spiekeroog were anoxic. For this reason, the delivery of aquifer derived metals to the ocean via fresh groundwater discharge appeared to be limited. Higher U, Mo, V, and Tl concentrations were observed in the presently developing young freshwater lens in the east of Spiekeroog Island. This suggests that less reducing groundwater lenses may be a source of these metals to the adjacent beach/coastal seawater. Especially for V, freshwater discharge from sandy coastal aquifers may be important, as groundwater concentrations exceeded seawater concentration under oxic as well as anoxic conditions. Regarding the suitability of the freshwater as drinking water, all measured trace metal concentrations were classified as uncritical.

The influence of bedrocks on groundwater chemistry in a crystalline basement complex of southwestern Nigeria

Groundwater in crystalline basement is highly mineralized. The chemistry of bedrocks in crystalline basement of Ogbomosho in relation to chemical composition of groundwater in the area was, therefore, investigated employing a combined approach involving hydrogeological, geological mapping and geochemical method. A total of seventy five (75) water samples collected from wells; shallow and deep, bore within rock exposures (6) and other locations within the study area were analyzed for elemental composition using Inductive Coupled Plasma-Mass Spectrometry (ICP-MS). Physico-chemical parameters, temperature, electrical conductivity (EC), total dissolved solids (TDS) and acidity (pH), were measured in situ using HANNA HI9813-6 hand-held meter. Six rock samples representative of the main geological units of the study area were also collected and analyzed for major oxides and rare earth elements (REE) concentrations using ICP-MS. Thin sections of the rock samples were prepared and observed under petrological microscope for mineralogical compositions of the rocks. Acidity ions/TDS model of groundwater in the area was also established to determine the reactivity of the rock minerals and groundwater evolution pattern. The results of in-situ physico-chemical tests of the water samples indicate the temperature, EC, TDS and pH ranged within 26.6–31.7 °C, 72–1491 mS/cm, 36–747 ppm and 6.15–9.3, respectively. The petrographical analysis revealed biotite, quartz, potassic feldspar (microcline), perthite, albite, hornblende, plagioclase, myrmekite, topaz and muscovite. The major oxides, SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5 and MnO, varied with median values of 72.21%, 14.99%, 1.17%, 0.375%, 2.245%, 4.215%, 3.46%, 0.16%, 0.045% and 0.015%. These indicate dorminance of SiO2 and Al2O3 suggesting acidic and metamorphic/acidic igneous rocks, respectively. The median percentage oxide compositions of the cations in rocks were, therefore, of the order: Na2O > K2O > CaO > MgO. The dissolved cations consequent upon weathering of the minerals were also of the order Na+ (26.9 mg/l) > K+ (4.69 mg/l) > Mg2+ (4.57 mg/l) > Ca2+ (4.23 mg/l) in groundwater based on their median values. These indicate solute concentration in groundwater is proportional to the reactivity of the bedrock minerals. The major anions, HCO3−, Cl− and NO3−, varied within 5.0–455 mg/l, 14.18–184.34 mg/l and 0.02–0.21 mg/l, respectively, in groundwater. The cross-plots of TDS against Ca2+, Na+, Cl− and HCO3− indicate the groundwater in the area is moderately to highly mineralized. The ionic reactivity based on pH ions/TDS plots indicated six-type models. The ionic concentration at low pH within 6.4–7.2 increases with pH for Type-1 (Ca2+), Type-5 (NO3−) and Type-6 (HCO3−,TDS), decreases for Type-2 (Mg2+, K+) but constant as pH increases for Type-4 (Cl−). At high pH, greater than 7.2 but less than 9, ionic concentration increases with pH for Type-1, Type-2 and Type-4, decreases with pH for Type-6 and constant for Type-5. However, the ionic concentration of Na+ (Type-3) momentarily increased, decreased and subsequently increased through low to high pH. These indicate varied degree of reactivity of the bedrock minerals and groundwater evolution pattern. Thus, the composition of dissolved ions in groundwater is controlled by weathering of Ca-feldspar (plagioclase), K-feldspar (orthoclase), Na-feldspar (albite) and biotite found in host rocks. The similarity in trend of cations in groundwater and their oxides in rock samples suggests the influences of local rock chemistry on the groundwater chemistry and hence, groundwater–rock interaction in the study area. The groundwater evolution pattern in the area depends on pH and reactivity of the ions produced from the weathering of the minerals. The chemical compositions of natural waters are, therefore, a direct indication of the geology of their catchment.

Groundwater Solute Transport Modeling: Effects of Transport and Fate

In this paper, the solution of the advection-dispersion equation with different sorption values is used for the prediction of solute concentration in groundwater. Sorption process in the groundwater is complex, due to increasing the groundwater pollution the effect of different chemical transport plotted. The fate and sorption process of different chemical different degradation constant. We used the analytical solution to evaluate the transport phenomenon and analysis of the chemical dissolved in groundwater. The solute transport model simulated with the analytical solution and final result obtained using MATLAB software. The solution of a test problem based on the sequential degradation of the different chemical in the groundwater. This solution of equilibrium and rate of sorption dynamics of processes is imperative for accurate fate and transport modeling. The present study shows the effects of advection, dispersion/diffusion, and sorption equation on the saturated media of soil. MATLAB software used for analyzing the solution of groundwater and showing the different case taking care of saturated aquifer and with different void ratio of soil its shows that the soil parameter is also impotent parameter and its effect can see in plot between concertation vs time. The solute transport model simulated with the analytical solution and final result obtained using MATLAB software. The solution of a test problem based on the sequential degradation of the different chemical in the groundwater. This study compares the solute concentration with respect to distance and its different hydraulics conductivity of dense sad and lose.

Sources of iron (Fe) and factors regulating the development of flocculate from Fe-oxidizing bacteria in regenerative streamwater conveyance structures

Regenerative streamwater conveyances (RSCs) are a relatively new stream restoration best management practice (BMP) being extensively implemented in degraded perennial streams and stormwater drainages throughout the mid-Atlantic. Although there is currently a great deal of interest in this type of BMP as a means of reducing nutrient and sediment export from disturbed catchments, little is known of its pollutant reduction capabilities and whether there are unintended ecological consequences associated with these structures. For example, dense accumulations of flocculate from iron-oxidizing bacteria (FeOB) have been observed at numerous RSC sites, yet it is unknown whether this flocculate is predominately natural, a consequence of leached iron (Fe) from the materials used in the RSC construction, or because of mobilized Fe from catchment soils influenced by higher groundwater levels that may occur after construction. We analyzed Fe and other solute concentrations in groundwater and perennial streams at RSC and control sites located in the Coastal Plain and Piedmont physiographic provinces of MD and DC, and conducted leaching experiments using RSC soils and construction materials. Iron flocculate from FeOB in RSCs is commonly localized and depends on several factors including a source of reduced Fe, the availability of dissolved organic carbon (DOC), and specific hydrological (i.e., low flow) and physical (i.e., warmer temperatures) conditions. Sources of reduced Fe are derived from both construction materials and catchment soils, which leach Fe in the presence of DOC originating from natural organic matter (OM) deposits and OM incorporated into the RSCs (i.e., wood chips, leaves, logs). Using construction materials that have relatively low Fe content in areas strongly influenced by OM will likely decrease the spatial and temporal presence of dense mats of Fe flocculate thereby improving stream habitat.

Including land use information for the spatial estimation of groundwater quality parameters – 2. Interpolation methods, results, and comparison

Summary Two dominant processes determine solute concentration in groundwater: vertical infiltration and horizontal advection. The goal of this paper is to incorporate both processes into a geostatistical model for spatial estimation of solute concentrations in groundwater. A multivariate copula-based methodology is demonstrated that considers infiltration via the marginal distribution and solute transport via the multivariate spatial dependence structure. The novel approach is compared to traditional methods as Ordinary- and External Drift Kriging. Leave-one-out cross-validation demonstrates that the novel approach estimates better both in concentration and in probability space, and improves the quantification and quality of uncertainty. The gain in uncertainty reduction is equivalent to at least a few hundred additional observations when Ordinary Kriging was used. Both censored and not-censored measurements are included. An ideal neighborhood size is estimated via cross-validation. The methodology is general and can incorporate other kinds of secondary information. It can be used to evaluate effects of land use changes.

Simulating reactive transport of selenium coupled with nitrogen in a regional-scale irrigated groundwater system

Summary The contamination or deficiency of Selenium (Se) has become a critical issue in watershed systems worldwide during the past half-century, with either elevated or deficient Se concentrations in groundwater, surface water, soils, and associated vegetation. Regardless of the reason for concern, there is a basic need for tools for use in assessing baseline concentrations and mass loadings in large-scale environmental systems and for exploring remediation strategies. This paper presents the application of UZF-RT3D, a multi-species variably-saturated reactive transport model for groundwater systems, and its recently developed Se agricultural and chemical reaction module to a regional study site (50,600 ha) along the irrigated Arkansas River Valley in southeastern Colorado. The study region has been monitored for Se contamination during the previous decade, with all segments of the Arkansas River impaired with respect to Se. The Se reaction module accounts for near-surface Se cycling due to agricultural practices, oxidation–reduction reactions, and sorption, and includes a nitrogen cycle and reaction module due to the dependence of Se transformation and speciation on the presence of nitrate (NO 3 ). Of prime importance is the role NO 3 plays in oxidation of residual Se from marine shale, which is prevalent throughout the study region, along with its inhibition of Se chemical reduction to less toxic forms. The model is corroborated against groundwater solute concentrations, mass loadings of solutes to the Arkansas River, relationships between solutes within the groundwater system, and overall regional statistics of groundwater solute concentration. Results indicate that the model is successful in replicating the major spatial and temporal patterns in Se and NO 3 contamination and transport, hence providing a tool that can be used with confidence to explore remediation schemes.

Spatial Moment Equations for a Groundwater Plume with Degradation and Rate-Limited Sorption

AbstractIn this paper, the authors analytically derive the solution for the spatial moments of groundwater solute concentration distributions simulated by a one-dimensional model that assumes advective-dispersive transport with first-order degradation and rate-limited sorption. Sorption kinetics are assumed to be governed by a first-order rate model, and degradation is assumed to occur in the aqueous (i.e., mobile) phase only. As an extension of a previously published moment analysis of a model with similar assumptions, an explicit analytical solution is presented for the zeroth, first, and second spatial moments, as well as equations that describe the behavior of the first and second moments at long times. Moment simulations show that when the degradation rate is relatively large compared to the sorption rate, there are time periods in which the first and second spatial moments of the solute concentration distribution (the mean and variance, respectively) decrease over time. At long times, it is seen tha...

A software tool for the spatiotemporal analysis and reporting of groundwater monitoring data

The GroundWater Spatiotemporal Data Analysis Tool (GWSDAT) is a user friendly, open source, decision support tool for the analysis and reporting of groundwater monitoring data. Uniquely, GWSDAT applies a spatiotemporal model smoother for a more coherent and smooth interpretation of the interaction in spatial and time-series components of groundwater solute concentrations. Data entry is via a standardised Microsoft Excel input template whilst the underlying statistical modelling and graphical output are generated using the open source statistical program R. This paper describes in detail the various plotting options available and how the graphical user interface can be used for rapid, rigorous and interactive trend analysis with facilitated report generation. GWSDAT has been used extensively in the assessment of soil and groundwater conditions at Shell's downstream assets and the discussion section describes the benefits of its applied use. Finally, some consideration is given to possible future developments.

Groundwater quality assessment and its suitability in Çeltikçi plain (Burdur/Turkey)

The Celtikci (Burdur) plain is located in the southwest of Turkey and is a semi-closed basin. Groundwater is densely used as drinking, irrigation and domestic water in the plain. Hydrogeochemical processes controlling groundwater chemistry and geochemical assessment of groundwater were investigated in the Celtikci (Burdur/Turkey) plain. In this study, groundwater samples for two seasons were analyzed and major ion chemistry of groundwater was researched to understand the groundwater geochemistry. Two major hydrochemical facies (Ca–HCO3 and Ca–Mg–HCO3) were determined in the area. Various graphical plots and multivariate statistical analysis were used for identifying the occurrence of different geochemical processes. In the study area, weathering is one of the key geochemical processes which controlled the solute concentration in groundwater. Chemical indexes such as sodium adsorption ratio, %Na, residual sodium carbonate, magnesium hazard and permeability index were calculated and results show that groundwater is suitable for irrigation purpose except for permeability index values. Concentrations of Mn, NO3 and total hardness exceed the prescribed limits of WHO and are the major limiting parameters of groundwater use for potable and domestic purposes.

Kriging groundwater solute concentrations using flow coordinates and nonstationary covariance functions

Interpolation of solute concentration measurements often yields disappointing results, especially when it fails to incorporate some knowledge relative to the underlying physics of groundwater flow and solute transport. Concentration maps, however, are required in several applications such as plume monitoring, evaluation of total dissolved mass, design of hydraulic containment and pump-and-treat systems, source identification and natural attenuation assessment. Kriged concentration maps can be improved by a coordinate transformation based on natural flow coordinates, as this allows to consider variations in the local anisotropy direction produced by heterogeneous groundwater flow. On the other hand, this does not consider nonstationarity induced by local dispersion. This paper presents a flexible kriging approach that combines a coordinate transformation based on groundwater flow and a class of nonstationary (NS) covariance functions that can be parameterized to account for the evolution of concentration correlation and variance with travel time/distance. We also propose an alternative flow coordinate transformation that does not rely on the stream function, enabling the computation of approximate 3-D flow coordinates. Nonstationary covariance parameters are estimated using maximum likelihood, and candidate parametrizations are compared by the likelihood ratio test and Akaike Information Criterion. The approach is tested on simple 2-D and 3-D synthetic plumes shaped by heterogeneous flow and local dispersion. Results show that: (1) NS covariance parameterizations improve significantly the likelihood compared to the stationary models, (2) the combined coordinate transformation and NS covariance approach enhances kriging of solute concentrations. Although the flow fields are considered known in the case studies, the approach can be modified to incorporate flow uncertainty and lends itself to various useful extensions, such as mass flux estimation.

Hydraulic Displacement of Dense Nonaqueous Phase Liquids for Source Zone Stabilization

Hydraulic displacement is a mass removal technology suitable for stabilization of a dense, nonaqueous phase liquid (DNAPL) source zone, where stabilization is defined as reducing DNAPL saturations and reducing the risk of future pool mobilization. High resolution three-dimensional multiphase flow simulations incorporating a spatially correlated, heterogeneous porous medium illustrate that hydraulic displacement results in an increase in the amount of residual DNAPL present, which in turn results in increased solute concentrations in groundwater, an increase in the rate of DNAPL dissolution, and an increase in the solute mass flux. A higher percentage of DNAPL recovery is associated with higher initial DNAPL release volumes, lower density DNAPLs, more heterogeneous porous media, and increased drawdown of groundwater at extraction wells. The fact that higher rates of recovery are associated with more heterogeneous porous media stems from the fact that larger contrasts in permeability provide for a higher proportion of capillary barriers upon which DNAPL pooling and lateral migration can occur. Across all scenarios evaluated in this study, the ganglia-to-pool (GTP) ratio generally increased from approximately 0.1 to between approximately 0.3 and 0.7 depending on the type of DNAPL, the degree of heterogeneity, and the imposed hydraulic gradient. The volume of DNAPL recovered as a result of implementing hydraulic displacement ranged from between 9.4% and 45.2% of the initial release volume, with the largest percentage recovery associated with 1,1,1 trichloroethane, the least dense of the three DNAPLs considered.

Assessment of temporal and spatial variation in chemical composition of groundwater in an unconfined esker aquifer in the cold temperate climate of Northern Finland

Samples of groundwater, surface water, snow and precipitation were collected to identify potential end-members for aquifer recharge in an unconfined esker aquifer connected to nearby surface waters. Concentrations of Ca2 +, Cl−, NO3–N and SiO2 and electrical conductivity were determined in these water samples. Time series, ANOVA followed by HSD test and biplots were used to investigate temporal and spatial variations in surface water and groundwater chemistry. Examination of the spatial and temporal variations in groundwater solutes revealed that water quality was similar in perched groundwater and the main aquifer. The exception was NO3–N concentration, which increased in the perched groundwater with rising groundwater level, indicating a nitrogen source from the overlying airport. In the main aquifer, solute concentrations generally decreased during and immediately after snowmelt periods, indicating the importance of snowmelt input for groundwater quality. Groundwater solute concentrations generally decreased with declining groundwater level. During the spring melt period, the surface water level rise higher than groundwater level and the Cl− concentration in groundwater decreased showing a seasonal surface water intrusion. During the anomalously warm period in winter (T > 0 °C) the solute concentration decreased, implying that groundwater quality may be sensitive to future warming.

Improvement of the solute transfer in a conceptual unsaturated zone scheme: a case study of the Seine River basin

AbstractFor predicting the evolution of solute concentrations in groundwater and testing the impact of remediation policies, a coupling between the agronomical model STICS and the hydrogeological model MODCOU was implemented. When applied to the Seine River basin, this model accurately represents the temporal evolution of average nitrate concentrations in the aquifer, but with large local errors.We propose an improvement to the simple unsaturated zone (UZ) scheme NonsatSW used in STICS–MODCOU. The modifications are based on a comparison with the mechanistic model Metis considered as a reference as it solves Richards' equation. A more realistic saturation profile and a varying percolation rate are integrated in NonsatSW. This new model, named NonsatVG, is assessed by comparing it with NonsatSW and Metis. In an ideal case, NonsatVG generates a solute transfer and a dispersion closer to that of Metis than of NonsatSW. In real cases, without additional calibration, NonsatVG and Metis simulate better the average transfer velocities of the observed nitrate profiles.Furthermore, modifications in NonsatVG give a direct relationship between the depth of the water table and the saturation profile. We obtain, therefore, as in Metis, an evolution of the solute transfer velocity depending on the piezometric level. These dynamics are not simulated in NonsatSW.Despite a modified water transfer through the UZ, NonsatVG is also as valid as NonsatSW in the modelling of water transfer to the saturated zone.Finally, an application to the Seine basin shows that solute transfer velocities are lower with NonsatVG than with NonsatSW, but are in better agreement with literature. Copyright © 2010 John Wiley & Sons, Ltd.

Nitrate retention in a sand plains stream and the importance of groundwater discharge

We measured net nitrate retention by mass balance in a 700-m upwelling reach of a third-order sand plains stream, Emmons Creek, from January 2007 to November 2008. Surface water and groundwater fluxes of nitrate were determined from continuous records of discharge and from nitrate concentrations based on weekly and biweekly sampling at three surface water stations and in 23 in-stream piezometers, respectively. Surface water nitrate concentration in Emmons Creek was relatively high (mean of 2.25 mg NO3–N l−1) and exhibited strong seasonal variation. Net nitrate retention averaged 429 mg NO3–N m−2 d−1 and about 2% of nitrate inputs to the reach. Net nitrate retention was highest during the spring and autumn when groundwater discharge was elevated. Groundwater discharge explained 57–65% of the variation in areal net nitrate retention. Specific discharge and groundwater nitrate concentration varied spatially. Weighting groundwater solute concentrations by specific discharge improved the water balance and resulted in higher estimates of nitrate retention. Our results suggest that groundwater inputs of nitrate can drive nitrate retention in streams with high groundwater discharge.

Anti-diffusion and source identification with the ‘RAW’ scheme: A particle-based censored random walk

This paper presents a novel methodology for time reversal in advective-diffusive pollutant transport in groundwater systems and other environmental flow systems (specifically: time reversal of diffusive terms). The method developed in this paper extends previous particle-based approaches like the Reversed Time Particle Tracking Method of Bagtzoglou [6]. The reversal of the ‘diffusive’ and/or ‘macrodispersive’ component of pollutant migration is especially under focus here. The basis of the proposed scheme for anti-diffusion is a continuous time, censored, non-local random walk capable of tracking groundwater solute concentration profiles over time while conserving the (reverse) Fickian properties of the anti-diffusing particle cloud in terms of moments. This scheme is an alternative to the direct solution of the eulerian concentration-based diffusion PDE, which is notoriously unstable in reverse time. Our analysis leads to the conclusion that an adaptive time stepping scheme—with decreasing time step—is necessary in order to maintain a constant amount of anti-diffusion (the reverse form of Fick’s law). Specifically, we study the relations between the following parameters: time step evolution vs. time (or vs. number of steps); variance evolution (decrease rate); total time (or number of steps) required to reach a fully anti-diffused solution. The proposed approach is shown to be quite efficient; typically, for every ten time steps, one to two orders of magnitude reduction of the dispersion width of the plume can be attained. Furthermore, the method is shown to be asymptotically exact for reverse Fickian diffusion. The method is applied with success to several situations involving the diffusive transport of a conservative solute in the following cases: (i) Single source recovery in one-dimensional space with constant diffusion parameters (this example serves as a validation test for the theory); (ii) Single source recovery in two-dimensional space with constant isotropic diffusion (this example also serves as a test for the theory); (iii) Multiple source recovery in two-dimensional space, assuming isotropic diffusion. It is expected that the methodology tested in this paper is applicable more generally to complex environmental pollution problems involving multiple sources, anisotropic hydrodynamic dispersion, and space-time variable advection-diffusion flow systems; the modeling of reverse diffusion/dispersion in such systems is currently under investigation.

Mineralogical sources of the buffer capacity in a granite catchment determined by strontium isotopes

Abstract The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The 87Sr/86Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering. The early phase of weathering was clearly dominated by base cation release from biotite weathering and to a lesser extent by apatite dissolution. Feldspars contributed to base cation release as well, but could not be regarded as endmembers of the mixing diagram. There was no clear evidence for a contribution from calcite. Correspondingly, base cation input by atmospheric deposition and liming had only minor effects on groundwater buffer capacity.