This study has been motivated by attempts to manage optimally the root zone of agricultural soils as well as by concerns about soil and groundwater pollution. A finite difference model to solve the one-dimensional water transfer equation is presented. The results obtained with this numerical model were compared with those calculated using an analytical solution, which considers a constant flow at the soil surface, and with those obtained using an optimum solution proposed in this study, in which a constant soil water content is considered to occur at the soil surface. Furthermore, a numerical scheme to solve the convection-dispersion equation is presented so as to simulate the transport of solute that does not interact with the soil particles. The numerical solution used to describe water and solute transport is also compared with experimental results reported by others authors. It was found that the simulate results obtained by the numerical solution generally are in good agreement with those using the approaches referred in this study, and with the experimental data utilized. Therefore, this numerical solution can be used for practical purposes. | El presente estudio fue motivado por la importancia que revisten las transferencias de agua y de sustancias quimicas en las capas superficiales del suelo en relacion con el manejo de los suelos agricolas y la contaminacion del suelo y del agua subterranea. Se presenta un modelo en diferencias finitas para la solucion de la ecuacion unidimensional de transferencia de agua en el suelo. Los resultados de la simulacion numerica se comparan con los de una solucion analitica para el caso de un flujo constante en la superficie del suelo y con los de una solucion optimal propuesta en este trabajo para el caso de una humedad constante en la superficie del suelo. Se presenta tambien una solucion numerica de la ecuacion de conveccion-dispersion para describir el transporte de los solutos que no interactuan con las particulas del suelo. Los resultados de la simulacion numerica para el transporte de agua y solutos se comparan con resultados experimentales de otros autores. En general, los resultados obtenidos con la simulacion numerica estan en buen acuerdo con los obtenidos mediante las soluciones referidas y con los resultados experimentales utilizados. Por lo tanto, esta solucion numerica puede ser usada para propositos practicos.

The Agua Amarga coastal aquifer has been the object of a succession of anthropogenic interventions over the last 90 years: (a) the operation of saltworks from 1925 to 1975; (b) the withdrawal, since 2003, of groundwater from the aquifer along the coast line; and (c) the programme of pouring seawater over the salt marsh, carried out since 2009, to recover the piezometric levels and the soil moisture conditions. For a better understanding of how these past and present human activities have affected the natural groundwater regime, and to validate certain hypotheses concerning the interpretation of experimental data on temperature depth profiles and piezometric and salinity changes, a numerical fluid flow and solute transport model was designed and applied to the period 1925–2010, using SEAWAT. This model reproduces, in a qualitative and quantitative way, the flow and transport processes that operated during this time, as well as the behaviour of the seawater wedge.

The Agua Negra drainage system (30 12'S, 69 50' W), in the Argentine Andes holds several ice- and rock-glaciers, which are distributed from 4200 up to 6300 m a.s.l. The geochemical study of meltwaters reveals that ice-glaciers deliver a HCO₃⁻----Ca²⁺ solution and rock-glaciers a SO₄²⁻----HCO₃⁻----Ca²⁺ solution. The site is presumably strongly influenced by sublimation and dry deposition. The main processes supplying solutes to meltwater are sulphide oxidation (i.e. abundant hydrothermal manifestations), and hydrolysis and dissolution of carbonates and silicates. Marine aerosols are the main source of NaCl. The fine-grained products of glacial comminution play a significant role in the control of dissolved minor and trace elements: transition metals (e.g. Mn, Zr, Cu, and Co) appear to be selectively removed from solution, whereas some LIL (large ion lithophile) elements, such as Sr, Cs, and major cations, are more concentrated in the lowermost reach. Daily concentration variation of dissolved rare earth elements (REE) tends to increase with discharge. Through PHREEQC inverse modelling, it is shown that gypsum dissolution (i.e. sulphide oxidation) is the most important geochemical mechanism delivering solutes to the Agua Negra drainage system, particularly in rock-glaciers. At the lowermost reach, the chemical signature appears to change depending on the relative significance of different meltwater sources: silicate weathering seems to be more important when meltwater has a longer residence time, and calcite and gypsum dissolution is more conspicuous in recently melted waters. A comparison with a non-glacierized semiarid drainage of comparable size shows that the glacierized basin has a higher specific denudation, but it is mostly accounted for by relatively soluble phases (i.e. gypsum and calcite). Meltwater chemistry in glacierized arid areas appears strongly influenced by sublimation/evaporation, in contrast with its humid counterparts.

The efficacy of different proportions of silt-loam/bentonite mixtures overlying a vadose zone in controlling solute leaching to groundwater was quantified. Laboratory experiments were carried out using three large soil columns, each packed with 200-cm-thick riverbed soil covered by a 2-cm-thick bentonite/silt-loam mixture as the low-permeability layer (with bentonite mass accounting for 12, 16 and 19 % of the total mass of the mixture). Reclaimed water containing ammonium (NH₄⁺), nitrate (NO₃⁻), organic matter (OM), various types of phosphorus and other inorganic salts was applied as inflow. A one-dimensional mobile–immobile multi-species reactive transport model was used to predict the preferential flow and transport of typical pollutants through the soil columns. The simulated results show that the model is able to predict the solute transport in such conditions. Increasing the amount of bentonite in the low-permeability layer improves the removal of NH₄⁺and total phosphorous (TP) because of the longer contact time and increased adsorption capacity. The removal of NH₄⁺and OM is mainly attributed to adsorption and biodegradation. The increase of TP and NO₃⁻concentration mainly results from discharge and nitrification in riverbed soils, respectively. This study underscores the role of low-permeability layers as barriers in groundwater protection. Neglect of fingers or preferential flow may cause underestimation of pollution risk.

When implementing remediation programs to mitigate diffuse-source contamination of aquifers, tools are required to anticipate if the measures are sufficient to meet groundwater quality objectives and, if so, in what time frame. Transfer function methods are an attractive approach, as they are easier to implement than numerical groundwater models. However, transfer function approaches as commonly applied in environmental tracer studies are limited to a homogenous input of solute across the catchment area and a unique transfer compartment. The objective of this study was to develop and test an original approach suitable for the transfer of spatially varying inputs across multiple compartments (e.g. unsaturated and saturated zone). The method makes use of a double convolution equation accounting for transfer across two compartments separately. The modified transfer function approach was applied to the Wohlenschwil aquifer (Switzerland), using a formulation of the exponential model of solute transfer for application to subareas of aquifer catchments. A minimum of information was required: (1) delimitation of the capture zone of the outlet of interest; (2) spatial distribution of historical and future pollution input within the capture zone; (3) contribution of each subarea of the recharge zone to the flow at the outlet; (4) transfer functions of the pollutant in the aquifer. A good fit to historical nitrate concentrations at the pumping well was obtained. This suggests that the modified transfer function approach is suitable to explore the effect of environmental projects on groundwater concentration trends, especially at an early screening stage.

What is the source of geogenic (natural or native) solutes in groundwater? The orthodox explanation suggests it is largely a function of water–rock interaction (weathering of the soil zone and aquifer mineral framework). It is proposed herein that atmospheric deposition (combination of wet and dry aerosols from ocean spray, smoke, volcanoes, continental dust, and lightning) is a significant source, and in many cases the dominant source, of the major and minor geogenic solutes in groundwater. Solute mass-balance analyses suggest that much of the mass of major and minor ions must be transported into the aquifer from an external source. Example case studies are presented: analysis of groundwater in a coastal marine aquifer located in an arid area (United Arab Emirates) suggests that over 50% of several major ions potentially originate from atmospheric deposition; in an alluvial fan in a semi-arid system (High Plains, USA), 100% of most solutes potentially originate from atmospheric deposition; and in a humid glacial aquifer system (Michigan, USA), 20–30% of many major ions are potentially from atmospheric deposition. These observations contrast with many hydrogeologic textbooks, which still propose the origin to be water–rock interaction—hence, the myth.

The maximum upward flux (Eₘₐₓ) is a control condition for the development of groundwater evaporation models, which can be predicted through the Gardner model. A high-precision Eₘₐₓ prediction helps to improve irrigation practice. When using the Gardner model, it has widely been accepted to ignore parameter b (a soil-water constant) for model simplification. However, this may affect the prediction accuracy; therefore, how parameter b affects Eₘₐₓ requires detailed investigation. An indoor one-dimensional soil-column evaporation experiment was conducted to observe Eₘₐₓ in the presence of a water table of depth 50 cm. The study consisted of 13 treatments based on four solutes and three concentrations in groundwater: KCl, NaCl, CaCl₂, and MgCl₂, with concentrations of 5, 30, and 100 g/L (salty groundwater); distilled water was used as a control treatment. Results indicated that for the experimental homogeneous loam, the average Eₘₐₓ for the treatments supplied by salty groundwater was larger than that supplied by distilled water. Furthermore, during the prediction of the Gardner-model-based Eₘₐₓ, ignoring b and including b always led to an overestimate and underestimate, respectively, compared to the observed Eₘₐₓ. However, the maximum upward flux calculated including b (i.e. Ebₘₐₓ) had higher accuracy than that ignoring b for Eₘₐₓ prediction. Moreover, the impact of ignoring b on Eₘₐₓ gradually weakened with increasing b value. This research helps to reveal the groundwater evaporation mechanism.

Groundwater flow is an important control on subsurface evaporite (salt) dissolution. Salt dissolution can drive faulting and associated subsidence on the land surface and increase salinity in groundwater. This study aims to understand the groundwater flow system of Gypsum Canyon watershed in the Paradox Basin, Utah, USA, and whether or not groundwater-driven dissolution affects surface deformation. The work characterizes the groundwater flow and solute transport systems of the watershed using a three-dimensional (3D) finite element flow and transport model, SUTRA. Spring samples were analyzed for stable isotopes of water and total dissolved solids. Spring water and hydraulic conductivity data provide constraints for model parameters. Model results indicate that regional groundwater flow is to the northwest towards the Colorado River, and shallow flow systems are influenced by topography. The low permeability obtained from laboratory tests is inconsistent with field observed discharges, supporting the notion that fracture permeability plays a significant role in controlling groundwater flow. Model output implies that groundwater-driven dissolution is small on average, and cannot account for volume changes in the evaporite deposits that could cause surface deformation, but it is speculated that dissolution may be highly localized and/or weaken evaporite deposits, and could lead to surface deformation over time.

The Svensk Kärnbränslehantering AB (SKB) has proposed the Forsmark site as a future repository for spent high-level nuclear fuel, involving disposal at about 470 m depth in sparsely fractured crystalline bedrock. An essential part of the completed inter-disciplinary site investigation was to develop an integrated account of the site and its regional setting, including the current state of the geosphere and the biosphere as well as natural processes affecting long-term evolution. First, this report recollects the integrated understanding and some key hydraulic characteristics of the crystalline bedrock at Forsmark along with a description of the flow model set-up and the methodology used for paleoclimatic flow modeling. Second, the protocol used for site-scale groundwater flow and solute transport modeling is demonstrated. In order to conduct a quantitative assessment of groundwater flow paths at Forsmark, the standard guide for groundwater flow modeling was elaborated on, to support both discrete and porous media flow approaches. In total, four independent types of data were used to confirm that the final groundwater flow model for the crystalline bedrock was representative of site conditions.

The McMurdo Dry Valleys (MDVs), Antarctica, exist in a hyperarid polar desert, underlain by deep permafrost. With an annual mean air temperature of −18 °C, the MDVs receive <10 cm snow-water equivalent each year, collecting in leeward patches across the landscape. The landscape is dominated by expansive ice-free areas of exposed soils, mountain glaciers, permanently ice-covered lakes, and stream channels. An active layer of seasonally thawed soil and sediment extends to less than 1 m from the surface. Despite the cold and low precipitation, liquid water is generated on glaciers and in snow patches during the austral summer, infiltrating the active layer. Across the MDVs, groundwater is generally confined to shallow depths and often in unsaturated conditions. The current understanding and the biogeochemical/ecological significance of four types of shallow groundwater features in the MDVs are reviewed: local soil-moisture patches that result from snow-patch melt, water tracks, wetted margins of streams and lakes, and hyporheic zones of streams. In general, each of these features enhances the movement of solutes across the landscape and generates soil conditions suitable for microbial and invertebrate communities.