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An analytical solution to calculate the recharge of unconfined aquifers with Dupuit‐Forchheimer type flow conditions is proposed. This solution is derived from an existing closed‐form analytical solution initially developed to determine groundwater travel time when the recharge of the aquifer is known. This existing solution has been modified to determine recharge when groundwater travel time is known. An illustration is given with a field case example for the Bonifacio aquifer of the island of Corsica (France), in the Mediterranean. In this aquifer, previously established differences in groundwater residence time between two water samples were determined from anthropogenic atmospheric gas (chlorofluorocarbons and sulfur hexafluoride) measurements. The time difference is entered into the new analytical solution to determine recharge. The calculations yield a value of average recharge that agrees with the results obtained by several other methods that were presented in previous studies to assess the recharge of the Bonifacio aquifer. Also presented in this study is a sensitivity analysis of the new analytical solution, to quantify the influence of different parameters that control recharge: hydraulic conductivity, effective porosity and the groundwater travel time. This study illustrates how geochemical data can be combined with physical models to measure recharge. Such an approach could be adopted in other homogeneous aquifers worldwide that satisfy Dupuit‐Forchheimer type flow conditions. Article Impact Statement: An analytical method is proposed for assessing recharge in unconfined aquifers based on groundwater travel times. [ABSTRACT FROM AUTHOR]

Elevated nitrate concentrations have been documented since the 1970s within the trans-national Abbotsford-Sumas aquifer, situated in the central Fraser Valley of southern British Columbia, Canada, and northern Washington State, United States. Nitrate concentrations in excess of 10 mg/L NO3-N are commonly observed in the monitoring wells. A groundwater model is used in this study to conduct a particle tracking analysis to estimate groundwater travel times and compare these with groundwater ages determined from 3H/3He concentrations measured in several monitoring wells. Groundwater ages estimated from particle tracking show excellent agreement with measured ages (slope=1), although there is scatter ( R2=0.76), reflecting the complexity of the travel pathways and ambiguities in groundwater ages because of mixing. An assessment of the efficacy of recommended nutrient management practices, which were implemented in 1992 to reduce nitrogen loading, suggests that wells shallower than about 20 m should be monitoring a decrease in nitrate concentration. However, persistent elevated nitrate concentrations even at shallow depths point to the lingering effects of a remnant manure-nitrate signature in combination with continued high loading. [ABSTRACT FROM AUTHOR]

The aim of this paper is to show how standard hydrogeologic software, usually used to model contaminant transport in groundwater under unsaturated conditions, can also be used to model gas transport in unsaturated porous media. Physical processes involved in the interaction between the atmosphere and the unsaturated soils are considered: transport by diffusion through the air and the groundwater, exchange between the liquid and gas phases and consumption. These physical processes are incorporated into the governing equations of a groundwater numerical code; by considering air, contained in the unsaturated soil, as water in the seepage numerical model, the air effectively becomes fluid within the numerical code. Then, the investigated gas is defined as the contaminant in the transport model, which is transported by -the air for the modeller-, and -water for the numerical code-. The over-riding assumption is that the air profiles and, therefore, water profiles of volume contents remain constant. The approach is illustrated using two examples, which consider the transport of oxygen. The first deals with oxygen distribution through a laboratory-cell diffusion containing reactive mining tailings. The second deals with the oxygen fluxes through the vadose zone, between the atmosphere and an unconfined aquifer's water table. Both examples consider different cases of oxygen consumption. [ABSTRACT FROM AUTHOR]

A new approach has been developed to detect, characterize, and quantify hydraulic short-circuits in boreholes with faulty seals. The methodology, applicable to an aquifer-aquitard-aquifer system, involves a series of successive, constant-rate pumping tests in the lower aquifer while determining the leakage rate with a simultaneous nonreactive tracer test. During each pumping step, the tracer is injected under constant concentration and constant hydraulic head from a piezometer in the upper aquifer. If a seal defect exists, the tracer will follow the leak and will be recovered from the pumped water. The theoretical equations relate the leakage rate, the pumping rate, the concentration of the injected tracer, and the recovered concentration. Leakage rates can be determined for any pumping rate. The theory is tested using numerical analysis and a full-scale field test. [ABSTRACT FROM AUTHOR]

An exact, closed-form analytical solution is developed for calculating ground water transit times within Dupuit-type flow systems. The solution applies to steady-state, saturated flow through an unconfined, horizontal aquifer recharged by surface infiltration and discharging to a downgradient fixed-head boundary. The upgradient boundary can represent, using the same equation, a no-flow boundary or a fixed head. The approach is unique for calculating travel times because it makes no a priori assumptions regarding the limit of the water table rise with respect to the minimum saturated aquifer thickness. The computed travel times are verified against a numerical model, and examples are provided, which show that the predicted travel times can be on the order of nine times longer relative to existing analytical solutions. [ABSTRACT FROM AUTHOR]