Effects of Microtopographically Concentrated Recharge on Nitrate Variability in a Confined Aquifer: Model Simulations

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Simulated recharge to a shallow confined aquifer indicates that locally concentrated recharge through microtopographically determined recharge areas (having elevation differences of less than 2 cm) can have a substantial effect on the spatial variability of nitrate-N concentrations [N-NO3] in the upper meter of an aquifer. For an aquifer that is confined and near pressure equilibrium with the watertable, hydraulic mounds formed under recharge areas can cause temporary large local increases in hydraulic gradient and subsequent flux from the aquitard to the upper aquifer. These hydraulic “surges” carry nitrates from the aquitard to the upper aquifer, where they move as local plumes and disperse according to the properties of the prevailing aquifer flow system, until discharged, reduced to ammonia, or denitrified. Under conditions of maximum local recharge the ratio of aquifer nitrate concentration to till nitrate concentration ([N-NO3]a/[N-NO3]t) may approach an upper limit of 1. Understanding these processes is important for interpretation of nitrate and other solute measurements near the aquifer-aquitard boundary.

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