IMPACTS OF VADOSE ZONE NITROGEN ON AQUIFER STORAGE AND RECOVERY SYSTEMS

Aquifer storage and recovery operations have become widespread in many arid and semi-arid regions where surface water supplies vary significantly with season. By infiltrating excess surface water during times of surplus, significant economic savings can be accomplished with, what was thought to be, minimal degradation in water quality. However, the presence of large stores of nitrogen (as nitrate) in vadose zone waters has resulted in elevated nitrate concentrations observed in monitoring wells is several projects in Arizona. Numerical simulation of nitrate and chloride transport are used in this presentation to clearly demonstrate the magnitude of the problem and time scales over which vadose zone nitrate may be a significant problem. Simulations show that ground water nitrate concentrations are strongly controlled by longitudinal dispersivity in the vadose zone, the saturated conductivity of the aquifer and the hydraulic gradient in the underlying aquifer. While both chloride and nitrate are elevated in the first pore volume of water passed through the vadose zone, the much lower maximum contaminant level (MCL) for nitrate results in violation of the MCL for significant periods of time and volumes of water injected.