A SIMPLE ANALYTICAL APPROACH FOR PREDICTING NITRATE CONCENTRATIONS IN PUMPED GROUND-WATER

A simple analytical expression is presented which predicts how solute concentrations evolve with time in a pumped well. The basic expression is for uniform recharge and uniform concentration of a conservative solute (e.g. nitrate) in the recharge. It shows that pumped concentrations are independent of pumping rates. The expression can be developed to allow for more complex patterns of recharge and solute loading, and an example is given with three zones of recharge and concentrations, induced river recharge, and a cross-boundary inflow. Predictions are made for Edgmond Bridge, a new pumping station which lies in a drift-filled valley in the Triassic sandstone aquifer of Shropshire, UK. The model predicts that concentrations of nitrate will stay below the drinking water limit for 78 years, although this would be substantially reduced if the aquifer were significantly layered or if nitrate loads from agriculture increased. Predictions were reevaluated after two years and found to be consistent with observed patterns once actual, rather than expected, conditions were incorporated in the model.