A possible solution to reduce nitrate concentrations in pumped groundwater is to case-out the section of the aquifer with a high nitrate content. This approach is best suited to a hydrogeological scenario where permeability and hydrochemistry are layered. A Chalk borehole at Swaffham, Norfolk was chosen and the upper layer cased-out. A considerable reduction in the concentrations of nitrate and other major ions was achieved immediately after completion of the engineering work, for example, nitrate and potassium concentrations were reduced by 50 and 58% respectively. A 2 year pumping test was designed to examine the long-term success of the technique and demonstrated that pumped groundwater nitrate values gradually increased as water with a high nitrate concentration leaked from the upper aquifer layer. After hydraulic equilibrium was reached the rate of increase in nitrate concentration reduced considerably. A water quality model, incorporating a mixing cell calculation, was used to simulate the pumping test results. A good simulation of the observed nitrate concentrations was achieved when the process of molecular diffusion was included in the model. Predictions for the next five years suggest that the nitrate concentration will not increase above 13.6 mg N l-1. The main disadvantage of the engineering solution is the significant loss in borehole yield experienced after casing-out the more permeable upper aquifer layer.