Combined use of digital aquifer models and field base-flow data to identify recharge-leakage areas of artesian aquifers

As a result of continuous pumping since the 1890's, a regional cone of depression encompassing 363 km² has developed within the artesian Miocene Cheswold aquifer at Dover, Del. The aquifer is not being recharged significantly by leakage near the center of the cone, nor is major recharge induced in the updip subcrop area. The source of pumped water is apparently an area of about 65 km² northwest of Dover, where vertical leakage is substantial. This area was delineated by use of a digital aquifer model and streamflow data. A model of the unconfined aquifer was used to compute the unconfined aquifer's natural discharge to streams, assuming no leakage to underlying artesian aquifers. The area of substantial leakage to the underlying Cheswold was delineated as being coincident with that area where model-computed values of stream discharge were substantially larger than actual values of stream discharge determined in the field.