Case History: Siting Shallow Groundwater Wells with the Aid Of Geophysics

Although the Colorado Division of Wildlife’s Bellvue Fish Hatchery site is relatively small, covering approximately 30 acres, groundwater drilling results across the site are variable and unpredictable; the site includes both productive, artesian wells as well as dry wells. The site is underlain primarily by the Lykins Formation, which includes interbedded siltstones, limestones, claystones, and evaporites. In preparation for new wells, geophysical surveys were done in an attempt to better understand the subsurface with respect to groundwater production. Both transient electromagnetic (TEM) and galvanic dipole-dipole resistivity lines were run, and substantial variations in resistivity within the Lykins are evident (as might be expected from the prior drilling results). The first test well after the geophysical survey was sited to test a locally conductive zone; the well was successful and flowed artesian. Similarly, a second test in a conductive zone also flowed artesian, while a third hole, not based on the geophysical survey (but sited primarily on the basis of permits and logistical considerations) was unproductive. The geophysical data confirm the heterogeneous nature of the Lykins at this site, agrees well with downhole logging, and has been clearly useful in successfully siting groundwater production wells.