Test-observation well near Davenport, Washington: description and preliminary results

Abstract

The 750-foot test-observation well drilled near Davenport, Wash., is one of several drilled to date (1972) to provide information on ground-water conditions in selected areas of the State. The well provides information on aquifer characteristics in this area which are not available from existing deep irrigation wells. The well was drilled by air-rotary methods and penetrates eight aquifer zones (A through H); the upper 75 feet of the well is cased, and the remainder of the hole is open in basalt. Test pumping during drilling showed that the well had specific capacities of (1) 4-8 gpm (gallons per minute) per foot of drawdown when at the 255-foot depth and open to aquifers A through D, (2) 5.6 gpm per foot of drawdown when at the 640-foot depth and open to aquifers A through G, and (3) 76 5 gpm per foot of drawdown when at the full 750-foot depth and open to all eight aquifers. The tests indicate that most of the water available to the well is from the deepest aquifer (zone H). Borehole geophysical logging supplemented the drillers log of the well and provided information on natural gamma radiation, water temperature and resistivity, and borehole diameter. In the completed well each aquifer zone is isolated by cement seals, and piezometer pipes installed to zones B through H allow a basis for defining the vertical hydraulic gradient and ground-water movement in the area. The pipes also permit chemical quality monitoring of water in the various aquifer zones. An additional pipe, installed for providing thermistor access, allows recording of the geothermal gradient in the well which provides a basis for estimating vertical ground-water movement in the area.