Test observation well near Almira, Washington: description and preliminary results

Abstract

A 750-foot test-observation well drilled bear Almira, Wash., is one of several drilled to date (1971) in selected areas if the State. The test wells provide information on ground-water conditions where large-scale pumping from wells is rapidly increasing or ids expected to increase, but where data available from existing wells are incomplete and fail to provide adequate guidance for management of the area;s ground-water resources. The upper 60 feet of the Almira well has a 10-inch casing below this depth the well is uncased in basalt. Four major aquifer zones are penetrated by the well. Geophysical logging during drilling provided data on hole diameter, water resistivity (mineralization) and temperature, and natural radiation of the rock units. These data supplemented the driller's log as a basis for final selection of the aquifers to be monitored. Two pumping tests showed that (1) when the well was 546 feet deep and was pumped at a rate of about 510 gpm (gallons per minute) for 9 1/2 hours, the specific capacity was 92 gpm per foot of drawndown; and (2) at the final 750-foot depth, with all four aquifers contributing water and the well being pumped at a rate of 525 gpm for 24 hours, the specific capacity was 96 gpm per foot of drawdown. Water from the well when drilled to its final depth of 750 feet is slightly warmer and somewhat more mineralized than when the well was only 546 feet deep. Four piezometer pipes were installed in the well so that each reflected the head in a specific aquifer zone that is hydraulically isolated from the other aquifer zones by cement seals. The maximum difference in water levels between the uppermost and lowermost aquifers after the zones were isolated was 180 feet; the difference may increase somewhat as the heads in the aquifers continue to adjust after isolation. An access pipe also was provided for an instrument to measure the geothermal gradient. The data that have been obtained from the test-observation well and that which will be available from it in the future are expected to provide invaluable guidance for management of the ground-water resources.