Water Supply in a Contaminated Basin

Situation

The quality of shallow groundwater supplies in a semi–arid central Arizona basin had been compromised byhuman activity.There is no other source of water in the basin than groundwater. Domestic water–supply wells are becoming contaminated, sources of potable water for use in industrial processes are disappearing, and a large–scale municipal supply well is planned to replace several other municipal supply wells that have become impacted by contaminated groundwater.

Hydro Geo Chem’s Approach

Water supply wells, as opposed to environmental monitoring wells, require design specifications that enhance water production. The design criteria include not only well construction materials, but also need to take advantage of a formation’s ability to yield water in sufficient quantity and quality to meet demand. Hydro Geo Chem, Inc. designed and supervised the installation of 4 large–scale remedial wellfields. The well locations were optimized to minimize interference while maximizing the extraction rate.


During the drilling of replacement wells, Hydro Geo Chem’s field hydrogeologists record drilling penetrationrates,clay sand and gravel fractionsin cuttings returned to the surface, calcite reaction strength,formation and groundwater pH, electrical conductivity, and lithology to optimize the location of a well screen in a given borehole. A drilling log is developed and maintained during replacement well drilling so that real time decisions on well screen placement can be made that result in production from a clean layer of water beneath the contaminated alluvial aquifer. In addition, Hydro Geo Chem’s extensive numerical modeling capabilities and proprietary Well Hydraulics Interpretation Program (WHIP) allow optimization of well location, so that production is not interfered with by existing or future wells in the vicinity of the supply wells.

Results

Hydro Geo Chem has designed and supervised the installation of over 100 domestic water–supply wells in the past few years. The wells are currently performing as designed. During the fall of 1998, Hydro Geo Chem designed and supervised the installation of a 1,600–foot, 18–inch diameter, municipal supply well. The production capacity met design specifications by producing almost 500 gallons per minute. Total depth and location of the well screen relied on exploration bore holes, formation hydraulic properties, and basin hydrostratigraphy so that pumping would not cause degradation of water quality due to capture of contaminated groundwater. Industrial water–supply wells were installed that also provided potable water using the same techniques.