McNeil Island, a 6.63-square-mile island located in southern Puget Sound, Washington,  is the former site of former federal and a state correctional facilities. Due to the aging and structurally compromised surface water system, the Washington State Department of Corrections sought to develop a new groundwater-based water supply system. Terraphase was part of the competitively selected project team tasked with developing a long-term groundwater solution for the island.

Scope of Services

Terraphase began with a review of available well data in the Washington State Department of Ecology (Ecology) database, finding limited groundwater potential from existing modern wells. Through extensive research, including review of unpublished U.S. Geological Survey (USGS) reports and regional sediment studies, our hydrogeologist identified the locally known “E Aquifer” at depths of approximately 700 feet below ground surface. While this aquifer had been reached by wells drilled in the 1920s, it had never been successfully developed due to construction issues, resulting in shallow well completions and seawater intrusion.

Terraphase facilitated site approvals with local and state health departments and provided water rights support with Ecology. Our hydrogeologist obtained security clearance and provided on-site oversight for drilling, construction, and testing of the well. A 741-foot-deep well was drilled using air- and fluid-rotary methods, and the final design included a stainless-steel, filter-packed well screen exposed to the formation from 700 to 735 feet below ground surface in a sand and gravel aquifer. Although water quality was generally good, treatment for iron and manganese was recommended–common for deep aquifers in the region.

Notable Accomplishments 

• Supported the Department of Corrections in transitioning from surface to groundwater supply
• Facilitated site approvals and water rights coordination with local and state agencies
• Oversaw drilling and testing of a 741-foot-deep well in a remote, secure island environment
• Successfully developed a new groundwater source under challenging logistical conditions