Deep Well Injectate Migration Assessment: A Case History

An assessment of deep injectate migration was initiated in 1989, beginning with the review of historical records of two wastewater injection wells, installation of four nested monitor wells (2-3 pipes per boring) during 1991 and one nested well during 2004, and periodic groundwater monitoring since 1995.  The injection wells were utilized for the disposal of ~85 million gallons of wastewater between 1963 and 1985.  The injection zone is ~970 to 1,600 feet below ground at the injection wells and is overlain by a 30-foot thick layer of clay that dips westward 3.5 degrees.  The injection wells were filled with cement grout in 1994 after well integrity testing found no defects. 

A total of 13 pipes were installed to depths between 800 and 1,380 feet in the five nested monitor wells.  Water level and water quality data are obtained from two permeable layers above the clay near the injection wells, including a compliance layer; and from the compliance layer and from one or two layers in the injection zone at four down-gradient locations.

The injection zone contained sodium-chloride groundwater – slightly saline (top) to very saline (bottom), prior to disposal operations.  The injectate was a sodium-sulfate brine with 1, 2-dichloroethane.  The assessment shows that injectate has not affected the compliance layer, which generally contains fresh sodium-chloride groundwater.

Depths to water have varied between 80 and 110 feet at the 13 wells since 2003.  Groundwater in these deep layers flows to the west and southwest under hydraulic gradients that are considerably smaller than overlying shallow fresh groundwater.  The dense injectate migrated 1,400 feet down-gradient (and down-dip) to the southwest in 40 years (35 feet per year), as shown by rapid increases in concentrations of sodium, sulfate, chloride, TDS, and 1,2-dichloroethane at the fourth well after 2002.