Deep Ground Water Investigation of Waste Water Injection Wells

A deep groundwater investigation was conducted on two wastewater injection wells during 1991.  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.  The injection wells were destroyed in 1994 after an investigation of well integrity and hydrogeology.

The hydrogeological investigation resolved several challenges:
            Cost-effective installation of deep monitor wells
            Adequate information to define injectate migration
            Safe working conditions
            Protection of freshwater aquifers

The scope of the investigation was set after 16 months of negotiations with US-EPA and California DTSC.  The deep hydrogeology was defined by four nested monitor wells (2-3 pipes per boring) with 11 pipes installed at depths between 800 and 1,380 feet.  These wells provide water level and water quality data for a compliance layer overlying the clay and two layers within the injection zone.To limit worker exposure to injectate, the clay was not pierced by the boring near the injection wells and the three deeper borings were located beyond the estimated distance of injectate migration, +0.3 miles down-gradient.  To protect the freshwater aquifers, conductor casings were installed in the borings to depths between 455 and 685 feet.

The cost of the four nested wells was $107 per foot of pipe in 2004 dollars, which was less than the cost of 11 single-completion wells.  A fifth nested well was built in the compliance layer and the first injection layer during 2004 at a cost of $66 per foot of pipe.  The lower cost of the fifth well was due to the elimination of the conductor casing because of injectate containment.  The injectate is migrating toward the southwest at 35 feet per year.