|Abstract:||Drilling of two test holes into the Fallon basalt aquifer commenced August 14, 2001. The basalt aquifer is located beneath the Carson Desert, near Fallon, Nevada, and is the sole source of drinking water for the City of Fallon, the Naval Air Station (NAS) Fallon, and the Fallon Paiute-Shoshone Tribe. Basalt comprising the aquifer is exposed at Rattlesnake Hill, an eroded volcanic cone, about 1 mile northeast of Fallon, and the remainder is buried beneath sediments deposited by the Carson River and ancient Lake Lahontan to depths of 600 feet near its edges (fig. 1). The basalt-aquifer system is a mushroom-shaped body of highly permeable volcanic rock. Viewed from above, the lateral extent of the basalt body is oval-shaped, about 4-miles wide and 10-miles long (fig. 1).
Drilling was part of a cooperative study between the U.S. Geological Survey (USGS), the Bureau of Reclamation, and NAS Fallon. The study was started because of concern about the continued viability of the basalt-aquifer system as a source of municipal water supply. Increased pumping from about 1,700 acre-feet per year (acre-ft/yr) in the 1970?s to over 3,000 acre-ft/yr in the late 1990?s has caused water levels in the basalt to decline as much as 12 feet (fig. 2). During this same time period, water pumped from the aquifer at NAS Fallon and the City of Fallon wells showed that concentrations of dissolved chloride increased, although chloride concentrations were well within the U.S. Environmental Protection Agency?s (EPA) drinking-water standards; at this rate of increase, it would take decades to exceed the present standard (Maurer and Welch, 2001, p. 46). Concentrations of arsenic in the aquifer are about 0.1 milligrams per liter (mg/L), exceeding the drinking-water standard of 0.01 mg/L, but show no apparent change over time (Maurer and Welch, 2001, p. 10 and 48; U. S. Environmental Protection Agency, 2001).
Increasing concentrations of chloride may be caused by increased pumping, that induces inflow of more saline water from aquifers surrounding or underlying the basalt, or from greater depths within the basalt itself. Prior to the drilling on August 14, 2001, few wells penetrated the basalt more than 70 feet below its upper surface (Maurer and Welch, 2001, p. 34). This prevented monitoring changes in water quality deeper in the aquifer that might be moving upward with continued pumping. Purposes of drilling were to fully penetrate the basalt, determine its hydrogeological character, the distribution of water quality in the basalt and in the underlying sedimentary aquifer, install monitoring wells.