Three 2-inch diameter monitoring wells were installed in 

the test hole, one each near the middle and bottom of the basalt, 

and one in the underlying sedimentary aquifer (fig. 4). Wells 

installed in the basalt produced water at rates of about 20 gallons 

per minute. Analyses of water pumped from the wells showed 

that the dissolved-solids concentration was 630 and 790 mg/L in 

the middle and bottom of the basalt, respectively, and was 3,800 

mg/L in sediments underlying the basalt (fig. 4). The drinkingwater standard for dissolved solids is 1,000 mg/L (U.S. Environmental Protection Agency, 2001). Dissolved arsenic concentrations in the middle and bottom of the basalt were 0.106 and 

0.113 mg/L, respectively, and 0.230 mg/L in sediments underlying the basalt, all exceeding drinking-water standards. Concentrations of dissolved chloride and iron in water from sediments 

underlying the basalt were 1,950 and 0.66 mg/L, exceeding 

drinking-water standards of 400 and 0.60 mg/L, respectively 

(U.S. Environmental Protection Agency, 2001). 

Results of the water-quality analyses indicate the underlying 

sedimentary aquifer could be a source for migration of poorquality water into the basalt. Periodic sampling of the completed 

wells will provide a means to monitor changes in water quality 

as pumping from the basalt continues.

The northern test hole, drilled about 3 miles northeast of 

Rattlesnake Hill (fig. 1), encountered massive basalt breccia, 

basalt breccia with clay-filled fractures, and massive basalt from 

depths of 146 to 487 feet below land surface (figs. 5 and 6). 

Again, three 2-inch diameter monitoring wells were installed in 

the test hole, one each near the middle and bottom of the basalt, 

and one in the underlying sedimentary aquifer (fig. 6). Wells 

installed in the fractured breccia and underlying massive basalt 

produced water at rates of less than 1 and about 2 gallons per 

minute, respectively. Analyses of water pumped from the wells 

showed that the dissolved solids concentration was 810 and 

2,100 mg/L in the middle and bottom of the basalt, respectively, 

and 680 mg/L in sediments underlying the basalt (fig. 6). 

Concentrations of arsenic were 0.060 and 0.045 mg/L in the 

middle and bottom of the basalt, respectively, and 0.210 mg/L

in sediments underlying the basalt, all exceeding drinking-water 

standards. Except for a dissolved chloride concentration of 840 

mg/L in the well installed near the bottom of the basalt, all other 

constituents analyzed were within drinking-water standards. 

The test drilling at the two sites (figs. 7 and 8) showed that 

near the southern part of the aquifer, the area where most public 

supply wells are located, ground water from depths as great as 

700 feet within the basalt aquifer were within the drinking-water 

standards except for dissolved arsenic. Ground water in sediments underlying the basalt near its southern extent greatly 

exceed drinking-water standards and could be a source for 

migration of poor-quality water into the basalt. The basalt

aquifer near the location of the northern test hole does not 

readily yield large quantities of water to wells, and contains 

ground water with lower concentrations of dissolved arsenic 

than elsewhere in the aquifer. Water in sediments underlying the 

basalt at the northern test hole, although low in dissolved solids, 

has high concentrations of dissolved arsenic. 

Additional test holes that would completely penetrate the 

basalt aquifer could be used to characterize the basalt and underlying sedimentary aquifers in terms of lithology, water quality, 

water levels, and hydraulic properties. This information would 

allow development of a numerical gound-water flow model of 

the basalt aquifer. The model would be used to refine estimates 

of the rates and sources of recharge to the basalt, and to determine long-term pumping rates that would maintain acceptable 

water levels and water quality in the aquifer.

———Douglas K. Maurer

ANNUAL PUMPAGE FROM BASALT

AQUIFER, IN ACRE-FT 

38

52

40

42

44

46

48

50

WATER LEVEL IN BASALT AQUIFER,

IN FEET BELOW LAND SURFACE

1,400

3,400

1,600

1,800

2,000

2,200

2,400

2,600

2,800

3,000

3,200

1970 1975 1980 1985 1990 1995 2000

Figure 2. Annual pumpage from basalt aquifer and water-level changes in basalt aquifer.
Figure 4. Lithology, well depths, and concentrations

of selected constituents in southern test hole.

Figure 3. Core sample of the porous fractured basalt from the 

southern test hole. 

Figure 5. Core sample of the basalt breccia from the 

northern test hole.

Figure 6. Lithology, well depths, and concentrations

of selected constituents in northern test hole.
REFERENCES CITED

Glancy, P.A., 1986, Geohydrology of the Basalt and 

Unconsolidated Sedimentary Aquifers in the Fallon Area, 

Churchill County, Nevada: U.S. Geological Survey WaterSupply Paper 2263, 62 p.

Maurer, D.K., and Welch, A.H., 2001, Hydrogeology and 

geochemistry of the Fallon basalt and adjacent aquifers, and 

potential sources of basalt recharge, in Churchill County, 

Nevada: U. S. Geological Survey Water-Resources 

Investigations Report 01-4130, 72 p.

U.S. Environmental Protection Agency, 2001, National Primary 

Drinking-Water Regulations: arsenic and clarifications to 

compliance and new source contaminants monitoring; final 

rule: Federal Register, U. S. Code of Federal Regulations, 

January 22, 2001, v. 66, no. 14, p. 6976-7066.

For more information on the U.S. Geological Survey

studies described in this Fact Sheet, or on other 

aspects of water resources in Nevada, please contact:

Public Information Assistant

U.S. Geological Survey

333 W. Nye Lane, Rm. 103

Carson City, Nevada

telephone: (775) 887-7649

fax: (775) 887-7629

email GS-W-NVpublic-info@usgs.gov

URL: http://nevada.usgs.gov

D.K. Maurer, U.S. Geological Survey

June 2002 Fact Sheet FS–048–02

D.K. Maurer, U.S. Geological Survey

Figure 8. Drilling of the northern test hole in the Fallon basalt 

Figure 7. Drilling of the southern test hole in the Fallon basalt