The presence of low-permeability Precambrian rocks near land surface limits ground-water availability in the northern two-thirds of Wood County. Sand and gravel deposits provide large amounts of water (more than 500 gallons per minute) to some wells in the southeastern part of the county. Fine-grained unconsolidated deposits generally are less than 20 feet thick in the northern two-thirds of the county, but sand and gravel deposits 40- to 100-feet thick underlie the extreme southeastern part of the county. Horizontal hydraulic conductivity of the sand and gravel deposits ranges from about 155 to about 280 feet per day. The horizontal hydraulic conductivity of fine-grained unconsolidated deposits in the northern part of the county ranges from about 0.02 to 2 feet per day. Where unconsolidated deposits do not yield dependable water supplies, wells are finished in Precambrian rocks. Fractures occurring at shallow depths are the primary source of water for wells finished in Precambrian rocks. Because the number of fractures tends to decrease with depth, the horizontal hydraulic conductivity of these rocks generally decreases from about 11 feet per day in wells less than 50-feet deep to about 0.02 foot per day in wells greater than 160 feet deep.

Estimates of ground-water recharge to sand and gravel deposits in the southeastern part of the county range from about 7 to 12 inches per year. Recharge estimates for the central and northern parts of the county range from about 1 to 4 inches per year.

The total dissolved-solids concentration in ground water in Wood County is relatively low. Concentrations in water samples from 124 wells ranged from 21 to 578 milligrams per liter, with a median concentration of about 190 milligrams per liter. Major dissolved constituents are calcium, magnesium, and bicarbonate; sodium, potassium, chloride, and sulfate are present in low concentrations. The most common water-quality problem in Wood County is elevated iron concentrations. Iron concentrations greater than 300 micrograms per liter were found in 54 of 124 samples, and 15 samples contained iron concentrations greater than 5,000 micrograms per liter.

Nitrate as nitrogen concentrations exceeded Wisconsin's drinking-water standard (10 milligrams per liter) in water from just 4 of 124 wells. The pesticide aldicarb was detected in 7 of 36 samples, and various volatile organic compounds were detected in 24 of 102 ground-water samples collected by the Wisconsin Department of Natural Resources since 1980. Wells in which these chemicals were detected are near irrigated agricultural fields and in commercially developed areas where buried gasoline-storage tanks and chemical spills are more likely to occur.

A reconnaissance approach combining electromagnetic surveys and sampling for water-quality indicators was used to assess effects of leachate on ground water near seven landfills. Results of the electromagnetic surveys were used to site water-quality observation wells. Total dissolved-solids concentrations and concentrations of volatile organic compounds, chloride, sulfate, iron, chemical oxygen demand, and organic carbon are some of the chemical constituents analyzed in samples collected from these wells.

The average rate of ground·water pumpage in Wood County in 1985 was 9.7 million gallons per day. Of this rate, about 6 million gallons per day is pumped from municipal-supply wells in seven communities.An additional 1.08 million gallons per day is pumped for agricultural irrigation.