Water samples were collected from 7 production wells and 28 private residential wells in central New York from August through December 2007 and analyzed to characterize the chemical quality of groundwater. Seventeen wells are screened in sand and gravel aquifers, and 18 are finished in bedrock aquifers. The wells were selected to represent areas of greatest groundwater use and to provide a geographical sampling from the 5,799-square-mile study area. Samples were analyzed for 6 physical properties and 216 constituents, including nutrients, major inorganic ions, trace elements, radionuclides, pesticides, volatile organic compounds, phenolic compounds, organic carbon, and 4 types of bacteria. Results indicate that groundwater used for drinking supply is generally of acceptable quality, although concentrations of some constituents or bacteria exceeded at least one drinking-water standard at several wells. The cations detected in the highest concentrations were calcium, magnesium, and sodium; anions detected in the highest concentrations were bicarbonate, chloride, and sulfate. The predominant nutrients were nitrate and ammonia, but no nutrients exceeded Maximum Contaminant Levels (MCLs). The trace elements barium, boron, lithium, and strontium were detected in every sample; the trace elements present in the highest concentrations were barium, boron, iron, lithium, manganese, and strontium. Fifteen pesticides, including seven pesticide degradates, were detected in water from 17 of the 35 wells, but none of the concentrations exceeded State or Federal MCLs. Sixteen volatile organic compounds were detected in water from 15 of the 35 wells. Nine analytes and three types of bacteria were detected in concentrations that exceeded Federal and State drinking-water standards, which typically are identical. One sample had a water color that exceeded the U.S. Environmental Protection Agency (USEPA) Secondary Maximum Contaminant Level (SMCL) and the New York State MCL of 10 color units. Sulfate concentrations exceeded the USEPA SMCL and the New York State MCL of 250 milligrams per liter (mg/L) in two samples, and chloride concentrations exceeded the USEPA SMCL and the New York State MCL of 250 mg/L in two samples. Sodium concentrations exceeded the USEPA Drinking Water Health Advisory of 60 mg/L in eight samples. Iron concentrations exceeded the USEPA SMCL and the New York State MCL of 300 micrograms per liter (ug/L) in 10 filtered samples. Manganese exceeded the USEPA SMCL of 50 ug/L in 10 filtered samples and the New York State MCL of 300 ug/L in 1 filtered sample. Barium exceeded the MCL of 2,000 ug/L in one sample, and aluminum exceeded the SMCL of 50 ug/L in three samples. Radon-222 exceeded the proposed USEPA MCL of 300 picocuries per liter in 12 samples. One sample from a private residential well had a trichloroethene concentration of 50.8 ug/L, which exceeded the MCL of 5 ug/L. Any detection of coliform bacteria indicates a potential violation of New York State health regulations; total coliform bacteria were detected in 19 samples, and fecal coliform bacteria were detected in one sample. The plate counts for heterotrophic bacteria exceeded the MCL (500 colony-forming units per milliliter) in three samples.