Ground-water samples were collected during the summer of 1995 from 29 wells in the western part of the Cambrian-Ordovician aquifer in the Western Lake Michigan Drainages study unit of the National-Water Quality Assessment Program. Analyses of ground-water samples from these wells were used to provide an indication of waterquality conditions in this heavily used part of the aquifer.

Ground-water samples from domestic, institutional, and public-supply wells were analyzed for major ions, nutrients, dissolved organic carbon (DOC), pesticides, volatile organic compounds (VOCs), radon-222, and tritium, as well as field measurements of temperature, pH, specific conductance, dissolved oxygen, and bicarbonate. The results of water-quality analyses indicate that the presence of the Maquoketa-Sinnipee confining unit has an important effect on the ground-water quality in the study area. Where the study area is overlain by the confining unit (that is, where it is regionally confined) sampled water was older (based on tritium analyses) and often contained relatively high concentrations of dissolved solids, up to 2,800 mg/L. Additionally, contaminants such as nitrate and pesticides were typically detected at lower concentrations and detected less frequently in samples from the regionally confined part of the study area.

The dominant ions in samples from the study area were calcium, magnesium, and bicarbonate which resulted from the dissolution of carbonate minerals such as dolomite and calcite. Sulfate was also a dominant ion in water from some of the deeper wells in the regionally confined part of the study area.

Radon-222 was detected in all samples and 66 percent (19 of 29) had concentrations that exceed the U.S Environmental Protection Agency (USEPA) proposed maximum concentration level of 300 pCi/L. Concentrations greater than 300 pCi/L were detected in samples from wells throughout most of the study area except the southwest. The higher concentrations were found in samples from a variety of geohydrologic conditions and do not appear to correlate to a particular formation or location.

Dissolved nitrate and ammonium were the most commonly detected nutrients. Dissolved nitrate concentrations were significantly higher in ground-water samples from the regionally unconfined part of the study area. The highest concentrations were detected in samples from the agricultural southwestern part of the study area from relatively shallow wells that produced modern water. Dissolved ammonium concentrations were significantly higher in samples from the regionally confined part of the study area and probably resulted from nitrate reduction.

Seven pesticides or metabolites were detected in ground-water samples, and at least one pesticide was detected in samples from 24 percent (7 of 29) of wells. Most of the pesticides were detected at low concentrations and were from wells in the regionally unconfined, agricultural, southwest part of the study area. Atrazine was the most commonly detected pesticide and was typically detected in samples from wells that produced modern water.