The quality of shallow alluvial ground water that is used for domestic supplies in the Wapsipinicon, Cedar, Iowa, and Skunk River Basins (Eastern Iowa Basins) is described. Water samples from 32 domestic-supply wells were collected from June through July 1998. This study of ground-water quality in alluvial aquifers in the Eastern Iowa Basins is part of the U.S. Geological Survey's National Water-Quality Assessment Program.

Calcium and bicarbonate were the dominant ions in solution, likely derived from the dissolution of carbonate minerals in the alluvial aquifer material. Concentrations of iron exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Level (300 micrograms per liter) for drinking water in 53 percent of the samples, and 50 percent of the samples exceeded the Secondary Maximum Contaminant Level for manganese (50 micrograms per liter). pH and alkalinity increased and sulfate concentrations decreased with increasing well depth.

Nitrite plus nitrate nitrogen was detected in 53 percent of the samples and exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 milligrams per liter for drinking water in 13 percent of the samples. Nitrite plus nitrate nitrogen concentrations were negatively correlated with well depth and positively correlated with percentage of oxygen saturation. Ammonia plus organic nitrogen concentrations were positively correlated with well depth, and ratios of nitrite plus nitrate to ammonia were positively correlated with percentage of oxygen saturation.

The majority of samples, 72 percent, contained water recharged since the early 1950's. The recharge date of water was earlier in deeper wells. Nitrite plus nitrate and total pesticide concentrations were greater in more recently recharged water.

Eight pesticides and eight pesticide metabolites were detected in ground-water samples. Atrazine was the most commonly detected pesticide, and metolachlor ethanesulfonic acid was the most commonly detected metabolite. No pesticide detections exceeded U.S. Environmental Protection Agency drinking-water Maximum Contaminant Levels.

The effects of land use on ground-water quality also were examined. There was a positive correlation between percentage of land used for soybean production and concentrations of metolachlor, metolachlor ethanesulfonic acid, and metolachlor oxanilic acid in ground-water samples.
Data from this study and from previous studies in the Eastern Iowa Basins were compared statistically by well type (domestic, municipal, and monitoring wells). Well depths were significantly greater in domestic and municipal wells than in monitoring wells. pH, calcium, sulfate, chloride, and atrazine concentrations were significantly higher in municipal-well samples than in domestic-well samples. pH and sulfate concentrations were significantly higher in municipal-well samples than in monitoring-well samples. Ammonia was significantly higher in domestic-well samples than in monitoring-well samples, chloride was significantly higher in monitoring-well samples than in domestic-well samples, and fluoride was significantly higher in domestic-well samples than in municipal-well samples.