Two water-quality studies were done on the outskirts of the Detroit metropolitan area to determine how recent residential development has affected ground-water quality. Pairs of monitor and domestic wells were sampled in areas where residential land use overlies glacial outwash deposits. Young, shallow waters had significantly higher median concentrations of nitrate, chloride, and dissolved solids than older, deeper waters. Analysis of chloride/bromide ratios indicates that elevated salinities are due to human activities rather than natural factors, such as upward migration of brine. Trace concentrations of volatile organic compounds were detected in samples from 97 percent of the monitor wells. Pesticides were detected infrequently even though they are routinely applied to lawns and roadways in the study area. The greatest influence on ground-water quality appears to be from septic-system effluent (domestic sewage, household solvents, water-softener backwash) and infiltration of stormwater runoff from paved surfaces (road salt, fuel residue). No health-related drinking-water standards were exceeded in samples from domestic wells. However, the effects of human activities are apparent in 76 percent of young waters, and at depths far below 25 feet, which is the current minimum well-depth requirement.Two water-quality studies were done on the outskirts of the Detroit metropolitan area to determine how recent residential development has affected ground-water quality. Pairs of monitor and domestic wells were sampled in areas where residential land use overlies glacial outwash deposits. Young, shallow waters had significantly higher median concentrations of nitrate, chloride, and dissolved solids than older, deeper waters. Analysis of chloride/bromide ratios indicates that elevated salinities are due to human activities rather than natural factors, such as upward migration of brine. Trace concentrations of volatile organic compounds were detected in samples from 97 percent of the monitor wells. Pesticides were detected infrequently even though they are routinely applied to lawns and roadways in the study area. The greatest influence on ground-water quality appears to be from septic-system effluent (domestic sewage, household solvents, water-softener backwash) and infiltration of stormwater runoff from paved surfaces (road salt, fuel residue). No health-related drinking-water standards were exceeded in samples from domestic wells. However, the effects of human activities are apparent in 76 percent of young waters, and at depths far below 25 feet, which is the current minimum well-depth requirement.