Water samples from representative shallow wells in the vicinity of Lincoln, Montana, were collected during March and June 1979 for analysis to evaluate changes in quality since 1974-75 and possible contamination by domestic or agricultural waste. All samples were suitable for drinking water with respect to the constituents tested. Tests for fecal coliform bacteria were negative. The concentrations of dissolved solids, composed mainly of calcium, magnesium, bicarbonate, and sulfate, were less than 300 milligrams per liter. Concentrations of sodium, potassium, and chloride were 3.5 milligrams per liter or less.
From March to June, the maximum concentration of nitrite plus nitrate as nitrogen decreased from 0.57 to 0.29 milligram per liter and the mean concentrations decreased from 0.16 to 0.11 milligram per liter, probably as a result of dilution of ground water by snowmelt. The concentrations in samples from the same wells generally were less during 1979 than during 1974-75. Isolation of sewage effluent in shallow zones by hardpan, consumption of nutrients by abundant phreatophytes, filtration of bacteria by the soil or aquifer, and dilution by underflow and recharge evidently combine to maintain the quality of the well waters within acceptable limits.
