This evaluation of the water resources on the Prairie Island Indian Reservation includes data collected from 8 surface-water sites and 22 wells during 1994–97 and historical data. The Mississippi River and the lakes and wetlands connected to it are separated from the Vermillion River and the lakes and wetlands connected to it by the surficial aquifer on Prairie Island and by Lock and Dam Number 3. These surface-water groups form hydrologic boundaries of the surficial aquifer. The aquifer is 130–200 feet thick, extends to bedrock (the Franconia Formation, which is also an aquifer), and is composed primarily of sand and gravel, but also contains thin, isolated lenses of finer-grained material. Flow in the surficial aquifer is normally from the Mississippi River to the Vermillion River (southwest). During spring snowmelt or heavy rains, a ground-water mound forms in the center of the study area and causes radial ground-water flow toward the surrounding surface waters.

Surface- and ground-water quality was generally similar, but the median ground-water nitrate concentration was 3.6-times greater than that for surface water. Water samples were dominated by calcium, magnesium, and bicarbonate ions, were usually oxygenated, and had a median dissolved solids concentration of 250 milligrams per liter (mg/L). Thirty-nine percent of groundwater samples showed evidence of anthropogenic nitrate. Most samples contained low concentrations of ammonia (less than 0.04 milligrams per liter as nitrogen). All 15 surface-water samples contained coliform or fecal streptococci bacteria, with 33 percent exceeding 100 colonies per milliliter. Two ground-water and two surface-water samples analyzed for trace metals contained natural concentrations except for one ground-water sample that contained 30 mg/L of lead (probably from a bullet). No volatile organic compounds were detected in 3 ground-water and 3 surface-water samples. Triazine herbicides and their degradation products were detected in one-half of the ground-water samples at concentrations below 1 microgram per liter (µg/L) except for one sample at 3 µg/L. Wells with initially high concentrations of nitrate or triazines continued to have high concentrations throughout the study. Several polycyclic aromatic hydrocarbons and monoaromatic chemicals were detected at low concentration (less than 89 micrograms per kilogram) in 4 samples of 1993 Mississippi River flood sediments deposited in the study area.

Ground-water recharge dates based on chlorofluorocarbon (CFC) concentrations indicate that sampled ground water was young (less than 2 decades old) and that all tritium contained in samples from this study can be explained by atmospheric sources. Most historical tritium concentrations can also be explained by atmospheric sources through recharge from spatially and temporally constant precipitation and snowmelt. However, samples from three wells within 800 feet of the Prairie Island Nuclear Power Plant contained tritium at concentrations that cannot be explained by such atmospheric sources. These concentrations decline to that explainable by atmospheric sources within 800 feet of the wells. Many samples contained CFC-113 concentrations higher than that possible from equilibrium with the atmosphere. This CFC-113 contamination is presumably from Mississippi River recharge and complicated the recharge date estimates.

The only surface-water constituents exceeding U.S. Environmental Protection Agency drinking water standards was coliform or fecal streptococci bacteria, which was exceeded in all samples. Thirteen percent of ground-water samples exceeded the nitrate maximum contaminant level (MCL), but this is probably higher than the percentage of the aquifer exceeding the nitrate MCL because most of the wells sampled were shallow. Surface-water recharge to and ground-water discharge from the surficial aquifer influence the water quality in both the aquifer and the surrounding surface water. However, surface water probably influences ground-water quality more because of the greater amount of surface water flowing through the study area.