EVALUATION OF GROUND-WATER CHEMISTRY AND AGE IN AQUIFER SYSTEMS IN LAGRANGE COUNTY, INDIANA

In 2002, a project was initiated to evaluate the chemistry and relative age of the ground water in LaGrange County, Indiana. The intent of the sampling was to acquire baseline data by analyzing for selected major ions, trace elements, and isotopes in ground water from wells that were not known to contain anthropogenic contaminants. Water samples were collected from 50 wells in the Topeka Fan glacial sequence, an outwash fan deposit of medium to coarse gravel, and the Oliver Lake glacial sequence, a lake deposit of fine-grained sediments. The Topeka Fan is an area of ground-water recharge; the Oliver Lake basin is an area of ground-water discharge. The wells that were sampled ranged in depth from 10 to 220 feet.

Nitrate concentrations in the water samples were generally low; 42 of the samples had concentrations below the detection limit (<1 mg/L). Only one of the water samples had a nitrate concentration above the U.S. Environmental Protection Agency Maximum Contaminant Level. Seven of the eight water samples containing nitrates were from wells in the Topeka Fan glacial sequence. Nitrogen isotope values determined for four sites with a minimum of 4 mg/L nitrate nitrogen ranged from 7.60 to 19.3⁰/₀₀ and indicated that the nitrate sources are largely animal waste. Tritium ranged from below the detection limit (<0.8 TU) to 17.0 TU; 35 of the samples had values above the detection limit. Tritium was generally not detected in waters from wells 80 feet or greater in depth.

In regard to potential contamination, the nitrate data suggest that the ground water in the Topeka Fan area is more prone to anthropogenic contamination; tritium data suggest that 60 to 70 percent of the ground water in wells sampled in the Topeka Fan and Oliver Lake areas has been recharged since 1952.