GROUND WATER UNDERGOES CHEMICAL CHANGES WITHIN 20 YEARS AFTER RECHARGE IN THE GLACIAL AND ALLUVIAL AQUIFER SYSTEM

Ground-water age, when based on measurement of the concentration of chlorofluorocarbons (CFCs) in water, refers to the time elapsed since recharge and isolation of the newly recharged water from the soil atmosphere. Many of the conditions and processes that affect CFC apparent age determinations are present in the glacial and alluvial aquifer system. Each sample is examined carefully and corrected to determine age reliability. Some general conclusions on the relation of water quality to apparent age can be determined for wells in the glacial and alluvial aquifer system underlying agricultural land use in the Upper Illinois River Basin (UIRB).

Apparent age of ground water increases 2 years for every foot of depth below the water table. The relation of apparent age to depth below the water table is evident from discrete water samples at four different depths at one site from the glacial and alluvial aquifer system in the UIRB. Yet, there is no relation between apparent age and depth below the water table for the 29 total samples from shallow wells (<50 feet deep). This result partially is the function of differences in the actual age of the water at various points in the UIRB resulting from differences in the distance from the wells to the recharge area and ground-water velocity. The relation between apparent age and geochemical conditions in the aquifer system is evidenced by dissolved oxygen and selected nutrient concentrations. Ground waters recharged since 1990 (recharge within the last 10 years) have dissolved oxygen concentrations above 0.3 mg/L, but ground waters older than 1990 have lower concentrations.

Pesticides or transformation products were detected at or above the respective detection limits, more than twice as frequently in ground water recharged prior to 1990 (82 percent of samples) than in ground water recharged after 1989 (33 percent). The highest concentrations of atrazine, alachlor, metolachlor, and their transformation products, were in samples from ground water recharged prior to 1990. A precise estimate of pesticide use over time would help in establishing the relation between pesticide occurrence and transport. The relation between geochemical conditions in the aquifer and water quality indicates, that within 20 years after recharge, the shallow ground water undergoes appreciable chemical changes.