DEVELOPMENTS AND COMPLICATIONS FOR USING RADIOGENIC (SUP)4(/SUP)HE AS A TRACER FOR YOUNGER (<10 KYA) GROUNDWATER

Several recent studies have shown that 4He may be applicable as a residence time or model calibration tool for younger (<10 kya) groundwater. Previous research in the Mahomet Aquifer, a buried valley aquifer of glaciofluvial origin in Central Illinois, demonstrated that 4He is accumulating on the time scale of 100s of years. The sources of 4He for the aquifer are recharge through the overlying glacial till, in situ production, and advection/diffusion from the underlying bedrock. We are currently working on determining the mechanism for the loss of atmospheric gases in the aquifer, the impact of the gas loss on 4He accumulation, and constraining the rate of in situ production from aquifer sediments.

In much of the aquifer, the concentration of atmospheric gases are less than calculated atmospheric equilibrium. This loss of gases is thought to occur as groundwater passes by pockets of methane (known as drift gas) in glacial till overlying the aquifer. Carbon isotopic evidence indicates the drift gas is produced by degradation of organic matter in the till which then dissolves in groundwater. Analysis of drift gas samples indicate that it contains up to 60% nitrogen, supporting the hypothesis that the gas pockets form by subsequent degassing of groundwater. Theoretical calculations of the gas composition of groundwater in equilibrium with drift gas pockets are similar to measured gas composition of groundwater samples from the till and the Mahomet Aquifer. Assuming equilibrium, the gaseous composition of groundwater in the Mahomet can be used to estimate the 4He concentration of groundwater recharging the aquifer through the overlying till.

Laboratory experiments are underway to directly measure the release rate of 4He from aquifer sediments. Previous release rate experiments on discrete interval samples indicated a remarkably constant release rate. Current release rate experiments are being performed on composite samples from other parts of the aquifer.

Currently, no wells are known to be screened in the bedrock directly underlying the aquifer. However, if the 4He flux from the overlying till and in situ production are well quantified, then transport modeling of the aquifer can be used to constrain the 4He flux from the bedrock, and the sources and residence time of water in the aquifer.