DISTRIBUTION AND ORIGIN OF 4HE TO DATE GROUNDWATER IN THE RECHARGE PORTION OF THE MAHOMET AQUIFER

The goal of this project is to further the development of ⁴He as a dating technique for young groundwater. Currently, there is no accepted technique for dating groundwater with a 50-400 year range and the ⁴He method may be applicable in this range. Originally, radiogenic ⁴He was thought to be limited to older groundwater (10³ to 10⁸ years) because the groundwater would need to be in contact with rocks releasing helium for thousands of years to get a sufficient concentration to analyze. Recent studies have shown that certain aquifers may be capable of releasing ⁴He at rates high enough to date groundwater as young as 10 years old. These studies show that recently eroded sediments have a much higher rate of ⁴He release. This preliminary survey evaluates spatial patterns of ⁴He concentrations in eighteen selected wells within the recharge area of the Mahomet Aquifer. This buried valley aquifer, underlying a part of central Illinois, is overlain by fine-grained till containing lenses of sand and gravel. The fact that this region is dominated with recently eroded sediments makes this aquifer an intriguing area to analyze ⁴He concentrations. In theory, the concentration of radiogenic ⁴He divided by the release rate from the aquifer materials will determine the age of the groundwater. For this study, total ⁴He concentrations were measured and compared to groundwater ages from other established dating techniques (tritium, CFC, and ¹⁴C). Using these other dating techniques will assist in calculating the ⁴He release rates from the sediments. Preliminary results indicate large horizontal and vertical variations in ⁴He resulting from the heterogeneity of the glacial deposits. These results also show an overall increase in ⁴He concentrations with depth. The ⁴He dating technique shows promise, but more data is required.