Paper No. 14
Presentation Time: 1:30 PM-5:30 PM
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.