Testing The Noble Gas Paleothermometer With A Year-long Study Of Groundwater Noble Gases In An Instrumented Monitoring Well

We report the results of a year-long noble gas study conducted in 2008 and early 2009, together with a wealth of continuous physical and chemical measurements collected in an instrumented monitoring well in the unconfined Glacial Drift aquifer in southern Michigan. Physical and chemical parameters at or near the water table are correlated with noble gas concentrations, corresponding noble gas temperatures (NGTs) and precipitation events. This year-long study is the first noble gas field test that has employed natural recharge, in situ monitored conditions, with minimal disturbance of the unsaturated zone. This study is unprecedented and demonstrates that there can be significant changes in physical and chemical conditions near the water table, over the space of a year, that can have profound effects on noble gas concentrations and hence, NGTs. The year-long record of conditions in the monitoring well show broad seasonal variations in pH, salinity, water temperature and water table depth. Results show that although precipitation events are detected within hours at the water table, there are significant pressure differences that persist for days at the water table, which argues for a very long time constant for gas transport within the unsaturated zone. There is strong evidence for the depletion of oxygen near the water table, which affects the noble gas air saturated water (ASW) component. When strongly reducing conditions prevail, as indicated by ORP, there is evidence for significant noble gas degassing. A major recharge event during the passage of the remnants of Hurricane Ike in the late summer 2008 caused a significant shift in H and O isotope ratios and it injected a large quantity of excess air into the groundwater. Hurricane Ike also appears to have caused a long-term change in the soil gas composition in the unsaturated zone, likely leading to a much more oxygen rich environment. Although individual competing NGT models can account for noble gas concentrations over portions of the record, no single NGT model can consistently account for all features observed over the entire year-long study. It is likely that the NGT temperature proxy must be viewed as an average of significant recharge conditions over the span of several years.