Unusually High Helium Fluxes in the Shallow Marshall Aquifer in Southern Michigan: Implications for Cross-formational Flow and Salinity Sources

A study of groundwater flow and salinity sources using combined dissolved helium data and major ion chemistry was carried out in the shallow Marshall aquifer in southern Michigan. He concentrations and isotopic ratios were measured for 14C-dated groundwater in conjunction with a noble gas temperature paleoclimate study of S. Michigan spanning the past 17 kyr. He excesses and isotopic ratios suggest the presence of tritiogenic 3He in young waters in the Marshall aquifer. He excesses in old groundwater samples are mostly of crustal origin although the presence of a significant mantle He component in some samples cannot be ruled out. He excesses in the Marshall aquifer are unusually high for such shallow depths (<=300m), and reach over 2-3 orders of magnitude above those of air-saturated water (ASW) for 3He and 4He, respectively. He isotopes require a source external to the aquifer, partly supplied by underlying formations within the sedimentary sequence, partly from the crystalline basement. Calibration of He concentrations observed in the Marshall aquifer using an analytical model require He fluxes of 1 × 10-13 and 1.6 × 10-6 cm3 STP cm-2yr-1 for 3He and 4He, respectively. These He fluxes are far greater than those reported in other multi-layered sedimentary basins around the world (e.g., Paris Basin, Gulf Coast Basin) at similar and far greater depths. Such high He fluxes present at such shallow depths within the Michigan Basin strongly suggest the presence of a dominant vertical water flow component, and further indicate that the impact of recharge water at depth is minor. Upward cross-formational flow is also likely responsible for the extremely high salinities (>200 gL-1) present in the shallow sub- surface (<=300m) of the Michigan Basin. The observed positive correlation between helium and bromide strongly suggests that these two very distinct conservative tracers originate both at greater depths, and further suggests that advection is the dominant transport mechanism within the basin. The occurrence of large-scale cross-formational flow is also consistent with the evolution of major ion chemistry throughout most of the sedimentary sequence, analyzed in conjunction with major element data sets from deeper and shallower water levels previously collected in this area, indicating that solutes from shallow levels carry the signature of deep formation brines.