Cross‐formational flow and salinity sources inferred from a combined study of helium concentrations, isotopic ratios, and major elements in the Marshall aquifer, southern Michigan

Abstract

Helium data and major ion chemistry are presented for the shallow Marshall aquifer in southern Michigan. This data set is subsequently analyzed in conjunction with major element data sets from deeper and shallower water levels previously collected in this area. 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 (≤300 m) and reach over two and three 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 requires He fluxes of 1 × 10−13 and 1.6 × 10−6 cm3 STP cm−2 yr−1 for 3He and 4He, respectively. These He fluxes are far greater than those reported in other 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 impact of recharge water at depth is minor. Upward cross‐formational flow is also likely responsible for the extremely high salinities present in the shallow subsurface of the Michigan Basin. The observed positive correlation between helium and bromide strongly suggests that these two very distinct conservative tracers both originate 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 displayed by the major ion chemistry throughout most of the sedimentary sequence, indicating that solutes from shallow levels carry the signature of deep formation brines.