HYDROGEOCHEMISTRY OF A CLAYEY TILL .1. SPATIAL VARIABILITY

The spatial variability of hydrogeochemical processes was studied within an 18-m-thick unit of clayey till beneath a flat prairie landscape. Models of major element mass transfer, constrained by chemical, mineralogical, and isotopic measurements on waters and solids, were developed for two end-member water quality types occurring within the unit. The observed pattern of chemical variation, which is predominantly lateral on a scale of tens of meters, is caused by microtopographically controlled differences in the ratio of vertical water flux to oxidation rate associated with depression-focused recharge. Such variation is to be expected where dynamic flow regimes develop in thin surficial tills containing chemically reduced constituents. Observed depletions of major elements from oxidized zones of the till support the mass transfer models and demonstrate that the time scales of persistence of hydrogeochemical variability in the till and of water quality evolution in the aquifer beneath range from 10(4) to 10(5) year. Na and S concentrations in such aquifers will depend primarily on ratios of influxes of end-member water quality types from the till above, while Ca, Mg, and inorganic C concentrations will remain high as long as partial pressures of CO2 in the till remain high. CO2 generation in the till is discussed in the companion paper (Keller, this issue).