VERTICAL DISTRIBUTION δ18O IN A CLAY-RICH AQUITARD CONSISTING OF PROGLACIAL LAKE SEDIMENT IN EAST-CENTRAL WISCONSIN: IMPLICATIONS FOR GROUNDWATER RECHARGE TO BEDROCK AQUIFERS

Thick sequences of fine-grained sediment deposited in large proglacial lakes in front of receding lobes of ice sheets can control recharge to bedrock aquifers. One example is Glacial Lake Oshkosh in east-central Wisconsin where silt and clay covers a 5000 km² area and limits infiltration of surface water to recharge important bedrock aquifers.

To constrain groundwater residence times, seven boreholes were drilled and pore water extracted from core samples to determine the vertical distribution of δ¹⁸O. In boreholes where the aquitard was less than 50 m thick, variation in the δ¹⁸O values was limited to -6 to -10 ^o/_oo, indicating that sufficient time had elapsed for modern-day pore water to move through the aquitard. However, in boreholes where the aquitard was at least 80 m thick, a bow-shaped curve δ¹⁸O profile was observed. Modern δ¹⁸O values (-9 ^o/_oo) near the surface gradually decreased with depth (-16 ^o/_oo to -18 ^o/_oo) before increasing toward the bedrock surface (-11 ^o/_oo to -12 ^o/_oo). These bow-shaped curves are typical of diffusion-dominated systems with limited advection driven by differences in hydraulic head. We interpret the lighter δ¹⁸O values in the middle of the aquitard to be an indication of relatively old pore water of glacial age. The heavier δ¹⁸O values near the top of the aquitard are probably the result of local modern-day precipitation. However, the heavier δ¹⁸O values near the bottom of the aquitard are probably the result of modern-day recharge to the bedrock aquifer that is occurring beyond the extent of the aquitard.

One-dimensional advection dispersion models were used to fit the δ¹⁸O bow-shaped profiles using the finite difference Crank-Nicolson approximation. The modeling results indicate that diffusion dominates with some discernable contribution from advection suggesting that where fine-grained glacial sediment is thick, recharge to bedrock aquifers is limited.