Ambient Flow and Heterogeneity in Multi-Aquifer Wells

Multi-aquifers wells, those wells that are open to more than one aquifer, have the potential to allow large quantities of flow between aquifers. Observed rates and direction of intra-borehole flow are often complex, reflecting the heterogeneity of the aquifers and variation of farfield heads. Spinner flow logs collected from several multi-aquifer wells in southern and eastern Wisconsin indicate the importance of flows through these wells in groundwater flow systems. The Paleozoic geology of Wisconsin, composed of more-or-less flat-lying sandstones, dolomites, and shales, gives rise to layered aquifer-aquitard systems where multi-aquifer wells are relatively common. A comparison of the flows in three multi-aquifer wells that cross the Wisconsin’s Paleozoic units showed heterogeniety in aquifers commonly thought to be homogeneous. Variation of the intra-borehole flow in a well gives an indication of heterogeneity and farfield heads in the aquifers. In the first example, the system was relatively simple, consisting of an aquitard (Eau Claire shale) between an upper aquifer (Wonewoc sandstone) and a lower aquifer (Mt Simon sandstone). Heads in the upper aquifer are higher than those in the lower aquifer. In this well, flows gradually increased with depth in the upper aquifer, remained constant in the aquitard, and then gradually decreased with depth in the lower aquifer. The gradual changes indicate relatively homogenous upper and lower aquifers. In the second example, the system also consisted of an aquitard (Tunnel City Group) between an upper aquifer (Sinnipee dolomite and the St. Peter sandstone) and a lower aquifer (Elk Mound Ground). As in the first example, heads in the upper aquifer are greater than those in the lower sandstone aquifer. In contrast to the first example, there were abrupt changes in intra-borehole flow in the upper aquifer, sometimes of more than 180 liters/minute over an interval of less than a meter. Caliper and television logging showed these high conductivity zones were due to fracturing, some type of dissolution or, perhaps, washout of poorly cemented sandstone. The third example was a more complex hydrogeologic system consisting of a stacked sequence of five hydrogeologic units: aquifer (St Peter sandstone)-aquitard (Tunnel City Group)-aquifer (Wonewoc sandstone)-aquitard (Eau Claire shale)-aquifer (Mt Simon sandstone). In this well, the very high conductivity zones were also present as shown by the abrupt changes in borehole flow in the upper aquifer. In addition, there was divergent flow out of the middle aquifer upward into the upper aquifer and also downward into the lower aquifer. This result was subsequently corroborated by packer tests that showed the highest farfield heads to be in the middle aquifer with lower heads in the upper and lower aquifers. The spinner flow meter logs characterize the variability of these aquifer and aquitard systems in Wisconsin. This knowledge can then be applied to groundwater issues such as contaminant transport, well head protection, or regional water supply issues.