RELATIONSHIP BETWEEN DEPTH AND HYDRAULIC CONDUCTIVITY WITHIN THE ST. FRANCOIS AQUIFER, MISSOURI

The St. Francois aquifer is a prolific groundwater source near its outcrop area in southeastern Missouri, but is rarely tapped in the southwest part of the state due to the presence of the shallower Ozark aquifer. Nevertheless, the St. Francois aquifer may become an important supplementary source of groundwater in areas experiencing severe drawdowns within the Ozark aquifer. This aquifer is also a target for CO2 injection in northern Missouri where the pore water is saline.

The main water-bearing unit within the St. Francois aquifer is the Lamotte Sandstone, the basal Cambrian sandstone above Precambrian basement. An overlying sandstone (Reagan), separated from the Lamotte by low-permeability siltstones and shales, is also present in portions of southwest Missouri. We measured the transmissivity and hydraulic conductivity of these sandstones at three sites with single-well pumping tests as part of the Missouri Carbon Sequestration Program. We also analyzed time-drawdown and specific-capacity measurements from 13 additional tests on file at the Missouri Division of Geology and Land Survey.

Transmissivity and hydraulic conductivity (K) vary inversely with depth. At depths less than ~ 370 m, K varies between 10^-3 and 10^-2 cm/sec; these wells generally yield > 600 gpm. Below this depth K decreases due to pervasive cementation. By 550 m, K decreases to ~10^-4 cm/sec, and this depth seems to mark the lower practical limit of these sandstones as an aquifer. By 650 m, K decreases to around 10^-5cm/sec and the sandstones become marginal for CO₂ injection, except in locations where the Lamotte fills in topographic lows on the Precambrian surface and is unusually thick.

This material is based on work sponsored by the Department of Energy National Energy National Energy Technology Laboratory under Award Number DE-NT0006642 to City Utilities of Springfield, MO