AQUIFER RESPONSE TO SWALLET INJECTION, FORT KNOX, KENTUCKY

In the course of installing monitoring wells for a landfill at Fort Knox, Kentucky, the connection of the wells/boreholes to the karst aquifer system was tested through injection of water in a nearby sinkhole swallet. This poster presentation illustrates the nature of the tests, the resultant data, and conclusions drawn from them.

The site is located on an upland sinkhole plain in the Pennyroyal Plateau on the north side of Fort Knox. The site is underlain by units of the Mississippian-aged St. Louis Limestone in which karst conduits appear to be locally developed in a series of steps leading to a baseline system on top of the underlying Salem Limestone.

Potable water was drained to the swallet from a 1,000-gallon polyethylene tank and aquifer response was monitored in the wells/boreholes using data loggers equipped with pressure transducers and probes for temperature and electrical conductivity. Hydraulic response testing was also conducted in a neighboring borehole as one borehole was flushed and significant drill water was lost to the formation.

The tests were conducted between August 23 and September 23, 1999, as the drilling schedule allowed. As a consequence, the controlling parameters, such as volume and rate injection, and status of wells/boreholes, varied between tests. The temperature and electrical conductivity of the injected water were significantly different from those the ambient groundwater (approximately 20°F warmer and 10% of the groundwater conductivity), suggesting that the injectate flow might be effectively traced as well as the hydraulic response.

Hydraulic responses in the range of 0.04 to 0.57 feet were recorded in all monitored points and subtle but distinct responses in temperature (0.55°F) and conductivity (250 µS/cm) were recorded at limited locations.

The data confirm the hydraulic connection of the final monitoring wells to the karst aquifer system represented by the sinkhole swallet, and suggest possible flow paths. However, the test responses were distinctly variable, suggesting that the localized conduit system may consist of a complex of minor channels through which the exact path of groundwater flow is subject to variable factors, including at a minimum the rate of flow.