THERMAL TRACING OF SUBSURFACE FLOW REGIMES IN A MANTLED KARST AQUIFER NW ARKANSAS

Thermal characteristics of epikarst, matrix, and conduit flow provide potentially useful information on the complex subsurface flow of a mantled karst aquifer. Thermal patterns are divided into two categories: flow with ineffective heat exchange and flow with effective heat exchange. Ineffective heat exchange occurs when groundwater flows through the aquifer before it can equilibrate with rock temperature. Thermally ineffective heat exchange is indicative of low storage and high transport rates, such as through conduits. Effective heat exchange occurs with increased residence time, enhancing equilibration between groundwater and rock temperature. Thermally effective heat exchange is indicative of high storage and low transport rates, such as flow through matrix or pore space. Project objectives are two-fold: a) determine thermal signature for each subsurface flow regime and b) categorize each flow regime as effective or ineffective heat exchange. From March to August 2011, a thermal investigation in NW Arkansas utilized temperature and pressure loggers, installed in 23 wells and six springs, to monitor groundwater temperature and discharge height. Temperature data produced distinct groundwater thermal patterns among epikarst water, shallow water, and deep water. Thermographs are used to statistically show distinct thermal patterns. Epikarst waters portray ineffective heat exchange (11-19°C temperature range). In contrast, deep aquifer locations produced effective heat exchange signals, portrayed by the constant 15.3 °C aquifer temperature. Overall, monitoring various stages along epikarst, matrix, and conduit flow paths coupled with monitoring spring response volume, temperature, and storm events produce unique thermal patterns for each specific subsurface flow regime.