Abstract:
The Bonne Femme watershed, located in central Missouri, is a karst watershed in a rapidly urbanizing area. This study was undertaken to characterize the hydrology of two karst aquifers within this watershed before significant increases in impervious surfaces take place. The specific objectives of this study were to: 1) use dye tracing to delineate the recharge area for Hunters Cave (HC); 2) quantify and summarize annual and monthly stream discharge at the resurgence of HC and Devils Icebox (DI) caves; and 3) characterize the chemical and physical status of the cave streams relative to temperature, pH, specific conductance, dissolved oxygen, and turbidity. The quantity and quality of the water at the resurgence of both cave streams was monitored from April 1999 to March 2002. Both recharge areas were determined to be of similar size (33.3 km² for HC and 34.0 km² for DI) and were formed in the same geologic strata. Average annual discharge was 55,900 m³ km-² at DI and 35,200 m³ km-² at HC. Relative discharge, as a percent of annual precipitation, averaged 6.1% at DI and 3.8% at HC. Average monthly discharge was 2,930 m³ km-² at HC and 4,650 m³ km-² at DI; however, median instantaneous discharge over the three years was about 18% higher at HC (74 m³ h-¹) compared to DI (63 m³ h-¹). Turbidity and pH showed the largest differences between sites over the three years. The higher turbidity and lower pH at DI reflected the greater magnitude and duration of runoff events for this system. The physical characteristics of the two recharge areas explained the observed differences in discharge. The HC recharge area is characterized by limited sub-surface conduit development, small conduits, short flow paths from surface to resurgence, and predominantly allogenic recharge. The DI recharge area is characterized by extensive sub-surface conduit development, large conduits, long flow paths to the resurgence, and autogenic and allogenic recharge.