The U.S. Army Corps of Engineers operates the Red Rock Dam on the Des Moines River in Marion County, Iowa. The dam consists of a gravity concrete control structure between two earthen embankments and has an impoundment storage capacity of 1,700,000 acre-feet. Since the impoundment of Lake Red Rock commenced during 1969, water seepage beneath the dam has been significant enough to cause continuing investigation by the Corps of Engineers of its source and implications.

The St. Louis Limestone, which consists of interbedded sandstones and carbonates with solution collapse features resulting from partial removal of a basal evaporite zone, forms the bedrock foundation of the dam in the river valley. The soluble gypsum and anhydrite in the evaporite zone have the potential to be removed in greater quantity with increasing seepage velocities and volumes. Solution channels may develop as material is removed from the bedrock foundation, which could result in the collapse of overlying strata, thereby threatening the integrity of the earthen dam.

The potentiometric surface in the overburden on the southwest side of the dam has an extremely steep hydraulic gradient from the reservoir through the dam to the downstream observation wells, which implies expected small permeability and minimal seepage through the dam and embankment materials. A lesser hydraulic gradient exists on the northeast side of the dam, which could indicate excessive seepage through embankment material from larger than expected hydraulic conductivity or underseepage through bedrock. Statistical analysis of water-level changes in the reservoir and in observation wells completed in the evaporite stratigraphic horizon on the northeast side of the dam indicates a hydraulic connection between the reservoir and the wells.

Direct evidence of the existence of a connection between the reservoir and the ground-water system is provided by chloride concentration data. Maximum chloride concentrations occurred in the reservoir water in the early spring of 1989. Chloride concentrations reached a maximum in ground water from bedrock and overburden observation wells on the northeast side of the dam 1 to 4 months after their maximum in the reservoir. Underseepage of reservoir water occurs through the basal evaporite zone of the St. Louis Limestone and through the glacial sands in the northeast bluff between the bedrock surface and the base of the dam fill.

The increased hydraulic head imposed on the system by the impounded waters of Lake Red Rock causes recharge and flow to the deeper bedrock aquifers in the immediate vicinity of the dam. This effect is manifested in the observation wells along and downstream from the dam axis, implying flow through the grout curtain in the bedrock foundation of the dam. There is potential for dissolution of the gypsum and anhydrite in the bedrock foundation, because reservoir water and shallow ground water in the vicinity of the dam are undersaturated with respect to these evaporite minerals.