Impact of Human Activities on Ground Water Quality in a Karst Formation

The City of Rapid City, South Dakota is located on the eastern flank of the Black Hills uplift, a 60 million year old elongated, domal structure with a core of metamorphic and igneous rocks surrounded by concentric rings of moderately dipping sedimentary rocks.  These dolomite, limestone and sandstone formations contain the principle groundwater aquifers that supply the drinking water sources for Rapids City and the surrounding area.

The City of Rapid City utilizes a combination of groundwater and surface water to meet the City’s water needs.  Water quality data of the City’s water supply collected from 1993 to 2007 clearly indicate an upward trend in nitrate concentrations, particularly in City wells that draw water from the Madison aquifer.  Research by the South Dakota School of Mines & Technology, South Dakota Department of Environment & Natural Resources, United States Geological Survey, and others strongly suggest that nitrate concentrations in the City’s water supply can be attributed to sources such as streams recharging the Madison aquifer at sinkhole locations, on-site wastewater systems, fertilizers from residential and/or agriculture applications, and explosives used for mining activities. Excessive concentrations of nitrates in drinking water have been linked to methemoglobinemia in infants (commonly known as blue-baby syndrome) and are a health concern for the elderly, pregnant women, and livestock.

A Source Water Protection Study developed and evaluated alternatives with respect to aquifer sensitivity, density of septic tanks per acre, and benefit vs. cost analysis.  Alternatives to solve the problem of increasing levels of nitrates from septic tanks in Rapid City’s municipal water supply includes proactive measures to maintain the current levels of nitrate below the EPA drinking water limit.