ARSENIC CONTAMINATION IN THE MOUNT RUSHMORE AREA AND PROPOSED REMOVAL TECHNIQUES

Naturally occurring arsenic has been detected in several wells in the area near Mount Rushmore, in the Black Hills of South Dakota. The geology of Mount Rushmore area includes Precambrian granitic and metamorphic rocks. The main aquifers are fractured schist, alluvium and colluvium. Arsenopyrite is a major contributor of arsenic to groundwater in the area. The U.S. Environmental Protection Agency’s maximum contaminant level (MCL) for arsenic is 10 parts per billion (ppb). Some individual wells in the area do not meet this criterion, and samples from the water supply at Mount Rushmore National Memorial have shown arsenic concentrations of about 13 ppb. Many treatment technologies have been tested for removal of arsenic from drinking water. Limestone-based methods are environmentally friendly and offer the advantage of low disposal costs of the waste product. Recent research funded by the U.S. Geological Survey has involved removal of arsenic from water with crushed Minnekahta Limestone. In this work, limestone removed 63% to 96% of arsenic from water. The limestone waste currently is being tested for thermal stability. If the method passes the test, the limestone waste could be used as a raw material for the manufacturing of cement. In related work, water was sampled from a city well in Keystone, two miles from Mount Rushmore. The results showed an average of 91.6% arsenic removal, and the final arsenic concentration was 0.006 mg/L, lower than the MCL. Further research is underway to encapsulate the limestone waste in concrete as aggregate, thereby minimizing the disposal cost.