INVESTIGATION OF WATER CHEMISTRY AFTER GROUNDWATER DRAWDOWN IN SOUTH-CENTRAL MISSOURI

The city of Springfield, Missouri depends on groundwater for 25% of its public water supply, while communities south of Springfield depend entirely on groundwater. Extensive groundwater pumping over the last 70 years has caused hundreds of feet of drawdown in some areas near Springfield. In this project, current and historical records of groundwater chemistry, as characterized by common ions, were investigated to determine if water chemistry had changed due to the increasing thickness of the unsaturated zone in the areas with high drawdown. Three time periods were considered: 1974, 1995, and 2016. For each time period, wells were classified as having significant drawdown (60 to 240 ft), moderate drawdown (10 to 60 ft), or water table rebound (-140 ft to 10 ft). Statistical analysis of these time periods and drawdown categories showed that there was little change in water chemistry as a function of drawdown, but variations did occur as a function of time across all drawdown categories. The most significant changes were a reduction of calcium and carbonate ions between 1974 and 1995 and an increase in chloride in 2016. The reduction in calcium carbonate may reflect the regional drop in the water table between 1974 and 1995, which could cause reduced dissolution of carbonate rock, even though specific connections to drawdown were not established. Pumping was decreased after 1995, and the water table rebounded in many areas. The increase in chloride in 2016 may be due to anthropogenic sources such as road salt, but the exact source is currently unknown.