Identification of groundwater sources for municipal wells using geochemical data on the Platte alluvial aquifer and underlying limestone at the Lincoln Water Well Field near Ashland Nebraska

Alluvial aquifer systems where pumping of municipal wells induces recharge from the adjacent river are the primary source of water for many cities. The city of Lincoln, NE has a primary water source in an alluvial aquifer adjacent to the Platte River. The Lincoln Water System manages the stream/aquifer system by using integrated models for a better understanding during periods of high stress such as a drought. The integrated models set the limestone aquifer as an impermeable boundary to understand the alluvial aquifer system without having a secondary water source. The limestone aquifer is permeable and the purpose of this study is to determine if water is being extracted from the limestone during a drought. Low river flows and groundwater exploitation during drought can cause water extraction from the underlying limestone aquifer due to high water demands. This study has focused on characterizing raw water entering the water treatment plant to determine if water from the underlying limestone is being pumped during dry periods. The cone of depression around a well would change with high stress levels in the aquifer, evidenced by decreased groundwater levels, can form what is known as up-coning in the limestone. The ion chemistry data used are from three different years, 2008 representing a wet year and 2012-2013 representing dry years. The ions analyzed are, manganese, bromide, fluoride and iron. Water chemistry data collected from treatment plants at the Lincoln Well Field near Ashland Nebraska have shown that especially during the summer manganese and bromide concentrations increased with increased stress caused by lower river and groundwater levels. A delay in groundwater movement was observed for all four ions studied, ion increases were shown one to two months after heavy pumping. During the winter months, when water is much colder manganese concentration increased significantly, this could be attributed to higher water viscosity affecting induced recharge and groundwater flow. Adviser: Darryll Pederso