HYDROGEOLOGIC INVESTIGATION INTO THE EFFECTS OF MANAGED RECHARGE ON WATER QUALITY, LOWER SOUTH PLATTE RIVER, CO

In order to meet in-stream flow requirements at the Colorado-Nebraska border, managed recharge along the lower South Platte River Basin is being used as a method for flow augmentation. The site is located on the Colorado Division of Wildlife's Tamarack Ranch State Wildlife Area in northeastern Colorado. During late winter/early spring, when there is no call for water from the South Platte, approximately 20 acre-feet per day of water is pumped from the alluvial aquifer near the river into a recharge pond approximately 1 km away. The goal is to have water return to the river during low flow periods prior to snow melt runoff in order to augment in-stream flows for endangered species in Nebraska. In this talk, we will present current findings on the areal and temporal distribution of water quality parameters (including nitrate, sulfate, alkalinity, DO, and specific conductance) within the alluvial aquifer between the recharge pond and the South Platte River. Data collected to date suggest 1) a zone of higher nitrate levels along the alluvial aquifer nearer the river; 2) the sulfate concentrations of the water pumped into the recharge pond is similar to that of the alluvial aquifer than to the river water; and 3) there appears to be a smaller contribution from the river during pumping than initially expected. In addition, we will present the preliminary results of a tracer test performed to address recharge pathways and timing. The understanding of the pathways and flow rates is important in determining the influence of the recharge water on groundwater and surface water quality, especially in light of the increasing use of flow augmentation along the length of the lower South Platte River.