WATER QUALITY CHANGES AT A MANAGED GROUNDWATER RECHARGE SITE ALONG THE SOUTH PLATTE RIVER, NORTHEASTERN COLORADO

Managed groundwater recharge is a common practice along alluvial river basins in Colorado. These systems pump water from alluvial aquifers during periods of low water demand from the river, discharge the water into upland ponds and increase groundwater discharge to the river during high-demand summer months. Thousands of groundwater wells in the South Platte River basin rely on managed groundwater recharge to enable junior groundwater appropriators to withdraw water from alluvial aquifers during high demand periods without harming senior surface water rights holders.

The Tamarack Ranch State Wildlife Area (TRSWA) Recharge Project is part of an overall groundwater retiming effort designed to provide extra water to the South Platte River during times of shortage to meet flow-related goals of the Platte River Recovery Implementation Program. A team of interdisciplinary researchers from Colorado State University have on-going investigations at TRSWA for over 7 years. The recharge project consists of 7 extraction wells in the alluvial aquifer ~1 km from the river. The pumped water is discharged into infiltration ponds located in an eolian deposit ~2 km from the river. As part of this research we have been quantifying the effects on water quality due to the mixing of river water with groundwater using spatial and temporal variations in water chemistry, using end-member mixing analyses, conducting a long-term applied tracer test, and re-calibration of the numerical model created for this site. Water quality monitoring has shown that the groundwater chemistry in wells located over one kilometer from the recharge pond (and perpendicular to the river) is gradually approaching that of surface water. The applied tracer (SF6) added over a 5 day period during recharge was detected in wells over one kilometer away from the recharge pond in three weeks.

The managed groundwater recharge project at TRSWA is an excellent field laboratory for a long-term study of an operating augmentation project. The multi-disciplinary approach created different data sets that interpreted together improve our conceptual and numerical modeling of long-term groundwater - surface water interactions at a managed groundwater recharge site.