Water quality changes at a streamflow augmentation project, Lower South Platte River, Colorado

Abstract

Flow augmentation projects utilizing managed groundwater recharge serve as a management tool for the conjunctive use of groundwater and surface water. The projects emphasize providing adequate water quantity at the right place and at the right time. However, water quality must be addressed. Mixing of different qualities within such a system can affect water quality both in the river and in the alluvial aquifer. The Tamarack Ranch Groundwater Recharge Project (Project) operates adjacent to the South Platte River in northeastern Colorado. The Project re-times excess flows in the South Platte River using managed groundwater recharge. Surface water, groundwater, and extraction water samples from the site were analyzed for water quality parameters and ionic composition. Water chemistry from the different sample locations determined the spatial and temporal influence of managed recharge activities. Two primary and distinct source waters are present in the system – groundwater and river water. The groundwater is dominated by calcium and bicarbonate. The river water is dominated by sodium / calcium and sulfate. The extraction water is a mixture of these two sources. The application of a simple batch mixing technique determined that the extraction water was about 80% groundwater. This research found that a streamflow augmentation project using managed groundwater recharge does affect water quality. As the system continues to operate, alluvial aquifer water quality will be affected by the surface water quality. A space for time substitution shows how groundwater quality is changing due to the effects of additional river water entering the alluvial aquifer system.