A LONG-TERM TRACER TEST AT A RECHARGE POND USING SULFUR HEXAFLUORIDE AND APPLIED DEUTERIUM

Flow augmentation is used frequently along the lower South Platte River in Colorado. In general, water is pumped from the alluvial aquifer near the river to recharge points off channel where return flow then augments stream discharge. Flow augmentation is often used to increase aquatic habitat during periods of low stream flow.

In March 2004, a tracer test using the dissolved gas sulfur hexafluoride (SF₆) and liquid deuterium (D) was used to track the return flow from a recharge pond to the South Platte River. The recharge pond is located in upland sand hills approximately 2 km from the river. A series of groundwater extraction wells are located 1 km from the main channel and are screened in the alluvial aquifer. The recharge pond is underlain by 1) an eolian sand layer up to 30m thick; 2) an approximately 30-m thick poorly sorted alluvial material; and 3) a shale bedrock at approximately 60m in depth.

For the tracer test, 1.9 L of 99.8% D was added as a spike at time zero and SF₆ was bubbled into the 0.8-ha recharge pond for 5 days, followed by 7 days of tracer-free recharge. During this time the average flow into the pond was 24,000 m³/day (20 acre-feet/day). Once recharge ceased. SF₆ was found in the first groundwater samples at 60 m beneath the pond taken less than 24 hours after injection started. Once recharge ceased, SF₆ concentrations beneath the recharge pond remained at approximately 20% of the tagged surface water for at least 4 months. Tracer was found near the extractions wells 1 km away in less than 3 weeks and in a well located 1 km parallel to the river in less than 4 weeks. The large concentration of tracer beneath the pond will be remobilized once recharge begins again in January 2005. The results from the tracer test will provide information on flow paths and return flow timing that will be used to improve numerical models for the site.