RESERVOIR SEDIMENT AS A RECORD OF HISTORIC MINING ACTIVITY IN THE ANIMAS RIVER WATERSHED

The City of Aztec Drinking Water Reservoir #1 in Aztec, NM was built in 1947 by the city of Aztec for use as a pre-treatment reservoir for drinking water. The reservoir is located downstream of historic mining activity and fed by the Animas River directly and via diversion channels. The reservoir serves as a settling basin for pre-treatment of water from the Animas River. As such, it has been collecting sediments and minerals transported from upstream mineralized zones in Colorado for the past 70 years.

Four trenches were excavated in the drained Aztec Reservoir in May 2018. A handheld x-ray fluorescence (XRF) instrument was used to gain a quick understanding of the chemical composition of the sediment layers in the trenches immediately following excavation. Sediment samples were collected from each trench and bulk chemical analyses were performed to evaluate the chemical distribution and mobility within and between each excavation. Samples from all four trenches show elevated concentrations of uranium (U), vanadium (V), and arsenic (As) within the bottom 10 cm of sediment. Uranium and V are found in the mineral carnotite, which is ubiquitous in the Colorado Plateau, where the Animas River has its headwaters, and is especially concentrated in the Morrison Formation along the Uravan Mineral Belt. Arsenic is known to be associated with these deposits. Sediment sample copper (Cu) concentrations decrease linearly from a high of about 1500 mg/kg near the bottom (or oldest) part of the trench and stabilize at 40-70 mg/kg at approximately 60 cm below the surface. The reason for this marked change is still under investigation.

The results from the chemical analysis of the sediment cores can aid in understanding trace element deposition and mobility. This information will ultimately inform land managers and water system operators about long-term trends in sediment chemistry in drinking water reservoirs and the potential effects of past or present upstream mining activities.