SUBAQUEOUS CARBON SEQUESTRATION IN ELEPHANT BUTTE RESERVOIR, NEW MEXICO

The American West is a semi-arid region where the hydrograph is controlled by spring snowmelt and the summer monsoon. To manage water resources, the region relies on many inland reservoirs. Despite covering less than 2% of the Earth’s surface, inland waters bury an estimated 0.15 Pg C/year, which is remarkable compared to oceans which bury 0.20 Pg C/year. Thus, reservoirs sequester 40% of the terrestrially-derived organic carbon (OC). In southern New Mexico, the Rio Grande is impounded by the Elephant Butte Dam, building the Elephant Butte Reservoir (EBR) for agricultural, recreational, and hydroelectric use. Summer monsoons (July–October) and spring snowmelts (April–June), bring sediment-laden, organic-rich water into the reservoir. These sediments build a reservoir delta and generate hyperpycnal plumes that drive sediment and OC deep into the lake. This research aims to quantify subaqueous carbon storage at EBR, providing insight into the importance of arid inland reservoirs as carbon sinks. In March 2024, we conducted a boat-based field campaign using sampling locations informed by the 2019 U.S. Bureau of Reclamation bathymetric survey at EBR and remote-sensing analyses to determine spatial inundation frequency. At each sample location, the team collected conductivity, temperature, and depth (CTD) data, water samples, and sediment samples. CTD data was used to determine the presence of hyperpycnal plumes. The sediment samples were analyzed for grain size distribution using a CILAS 1190 and organic matter content was determined by loss on ignition. The water samples were filtered to determine suspended sediment concentration. Preliminary results indicate that the median grain size of the EBR lake bed ranges from 3.34 μm to 123.37 μm and organic matter content (OM%) ranges from 1.56–9.95% with an average of 7.5%. During our study period, suspended sediment concentrations ranged from 4 to 12 mg/L. We find that OM% of the subaqueous reservoir sediment exceeds values reported for the subaerial EBR delta. Ultimately, this study will inform the magnitude and spatial distribution of carbon sequestration at EBR which holds important implications for carbon storage in dams across the Western United States.