DETERMINATION OF GROUND AND SURFACE WATER INTERACTION IN EPHEMERAL WETLANDS THROUGH CHEMICAL AND ISOTOPIC CHARACTERIZATION, SAN LUIS VALLEY, COLORADO

Mishak Lakes is an ephemeral wetlands complex located in the northern San Luis Valley of south-central Colorado. It consists of a series of shallow (<1 meter deep) basins connected by natural channels. Groundwater occurs between 0.7 to 2 meters below the bottom of the basins. Historically, the wetlands received water primarily from Russell Creek, which is fed by Russell Springs (located 15 kilometers west of Mishak Lakes) and by snowmelt from the San Juan Mountains. Several artesian wells have been completed in the vicinity of Russell Springs, substantially increasing flow in Russell Creek. A large amount of that water is diverted between May and November for irrigation, causing natural streamflow conditions to be mimicked. That is, the wetlands receive water in the spring as they would from snowmelt, then the water supply terminates and the basins become completely dry by the end of summer. Infiltration appears to be limited by a clay layer that exists near the soil surface in this part of the valley. Stratigraphic control on the hydraulic connection between ground and surface water was evaluated by analyzing water samples for major cation/ anion chemistry, and stable isotope enrichment (18O and 2H). Samples from both features were collected simultaneously at specific locations to ensure spatial and temporal consistency. The arid rainfall average and abundant artesian wells in the San Luis Valley create a unique opportunity from a research perspective to use stable isotopes. Samples are collected directly from the artesian wells, giving accurate initial isotopic ratios as opposed to an average ratio of precipitation. Thus, it is possible to measure total isotopic enrichment between discharge from the well pipe and complete wetland desiccation. Chemical and isotopic analyses were performed on groundwater from existing monitoring wells at the wetland boundaries. This allowed for identification and evaluation of mixing between infiltrating surface water and regional unconfined groundwater. Preliminary data indicates that, although some groundwater mounding occurs while the basins are flooded, infiltration to the regional unconfined aquifer is limited, and evaporation is the primary surface water discharge mechanism from Mishak Lakes.