HYDROGEOCHEMISTRY OF A HETEROGENEOUS AQUIFER LOCATED IN UNAWEEP CANYON, COLORADO

A confluence of population pressure, climate change, pollution, and resource scarcity create high demand for water resources in arid regions. Groundwater in arid environments is particularly susceptible to overuse, and therefore a thorough understanding of groundwater sources and its contribution to sensitive ecosystems is vital. Unaweep Canyon, in the arid region of Colorado’s western slope, is a geologically unique site that harbors a buried paleovalley. The modern valley contains up to ~500 m of sediment fill comprising unconsolidated lacustrine, fluvial, and mass-wasting deposits. We hypothesize that the unconsolidated layers in Unaweep Canyon aquifer are not hydro-geochemically linked and represent different, unique sources of groundwater. Our study focused on integrating published geologic information, geophysical data, and seasonal geochemical properties of groundwater and surface water to identify another sustainable source of potable water for both humans and wetlands. Several streams, seeps, springs, mine tunnels, and several domestic wells were sampled for metals, anions, and stable isotopes (O¹⁸ and D) during spring, summer, and fall 2020 to identify seasonal effects. Geochemical characteristics, for example, the average mass ratio of Ca:Mg for the streams, groundwater, seeps, spring, and mine tunnel samples which are 6.3, 7.4, 8.1, 19.3 and 460 respectively, suggest three distinct water types. Piper plots also revealed that the mine tunnel may be a sodium chloride or mixed type water, the spring is a sodium chloride type water, and the surface and groundwaters are magnesium-bicarbonate type waters. Furthermore, deuterium (D) ranges from -99.3 to 111.3^o/_oo and ¹⁸O from -12.89 to -14.59 ^o/_oo in streams and groundwater with streams having more enriched isotopic values when compared to groundwater. Seasonal in-situ water chemistry, metal and stable isotope results showed very little variation, likely attributable to the drought conditions of 2020. Our results suggest the presence of multiple sources of groundwater within Unaweep Canyon, leading for the potential to utilize each source in a sustainable fashion. The findings and methodology used in our study may have applicability in similar hydrogeologic settings where alternate water sources are needed to offset population growth.