Waiting on a mule deer crossing…
Climate and land-use change may alter the hydrologic regime of a watershed by changing the frequency, intensity, duration, and annual timing of water input to the landscape. These changes can affect water table dynamics, streamflow volumes, and hydrologic flowpaths through the landscape. Water serves to drive geochemical reactions, weathering minerals, and releasing solutes to terrestrial and aquatic ecosystems, and changes in hydrologic regime can impact not only water quantity but also the quality of water reaching streams or aquifers. Thus, there is a need to understand how hydrologic conditions relate to stream and groundwater chemistry to better anticipate possible water-quality changes under varying hydrologic conditions.\nWater chemistry, hydrologic, and solid-phase geochemical data were analyzed in two watersheds in Colorado, USA to better understand processes controlling hydrochemistry in two landscapes characterized by variable hydrologic conditions. The Lower Uncompahgre River Basin (LURB) is located on the arid Western Slope of Colorado and has been intensively irrigated for agricultural development over the last century. Application of irrigation water to the arid landscape has increased weathering of the underlying soils and bedrock and mobilized salt and selenium into the shallow groundwater system. Shallow groundwater in the LURB eventually discharges to downgradient surface water systems, resulting in considerable degradation of aquatic ecosystems in the LURB and in downstream rivers. Major ion and selenium concentrations were measured in aquifer sediments and groundwater from 30 wells randomly distributed across the LURB. Soluble salts appear to be the primary source of mobile Se in the aquifer sediments. Selenium concentrations were highly variable among wells and were generally controlled by redox processes driven by elevated nitrate concentrations derived from geologic sources. Mitigation of selenium in the LURB will be particularly problematic because irrigation water not only mobilizes selenium but also nitrate, which greatly increases selenium mobility in the low dissolved oxygen groundwater system.\nThe Gordon Gulch watershed in the Boulder Creek Critical Zone Observatory is a semi-arid, montane, forested watershed located on the Colorado Front Range. The hydrology of Gordon Gulch is sensitive to the accumulation and melt of snow deposited over the winter as well as periodic rain events during the summer and fall. Two studies were conducted in Gordon Gulch to better understand the hydrochemistry of the watershed. One study analyzed the major ion chemistry of stream, spring, and groundwater samples along with hydrologic and precipitation data to quantify solute fluxes and better understand streamflow dynamics. Solute fluxes from Gordon Gulch were low compared to other montane and alpine catchments, and alkalinity and calcium concentrations in the watershed were controlled largely by weathering of accessory calcite.\nEnd-member mixing analysis was used to investigate streamflow dynamics. No combination of measured end-members could explain observed variations in stream chemistry due to elevated chloride and sulfate concentrations in the stream over those measured in end-members, particuarly during fall storm events. Spikes in chloride and sulfate were attributed to flushing of evapoconcentrated soil moisture rich in chloride and sulfate accumulated from atmospheric deposition. An additional end-member was invoked by modelling evaporation of a solution with composition equivalent to measured bulk precipitation chemistry, and the modelled end-member successfully constrained the stream samples along with groundwater and dilute bulk precipitation. End-member mixing analysis revealed two chemically distinct baseflow periods: late summer baseflow and winter baseflow. Late-summer baseflow was composed primarily of groundwater, whereas winter baseflow appeared to have a large contribution
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