Snowpack Variability and Dissolved Oxygen Dynamics in the Soils and Underlying Sediments of an Alpine Hillslope: Insights from a Continuous Time-series Spanning Four Water-years

How does snow drought affect subsurface oxygen dynamics in an Alpine watershed? Dissolved oxygen is a master variable of biogeochemical function in ecosystems. Oxygen controls redox state, affecting mobility/cycling of chemical species in soils and groundwater. Over four consecutive years spanning normal to snow-drought conditions, we report on oxygen dynamics in the root zone and soil/shale interface. In-situ DO sensors directly in contact with the soil/sediment were deployed in a hillslope to obtain high frequency (5min), continuous data. As the sensor system can survive winter, we obtained real-time "movies" of DO, focusing on the period around snow-melt to investigate the role of temporal variations in groundwater and prior winter snowpack on oxygen dynamics at the soil/bedrock boundary.

Our study was conducted in lower montane portion of the East River watershed (Colorado, USA) during WY19, WY20, WY21 and WY22. We monitored DO in root zone (20cm depth) and soil/shale interface (100cm depth) near the midpoint of the hillslope (~22m above the flood plain). The root zone remained oxic year round, with diurnal variation in DO during non-winter conditions. For WY19 (normal snowpack), WY20 (snow deficit), and WY22 (normal snowpack), the soil/shale interface is oxic in winter, drops to anoxia for a month, during incursion of anoxic groundwater at snowmelt, then rapidly (~18hours) returns to root zone DO in late spring. This phenomenon was not observed in WY21 (2nd consecutive snow drought). The temporal DO profile is correlated with intrusion followed by recession of ground water at the sensor location. Additionally, at the toe-slope, specific conductivity indicates incursion of surface water at a depth of 2 meters. These results suggest that antecedent snowpack controls the depth of anoxia in soils as well as spatial redox gradients over the entire hillslope. Mn, Fe and nitrate profiles are in progress to elucidate role of metal chemistry in the upper 100cm of the soil. Returning to the root zone, preliminary analysis suggests that diurnal DO signal is primarily influenced by solar radiation and air (not soil), temperature; indicating that the top 20cm of the soil is in good contact with the atmosphere. We anticipate that these measurements will shed light on the role of snow pack cover for alpine watershed management.