DISSOLVED OXYGEN SYSTEMATICS AS AN INDICATOR OF WHOLE SYSTEM COUPLING OF SURFACE AND GROUNDWATER

Fluvial aquifers in mountain valleys can have extensive kilometer-scale lateral hyporheic exchange. We characterized subsurface-surface coupling for a 20 km2 aquifer at the ecosystem level, based on dissolved oxygen (DO) systematics and sediment respiration rates. DO and temperature were strongly correlated in the main river channel. This correlation persisted for most well sites, although the slope was site specific, and slope approached zero for wells on longer flowpaths. Respiration rate in the aquifer recharge zone was similar to expected riverine patterns, and was positively correlated with temperature: low winter respiration, a rise during spring snowmelt and flood pulse, and a maximum from July through September. On longer flowpaths, respiration showed depressed summer rates and two maxima: one in the spring, and a second larger peak in the fall and winter. This heretofore unreported winter respiration peak suggests that energy from the aquifer is transported longer distances when the river and recharge area temperatures are low. Modeled oxygen concentration based on the seasonal respiration pattern corresponded with the annual oxygen waveform at well sites. The overall DO patterns indicate the dynamic influence of the river on floodplain groundwater and subsurface energetics at large scales. Moreover, the spatial and temporal patterns of this energetic coupling can be inferred at the ecosystem level from two relatively simple indices: (1) the slope of DO/temperature relationship; (2) by comparing DO waveforms between source and end points over the annual cycle.