APPLYING PHYSICAL MEASUREMENTS OF HYPORHEIC EXCHANGE TO UNDERSTAND GEOCHEMICAL VARIABILITY OF HYPORHEIC WATERS

Groundwater/surface water interactions in the hyporheic zone create a chemically unique area where the variation of dissolved oxygen concentrations cause closely spaced oxic and anoxic zones to develop.  Enhanced biogeochemical cycling in these zones can significantly impact stream water chemistry and physical exchange metrics may not fully address the spatial variability of these processes.  It is important to identify streambed characteristics that control hyporheic exchange and how they relate to differences in the chemistry of streambed waters.  Here we use field-based physical measurements of hyporheic exchange, including temperature gradients and Darcian flux calculations, to assess the rate and extent of surface-groundwater interaction in a Wyoming stream.  We compare those results to chemical indicators of oxic and anoxic conditions in the streambed. A 5-m by 15-m reach, which includes a 1.5-m. high log dam, was instrumented with 36 in-stream mini-piezometers.  At each piezometer, we measured streambed hydraulic conductivity and hydraulic gradient, collected a water sample and measured streambed temperatures in the morning and evening.

Results indicate the stream is losing water at flux rates of between 0.16 and over 17 ft³/s per ft².  Daily streambed temperature fluctuations ranged from 0^oC to almost 5^oC.  Lower flux rates and low daily temperature variability measured at some piezometers suggest the subsurface is not well connected to the stream.  Lower sulfate concentrations with higher concentrations of soluble iron and manganese were found at the same sites and are chemical indicators of anoxia.  Conversely, higher flux rates and daily temperature variability measured at other sites indicate interstitial streambed water is more connected to the stream.  High concentrations of nitrate, possibly due to the release of inorganic nitrogen via mineralization of organic matter, and low concentrations of manganese and iron indicate a connection to an oxygen source (i.e., the stream).  The presence of the log dam generates hyporheic zones that are well connected to the stream, as indicated by Darcian flux rates, temperature variability and geochemistry of the interstitial water.