Influence of groundwater-surface water interactions on trace element concentrations in a semiarid mountain catchment (Red Canyon, Wyoming)

The interface between oxic surface water and anoxic groundwater plays an important role in trace element cycling in stream ecosystems. In this investigation, we sampled stream water and shallow groundwater in a semiarid mountain catchment at Red Canyon Creek, Wyoming, to compare trace element concentrations in the two compartments. Here we present the results of a sampling campaign that included seven stream samples and nine wells collected during July 14-15, 2013, when the stream was at a baseflow condition. Groundwater samples were collected along a ~100 m long transect of wells (maximum screen depth of 3 m) that were installed on the inside of a meander in Red Canyon Creek. At each site, we measured field parameters and collected samples for trace elements, H and O stable isotopes, and major ions. Field measurements showed high dissolved oxygen concentrations for stream water, in contrast to anoxic/reducing groundwater. The stream water had higher pH relative to groundwater (8 and 7, respectively), higher temperature (>15°C and <12°C, respectively), and lower specific conductance (ranging from 500 to 800 μS/cm and from 700 to 1600 μS/cm, respectively). H and O isotopes indicate strong connection between the surface water and groundwater. Concentrations of most elements were a factor of two higher in groundwater relative to surface water (Li, Be, Na, Mg, K, Ca, Ti, Co, Ni, Zn, Sr, Y, Ce, Pb, and U), and Mn and Fe concentrations were a factor of 100 higher in groundwater relative to surface water. In contrast, As, Se, and V concentrations were factors of 3 to 10 higher in surface water relative to groundwater. These contrasting results can potentially be explained by a number of mechanisms, including dissolution of Mn and Fe oxides in groundwater and precipitation of Mn and Fe oxides in surface water. This contribution will discuss preliminary geochemical modeling results to describe observed trace element concentrations, with implications for furthering understanding the key biogeochemical processes that occur during GW-SW interactions.