TRACE ELEMENT DYNAMICS IN SURFACE WATER AND SHALLOW GROUNDWATER AT 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, Wyoming, to evaluate trace element concentrations and other parameters. Samples were collected in July 2013 during summer baseflow and May 2014 during snowmelt runoff. Groundwater samples were collected along a ~100 m long transect of shallow wells (maximum screen depth of 3 m) located on the inside of a meander in Red Canyon Creek. Stream samples were collected upstream and downstream of the well transect. At each site, we measured field parameters and collected samples for trace elements, stable water isotopes, and major ions. Field measurements showed dissolved oxygen-saturated stream water and 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). Nearly identical stable water isotope values indicate strong connection between the surface water and groundwater (δ¹⁸O and δD ranged from -18.1 to -18.9‰ and from -140 to -147‰, respectively). 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). Notably, Mn and Fe concentrations were 100-fold higher in groundwater relative to surface water. In contrast, As, Se, and V concentrations were 3- to 10-fold higher in surface water relative to groundwater. Groundwater and surface water chemistry were remarkably similar during both the summer and spring sampling events, indicating overall stability of the oxic/anoxic transition and trace element concentrations. Geochemical modeling was used to understand observed trace element measurements. This study is important for characterizing trace element cycling in mountainous areas where measurements are rare, and has implications for furthering understanding the key biogeochemical processes that occur during groundwater-surface water interactions.