OBSERVING SURFACE AND GROUNDWATER CONNECTIONS THROUGH TRACE METAL AND STABLE ISOTOPE GEOCHEMISTRY: ANCHORAGE, ALASKA

Surface and shallow ground waters are typically connected water resources. This connection is important because of water quantity and quality issues that are associated with most aquifer systems used to supply drinking water. The complex glacio-stratigraphy in the Anchorage aquifer make it difficult to understand the physical connections between surface water and shallow groundwater. The project objectives are to use major and trace elements and stable isotope data from surface and ground water to try to understand the seasonal variations in the connection between surface and groundwater.

Biweekly water sampling was conducted between April 2009 and April 2010 during which time measurements of physical and chemical parameters of the stream and groundwater were collected. Groundwater tended to have higher pH (7.4-8.42), higher alkalinity, consistent temperatures, and overall reduced conditions relative to Chester Creek. Chester Creek water showed much higher DO concentrations (9.88-15.65 mg/L) than groundwater and a wider range in temperature (0.26-15.3°C).

During late winter and early spring of 2010, Cr, Cu, Pb and V were observed in elevated concentrations in stream water with lesser concentrations in groundwater. Manganese, Fe and As are correlated with groundwater levels as the concentrations and water levels decrease into summer, then recover in fall, winter and spring. In Chester Creek Mn and As were in higher levels during low flow conditions and respond to high flow by dilution. Concentrations of As in groundwater for spring of 2010 were above EPA drinking water standards for two and a half months, but are consistently low to non-detect in stream water during 2010. Molybdenum, Ca, Na and Ni in both water sources increased in concentration in the summer of 2009 and stayed at elevated levels throughout the sampling period. An overall decreasing trend in Zn concentration was noticed in both water sources from the onset of sampling in 2009. δ¹⁸O and δD values in the stream water had an enriched isotopic composition relative to groundwater during the summer, with similar compositions the rest of the year.

By studying these major and trace elements and water isotope composition, the strongest hydraulic connection is observed during spring, fall and winter with moderate to little connection during summer.