PHYSICAL AND CHEMICAL CONNECTIONS BETWEEN SURFACE AND GROUND WATER IN THE ANCHORAGE WATERSHED, ALASKA

The chemical composition of water is effected by meteoric water, rock-water interactions and the physical connectivity between surface and ground water. In the Anchorage watershed, the relationship between stream and ground water has not been quantified. The municipality of Anchorage has approximately 280,000 residents, of which 47,600 people use groundwater as their primary source for consumption. Therefore, understanding the seasonal cycle of this resource as well as the geochemical cycles of environmentally significant trace elements is imperative. Arsenic has already been identified to be above EPA drinking water standards in the groundwater of Anchorage which is incentive to conduct further studies of trace element cycles in the groundwater. A study of stream and shallow groundwater interactions began in April, 2009. The aims of this study are to understand the seasonal connections between the shallow aquifer and stream water. Water from Chester Creek and groundwater from a nearby well were sampled bimonthly. Field measurements of static water level (SWL), discharge, pH, DO, SC, ORP, T, alkalinity, Fe²⁺, and NO₃^- were measured. Field data for the groundwater is as follows: SWL (26.2-62.03 ft), pH (7.4-8.38), DO (0.16-7.74 mg/L), SC (255-262.3 μS/cm), ORP (-261.5 to 32.7), T (3.7-7.77°C), alkalinity (45-120 mg/L CaCO₃), Fe²⁺ (0.03 - 1.55 mg/L). For Chester Creek: discharge (3.44-8.55 m³/s), DO (10-13.54 mg/L), SC (185-241 μS/cm), ORP (40.6-205.7), T (6.1-15.3°C), alkalinity (26-58 mg/L), Fe²⁺ (0.01 mg/L), NO₃^- (1.07-1.77 mg/L). In general, the groundwater is more reduced and has a higher SC. To help verify the surface-ground water connections a resistivity survey was also performed from the stream to the well and indicates the local water table. Elemental concentrations were measured by ICP-MS and anions by IC. In the groundwater major elements K, Mg, and Na increased with time as SWL decreased except for Fe and Ca. In stream water all major elements declined as discharge decreased with the exception of Mg. Minor elements including Ba, Ni and Zn all decreased in surface water over time but As increased. As, Ba, Ni and Zn all decreased in groundwater as Mo concentrated. These results show that surface and ground water are dominated by meteoric inputs in the spring whereas dilution effects dominate during summer.