Stable Isotope Hydrology of the Anchorage, Alaska Area: Precipitation, Streamwater, and Groundwater Connections

In order to assess the contributions of precipitation, glacial meltwater, and groundwater to streamwater in the area of Anchorage, Alaska, weekly to bi-weekly samples of precipitation and streamwater and several groundwater samples were collected from the Anchorage area of southcentral Alaska from Spring 2005 through early 2006. Additionally, several winter precipitation, glacial ice, and streamwater samples were collected throughout the region. As expected, oxygen and hydrogen isotope results are more variable and relatively enriched for precipitation samples then for stream samples. δ 18O of summer precipitation samples vary by approximately 9‰, and stream samples vary by only approximately 0.5‰. Therefore, non-glacial streams in the study area appear to be influenced more by groundwater than precipitation. Mean isotope results for small, spring-sourced streams are quite similar to mean results from precipitation samples, with a mean δ 18O of approximately of -18‰. Glaciated stream results suggest the isotope composition of the stream is greatly influenced by glacial melt-water and did not vary by more than a few tenths of a permil over the duration of the study. Additionally, differences among winter and summer samples reflect the greater influence of groundwater in winter stream water as compared with summer. Simple isotopic mixing calculations suggest glacial melt contributes approximately 25 percent of the water in small, glaciated streams during summer peak flow conditions. Isotopic results suggest the isotopic composition of the glaciated streams contains more isotopically depleted water than expected if only current climate data and local isotope hydrology is considered.