d15N AND d18O OF NITRATE AND THE INFLUENCE OF ANTHROPOGENIC INPUTS ON THE SURFACE WATER NITRATE BUDGET IN ANCHORAGE, ALASKA, USA

Anchorage, Alaska has a large percentage of private drinking water wells and septic systems for a city of its size (population 275,000). In sections of the city where the concentration of private septic systems is high, nitrate concentrations tend to be elevated in shallow groundwater relative to neighborhoods with municipal sewer systems. Several recent studies have made use of nitrogen and oxygen isotope ratios of stream water nitrate to identify anthropogenic or natural nitrate sources to surface water systems. In order to determine if anthropogenic nitrate from septic systems and other sources reaches the surface water and is a significant contributor to the stream water nitrate budget, stream water nitrate was collected from three streams that run through the city (including those areas with a high concentration of private septic systems) and analyzed for nitrogen and oxygen isotopic composition. Sampling was initiated during the spring 2007 snowmelt/breakup event and continued through the summer. Samples were collected upstream of urbanized areas and very near the stream mouths in order to assess nitrate concentration increase or decrease within the urbanized area. Additionally, upstream sites serve as non-urban, background sample locations for stream water nitrate isotopic composition. During the snowmelt/breakup event, stream water nitrate concentrations were elevated by several tenths of a ppm between upstream and downstream sites, and nitrate concentrations ranged between 0.3ppm and 1.1ppm. In later spring and early summer, nitrate concentrations remained constant or in some cases decreased from upstream to downstream sites. This may suggest nitrate sinks in the urbanized zone or dilution by nitrate-poor ground or surface waters. Preliminary isotopic analyses may suggest some input of anthropogenic nitrate to the total nitrate budget during the snowmelt/breakup period. Isotopic forensics of stream water nitrate in Anchorage may be complicated by ubiquitous alnus-related nitrogen fixing soil bacteria that provide the ecosystem with abundant available nitrate. Additionally, marine-derived nitrogen via the decay of anadromous fish carcasses during the summer and fall may be a source of nitrate that is isotopically similar to anthropogenic-sourced nitrate.