The goal of this project is to identify sources of excess nitrogen impacting wetlands (Pine Draw) within the Turnbull National Wildlife Refuge (TNWR, Washington State, USA). Pine Draw is located in the Channeled Scablands, an arid region with poor soil development and shallow aquifers within fractured basalt. Most of the watershed is within TNWR and has little development. However, high nitrogen loading has been documented since 1991. Excess nutrients may be traveling through shallow groundwater flow from an adjacent watershed dominated by conventional dryland wheat farming. Alternatively, excess nitrogen may derive from livestock, septic systems (including those on TNWR) or migrating waterfowl. We will use nutrient concentrations in surface and ground water, as well as analysis of nitrogen and oxygen isotope ratios in nitrate, to investigate the source of nitrogen loading. Nitrogen from fertilizer has relatively low levels of 15N compared to animal waste sources. We have monitored common forms of nitrogen within Pine Draw, the adjacent agricultural watershed, and groundwater springs monthly since March 2016 and will continue sampling through spring 2018, including the full 2016-2017 water year. We will analyze nitrogen and oxygen isotope ratios within dissolved nitrate from these sources during winter, spring runoff, summer, and fall.
The most common form of nitrogen in Pine Draw surface water from March 2016 through January 2017 was nitrate. Nitrate in surface water ranged from below detection to 5.7 ppm nitrate-N, (mean of 1.2 ppm). We observed the highest nitrate in surface water during January 2017 (mean 3.3 ppm) and the lowest during September 2016 (mean 0.32 ppm). Nitrate levels in groundwater springs ranged from 0.31 ppm to 6.1 ppm (mean 2.8 ppm). Highest nitrate levels in groundwater occurred during September 2016 (mean 3.6 ppm) and lowest during April 2016 (mean 1.3 ppm). The adjacent agricultural drainage is inundated only seasonally, so data are less complete, but include nitrate concentrations over 10 ppm.
These data are consistent with groundwater as the source of nitrogen loading to Pine Draw; concentrations were consistently higher in springs than surface water. In addition, nitrate concentrations in surface water were highest just downstream of springs, and decreased with distance downstream. Preliminary data indicate high nitrogen runoff in the adjacent agricultural watershed, but cannot determine if this is the source of groundwater nitrogen. Analysis of stable nitrogen isotope ratios (in progress) will help determine if the nitrogen originates from fertilizer, or other sources.