Stable Isotopes and Sources of Groundwater Nitrate in Colorado.

Groundwater impairment due to elevated levels of nitrate-nitrogen (NO₃-N) is found in many areas of world including eastern Colorado. In portions of the South Platte alluvial aquifer, greater than 58 percent of monitoring wells sampled from 1989 – 2013 contained NO₃-N in excess of the USEPA drinking water standard of 10 mg L^-1. Trend analyses of continuously monitored wells from this time period shows 29.4% wells are decreasing, 17.7% increasing, and 52.9% showing no trend. Median values of well network NO₃-N concentrations have not statistically changed through time. During the summer of 2013 and 2014, samples were collected from 22 groundwater wells and 10 – 12 surface water sites at three time periods each season. Surface water sites included locations on the South Platte and its tributaries and irrigation canals that divert water from these rivers. Samples were analyzed for nitrate concentrations and isotopes of nitrogen (δ¹⁵N), oxygen (δ¹⁸O) and boron (δ¹¹B) to better understand potential nitrate sources, trends and how proximity to potential sources of pollution affects nitrate contamination. Using isotopic ranges from the literature, samples were categorized as having an inorganic, organic manure, organic sewage and/or soil nitrogen signature. 2013 results indicated that 45% of samples had a signature suggesting an organic nitrate origin. Of those, 80% had δ¹¹B signatures that indicated that the nitrate source was most likely from an animal manure source and 20% from human sewage. Only one sample from a surface water site had an isotopic signature indicating a nitrate fertilizer source. The remainder of the samples had signatures that could be interpreted as nitrate from the soil nitrogen pool, an undetermined organic source, and/or from nitrified ammonium fertilizer. Levels of the isotopes δ¹⁵N and δ¹¹B in groundwater were found to be positively correlated geospatially to confined animal density and static water level.