Summary Geochemical and isotopic tools were applied at aquifer, transect, and subtransect scales to provide a framework for understanding sources, transport, and fate of dissolved inorganic N in a sandy aquifer near La Pine, Oregon. NO 3 is a common contaminant in shallow ground water in this area, whereas high concentrations of NH 4 -N (up to 39 mg/L) are present in deep ground water. N concentrations, N/Cl ratios, tracer-based apparent ground-water ages, N isotope data, and hydraulic gradients indicate that septic tank effluent is the primary source of NO 3 . N isotope data, N/Cl and N/C relations,
3H data, and hydraulic considerations point to a natural, sedimentary organic matter source for the high concentrations of NH 4 , and are inconsistent with an origin as septic tank N. Low recharge rates and flow velocities have largely restricted anthropogenic NO 3 to isolated plumes within several meters of the water table. A variety of geochemical and isotopic data indicate that denitrification also affects NO 3 gradients in the aquifer. Ground water in the La Pine aquifer evolves from oxic to increasingly reduced conditions. Suboxic conditions are achieved after about 15-30 y of transport below the water table. This article is a U.S. government work, and is not subject to copyright in the United States.denitrified near the oxic/suboxic boundary. Denitrification in the La Pine aquifer is characterized well at the aquifer scale with a redox boundary approach that inherently captures spatial variability in the distribution of electron donors. ª