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. NO3 is a common contaminant in shallow ground water in this area, whereas high concentrations of NH4–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 NO3. 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 NH4, and are inconsistent [...]
