Nitrate Source, Transport and Fate in Ground Water Near La Pine, Oregon

A shallow, sandy aquifer serves as both the source of drinking water and the receptor of septic tank effluent for most residents in the vicinity of La Pine, Oregon. High concentrations of NO₃^- (>10 mg NO₃^--N/L) first were observed in study area ground water in the early 1980s. A framework for understanding NO₃^- dynamics and a conceptual model in support of a numerical NO₃^- transport model are described here. Geochemical and hydrogeologic data were collected at a variety of scales to develop an aquifer scale (640 km² area, 37-m thickness) understanding of NO₃^- source, advection, dispersion, and fate. A network of 193 existing wells, two transects of monitoring wells installed along ground-water flowpaths, a dense array of direct-push wells installed perpendicular to one of the transects, and three wells installed in septic tank effluent plumes were sampled and variously analyzed for common ions, nutrients, dissolved organic carbon, field parameters, dissolved gases, isotopes of water and nitrogen, and age-dating tracers (CFCs, ³H, ³H/³He). Nitrogen isotopes, N/Cl^- relations, age gradients, and hydraulic considerations indicate that septic tank effluent is the dominant source of NO₃^- in the aquifer. Most NO₃^- currently resides within the upper 5 m of the aquifer, due in large part to low recharge rates (CFC-based ground-water age gradients indicate a median recharge rate of 5.1 cm/y) and low hydraulic gradients that limit advection. High concentrations of NH₄⁺ (up to 39 mg NH₄⁺-N/L) were observed in deep (generally > 37 m) ground water (water that, for the most part, resides beneath the primary aquifer). Nitrogen isotopes, N/Cl^- and N/C relations, ³H data, and hydraulic considerations point to a natural, sedimentary organic matter source for this NH₄⁺. Relations between NO₃^-, Cl^-, and geochemical indicators of redox conditions, and relations between concentrations and isotopes of N₂, indicate that denitrification is extensive in the study area. Denitrification occurs near the oxic/suboxic boundary. Laboratory denitrification experiments with aquifer sediments confirm the existence of a denitrification capacity in sediments currently exposed to NO₃^-, and also demonstrate a latent denitrification capacity in sediments collected from what is currently NO₃^--free ground water. Our data allowed development of a framework and a conceptual model for the NO₃^- transport model. Septic tank effluent is the dominant NO₃^- source term; census data were combined with study area septic tank effluent data to estimate NO₃^- loading terms. Concentration data from a dense array of wells facilitated estimation of dispersion. Advection of NO₃^- occurs until NO₃^- reaches the oxic/suboxic boundary, at which point denitrification quickly results in reduction of NO₃^- to N₂. Age-dating data were combined with hydraulic head, slug test, and ground-water/surface-water interactions data to constrain and calibrate the transport model. Results from the transport model are being presented by Morgan et al. in a separate Fall, 2002 AGU session.