Groundwater Under
the Pacific Northwest • November 2-3, 2005 •
Presenter:
Stephen R. Hinkle,
Co-Authors: David S. Morgan, Barbara J.
Rich, Rodney J. Weick
Geochemical and
isotopic tools were applied at aquifer, transect, sub-transect, and laboratory
scales to provide a framework for an aquifer-scale NO3-
transport model in a sandy aquifer near La Pine, Oregon. Nitrogen isotopes, N/Cl-
relations, age gradients, and hydraulic considerations indicate that septic
tank effluent is the dominant anthropogenic source of nitrogen to the
aquifer. 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+ and are inconsistent with an origin from
septic tank nitrogen. Most NO3-
currently is in isolated plumes within several meters of the water table. Chlorofluorocarbon dating constrains recharge
to about 5.4 cm/y along the most intensively- studied ground water transect; at
this rate of recharge, vertical velocity near the water table is about 0.18
m/yr. Most residential building in the
La Pine region has occurred since 1960.
Population trends would suggest that most NO3-
should be found near the water table.
However, denitrification also affects NO3-
gradients in the aquifer. Denitrification was identified (1) at the aquifer scale by
NO3-/Cl- relations, (2) at the ground water
transect scale by the presence of excess N2 enriched in 15N,
the presence of N2O, NO3-/Cl-
relations, and the position of denitrification
reactions within the context of progressive geochemical evolution and redox zonation along ground water
transects, and (3) at the laboratory scale by results of denitrification
experiments with aquifer sediment.
Ground water in the La Pine aquifer evolves from oxic
to increasingly reduced conditions. Suboxic conditions are achieved in 15 to 30 years, and the
boundary between oxic and suboxic
ground water is sharp. Nitrate is
reduced near the oxic/suboxic boundary. To account for denitrification
in the La Pine aquifer, kinetic and electron-limited (redox
boundary) approaches were evaluated.
Quantification of denitrification rates for
use at the aquifer scale might be successful in aquifers with homogeneous
distributions of slowly reacting electron donors. However, in the La Pine aquifer, a redox boundary approach provided a more reliable means of
accounting for NO3- reactivity because redox zonation implicitly
captures spatial variability in the distribution of reactive electron
donors. This redox
boundary approach was implemented in a transport model that is currently being
used for regional planning.