U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
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Chemical, Isotopic, and Microbiological Evidence for Denitrification
During Transport of Domestic Wastewater Through a Thick Unsaturated Zone
in the Mojave Desert
by
Roy A. Schroeder (U.S. Geological Survey, San Diego, Calif.),
Peter Martin (U.S. Geological Survey, San Diego, Calif.), and J.K. Bohlke
(U.S. Geological Survey, Reston, Va.)
Abstract
Septic-tank wastewater disposed in 30-foot-deep seepage pits (dry wells)
at 46,000 residences is estimated to equal about 18 percent of natural recharge
to the sole-source aquifer beneath the upper Mojave River Basin, which is
rapidly becoming urbanized, in the high desert northeast of Los Angeles.
Nitrogen in the downward-infiltrating wastewaters represents a significant
potential source of nitrate contamination to underlying ground water, but
increases in nitrate concentration in ground water have not yet been observed.
The low nitrate concentration in the ground water may be the result of lateral
dispersion in the unsaturated zone, dilution below the water table, or denitrification
of wastewater nitrate in the unsaturated zone.
Measured vertical rates of movement of wastewater fronts through
the unsaturated zone at three newly occupied residences
ranged from 0.07 to 1 foot per day. Those measurements,
along with moisture-content profiles at older residences, indicate
that some wastewater has reached the water table beneath communities
that are older than 5 to 10 years. As wastewater percolates
from seepage pits into the unsaturated zone, reduced nitrogen
is converted rapidly to nitrate at shallow depths. Analyses
of water extracts of soil cores and of soil moisture from suction
lysimeters deep beneath seepage pits at several residences
indicate that nitrate concentrations commonly decrease
with depth. The largest nitrate decreases seem to coincide with
increased content of fine-grained sediments or proximity to the
water table. Nitrate-reducing bacteria were found in soil
cores collected from two residences. Between lysimeters at 160
and 199 feet at one residence, the decrease in nitrate
concentration coincided with a large increase in sulfate, decrease
in alkalinity, and increase in 15N in
nitrate. Those data are consistent with denitrification by oxidation
of iron sulfide to produce ferric oxides; but if such
a reaction occurs, it must be in domains that are small in comparison
with the sampled volumes because the waters also contain
substantial quantities of dissolved oxygen. The predominantly
low nitrate concentrations in the area's ground water are
consistent with the operation of a nitrogen- removal mechanism,
possibly denitrification, as wastewater moves through an unsaturated
zone that averages 150 feet in thickness. However, the reducing
capacity of the sediments to maintain denitrification is
not known.
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