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

Chemical, Isotopic, and Microbiological Evidence for Denitrification During Transport of Domestic Wastewater Through a Thick Unsaturated Zone in the Mojave Desert

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.