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
Depletion of Nitrogen-Bearing Explosives Wastes in a Shallow
Ground-water Plume near Hawthorne, Nevada
by
A.S. Van Denburgh (U.S. Geological Survey, Carson City, Nev.),
D.F. Goerlitz (U.S. Geological Survey, Menlo Park, Calif.), and E.M. Godsy
(U.S. Geological Survey, Menlo Park, Calif.)
Abstract
Liquid explosives wastes from the emptying and washout of military projectile
and bomb casings were discharged into unlined disposal pits north of
Hawthorne, Nevada. The explosives involved at this arid study area were
ammonium picrate, during 1952-58, and trinitrotoluene (TNT) plus ammonium
nitrate, during 1964-68. Percolation from the pits entered unconsolidated,
heterogeneous sedimentary deposits in which clay- to gravel-size particles
predominate. The water table at and downgradient from the pits was from
1-1/2 to 6-1/2 meters below land surface during the study. The shallow ground
water moves northwestward at an average linear velocity of about 130 meters
per year.
A shallow plume of ground water containing 3 to 130 milligrams per
liter of dissolved-nitrogen species, as nitrogen, extended more than 1,700
meters northwest of the northernmost pits. Either ammonium or nitrate dominated
at sample sites within 300 meters of the pits; farther downgradient in the
plume, nitrate was the most abundant. Overall, the plume contained about
3 megagrams of dissolved nitrogen downgradient from the pits. The proportions
of sodium and bicarbonate ions increased downgradient, relative to chloride
and sulfate ions. Saturated sedimentary deposits within the plume generally
had greater concentrations of organic nitrogen, ammonium, and organic carbon
than did the adjacent sediments.
The maximum linear velocities for the dissolved explosives-waste components
(a measure of their mobility) ranged from more than 77 meters per year for
nitrate to less than 20 meters per year for picrate, with velocities of
TNT and ammonium in between. The downgradient movement of nitrate may not
have been impeded by microbiological or geochemical processes. For the other
dissolved components, in contrast, the following depletion mechanisms are
proposed: TNT, microbial transformation to inorganic nitrogen and carbon
species, and probable sorption of organic transformation products; ammonium,
sorption, and probable oxidation by nitrifying bacteria; and picrate, microbial
alteration to picramic acid, then mineralization to inorganic nitrogen and
carbon species. The reactions that depleted TNT, ammonium, and picrate in
the ground water may also have enriched the proportions of sodium and bicarbonate
ions in the plume. Overall, the Hawthorne study shows that natural microbiological
and geochemical processes can degrade or retard the transport of large quantities
of toxic, chemically complex, nitrogen-rich explosives wastes over distances
of hundreds of meters, leaving nitrate as the only residual nitrogen-bearing
component of the ground-water plume.
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