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

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.