Laboratory and Field Scale Evaluation of Geochemical Controls on Groundwater Transport of Nitroaromatic Ammunition Residues

Sorption to soils and sediments of nitroaromatic explosives may be due to interactions with natural organic matter (NOM) or complex formation with clays, which strongly depends on the type of exchangeable cations, i.e., the base saturation of the clays. We examined the relative importance of these two processes for NAC sorption at aquifer material and evaluated the potential of decreasing or enhancing the mobility of NACs in contaminated aquifers by stimulated cation exchange. Generally, sorption on NOM of 2,4,6-trinitrotoluene (TNT) and related nitroaromatic compounds (NACs) was low compared to sorption at clays, and no evidence for specific interactions with NOM was found. Adsorbed NOM hardly affected the complex formation of NACs with clays. NAC sorption at pure clays and at aquifer material depended on the K + -saturation of these materials. Typical aquifer material containing ≤1% NOM and 3−5% clays exhibited similar sorption features than pure clay minerals, suggesting that NAC sorption to the bulk aquifer matrix was dominated by complex formation at clays. We applied these laboratory findings to a two-step field test designed to control the groundwater transport of NACs by the injection of electrolytes. 4-Nitrotoluene (4-NT) and KCl were injected into a sandy aquifer, and their groundwater transport was monitored at an extraction well. Subsequent injection of CaCl 2 remobilized the previously adsorbed 4-NT due to ion exchange of Ca 2+ for K + at clays present in the aquifer matrix. The susceptibility of NAC sorption to the composition of exchangeable cations at clays was confirmed for TNT and related NACs. Controlling the base saturation of the aquifer matrix by electrolyte injections thus opens new perspectives for gentle remediation of sites contaminated with nitroaromatic explosives.