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 i nteractions with natural organic matter (NOM) or complex formation with cla ys, which strongly depends on the type of exchangeable cations, i.e., the b ase saturation of the clays. We examined the relative importance of these t wo processes for NAC sorption at aquifer material and evaluated the potenti al of decreasing or enhancing the mobility of NACs in contaminated aquifers by stimulated cation exchange. Generally, sorption on NOM of 2,4,6-trinitr otoluene (TNT) and related nitroaromatic compounds (NACs) was low compared to sorption at clays, and no evidence for specific interactions with NOM wa s found. Adsorbed NOM hardly affected the complex formation of NACs with cl ays. NAC sorption at pure clays and at aquifer material depended on the K+- saturation of these materials. Typical aquifer material containing less tha n or equal to 1% NOM and 3-5% clays exhibited similar sorption features tha n pure clay minerals, suggesting that NAC sorption to the bulk aquifer matr ix was dominated by complex formation Bt clays. We applied these laboratory findings to a two-step field test designed to control the groundwater tran sport of NACs by the injection of electrolytes. 4-Nitrotoluene (4-NT) and K CI were injected into a sandy aquifer, and their groundwater transport was monitored at an extraction well. Subsequent injection of CaCl2 remobilized the previously adsorbed 4-NT due to ion exchange of Ca2+ for K+ at clays pr esent in the aquifer matrix. The susceptibility of NAC sorption to the comp osition 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 cont aminated with nitroaromatic explosives.