Fate and transport of TNT, RDX, and HMX in streambed sediments: Implications for riverbank filtration

Riverbank filtration (RBF) refers to the process of capturing surface water passing through the river-sediment-aquifer system by using a collection technique such as a well or an infiltration gallery. RBF removes nearly all suspended and a large number of dissolved contaminants from the surface water. Therefore, it can function as an effective pretreatment process in drinking-water production. TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) are three military explosive chemicals that are considered of concern to human health when present in source waters. This study is to evaluate the ability of the filtration media in RBF systems to remove these chemicals. The results from an anoxic batch test showed that all three chemicals will degrade while passing through streambed sediments. The pseudo first-order degradation-rate constants for TNT, RDX, and HMX were measured to be 0.33, 0.055, and 0.033 d^-1, respectively. Under aerobic conditions only TNT showed significant degradation. Results from a model RBF system showed that the mobility of the three chemical contaminants in streambed sediments was in the order: HMX > RDX > TNT. The results suggest that RBF is capable of removing TNT and RDX but HMX levels may continue to be of concern-especially when collector wells use laterals running directly beneath the stream or riverbed.