GROUNDWATER TRANSPORT OF ARSENIC AND CHROMIUM AT A HISTORICAL TANNERY, WOBURN, MASSACHUSETTS, USA

The geochemical environment in hide piles at a historical tanning and rendering site (Woburn, Massachusetts, U.S.A.) is strongly reducing, a s reflected by the presence of H2S and CH,HS in the pile offgas. The p resence of a reducing environment in the subjacent groundwater, along with DOC (>100 mg/l) from hide breakdown, results in reduction of As(V ) to As(III), and subsequent methylation to monomethylarsonic acid (MM AA) and dimethylarsinic acid (DMAA). The reducing conditions also resu lt in precipitation of FeS(am), while hydrophilic organic acids have i ncreased Cr(III) solubility. Three spatially sequential geochemical re dox facies were recognized in groundwater downgradient from the hide p iles. Typically, a reduced core zone was present adjacent to the hide piles, characterized by S2- greater than or equal to 1 mg/l, Fe2+ <5 m g/l, NH3 greater than or equal to 200 mg/l and the presence of MMAA in conjunction with DOC (greater than or equal to 30 mg/l). This facies transitions through an intermediate zone, represented by Fe2+ > 20 mg/ l, NH3 (5-200 mg/l) and the sporadic presence of measurable S2- (1-2 m g/l), to an oxidizing peripheral zone characterized by conditions repr esentative of background (i.e. DO > 1 mg/l, Eh > 0 mV, Fe2+ < 20 mg/l, S2- < 1 mg/l, NH3 < 5 mg/l and NO3- > NH3), accompanied by precipitat ion of amorphous ferric hydroxide, sorption of As and co-precipitation -sorption of Cu, Pb and Zn. Electron microprobe analysis of hide-pile materials demonstrated authigenic precipitation of amorphous ferric hy droxide and gypsum, in agreement with the results of geochemical model ing.