Groundwater transport of arsenic and chromium at a historical tannery, Woburn, Massachusetts, U.S.A

The geochemical environment in hide piles at a historical tanning and rendering site (Woburn, Massachusetts, U.S.A.) is strongly reducing, as reflected by the presence of H 2S and CH 3HS in the pile offgas. The presence of a reducing environment in the Subjacent groundwater, along with DOC (≥ 100mg/l) from hide breakdown, results in reduction of As(V) to As(III), and subsequent methylation to monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA). The reducing conditions also result in precipitation of FeS (am), while hydrophilic organic acids have increased Cr(III) solubility. Three spatially sequential geochemical redox facies were recognized in groundwater downgradient from the hide piles. Typically, a reduced core zone was present adjacent to the hide piles, characterized by S 2− ≥ 1mg/l, Fe 2+ 20mg/l, NH 3 (5–200 mg/l) and the sporadic presence of measurable S 2− (1–2 mg/l), to an oxidizing peripheral zone characterized by conditions representative of background (i.e. DO> 1mg/l, Eh> 0mV, Fe 2+ < 20mg/l, S 2− < 1mg/l,NH 3 < 5mg/l and NO 3 − >NH 3), accompanied by precipitation 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 hydroxide and gypsum, in agreement with the results of geochemical modeling.