Fixation and redistribution of arsenic during early and late diagenesis in the organic matter-rich members of the Lockatong Formation, Newark basin, USA: implication for the quality of groundwater

Résumé

The Byram and Walls Island members in the lower and upper sections, respectively, of the Lockatong Formation in the Newark basin near the border between Pennsylvania and New Jersey were chosen to assess (i) the role of euxinic/anoxic conditions in sequestering arsenic (As) and other trace elements and (ii) the redistribution of these elements during catagenetic transformations. ἀese members are rich in organic matter and host pyrite which occurs as disseminations, small patches, and subparallel veins. ἀe sulfur isotope values of pyrite samples range between -7.5 and 0.5 ‰CDT (average = -3.5‰CDT). ἀe negative δ34S values are indicative of Bacterial Sulfate Reduction (BSR) under low temperature and euxinic/anoxic conditions. ἀe total organic carbon (TOC) values in this member ᴀuctuate between 0.5 and 2.1%. ἀese euxinic/anoxic conditions enhanced the incorporation of As and other trace elements in both organic matter and pyrite. ἀe As concentrations range from 13 to 800 mg/kg and from 1.4 to 34 mg/kg in pyrite and black shale samples, respectively. Rock Eval analyses reveal that organic matter is over-mature which altered the correlation between TOC and As. ἀe thermal cracking of organic matter resulted in the removal of these elements from organic matter and their subsequent incorporation in pyrite and bitumen. Organic matter- and pyrite-rich anoxic black shale layers and bitumen veins are potential sources of arsenic in groundwater in the Newark basin, with arsenic values that reach up to 215 μg/L.