CHEMICAL FORMS OF INORGANIC ELEMENTS IN WASTEWATER FROM A MARCELLUS SHALE-GAS WELL

We are examining the chemical forms of elements with potential environmental health impacts in wastewater from a shale-gas well. Produced water samples were collected from the separator at well 5H near Morgantown, West Virginia as part of the Marcellus Shale Energy and Environment Laboratory (MSEEL) project. All wastewater samples were circumneutral Na-Ca-Cl brines with Na/Ca ratios of 6-8, Cl concentrations up to 1.5 moles per kilogram H₂O (mol/kgw), total dissolved solid concentrations up to 100 g/L, and Mg, Ba, Sr, and NH₄ concentrations up to 35, 26, 20, and 6 millimoles/kgw (mmol/kgw), respectively. Dissolved Fe(II) concentrations in wastewater filtered near the time of collection ranged from 1 to 8 mmol/kgw. Exposure to oxygen produced hydrous ferric oxide (HFO) solids in wastewater samples. HFO formed rapidly at first but complete oxidation of Fe(II) required weeks to months, possibly owing to inhibition by abundant organics in the samples. Total Fe concentrations (dissolved plus Fe in filtered solids) were ~ 0.1 mol/kgw. Two-line ferrihydrite and quartz were the only minerals discernable in X-ray diffraction patterns of the solids filtered from wastewater samples. Cobalt, Cu, Ni, and Zn concentrations in the range 0.1 to 5 micromoles/kgw (μmol/kgw) were determined in filtered wastewater by pre-concentrating on Chelex. Aluminum, As, Cr, Pb, P, and U could not be detected in filtered wastewater but were detected in nitric-acid digests of the solids. Copper was detected both in aqueous samples and nitric-acid digests of the solids, with solid-bound Cu accounting for 99% of the total. Solid-bound As concentrations scaled by the solid-liquid ratio were ~ 1 μmol/kgw, nearly 10 times the U. S. EPA drinking water standard (0.13 μmol/L). The ratio of total arsenic to total S was ~ 0.0005, similar to As/S ratios reported for Marcellus Shale. Trends in the degree of partitioning of all elements between the aqueous and solid phases were consistent with those expected based on computations with models for sorption on synthetic HFO. Results suggest that many elements partition onto HFO that forms in these wastewaters. Any environmental health impacts from wastewater spills associated with these elements may be limited to the area near the spill site and linked to the fate of HFO trapped in soils or sediments.