ORGANIC SUBSTANCES IN PRODUCED WATER FROM UNCONVENTIONAL NATURAL GAS PRODUCTION - POTENTIAL HEALTH CONCERNS

Large volumes of water are produced during unconventional gas extraction in coal and shale, and these fluids represent a significant safe disposal issue. Chemical substances in the produced water potentially pose an environmental hazard and human health concern. In this study, organic substances in produced water from natural gas-producing coals and shales were examined, with the produced water salinity ranging from fresh (Powder River Basin coal), to saline (black Warrior Basin coal), and hypersaline (Marcellus Shale). Dissolved organic carbon (DOC) concentrations in water from the coals ranged from <1 to 6 mg/L, compared to values up to 3,000 mg/L in water produced from the Marcellus shale. High DOC concentrations in the Marcellus likely originate from organic substances in hydrofracturing fluids.

In produced water from the coals, extractable organic compounds identified included: phenols, biphenyls, heterocyclic compounds, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and long-chain fatty acids. PAHs, such as naphthalene, phenanthrene, and their alkylated derivatives were the most prevalent compounds identified. Many more unidentified compounds are also present. Some identified compounds are toxic, but are unlikely to have acute health effects at the observed concentrations (0.1-10 µg/L). Chronic health and environmental effects from long periods of low-level exposure are unknown. In the Marcellus shale and overlying sandstone, extractable hydrocarbons in produced water include alkanes, biocides, anti-corrosion chemicals, ethylene glycol, and heterocyclic compounds. Many of the heterocyclics, and other identified organics (biocides, ethylene glycol) originate from chemicals added during hydrofracturing operations. The highest concentrations of organics and volumes of water from wells occurred in the first ten days following hydraulic fracturing in the flowback water. However, traces of hydrofracturing organic chemicals were seen in the produced water even months after hydraulic fracturing. Because many of these organic compounds are toxic, care should be exercised in the disposal of these fluids.