BROMIDE, CHLORIDE, AND ASSOCIATED BRINE CONSTITUENTS IN WATERS FROM COAL-BEARING ROCKS IN PENNSYLVANIA

Various halogen (Cl, Br, I) and alkali earth (Na, K, Li) elements are commonly elevated in produced waters from oil, gas, and coalbed methane (CBM) deposits, and have been documented at moderate concentrations in coal-mine drainage (CMD), groundwater, and surface water in the northern Appalachian Coalfield. Moderate concentrations of bromide (Br^-) in source water for human consumption are cause for concern because of its potential to oxidize to bromate (BrO₃^-) or to interact with organic carbon forming brominated trihalomethane disinfection byproducts. The drinking-water maximum contaminant levels for bromate and total trihalomethanes are 0.010 and 0.080 mg/L, respectively.

Historical data on Br^-concentrations in 483 groundwater samples from springs and wells in the northern Appalachian region collected in 1977 – 1978 (NURE; Br^- median 0.032 mg/L, range <0.003 – 5.9 mg/L) and more recent data for 182 untreated CMD samples from western Pennsylvania collected in 1999, 2003, and 2011 (Br^- median 0.036 mg/L, range <0.003 – 12.8 mg/L) indicate similar medians. The median Br^- of 0.036 mg/L in CMD is near the upper limit assumed for natural background levels in Pennsylvania waters. Several CMD samples from deep mines or associated waste rock piles had elevated concentrations of Br^-, Cl^-, I^-, Na⁺, K⁺, and Li⁺. Although these brine constituent concentrations are dilute in CMD compared to oil and gas brines or other sources of salinity, the Br^-/Cl^- ratios indicate many of the deep mine waters have enriched Br^- compositions that are consistent with residual brine diluted with meteoric water. Shallow rocks are more fractured and thus allow for greater flushing by meteoric water than deeper portions of the flow system. In general, the deeper rocks retain greater amounts of residual brine, including residual Na on cation exchange sites in the intermediate flow system, and Na⁺ and Cl^- in the regional system. Although injection of oil or gas-well produced water or CBM wastewater into mine(s) could be a source of locally elevated brine constituents in some samples, the historical sampling results indicate that moderate concentrations of Br^- and Cl^- were present in groundwater and CMD in western Pennsylvania prior to the recent, extensive production of natural gas from the Marcellus Shale and overlying coalbeds.