Abstract
Unconventional gas flowback-produced waters, particularly those of marine origin from shale gas, can contain elevated concentrations of barium (Ba) and sodium chloride (NaCl). There are limited Ba water quality guideline values to help assess the potential risk of Ba exposure to freshwater biota. Barium toxicity is heavily influenced by sulfate concentrations as Ba and sulfate react, forming the highly water-insoluble and thus less bioavailable Ba sulfate. To determine survival and reproductive impacts, the water flea Ceriodaphnia dubia was exposed to dissolved Ba, NaCl, and binary combinations of dissolved Ba and NaCl. No chronic lethal concentration (LC) endpoints could be determined for dissolved Ba-only exposure up to 16 mg/L due to near 100% parent survivorship across all treatments. The NaCl LC50 (95% credible intervals) = 708 (502-939) mg/L. The dissolved Ba reproductive effect concentration (EC) at EC20 was 0.95 (0.19-3.22) mg/L. Meanwhile, for NaCl, the EC10 and EC20 concentrations were 365 (149-651) mg/L and 510 (282-760) mg/L, respectively. The binary exposure of Ba and NaCl had limited meaningful data due to some experimental shortcomings (lack of Ba-only and NaCl-only controls). Despite this, at 410 mg/L NaCl, the dissolved Ba reproductive EC10 and EC20 were determined to be 8.87 [3.58-11.7] mg/L and 10.1 (5.64-11.8) mg/L, respectively. These results suggest that marginally increased NaCl concentrations alleviate Ba toxicity, particularly when Ba is at low concentrations. Further chronic studies are needed to account for Ba toxicity in dissolved and precipitated forms and derive a Ba guideline value for freshwater biota.