Coalbed Methane Produced Water In The Western Us

effluents was based on laboratory results, the direct relevance of these findings to field conditions is somewhat uncertain. However, the results indicate that high TDS effluents have the potential to be highly toxic to standardized test organisms under controlled conditions.

Certain limitations to the application of these modeling and laboratory studies to examine the effects of CBM produced water include (1) the use of mean concentrations and discharges in a system with natural geochemical and hydrogeological variability; (2) the fact that permitted discharges of CBM produced water in many cases require treatment before discharge as well as a defined mixing zone (zone of mixing between CBM produced water and receiving water that dilutes the concentration of the CBM water; see Chapter 3 for details), leaving a small likelihood of direct exposure to undiluted CBM produced water; (3) ionic concentrations in surface water that vary with stream discharge and may increase during the beginning of storm and snowmelt events and during low-flow conditions (Sharma and Frost, 2008). CBM produced water may comprise a significant portion of total stream discharge during periods of low summer flow, and concentrations of major ions may vary during these low-flow periods. These temporal patterns of stream discharge and conductivity also depend on the source of water. For example, streams and rivers that originate in the mountains typically show a single peak in discharge during spring runoff (Clark et al., 2001). In contrast, streams and rivers originating in the plains are much more variable and may have little or no flow during late summer to early winter.

Laboratory Studies on Toxicity of Sodium Bicarbonate

The USGS examined acute and chronic toxicity of sodium bicarbonate to fathead minnows (Pimephales promelas), a standardized test species used in aquatic toxicology studies (see Table 5.2; Skaar et al., 2006; Farag et al., 2010). Laboratory tests simulating water characteristics in the Powder and Tongue rivers implicated bicarbonate, rather than sodium, as a cause of significant acute toxicity to the minnows (Farag et al., 2010). The study additionally included assessment of responsiveness of other fish species, amphibians, and invertebrates (see Table 5.3). As shown in Table 5.2, the 50th and 75th percentile concentrations of bicarbonate from groundwater samples collected from the Powder River Basin were 712 mg/L and 1,103 mg/L, respectively. Minnow survival was significantly lower in all treatments having sodium bicarbonate concentrations exceeding 400 mg/L (291 mg/L bicarbonate) and was reduced from 89 percent survival in controls to 2.4 percent at sodium bicarbonate concentrations of 1,400 mg/L (1,017 mg/L bicarbonate). Researchers also reported that the incidence of gill lesions and kidney damage increased as sodium bicarbonate concentrations and exposure time increased.

Acute LC50 values for several fish species after 96 hours of exposure to treatment water ranged from 1,158 to 5,526 mg/L sodium bicarbonate (841 to 4,014 mg/L bicarbonate), with significantly greater effects on younger fish (Table 5.3). Results also showed that an

TABLE 5.3 Results of Acute and Chronic Toxicity Tests Showing Effects of Sodium Bicarbonate on Fish, Amphibians, and Invertebrates.

Acute Tests

TABLE 5.3 Results of Acute and Chronic Toxicity Tests Showing Effects of Sodium Bicarbonate on Fish, Amphibians, and Invertebrates.

Acute Tests

Age of Test Species

Endpoint

Mean LC50a for

Equivalent HCO3

Species

(posthatch)

Measured

NaHCO3 (mg/L)

LOECb,c (mg/L)

Fathead minnow

4 days

Survival

1,643

1,118

Pallid sturgeon

4 days

Survival

1,158

788

Chironomus

4 days

Survival

Z,920

5,391

Fathead minnow

2 days

Survival

1,793

1,220

Pallid sturgeon

4 days

Survival

1,828

1,244

Hyallela azteca

4 days

Survival

6,384

4,345

African clawed frog

4 days

Survival

1,700

1,157

Chronic Tests

Endpoint

NaHCO3 LOECc

Equivalent HCO3-

Species

Length/Type of Test

Measured

(mg/L)

LOECb (mg/L)

Fathead minnow

60 days

Survival

500

340

White sucker

53 days

Growth

450

306

Ceriodaphnia

7 days

Reproduction

510

347

African clawed frog

Modified FETAX embryos

Malformations

1,108

754

aLC50 is defined as the concentration that resulted in 50 percent mortality of test species. fcLOEC is the lowest observed effects concentration (higher concentrations resulted in adverse effects noted in Endpoint Measured).

cFor comparison to bicarbonate values in CBM produced water and the Powder River (Table 5.1). SOURCES: Skaar et al. (2006); Farag et al., 2010.

amphibian species (African clawed frog) was highly sensitive to sodium bicarbonate (LC50 = 1,700 mg/L or 1,235 mg/L bicarbonate). However, acute toxicity was much lower for two of the invertebrate species tested (Chironomus and Hyalella), with LC50 values ranging from 6,384 to 7,920 mg/L sodium bicarbonate (4,637 to 5,753 mg/L bicarbonate).

Chronic (longer-term) toxicity was observed for fish and invertebrates at much lower sodium bicarbonate concentrations, with lowest observed effect concentrations (LOECs) ranging from 450 to 510 mg/L sodium bicarbonate, or 327 to 370 mg/L bicarbonate. These laboratory findings are relevant to the Powder River Basin because the median concentration of bicarbonate in produced water from CBM wells is 712 mg/L and concentrations can exceed 3,000 mg/L (see Table 6 in Bartos and Ogle, 2002). In situ toxicity tests conducted in several tributaries of the Powder and Tongue rivers showed significant mortality when levels of sodium bicarbonate exceeded these laboratory thresholds. However, the commit-

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