Coalbed Methane Produced Water In The Western Us

ment for the Tongue River through water year 2005. The study also noted that none of the assessed tributaries of the river met water quality standards for specific conductance either before or after CBM development. Surface water measured at the two mainstem Tongue River stations in Montana met applicable SAR and specific conductance standards before and after CBM development.

Dawson (2007b) used specific conductance and SAR data to determine if water quality in the Powder River at Moorhead, Montana, had changed since CBM production began in the Powder River Basin. When Powder River water quality data were considered in aggregate, with adjustments for wet and dry periods, no statistically significant effects on SAR and specific conductance values from CBM operations were evident. The results of Dawson's Powder River water quality analysis were influenced by variations in climatic conditions during the years of record that were available for comparison and the influence of the quality of produced water associated with historical conventional oil and gas operations prior to CBM development on Powder River water quality.

Another study by the U.S. Geological Survey (USGS) in conjunction with the Wyoming DEQ(Clark and Mason, 2007) compared long-term trends in water quality from 1975 to 1981 with those from 2001 to 2005. Concentrations were corrected for the influence of changes in flow. The researchers found statistically significant increases in SAR in the Powder River downstream of CBM produced water inputs and decreases in SAR values in the Powder River downstream of Clear Creek (due to diluting effects from a non-CBM-influenced tributary near the Montana border). However, the effects of CBM discharges on Powder River water quality were difficult to discern because of the effect of inputs from Salt Creek, a Power River tributary with traditional oil and gas operations.

A study by Wang et al. (2007) examined even longer-term water quality trends (1946— 2002) at four USGS gauging stations on the Powder River in Wyoming and Montana. The researchers used statistical methods to examine trends in flow-corrected water quality before and after 1990 (the beginning of CBM development in the Powder River Basin) and found little change in salinity but statistically significant increases in sodicity as measured by SAR. The study also found smaller differences in water quality among downstream stations after CBM development and increasing differences in water quality between downstream stations and the most upstream station after CBM development began.

ALL Consulting (2008) used specific conductance and SAR data to evaluate changes in water quality and streamflow for five watersheds in the Powder River Basin with CBM development and produced water discharge. Conclusions about the effects of CBM produced water discharge were complicated by the influence of drought and by limited data at certain stream stations that preceded or postdated CBM development in the area. The study interpreted any observed changes in surface water quality as being due to prolonged drought rather than CBM production or produced water discharges.

A comparison between the major ion chemistry for the Powder River and CBM pro duced water by Brinck et al. (2008) showed that Powder River water and CBM produced water have similar TDS and sodium contents, but that the Powder River has lower SAR values due to higher calcium and magnesium concentrations than CBM produced water. Because the natural salinity of the river is similar or higher than the salinity measured in the CBM produced water, TDS was suggested not to be an effective tracer of produced water contributions to the Powder River by the authors.

Smith et al. (2009) evaluated changes in nitrogen compounds (ammonium, nitrate, and nitrite) in streams and rivers receiving CBM produced water discharges in the Powder River Basin. Ammonium, at concentrations in the range of 1 to 3 mg/L, is frequently present in CBM produced water at the wellhead. In unimpaired surface waters, ammonium is seldom present in concentrations exceeding 0.1 mg/L. Ammonium concentrations decreased with distance from the discharge source while concentrations of nitrate and nitrite increased downstream of discharge points. The extent of these changes in concentration varied, depending on the ephemeral channel type. Collectively, the nitrogen introduced into the Powder River from CBM sources resulted in substantial increases in total dissolved inorganic nitrogen (DIN) loads downstream of the point of permitted discharge of CBM water directly into the Powder River or into the conducting channel.

Rapid development of the CBM industry and discharges of large volumes of produced water into ephemeral and perennial streams and rivers have stimulated much interest in capabilities to track or trace produced water from the point of discharge to downstream locations. A similar interest has been expressed with regard to tracking the fate of produced water discharged to impoundments. These interests have been particularly expressed in the Powder River Basin, and recent studies of isotopes of strontium and isotope ratios of carbon have identified unique isotope signatures in CBM produced waters of the basin. These signatures, much like fingerprints, have been used to uniquely identify CBM produced water, assess connectivity and comingling of waters produced from differing coal deposits, and determine the presence of CBM produced water in surface water in the Powder River Basin (Sharma and Frost, 2008; Brinck and Frost, 2009; Frost et al., 2010). During formation of biogenic methane, 12C is preferentially removed by methanogenic bacteria, leaving the dissolved inorganic carbon (DIC) of the formation water enriched in 13C. The resulting high (13C/12C) ratio of DIC for CBM produced water is distinct from the ratio of the same inorganic carbon ratio (13C/12C) of surface water or groundwater which does not contain CBM produced water. For ease of comparison and explanation, the carbon isotope ratios of water samples are compared to a defined international standard ratio. The difference between the carbon isotope ratio of the sample in question and the international standard is referred to as "delta 13," with a notation of 513Cdic (see Figure 5.3). Because of the relatively small differences that are measurable between the carbon isotope ratio of the sample in question and the international standard, the differences are expressed as tenths of percentages, with a notation of "per mil."

COALBED METHANE PRODUCED WATER IN THE WESTERN U.S

PR11

River now direction

HEADWATERS

PR11

O 2006 low flow A 2007 high flow

River now direction

HEADWATERS

Relative position along the Powder River

CONFLUENCE

FIGURE 5.3 813Cdic values of DIC of water samples from along the Powder River and its tributaries, under low- and high-flow conditions (September 2006; June-July 2007). Locations: PR1 (farthest upstream sampling location, WY); PR30 (farthest downstream sampling location, confluence of Powder River with Yellowstone River, MT). PR8 (Beaver Creek), PR11 (Flying E), and PR24 (Little Powder River) are tributary locations. Sample sites PR1 through PR15 were located in Wyoming; all other sample sites were located in Montana. A single sample was collected at each lopation during the low-flow or high-flow sampling times. In total, 17 samples were collected each time—14 from the main stem and three from tributaries (samples PR8, PR11, and PR24). Note that carbon isotope signatures can only be used as a fingerprint in this way in locations where methane is produced biogenically (see Chapter 2). SOURCES: Sharma and Frost (2008), Frost et al. (2010).

Sharma and Frost (2008) found that the 813CdiC for produced water samples collected from different coal zones and from different parts of the Powder River Basin were enriched in 813CdiC, ranging from +12 per mil to +22 per mil as a result of the biogenic production of methane, which preferentially removes 12C. In contrast, water samples not influenced by CBM produced water typically have negative 813CdiC values. Sharma and Frost subsequently collected water samples from the entire length of the Powder River for two different flow conditions (low and high). Values of 813CdiC for all samples ranged from -11.4 per mil to +16.4 per mil, as shown in Figure 5.3. Sharma and Frost concluded that samples with significantly positive 813CdiC values reflected inputs of CBM produced water.

The headwaters area of the Powder River in Wyoming, represented by sample sites PR1 through PR5, is considered upstream of CBM development. Samples from these locations had 813CdiC values ranging from between -8.3 and -11.4 per mil, suggesting that the water in this section of the river was relatively uninfluenced by CBM produced water. Samples collected progressively downstream (PR6 and PR7) had 813CdiC values that were less negative (-4.7 per mil and —1.4 per mil, respectively). Sharma and Frost proposed that "these values may reflect incorporation of CBNG [CBM] water discharged from production in this area." Downstream of this point (i.e., PR8 through PR15), water samples had significantly positive 513Cdic values, reflecting "an area of more intense CBNG [CBM] development" and likely a predominance or relative abundance of CBM produced water in the river. The authors reported that "highly positive 513CDIC of Powder River samples in Wyoming . . . from . . . (PR9 to 15) suggests the presence of CBNG [CBM]-produced water in the river related to local CBNG [CBM] production."

Again referring to Figure 5.3, the authors report that samples collected in Montana all had negative 513CDIC, further noting "that surface water in Montana is little to unaffected by CBNG [CBM] production during low-flow conditions." Similar patterns were observed for samples collected during high-flow conditions.

In interpreting the data for the Powder River, it is important to recognize that 513CDIC values can be changed or influenced by a number of processes, including dilution by addition of another source of water with a different 13C/12C ratio, such as at the confluence of a major tributary like Clear Creek. Clear Creek discharges to the Powder River between sampling points PR14 and PR15. Below the confluence of the tributary and the mainstem of the river, the 13C/12C ratio will be somewhere between the 513CDIC values of the Powder River and the tributary inflow. Thus, the change in 513CDIC value between PR14 and PR23 (i.e., in crossing between the Wyoming-Montana border) reflects the diluting effect of inflows from Clear Creek, a Wyoming-originated tributary that is relatively uninfluenced by CBM produced water discharges.

Ephemeral Drainages and Impoundments

Several studies have documented increases in concentrations ofTDS, sodium, and trace elements and the pH of CBM produced water that is discharged to ephemeral drainages in the Powder River Basin. Recalling that the outfall which discharges CBM produced water into an impoundment usually represents a combination of CBM product water combined from several CBM wells (see Chapter 4), water in the impoundments reflects changes in the chemistry (1) between the end-of-pipe discharge and impoundment and (2) after the water has been sitting in the impoundments. McBeth et al. (2003) assessed changes in CBM produced water composition between discharge points and associated holding ponds within the Powder River Basin. Consistent with data reported by Rice et al. (2000), they reported that pH, specific conductance, SAR, and concentrations of TDS, alkalinity, sodium, calcium, magnesium, and potassium in CBM discharge water increased significantly as discharged water traveled downgradient in ephemeral stream channels. These findings were further substantiated by Jackson and Reddy (2007).

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