Coalbed Methane Produced Water In The Western Us

and sulfate concentrations at some point in their history (flushed or upward trend), and 6 percent showed improved groundwater quality (see Figure 5.2). Eight of the wells did not clearly fit into any category. Of the 170 wells, 12 exceeded Class III standards (changed from Class III to IV): seven of the monitored wells exceeded standards for sulfate or TDS and five exceeded standards for selenium only. Confined artesian aquifers6 generally had greater depths to groundwater and lower percentages of wells exhibiting a decrease in water quality. In analysis of some of the same data, the ALL Consulting (2008) study concluded that impacts of CBM produced water impoundments on shallow groundwater were site specific and influenced in large part by the shallow subsurface geology of the area (on-chan-nel versus off-channel). Data gaps identified by the 2008 study included lack of knowledge of the volumes of water discharged into impoundments; absence of analysis of groundwater and CBM produced water for major cations and anions such as calcium, magnesium, sodium, sulfate, chloride, and bicarbonate; and need for evaluation of impoundment inflows to deeper groundwater in order to continue to monitor the effects of CBM produced water infiltration.

Summary of Groundwater Studies

Primary considerations with respect to CBM produced water and effects on groundwater are (1) drawdown of groundwater levels in coalbeds as a result of pumping water during CBM extraction and (2) changes in groundwater quality beneath surface impoundments associated with leakage of stored CBM produced water. Groundwater drawdown in any shallow groundwater aquifer as a result of water and methane extraction from CBM operations is a function of the depth to the target coalbeds and the degree of hydraulic connection between CBM targets and other local or regional aquifers. Due to the great distance between the deep coalbeds and shallow groundwater aquifers and to aquifer compartmen-talization, pumping water during CBM extraction in basins with deep methane-bearing coals (e.g., the San Juan and Raton basins) is unlikely to cause lowering of the water table of shallow alluvial aquifers.

Groundwater monitoring networks established for coalbeds in the Powder River Basin in Montana and Wyoming have measured the degree to which CBM production has affected water levels in coalbed aquifers, either in proximity to areas of CBM development or near the fringes of the coalbed outcrops. Measured drawdowns ranged between 20 and 625 feet below prepumping levels. These coalbed aquifers are not necessarily the same as shallow alluvial aquifers used frequently as the principal source of water in the area. On the edge of

6An artesian aquifer is a confined aquifer (bounded by impermeable geological strata) that contains groundwater that can flow upward through a well (an "artesian well") without pumping.

FIGURE 5.2 Graphical distribution of the classification of groundwater data from 162 compliance monitoring wells associated with 144 CBM produced water impoundments. The data showed stable, upward, flushed, or improved geo-chemical trends in shallow groundwater beneath impoundments. "Improved" indicates reductions in TDS and sulfate concentrations in groundwater over time. Importantly, qualitative classifications based on trend analyses do not imply magnitude or cause of changes to groundwater quality. Another eight wells did not fit clearly into any of the four categories. SOURCE: Adapted from C. Steinhorst, Wyoming DEQ Water Quality Bureau (WQB), personal communication, Dec. 22, 2009 and August 23, 2010.

the basin in Montana, near recharge areas, 75 percent recovery of the water levels in one of these coalbed aquifers occurred within five years when pumping was discontinued. In the center of the area monitored, where pumping was most aggressive, groundwater levels in the affected coalbed for which data were available have recovered 87 percent in 10 years.

Observed drawdowns were less than those predicted in modeling. Although model results predict that recovery to original water levels in the absence of pumping may take decades, the extent of water level drawdown in the coalbeds and the time to recovery depend on proximity to CBM production wells, site-specific aquifer characteristics, and proximity of drawdown monitoring sites to recharge areas. The water in coalbeds used for methane extraction in the San Juan and Raton basins, and in at least some portions of the Powder River Basin, has been documented to be nonrenewable fossil water (see Chapter 2). The long-term implications of mining fossil water, or the degree to which waters may be considered fossil, have not been thoroughly studied nor included as part of the discussion of management approaches for CBM produced water.

About 83 percent of the impoundments in the Powder River Basin ofWyoming are on-channel and about 6 percent are unlined and off-channel, with intent to recharge groundwater beneath impoundments. The remaining impoundments are lined and off-channel, with the aim to reduce or prevent leakage and infiltration of CBM produced water into underlying shallow alluvial groundwater. The natural and human-influenced differences between individual impoundments—including the substrate (e.g., soil or bedrock) on which

0 0

Post a comment