Coalbed Methane Produced Water In The Western Us

The initial quality of water in the impoundments reflects the chemistry of the produced water being discharged to the impoundments, which may or may not have been treated prior to disposal depending upon initial water quality and discharge requirements. However, as numerous studies have shown, impoundment water chemistry generally changes over time, with subsequent increases in salinity and trace element concentration (see Chapter 5). As noted in Table 4.2, some of the proposed ancillary benefits of disposal of CBM produced water in surface impoundments include livestock or wildlife watering, or infiltration to shallow alluvial aquifers. Livestock and wildlife watering are described in the next section under "Beneficial Use Options." The committee was unable to find documented evidence of measured alluvial aquifer recharge consequent to introduction of CBM produced water to impoundments.

Land-Applied Disposal Through water spreading and Managed Irrigation

During early stages of development of the CBM industry in the Powder River Basin, a technique referred to as "land-applied disposal" was adopted by several of the principal gas and water producers. Land-applied disposal was the term used to describe spreading of large volumes of untreated produced water across agricultural fields using sprinkler irrigation systems, with the expectation of increasing rangeland or cultivated forage production while simultaneously disposing of large volumes of produced water. Studies of this practice revealed that the technique was not sustainable in many locations, due to substantial deterioration in soil structure caused by the effect of applied salts and sodium on some soils of the basin (see Chapter 5). As a result, operators and water resource managers recognized the need for either preventive or intervention soil management actions, including the use of soil amendments (primarily gypsum as a calcium source and sulfur as an acidifying agent), in order for land-applied disposal to remain sustainable. Subsequently, the technique of landapplied disposal was relabeled as "managed irrigation," which combines the simultaneous application of amendments2 with water spreading.

Irrigation or land spreading of saline-sodic water as a mechanism to use or disperse produced water can be feasible. However, the requirements for management and sustain-ability of this practice are likely to be unachievable in marginally productive areas, in areas where scientific irrigation water management and monitoring have not previously been used, and in areas where irrigated crop production is marginally economical, except when used as a means of water disposal in comparison to water treatment or other water disposal costs. Under careful management, ancillary benefits of land spreading of CBM produced

2Soil "amendments" such as gypsum and elemental sulfur may be added to agricultural soils to liberate sodium. This release of sodium, accompanied by a supply of calcium, enhances improvement in soil structure, and sodium-affected soils can be restored to agricultural productivity. Soil amendments are sometimes called "soil conditioners."

water are rangeland habitat improvement, increased forage production, and shallow alluvial aquifer recharge.

Beneficial Use Options

In the arid and semi-arid landscapes of the study area, water, or lack thereof, is often the single most influential factor in land suitability for multiple uses. Under most circumstances, the addition of water is presumed to result in enhanced landscape quality, whether as a result of increased forage production for livestock and wildlife grazing and habitat, sustained instream flows during dry periods, or sustainability of diverse communities of native plant species. At present, however, little evidence or concerted effort exists to document that CBM produced water has been put to beneficial use for rangeland, wildlife, or stream augmentation. Although the long-term effects of putting CBM produced water to widespread beneficial use in these specific applications are not known, the next sections describe both known (and practiced) beneficial uses as well as those that are not widely applied or documented.

Surface Irrigation

Livestock production is the most economically significant agricultural land use in many locales of the western United States where CBM production has expanded rapidly in the past decade. Most of these areas are characterized by semiarid climates, where evaporative demand far exceeds annual precipitation. Correspondingly, with the exception of stream and river floodplains and mountain valleys, most of the associated landscapes are "rangelands," dominated by sparsely growing native grasses, forbs,3 shrubs, and drought-tolerant woody plant species. Livestock production is sustained by rangeland and forest grazing, supplemented by winter feeding of grass and alfalfa hay reserves harvested along stream and river corridors during the summer growing season. Where water of sufficient quantity and quality is available, irrigation has been developed to expand livestock forage production as a source of winter feedstocks. In 2007, Montana and Wyoming produced approximately 6 million tons of hay (for livestock feed) with a gross economic value of nearly $630 million.4

Correspondingly, irrigation is a mainstay of the agricultural industry tied to livestock production in the western United States. Abundant supplies of water with salt concentrations low enough to meet water quality requirements of irrigated croplands offer the potential to supplement and replace existing water supplies, while doubling or tripling the capacity of arid landscapes to produce feed for livestock. However, neither all water nor all

3Forbs are herbaceous flowering plants.

4Statistics are sourced from the National Agricultural Statistics Service, available at www.nass.usda.gov/QuickStats/in-dexbysubject.jsp?Pass_group=Livestock+%26+Animals (accessed January 27, 2010).

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