Coal Bed Methane Product Water

The quantity of the CBM product water: 
Extraction of CBM involves pumping large volumes of water from the saturated coal seam in order to release the water pressure holding the gas in the coal seam. What to do with this volume of often marginal-quality CBM product water is a source of much debate. Each well produces 5 to 20 gallons of water per minute. At 12 gallons per minute, one well produces a total of 17,280 gallons of water per day. It is common to have to have one well every 80 acres, and in the Powder River Basin, there are up to three methane-bearing coal seams. Therefore, there may be up to three wells per 80 acres.

The quality of CBM product water and its effects on soil:  CBM product water has a moderately high salinity hazard and often a very high sodium hazard based on standards used for irrigation suitability. Irrigation with water of CBM product water quality on range or crop lands should be done with great care and managed closely. With time, salts from the product water can accumulate in the root zone to concentrations which will affect plant growth. Saline conditions stunt plant growth because plants must work harder to extract water from the soil.
The sodium hazard of CBM product water poses additional threats to certain soil resources. Sodic irrigation water causes soil crusting and impairs soil hydraulic conductivity, adversely affecting water availability and aeration and subsequent crop growth and yield. Upon wetting of soils containing swelling clay, sodium causes the degree of swelling in the clay to increase, leading to dispersion and migration of clay particles. Current research at Montana State University shows that water with sodium levels equal to typical Montana CBM product water can degrade the physical and chemical properties of heavier, clay soils, making such soils completely unsuitable for plant growth.
The risk of sodium degradation has been observed in other soil textures. Jim Oster (personal com.) observed crusting, poor soil tilth, hardsetting and aggregate failure on a sandy loam soil irrigated with water with EC ~ 1 and SAR ~ 7. Minhaus (1994) saw irreversible and severe reduction in infiltration on sandy loam soil with long term irrigation under high SAR water followed by monsoon rain. 
There are many factors in addition to soil textures that affect infiltration rates. Mineralogy, lime, sesquiozides, organic matter content, cultivation, irrigation method, wetting rate, antecedent water content and time since cultivation all play a roll in infiltration. The only way to be certain of the impacts of saline/sodic irrigation water on the soil is to periodically sample and test the irrigation water and the soil.


The quality of CBM product water and its effect on plants: 
Disposal of the quantities of CBM product water into stream channels and on the landscape poses a risk to the health and condition of existing riparian and wetland areas. High salinity and sodium levels in product water may alter riparian and wetland plant communities by causing replacement of salt intolerant species with more salt tolerant species. It is well recognized that encroachment of such noxious species as salt cedar,
Russian olive, and leafy spurge is enhanced by saline conditions.

What are the current management practices for disposal of CBM product water? 
Currently, CBM product water in the Powder River Basin is managed by the following methods: 
•  Discharged into a stream channel - Although direct stream discharge is no longer permitted on new wells, existing operations were "grandfathered" and are still discharging directly into streams. Also, proposals are being advanced to allow regulated discharges during certain flow conditions. 
•  Impounded - This method involves constructing a pond in which CBM product water is stored or allowed to infiltrate to the subsurface. There are several terms for these impoundments: "holding ponds", "zero discharge ponds" or "infiltration ponds". Although they do not directly discharge water on the land surface, most impoundments are not lined and do discharge to the subsurface. Some percentage of seepage flow from impoundments is likely to reach stream channels via subsurface flow.
•  Land applied to crop or rangeland - through some form of irrigation equipment. 
•  Other uses - CBM product water is also used for dust control and, in some cases, is being used by coal mines. 
Another option proposed for disposal of CBM product water in eastern Wyoming and Montana is to reinject the CBM product water back into an aquifer(s). This practice occurs in the southwest U.S., where CBM product water is injected into formations below CBM-bearing coal. This approach avoids surface discharge. Many opinions exist, and the feasibility - economic, physical, and environmental - of either reinjecting CBM product water to the coal seam from which it was pumped or injecting it into an aquifer above or below the CBM-bearing coal seam is being investigated.

Can CBM product water be treated to make it more usable?  The only ways to lower the salt concentration in saline and/or sodic water are through dilution with non-saline water, reverse osmosis, or salt precipitation with an evaporation process that leaves salt behind and traps evaporated water. Reverse osmosis is expensive, and evaporation and salt precipitation treatment is neither economical nor feasible with large quantities of saline CBM water. Dilution of CBM product water is only possible if there is a large source of non-saline water with which to dilute the saline water. 
It is possible to alter the chemistry of sodic water by adding calcium and magnesium. This does not eliminate or reduce sodium, but changes the ratio of sodium to other salts, thus decreasing the sodium adsorption ratio (SAR). The net result is more saline water with the sodium salt still dissolved in the water. This approach is not likely to work with CBM product water because the added calcium will combine with carbonate from the
CBM water and precipitate out as calcium carbonate (lime). To make this process work,CBM product water must be de-gassed of carbonate by addition of acid, or additional calcium must be made available in the soil by acidification from sulfur additions.Unfortunately, addition of more salts to water or soil may result in conditions too saline for plant growth.

Source:
Frequently Asked Questions Coal Bed Methane (CBM) 
Kristin Keith and Jim Bauder, Montana State University-Bozeman 
John Wheaton, Montana Bureau of Mines and Geology (2003)