The Powder River Basin in Wyoming and southern portions of Montana hosts extensive reserves of natural gas in coal seams within near-surface sediments of the Fort Union Formation. The coal seams must be de-pressurized by pumping water to facilitate release of the coalbed natural gas (CBNG) or methane contained in the coal. This produced water naturally contains moderate levels of dissolved ions in which sodium is the dominant cation and bicarbonate the primary anion. The electrical conductivity (EC) and sodium adsorption ratio (SAR) typically range from 1,000 to 2,500 µS/cm (µmhos/cm) and 10 to 60 respectively. The produced water is among the better quality groundwater in southeastern Montana for domestic and stock water uses.
Irrigators that rely on Tongue River water for crop and forage production have expressed concern about the potential adverse impact that CBNG development may have on irrigation water quality. Currently, the Tongue River enjoys good quality water that is used to irrigate more than 20,000 acres of land while supporting a healthy fishery within and just below the Tongue River Reservoir. Recently, numerous programs have been implemented to protect water quality for irrigation and other uses in southeastern Montana including development of stringent water quality standards for electrical conductivity and sodium adsorption ratio, extensive surface water monitoring, and development of basin wide surface water models and water quality control programs.
The AMPP was designed by Neal Fehringer, Certified Professional Agronomist; Kevin Harvey, Certified Professional Soil Scientist; and, Dr. Bill Schafer, Soil Scientist. Fidelity Exploration & Production Company, a coalbed natural gas producer operating in Montana, is the sponsor of the program. The soil and crop testing program will help irrigators better understand the potential effects of CBNG development on their irrigated crops. This package of soil sampling and analysis, cropping system evaluation, and interpretation is being provided at no cost to cooperating irrigators who use Tongue River water.
The purpose of this program is to measure baseline soil characteristics. Subsequent annual soil sampling will also help identify and manage any soil chemical trends related to CBNG development that could impair future crop yields. The program consists of three tiers of sampling including:
Tier 2 - Soil Sampling
In selected fields spaced at intervals along the Tongue River (and potentially on its major tributaries Hanging Woman Creek, Otter Creek, and Pumpkin Creek), additional soil sampling will be performed to determine seasonal changes in soil chemistry, and to assess soil characteristics at depths of up to 8 feet. Approximately 8 to 12 fields will be sampled in the Tier 2 program. Typical soils that will be targeted for sampling in Big Horn and Rosebud County will include the Havorson, Glenberg, Havre, Yamac, Busby, Glendive and Kobar series. In Custer County (including the T&Y Irrigation District east of Miles City along the Yellowstone River) typical soils that will be targeted for sampling include the Kobase, Marvan, Vanda, Hanley, Glendive, Havre, Harlake, and Yamacall series. Two sets of samples will be collected at each Tier 2 location and a third set of samples will be collected at two sites. Each set of samples will address a specific issue.
Composite Samples: First, a representative number of composite sub-samples will be collected from a portion of the field that consists of a single soil mapping unit from the County Cooperative Soil Survey. Composite samples will be collected from the following depth intervals: 0 to 6, 6 to 12, 12 to 24, 24 to 36, 36 to 60, and 60 to 96 inches. Samples will be air-dried and prepared in a flail-mill, then sieved through an ASTM 10 mesh sieve. Analysis will be performed on the fine earth (< 2 mm) fraction. Coarse fragment content and percent moisture content will also be reported. Analytical methods will be taken from Agronomy Monograph Number 9.
Quality assurance samples will consist of 1 blind field duplicate at a frequency of 1 in 20. Laboratory quality assurance will consist of 1 lab duplicate per batch but not less than 1 in 20 samples, 1 lab control sample per batch but not less than 1 in 20 samples, and analyte spike recoveries as appropriate.
Reference Pedon Samples: A backhoe pit will be excavated in the same field sampled above. A detailed soil profile description will be prepared of the soil using methods and nomenclature described in the National Soils Handbook. Samples will be collected from each genetic horizon described, and samples will extend to at least 96 inches in depth. Samples will be prepared and analyzed as described above. The quality assurance program will also be similar for these samples. Clay mineralogy will be performed on the clay-sized particles of the fine earth fraction from up to 2 selected horizons.
Grid Samples: A final set of samples will be collected to assess the spatial variability of soil properties. At two fields, samples will be collected from three depth increments at about 20 locations within a field. Each individual sample will be submitted for analysis without compositing. In this way, the spatial variability of each soil property can be described.
Tier 3 - Irrigated Crop and Forage Test Plots
Numerous water management strategies have been developed by petroleum companies to store, utilize, or discharge CBNG production water. Some of the water management options may entail discharge of production water into surface waters, so long as the receiving water can comply with irrigation water quality standards. Consequently, irrigators should not expect to apply undiluted CBNG production water except in special circumstances where “managed irrigation” programs are developed near the CBNG fields. Under managed irrigation, texturally suitable soils will be amended with chemicals such as gypsum to reduce the exchangeable sodium percentage in the irrigated soils.
Irrigators using water from the Tongue River may experience slight changes in the EC and SAR in their water supply if CBNG development expands in the Tongue River basin, but only within the prescribed water quality limits adopted by the State of Montana, which were developed to protect irrigation uses of water. In order to evaluate the potential effects associated with blending CBNG production water with Tongue River water, a series of irrigated test plots will be constructed.
Two sets of test plots will be developed, one on the upper river on medium textured soils and one set on the lower river on finer textured soils. Each test plot area will include sprinkler and flood-irrigated plots. The experimental design will consist of seven mixtures of water ranging from Tongue River water to CBNG dominated water. While water quality criteria will likely limit CBNG discharge to a dilution ratio on the range of 1 to 10 or less, the plots will evaluate water mixtures with proportionally greater amounts of CBNG water so that a minimum effects threshold can be determined. Each plot will be replicated three times at each site. Additionally, a split plot design will be used so that two rotations can be assessed (e.g. continuous alfalfa vs. hay barley – alfalfa or beans – beets – spring wheat). Soil and crop/forage samples will be collected annually to assess trends in soil chemistry, yield or quality.