Interest in coalbed methane (CBM) extraction in Montana and current development of the industry in Wyoming has resulted in significant focus on the issue of soil x plant community x irrigation water quality interactions/compatibility. Coalbed methane extraction is typically accompanied by the necessity to dispose of significant volumes of water. Currently proposed approaches to disposal of CBM product water include impoundment, discharge into and blending with existing surface waters or ephemeral stream channels, land spreading, and various configurations of irrigation. Although the chemical characteristics of CBM product water vary significantly from discharge point to discharge point, it is generally agreed that the two principal water quality parameters of significance with respect to disposal of CBM product water in association with soil material are salinity and sodicity. Numerous instances of elevated salinity and sodicity levels in discharge water have been reported in the Powder River basin and, correspondingly, questions have been raised regarding the potential interaction of CBM product water with soil representative of land surfaces where CBM product water may be dispersed. The objective of the research summarized here is to assess the consequence of periodic flood irrigation with simulated CBM product water on: 1) soil physical and chemical properties, 2) soil drainage water quality, and 3) the production capability of two different forage plant communities. Assessment of plant survival and productivity, preservation of soil structure, and the reduction of salinity and sodicity in water after it has passed through the root zone are also being completed.

^1/J. W. Bauder, Professor, ^2/K. Pearson, ^3/S.D. Phelps, and ^3/N. Warrence, Research Associates, and ^4/J. Drake, Research Technician;
Department of Land Resources and Environmental Sciences, Montana State Univeristy-Bozeman.

Six growth boxes 36" long x 24" wide x 22" deep were constructed in the Plant Growth Center (PGC) at Montana State University. Each box was instrumented with a drainage system consisting of 1" PVC slotted pipe connected to a central drainage outlet. A layer of 3/4" pit run coarse gravel was placed over the drainage pipe, to a depth of approximately 2". A single sheet of weed barrier fabric cloth was placed over the gravel. A 1" layer of river washed sand was placed on top of the weed barrier. Soil material was then placed over the top of a sand layer to a standing depth of 20" of fine soil material. Soil material used to fill the boxes was collected from the 0-6" and 6-20" depths of a Haverson silty clay/silt loam soil at the Gay Ranch, Powder River, Broadus, MT, separated, transported to MSU, and placed in the boxes in 6-20" and 0-6" depths. The soil was placed in the boxes so that the B horizon material constituted the 6-20" depth, while the Ap material made up the 0-6" surface depth.

Haverson silty clay loam is an alluvial soil occurring on flood plains, stream terraces and foot slopes. On foot slopes a clay alluvium underlies the surface soil at a depth of more than 60 inches. The soil has a slow runoff and moderately slow permeability. It has an available water capacity of 10-12" per five feet soil depth. Its organic matter content is generally low, and it has a medium fertility.

Figure 1. Stages of chia box construction.

After the soil was placed in the boxes, each box was saturated with PGC water, drained, and resaturated 5 times. The boxes were then allowed to drain 96 hours. Watermark soil moisture sensors were installed at 20" and 6" in two of the boxes and connected to Hansen AM400 continous soil moisture monitors.

Bulk soil samples were collected, prepared and shipped to MDS Harris Laboratories, Lincoln, NE for chemical baseline characterization. Analyses requested included: pH, EC, base cations, carbonate, bicarbonate, sulfate, N, P, K and cation exchange capacity. Four replicates of each depth (0-6", 6-24") were sent to the lab.

Table 1. Baseline Soil Characterization (Average of 4 Subsamples)
Parameter	0-6"	6-20"
pH	7.7	7.5
EC (mmhos/cm)	1.63	1.03
% OM	2.75	3.075
Olsen P	17.5 ppm	11.5 ppm
K	256 ppm	343 ppm
Mg2+ (exchangeable)	830 ppm	574 ppm
Mg2+ (extractable)	3.6 meq/L	2.4 meq/L
Ca2+(exchangeable)	5147 ppm	3943 ppm
Ca2+(extractable)	7.0 meq/L	5.3 meq/L
Na+(exchangeable)	288 ppm	64.8 ppm
Na+(extractable)	7.0 meq/L	1.6 meq/L
CEC (MDS)	34.6	25.7
CEC (MSU)	26.5	18.5
ESP (soil)	3.6%	1.1%
Bulk Density (g/cc)	based on dry volume = 1.632/ based on wet volume = 1.33 (@23% shrinkage)

Site/soil conditioning: Following initial soil conditioning by means of the repeated wetting/drying/drain down process, all boxes were planted to McNeal spring wheat (Triticum aestivum L.) at a seeding rate of 60 seeds/square foot. The purpose of the high density spring wheat seeding was to provide rapid growth and full plant cover to facilitate re-establishment of both a root system and the columnar structure characteristic of the study soil. Wheat was grown to early boot stage stage (stage 10 Large and FAO, Feekes stage 10) with the specific intent that the plants would also deplete the available soil moisture within the soil boxes. Wheat biomass was harvested, dried, and weighed to determine total dry matter yield from each box and to determine uniformity of production. Individual dry biomasses per box were 243, 223, 230, 212, 2116, and 218 g/box. Average biomass production per box was 224 g, equivalent to 1.78 tons/acre.

Figure 2. Chia boxes planted to spring wheat.	Figure 3. Hansen AM400 soil moisture monitor.

Figure 4. Dr. James Bauder with a Hansen AM400 soil moisture monitor.

Following spring wheat harvest, the soil material in the boxes was again sampled using a 2" coring technique. Soil core samples were obtained from each box, retained for integrity, and partitioned for chemical analyses and physical characterization. Chemical characterization consisted of EC (saturated extract), pH, base cations, N, P, and K. Physical characterization included bulk density (from cores), falling head double ring infiltration rate into dry soil, theta-h relationship (soil moisture characteristics on shaved samples), modulus of rupture and COLE on ground samples. These data constitute baseline conditions.

Experimental water qualities were chosen based on existing aggregate CBM product water data from the Powder River Basin and from the 30-year data from the Powder River at Moorhead, MT. The values represented average concentrations, based on 10-90 percentile values or ranges, i.e., the thresholds were derived by deleting the outliers falling in the < 10 percentile group and the outliers falling in the > 90 percentile group. Table 2 summarizes the characteristics of Powder River Basin CBM product water. Figures 5-8 display various relationships between assessed parameters.

Table 2. Powder River Basin CBM Product Water Criteria

Constituents	Average Concentration (mg/L)
Sodium (Na+)	311.25
Calcium (Ca2+)	34.84
Magnesium (Mg2+)	18.65
Approximate EC (dS/L)	1.80
Approximate TDS (mg/L)	1245
Approximate SAR	12

Figure 5. EC versus TDS of Powder River and CBM product water.

Figure 6. EC versus SAR of Powder River and CBM product water.

Figure 7. EC versus sodium concentration of Powder River and CBM product water.

Figure 8. Sodium concentration versus SAR of Powder River and CBM product water.

Additional instrumentation: A third set of Watermark sensors will be installed in one of the remaining boxes and these sensors will be connected to the existing box. A second Hansen AM400 monitoring box will be installed and Watermark sensors installed at 6" and 20" in each of the three remaining boxes. The boxes will be monitored continuously for moisture characteristics.

Replication: three replications of the two plant community treatments. All boxes will be treated with the same water quality, i.e., simulated CBM product water, SAR ~ 12, TDS ~ 1245 mg/l.

Irrigation strategy: Irrigation applications will be based on a soil moisture status of 100 cb (1.0 bars), i.e., when average soil moisture drops to 100 cb, irrigation water (CBM product water) will be applied. On the basis of measurements of bulk density, a determination will be made of the volume of water required to bring the moisture content in each box from the equivalent at 100 cb (1.0 bars) to 10 cb (1/10 bar). This value will be determined as a volume application. On the basis of this value, an irrigation leaching fraction of 25% will be used to determine the gross irrigation application during each event. CBM product water will then be added accordingly, using a regulated drip system. The purpose of the drip system will be to allow for water to be added at the rate of surface infiltration but to minimize the potential of boundary flow along the boundary between the soil and the box wall. Water will be applied in a concentric pattern, from the center outward at a rate that equals the infiltration rate, i.e., incipient ponding. The total volume of water applied will be equivalent to the soil moisture deficiet (between 1.0 bars and 0.1 bars) plus 25%; and the length of time for application will be recorded as well.

Plant communities: A native community consisting of Pryor slender wheatgrass (Elymus trachycaulus), Shoshone beardless wildrye (Leymus triticoides) and Basin wildrye (Elymus cincereus) and an agricultural community consisting of sugar beets (Beta vulgaris), cultivated oats (Avena sativa), and Haybet or Valier hay barley (Hordeum vulgare) will be used thoughout the experiment. Each community will be planted after harvest of the springwheat and baseline soil sampling. Immediately following planting, each box will be irrigated with a volume of water sufficient to bring the soil moisture level from 2 bars to 0.1 bar or with an appropriate volume to bring the soil back to field capacity. Soil moisture monitoring will begin at this point.

Outflow sampling: Each box will be instrumented such that outflow drainage samples can be collected on a routine basis. Total outflow will be collected and monitored on a 12 hour schedule following each irrigation event until drainage ceases. Total outflow volume will be measured and the entire sample will be collected during each sampling, subsampled, preserved to avoid atmospheric contamination, archived and subsequently analysed for pH, EC, base cations, sulfates, carbonates, and bicarbonate.

Plant measurements: Parameters to be measured include yield, forage quality, and salt and sodium uptake.

Post irrigation cycle measurements: Following the irrigation cycling, the soil material in the boxes will again be sampled using a 2" coring technique and analysed for chemical and physical characterization. Replicate core samples will be obtained from each box, retained for integrity, and partitioned for chemical analysis and physical characterization. Chemical characterization wil consist of EC, pH, base cations, N, P, and K. Physical characterization will include bulk density (from cores), falling head double ring infiltration rate into dry soil, theta-h relationship (soil moisture characteristics on shaved samples), modulus of rupture and COLE on ground samples.

Calculations: ESP, SAR at sampling times, change in soil salinity, salt outflow, salt uptake, plant performance.