TRACKING SALT AND SODIUM BUILD UP DUE TO IRRIGATION WITH COALBED NATURAL GAS PRODUCT WATER: SOIL SOLUTION LYSIMETER AND SOIL SATURATED PASTE EXTRACT STUDIES

摘要

Irrigation with coalbed natural gas (CBNG) co-produced water is a popular management option used by many gas companies operating in northwestern Powder River Basin (PRB), Wyoming. Depending upon local conditions and production rates, a CBNG well may be productive for 2 to 20 years, with an average lifespan of 7 years. At present there are over 20,000 CBNG wells permitted or drilled in the PRB region and it is estimated that another 50,000 to 100,000 new wells will be drilled in the future. The total CBNG water production in the PRB is expected to peak at about 47,000 ha-m in 2006 and the cumulative CBNG-water production during the period 2002-2017 is estimated to be 366,000 ha-m. CBNG water is dominated by sodium (Na~+) and bicarbonate (HCO_3~-) ions and the average discharge of a single CBNG well ranges from <1 to 100 liter per minute, with pH ranging from 6.8 to 9.0, electrical conductivities (EC) from 0.4 to 4 dS/m, Na adsorption ratio (SAR) from a low of 5 to an extreme high of 70 and total dissolved solids (TDS) concentrations from 270 to 2720 mg/L. Application of poorer quality CBNG water can have significant impacts on soil physical and chemical properties. Changes in soil chemistry due to land application of CBNG waters were investigated using lysimeters installed at depths of 15, 30, and 60 cm. Soil solutions collected during June to August 2004 from soil solution lysimeters were analyzed for EC and SAR. Soil solution chemistry data were compared with EC and SAR data from saturated paste extracts of CBNG irrigated soil samples collected at the same depth. Preliminary data indicate the build up of salts and Na in the upper horizons of irrigated fields. The EC values of lysimeter soil solution samples were greater than those of saturated paste. However, SAR of lysimeter soil solution and saturated paste extracts were comparable. The results of this study will be useful to understand potential changes in soil properties due to land application of CBNG waters and to develop possible mitigating criteria for reclaiming impacted PRB ecosystems.