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TECHNICAL REPORTS

Ecological Risk Assessment

Soil Chemical Changes Resulting from Irrigation with Water Co-Produced with Coalbed Natural Gas

Department of Renewable Resources, 1000 East University Avenue, University of Wyoming, Laramie, WY 82071-3354

^* Corresponding author (gfv{at}uwyo.edu)

Received for publication January 19, 2005. Land application of coalbed natural gas (CBNG) co-produced water is a popular management option within northwestern Powder River Basin (PRB) of Wyoming. This study evaluated the impacts of land application of CBNG waters on soil chemical properties at five sites. Soil samples were collected from different depths (0–5, 5–15, 15–30, 30–60, 60–90, and 90–120 cm) from sites that were irrigated with CBNG water for 2 to 3 yr and control sites. Chemical properties of CBNG water used for irrigation on the study sites indicate that electrical conductivity of CBNG water (EC_w) and sodium adsorption ratio of CBNG water (SAR_w) values were greater than those recommended for irrigation use on the soils at the study sites. Soil chemical analyses indicated that electrical conductivity of soil saturated paste extracts (EC_e) and sodium adsorption ratio of soil saturated paste extracts (SAR_e) values for irrigated sites were significantly greater (P < 0.05) than control plots in the upper 30-cm soil depths. Mass balance calculations suggested that there has been significant buildup of Na in irrigated soils due to CBNG irrigation water as well as Na mobilization within the soil profiles. Results indicate that irrigation with CBNG water significantly impacts certain soil properties, particularly if amendments are not properly utilized. This study provides information for better understanding changes in soil properties due to land application of CBNG water. These changes must be considered in developing possible criteria for preserving fragile PRB ecosystems.

Abbreviations: CBNG, coalbed natural gas • CEC, cation exchange capacity • EC_e, electrical conductivity of soil saturated paste extracts • EC_w, electrical conductivity of CBNG water • PRB, Powder River Basin • SAR_e, sodium adsorption ratio of soil saturated paste extracts • SAR_w, sodium adsorption ratio of CBNG water