Subsurface drip irrigation application of coalbed methane produced waters: a three-way analysis of the impacts to shallow groundwater composition and storage

Since 1987, coalbed methane (CBM) production in the Wyoming portion of the Powder River Basin has generated 1.2 x 1011 m3 (4,240 bcf) of natural gas and 1.0 x 109 m3 (35.3 bcf) of co-produced water. Year-round introduction of the produced waters with potentially soil-damaging Na-rich composition into infiltration impoundments and ephemeral hydrologic systems have led to serious concerns related to the handling of the water. An alluvial aquifer site where treated Na-HCO3- to Na-SO4-type CBM water is added into the unsaturated zone (~0.9m depth) through the use of a subsurface drip irrigation (SDI) system has been studied to assess the impact to groundwater levels and composition. The SDI system is designed to provide water for alfalfa, which has roots that can reach the depth of the SDI emitters, whereas the Na-rich solutes are stored below the more Na-sensitive upper layers of the soil column. In the first two years of SDI operation, little net change in groundwater levels in wells outside of the SDI areas was observed, whereas groundwater levels have increased in some SDI areas of the site by more than 0.6 m. Changes in groundwater specific conductance, an indicator of solute load, have varied substantially, with both increasing and decreasing trends observed within SDI and non-SDI wells. To better understand the nature of these changes, concentration data (Ba, Cl, Fe, HCO3, H2O, Mg, Na, Si, SO4, and Sr) for water samples collected from 14 monitoring wells during eight rounds of quarterly groundwater sampling were arranged in a three-way array (wells x constituents x sampling events). Because the focus of this study is the chemical composition of groundwater samples, analyses focused on the molar proportions of the chemical constituents, rather than the raw molar concentration data. In attempt to find underlying multivariate structure and to identify processes controlling the data, the three-way array was examined using the recently developed Tucker models for compositional data. Results from this investigation show the potential controls and impacts of SDI application of CBM waters on a shallow groundwater system.