Compared to other natural waters, water associated with biogenic natural gas is enriched in 13-carbon. Shallow coal seams regularly contain abundant resources of biogenic gas; as such water associated with biogenic gas in these coal beds is isotopically distinct from other waters. The production of gas from coal beds requires the removal of large volumes of produced water. Thus a method of discerning coalbed reservoir water from other natural waters (surface and groundwater) is important to both the coalbed natural gas (CBNG) industry and associated environmental and regulatory agencies. Although isotopic tracers have been employed to identify coalbed natural gas produced waters, the isotopic variability within the reservoir has not been documented and explained. In this study, we present the isotopic compositions of dissolved inorganic carbon, oxygen and hydrogen for water produced from 197 CBNG wells in the Powder River Basin of Wyoming and Montana. This extensive database allows us to distinguish variations in isotopic compositions that may occur by multiple processes. These include variations that identify efficient dewatering of coal beds, variations characterizing incomplete hydraulic isolation of coal beds from adjacent strata and the subsequent mixing of groundwaters, variations related to well completion design, and variations associated with geochemical and biogenic processes that occur along groundwater flow paths. These data suggest that little change in delta C-13(DIC) occurs within the reservoir as a result of water and gas production; thus, the carbon isotopic composition informs other processes within the reservoir unrelated to coalbed natural gas recovery. The delta C-13(DIC) and delta D of groundwater vary along flow-path across the basin, reflecting different methanogenic pathways that are associated with different isotopic fractionations, and the pathways that dominate in different areas within the basin. In areas where several producing coal seams are present, the delta C-13(DIC) and delta D of produced waters from each seam are distinct. Therefore on a local scale, the isotopic composition of produced water can identify the particular coal seam from which water and gas are withdrawn. The methods and results presented in this case study provide examples that illustrate how water quality and isotopic data can be used to determine the hydraulic connectivity between coal and non-coal strata, identify and quantify water from individual coal horizons, as well as predict and understand the isotopic variability of the reservoir. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.