Produced fluids and shallow groundwater in coalbed methane (CBM) producing regions of Alberta, Canada: Trace element and rare earth element geochemistry

The production of coalbed methane (CBM) represents a vital new source of natural gas supply in Western Canada. There are, however, concerns over potential negative environmental impacts on groundwater resources caused by potential contamination with fluids and gases from coal-bearing strata. This paper provides a brief description of the geochemistry and a detailed discussion of trace element and rare earth element (REE) concentrations of the produced fluids from two major coal deposits in Western Canada, the Mannville Formation and the Horseshoe Canyon/Belly River Group (HSCN/BRG), and shallow groundwater (SGW) in this region. It evaluates how the depositional environment, redox conditions, and water–rock interactions influence the trace element and rare earth element distributions in the produced fluids and SGW of the study area.Produced waters from the Mannville Formation are a saline Na–Cl type and a high boron content is indicative of a brackish/marine depositional environment. In contrast, the HSCN/BRG produced fluids are a Na–HCO3 water type and have lower total dissolved solids (TDS) and lower boron concentrations. Shallow groundwater has a Na–HCO3 to Na–HCO3–SO4 water type with often high concentrations of sulphate and low TDS. Some groundwater samples, many HSCN/BRG and all Mannville fluids had low sulphate concentrations often with elevated δ34S values indicating that bacterial sulphate reduction had occurred making the redox environment suitable for methanogenesis. Dissolved gas from the Mannville Formation had a notable thermogenic component, whereas dissolved gas from the HSCN/BRG and free gas in shallow groundwater contained methane predominantly or exclusively of biogenic origin.Mannville produced waters are up to 70 times more concentrated in trace elements compared to the HSCN/BRG waters and up to 300 times more concentrated than shallow groundwater. In the Mannville fluids, trace elements, such as As, Se and Pb occurred in concentrations often exceeding drinking water guidelines (5 μg/L, 10 μg/L and 10 μg/L, respectively), whereas most samples in the HSCN/BRG contained only Se above drinking water guidelines, and a few samples contained As and Pb slightly above the maximum allowable concentrations for drinking water. Therefore, the potential risk of trace metal contamination of shallow groundwater with HSCN/BRG swabbing fluids is low because of their comparatively low trace metal content. Rare earth elements in the Mannville produced fluids indicate interaction with silicate dominated host rocks. In contrast, the REEs of the HSCN/BRG produced fluids show interaction with carbonate-rich material. The shallow groundwater shows a similar pattern to the HSCN/BRG, suggesting that REEs are not a suitable parameter to monitor potential contamination of shallow groundwater with CBM fluids from the HSCN/BRG. However, several other geochemical and isotopic parameters are sufficiently distinct between shallow groundwater, HSCN/BRG, and Mannville fluids so that cross-contamination with produced fluids or gases should be identifiable with a suitable geochemical monitoring program.