The effect of long-term regional pumping on hydrochemistry and dissolved gas content in an undeveloped shale-gas-bearing aquifer in southwestern Ontario, Canada

Stewart M Hamilton,Jennifer C Mcintosh,Stephen G Osborn,Stephen E Grasby

doi:10.1007/s10040-014-1229-7

Abstract

Baseline groundwater geochemical mapping of inorganic and isotopic parameters across 44,000 km2 of southwestern Ontario (Canada) has delineated a discreet zone of natural gas in the bedrock aquifer coincident with an 8,000-km2 exposure of Middle Devonian shale. This study describes the ambient geochemical conditions in these shales in the context of other strata, including Ordovician shales, and discusses shale-related natural and anthropogenic processes contributing to hydrogeochemical conditions in the aquifer. The three Devonian shales—the Kettle Point Formation (Antrim equivalent), Hamilton Group and Marcellus Formation—have higher DOC, DIC, HCO3, CO2(aq), pH and iodide, and much higher CH4(aq). The two Ordovician shales—the Queenston and Georgian-Bay/Blue Mountain Formations—are higher in Ca, Mg, SO4 and H2S. In the Devonian shale region, isotopic zones of Pleistocene-aged groundwater have halved in size since first identified in the 1980s; potentiometric data implicate regional groundwater extraction in the shrinkage. Isotopically younger waters invading the aquifer show rapid increases in CH4(aq), pH and iodide with depth and rapid decrease in oxidized carbon species including CO2, HCO3 and DIC, suggesting contemporary methanogenesis. Pumping in the Devonian shale contact aquifer may stimulate methanogenesis by lowering TDS, removing products and replacing reactants, including bicarbonate, derived from overlying glacial sedimentary aquifers.

Highlights

A groundwater geochemical mapping project by the Ontario Geological Survey (OGS) has been completed in a 44,000 km2 area of southwestern Ontario, Canada
As is usual in Ontario, wells in bedrock are uncased with no well screens, whereas wells finished in the overburden part of the contact aquifer typically have short (1–1.5 m) well screens
The four Devonian shale units that subcrop in southwestern Ontario include the Port Lampton Group, the Kettle Point Formation, the Hamilton Group and the Marcellus

Summary

Introduction

A groundwater geochemical mapping project by the Ontario Geological Survey (OGS) has been completed in a 44,000 km area of southwestern Ontario, Canada. The geochemical data, which include dissolved gas concentrations, have delineated an area of elevated natural gas (CH4) in groundwater that is coincident with the subcrop of Devonian-aged shale formations in southwestern Ontario. Study area The detailed study area encompasses all of the on-land exposure of Devonian shale in southwestern Ontario (Fig. 1). The broader study area encompasses all of southwestern Ontario and is intended to provide context for the geochemical, hydrogeological and isotopic investigation of the shale region. For the sake of geological and hydrogeological completeness, 50 samples were borrowed from the 2011 study area and serve to infill a small part of southwestern Ontario (Fig. 1) that had been excluded from the 2007 to 2010 work. The 2011 study (Hamilton et al 2011) used the same sampling and analytical protocols as previous years (Hamilton et al 2007; Hamilton and Brauneder 2008)

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