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Eastern Section Meeting

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Freshwater Oxygen and Hydrogen Isotope Baseline for the Great Lakes Region

Abstract

Leaked or spilled fluids from petroleum production operations, hydrofracture treatments, and orphan wells can pose a threat to shallow groundwater and the surface environment. The geochemistry of fluids from a known or suspected leak or spill can be used to identify their origin(s), provided that the compositions of the various end-member components are known. Southwestern Ontario, for example, contains abandoned hydrocarbon wells that leak on occasion. Fluids reaching the surface are commonly mixtures of deeper formation waters and shallow freshwater. This paper describes the hydrogen and oxygen isotopic systematics of the latter end-member, which is derived from precipitation in the Great Lakes region.

The monthly average hydrogen (δ2H) and oxygen isotopic (δ18O) values of monthly precipitation were measured from 1996 to 2011 for 10 Canadian sites located close to the shoreline of the Great Lakes. These data yield a Great Lakes Meteoric Water Line described by:

δ2H = 7.1 × δ18O + 1.0

There is considerable variation in this relationship around the Great Lakes. The slope of Local Meteoric Water Lines, which characterizes each of the sampling stations, varies from 5.5 to 7.9, as compared to the regional average (7.1). The slopes are controlled by interplay among various air masses, which vary regionally and seasonally. The largest seasonal variations in δ-values occur in colder Great Lakes regions, which are also characterized by lower mean precipitation δ-values. There is a robust correlation between monthly mean surface temperature and monthly average precipitation δ18O values around the Great Lakes, ranging from 0.23‰/°C in warmer regions to 0.47‰/°C in cooler regions. The d-excess parameter, which is derived from the water isotope data, also varies seasonally; the lowest values occur during summer and the highest during winter. The d-excess data suggest that re-evaporation from the Great Lakes can comprise 15-20% of regional precipitation.

Water samples of low salinity obtained from shallow Devonian and uppermost Silurian aquifers in southwestern Ontario have δ2H ranging from −122 to −39 ‰ and δ18O ranging from −16.7 to −6.5 ‰, depending on location. Almost all of these results plot directly along the Great Lakes Meteoric Water Line. The isotopic data thus provide a robust signature of their origin that can be used in mixing models, in combination with data for other isotopic systems, to identify freshwater involvement in fluids from abandoned wells.