Abstract

The expansion of oil and gas development, as a result of advances in horizontal drilling and well stimulation techniques, has raised concerns about the impact of these activities on groundwater resources. Oil and gas development activities have the potential to introduce contaminants such as hydrocarbons and brines into freshwater aquifers through a variety of pathways, including improper disposal/surface spills, leaking wells, underground injection, and induced fracturing. However, in basins with a long history of oil and gas extraction, industrial, agricultural, and urban activities, distinguishing sources and pathways of groundwater contamination is difficult. To solve this complex problem, a broad suite of constituents, including major, trace, and radioactive elements, isotopes, organic compounds, dissolved gases, and hydrocarbons need to be investigated. Careful analyses of these data, coupled with an independent characterization of plausible pathways for gas and/or fluid flow, can provide a means of distinguishing the mechanisms and sources of contamination in groundwater. Examples from studies across the United States, and regional reconnaissance data from groundwater samples collected near oil and gas fields in California, illustrate the potential utility of a multi‐tracer (constituent) approach to identify the dominant pathways that lead to groundwater contamination. Constituents investigated as tracers may have unique isotopic systematics that provide a geochemical 'fingerprint' identifying the source of the contaminant, while the saturation of gases (hydrocarbon, noble, and atmospheric gases), and the transport of organic and inorganic constituents, provide insight into the pathways and interactions between oil and gas fluids and groundwater. These tracers, combined with a detailed characterization of formation geometry, well depths, perforations and locations, faults, injection and disposal history, and groundwater flow characteristics provide the framework for assessing sources and pathways for groundwater contamination from oil exploration and development.