Surface vs. subsurface groundwater contamination at legacy oil site

The ability to identify the source of oilfield groundwater contamination, such as surface releases, wastewater injection, or subsurface well leakage, is critical in selecting proper remedial actions. For example, contamination from historical operations at the land surface can be remediated to prevent off-site migration and the contamination of the local aquifer. However, if the source of contamination is an ongoing subsurface leak from a well, the leak needs to be addressed before groundwater remediation can be successful.

In a case study with Chevron Technical Center, staff scientist in the Nuclear and Chemical Sciences division Ate Visser tested samples from two brine plumes within 1,000 feet of each other at a legacy oil and gas (O&G) production site in the American Midwest. Brine, water that is saltier than seawater, is one of the most common groundwater contaminants found within O&G fields. Brine contamination may originate from surface or subsurface releases of oilfield-produced water, which is the water produced along with oil that is typically naturally enriched with salts.

The objective of the study, published in Groundwater, was to identify the primary source of the plumes, both surrounding an abandoned oil well, using a combination of dissolved noble gas analysis and geochemical and geophysical investigations. Dissolved noble gases (helium, neon, argon, krypton, and xenon) have distinct source signatures in groundwater, are chemically inert, and fractionate via physical processes in predictable ways. The analysis of dissolved noble gas concentrations can differentiate subsurface from surface sources of groundwater contaminants at O&G sites because surface spills cause re-equilibration of dissolved concentrations with the atmosphere. The result is a loss of terrigenic helium that is typically found in high concentrations in produced water.

From their analyses, the researchers demonstrate that only a historic surface release can explain the dissolved noble gas signature of groundwater in monitoring wells contaminated with brine near an abandoned O&G well, rather than subsurface leakage from the well. Identifying the correct source of contamination will help allocate responsibility and determine the appropriate remedial action.

This study demonstrates noble gas analysis as a powerful tool and recommended method for differentiating surface from subsurface sources of oilfield groundwater contamination, especially in cases where other lines of evidence are inconclusive.

[D.C. Segal, A. Visser, C. Bridge, Noble Gas Analyses to Distinguish Between Surface and Subsurface Brine Releases at a Legacy Oil Site, Groundwater (2024), doi: 10.1111/gwat.13412.]

–Physical and Life Sciences Communications Team