Groundwater uranium contamination from produced water disposal to unlined ponds in the San Joaquin Valley.

In the southern San Joaquin Valley (SJV) of California, an agriculturally productive region that relies on groundwater for irrigation and domestic water supply, the infiltration of produced water from oil reservoirs is known to impact groundwater due to percolation from unlined disposal ponds. However, previously documented impacts almost exclusively focus on salinity, while contaminant loadings commonly associated with produced water (e.g., radionuclides) are poorly constrained. For example, the infiltration of bicarbonate-rich produced waters can react with sediment-bound uranium (U), leading to U mobilization and subsequent transport to nearby groundwater. Specifically, produced water infiltration poses a particular concern for SJV groundwater, as valley-fill sediments are well documented to be enriched in geogenic, reduced U. Here, we analyzed monitoring well data from two SJV produced water pond facilities to characterize U mobilization and subsequent groundwater contamination. Groundwater wells installed within 2 km of the facilities contained produced water and elevated levels of uranium. There are >400 produced water disposal pond facilities in the southern SJV. If our observations occur at even a fraction of these facilities, there is the potential for widespread U contamination in the groundwaters of one of the most productive agricultural regions in the world.

Chemical Characterization of Natural Gas Leaking from Abandoned Oil and Gas Wells in Western Pennsylvania.

The U.S. Environmental Protection Agency estimates that there are over 3.2 million abandoned wells in the United States. Studies conducted on gas emissions from abandoned wells have been limited to methane, a powerful greenhouse gas, due to concerns regarding climate change. However, volatile organic compounds (VOCs), including benzene, a known human carcinogen, are known to be associated with upstream oil and gas development and hence could also be released when methane is emitted to the atmosphere. In this investigation, we analyze gas from 48 abandoned wells in western Pennsylvania for fixed gases, light hydrocarbons, and VOCs and estimate associated emission rates. We demonstrate that (1) gas from abandoned wells contains VOCs, including benzene; (2) VOCs are emitted from abandoned wells, the magnitude of which depends on the flow rate and concentration of VOCs in the gas stream; and (3) nearly one-quarter of abandoned wells are located within 100 m of buildings, including residences, in Pennsylvania. Together, these observations indicate that further investigation is necessary to determine whether emissions from abandoned wells pose an inhalation risk to people living, working, or congregating near abandoned wells.

Historic and Contemporary Surface Disposal of Produced Water Likely Inputs Arsenic and Selenium to Surficial Aquifers.

Oil and gas development generates large amounts of wastewater (i.e., produced water), which in California has been partially disposed of in unlined percolation/evaporation ponds since the mid-20th century. Although produced water is known to contain multiple environmental contaminants (e.g., radium and trace metals), prior to 2015, detailed chemical characterizations of pondwaters were the exception rather than the norm. Using a state-run database, we synthesized samples (n = 1688) collected from produced water ponds within the southern San Joaquin Valley of California, one of the most productive agricultural regions in the world, to examine regional trends in pondwater arsenic and selenium concentrations. We filled crucial knowledge gaps resulting from historical pondwater monitoring by constructing random forest regression models using commonly measured analytes (boron, chloride, and total dissolved solids) and geospatial data (e.g., soil physiochemical data) to predict arsenic and selenium concentrations in historical samples. Our analysis suggests that both arsenic and selenium levels are elevated in pondwaters and thus this disposal practice may have contributed substantial amounts of arsenic and selenium to aquifers having beneficial uses. We further use our models to identify areas where additional monitoring infrastructure would better constrain the extent of legacy contamination and potential threats to groundwater quality.

An examination of onshore produced water spills in the state of California: incident frequency, spatial distribution, and shortcomings in available data.

Accidental releases (i.e., spills) of produced water can occur at any point during oil and gas development operations, potentially resulting in chronic and/or catastrophic loadings of produced water to nearby ecosystems and exposures of human populations to toxic constituents including trace metals (e.g., arsenic), organic compounds (e.g., benzene), and/or radionuclides (e.g., radium). Despite California being one of the largest oil and gas producing states in the USA, no comprehensive reviews of produced water spills in the peer-reviewed literature have been published. To address this knowledge gap, produced water spill incident data contained within the California HazMat database were synthesized to elucidate trends in produced water spills in California. During the period of 2006-2020, a total of 1029 incidents involving produced water spills were reported. Despite the potential threat to environmental and human receptors, there are significant knowledge gaps concerning these incidents. Specifically, only ~ 6% of spill incidents contained geographic coordinates, greatly hindering assessments of the impacts of these events to public health and the environment. Moreover, updated spill volumes are not rapidly retrievable from the HazMat database, and during the years 2018-2020 volumes of produced water spilled were underreported in initial reports anywhere from 35-2750%. Further, it is unclear if groundwater monitoring is performed following spill events. This study highlights significant shortcomings in produced water spill reporting in California and recommends improvements to aid future investigations that assess the environmental and public health impacts of spill incidents.

Vulnerability of Groundwater Resources Underlying Unlined Produced Water Ponds in the Tulare Basin of the San Joaquin Valley, California.

The San Joaquin Valley (SJV) in California is one of the most agriculturally productive regions in the world relying in part on groundwater for irrigation and for domestic or municipal water supply for nearly 4 million residents. One area of growing concern in the SJV is potential impact to groundwater resources from ongoing and historical disposal of oilfield-produced water into unlined produced water ponds (PWPs). In this investigation, we utilized available information on composition of produced water disposed into unlined PWPs and levels of total dissolved solids in underlying groundwater to demonstrate that this disposal practice, both past and present, poses risks to groundwater resources, especially in the Tulare Basin in the southern SJV. Groundwater monitoring at unlined PWP facilities is relatively sparse, but where monitoring has occurred, impact to aquifers used for public and agricultural water supply has been observed and has proven to be too expensive to actively remediate. Results of this investigation should inform policy discussions in California and other locations where disposal of produced water into unlined impoundments occurs, especially at locations that overlie groundwater resources.

Impact to Underground Sources of Drinking Water and Domestic Wells from Production Well Stimulation and Completion Practices in the Pavillion, Wyoming, Field.

A comprehensive analysis of all publicly available data and reports was conducted to evaluate impact to Underground Sources of Drinking Water (USDWs) as a result of acid stimulation and hydraulic fracturing in the Pavillion, WY, Field. Although injection of stimulation fluids into USDWs in the Pavillion Field was documented by EPA, potential impact to USDWs at the depths of stimulation as a result of this activity was not previously evaluated. Concentrations of major ions in produced water samples outside expected levels in the Wind River Formation, leakoff of stimulation fluids into formation media, and likely loss of zonal isolation during stimulation at several production wells, indicates that impact to USDWs has occurred. Detection of organic compounds used for well stimulation in samples from two monitoring wells installed by EPA, plus anomalies in major ion concentrations in water from one of these monitoring wells, provide additional evidence of impact to USDWs and indicate upward solute migration to depths of current groundwater use. Detections of diesel range organics and other organic compounds in domestic wells <600 m from unlined pits used prior to the mid-1990s to dispose diesel-fuel based drilling mud and production fluids suggest impact to domestic wells as a result of legacy pit disposal practices.

Geochemical impacts to groundwater from geologic carbon sequestration: controls on pH and inorganic carbon concentrations from reaction path and kinetic modeling.

Geologic carbon sequestration has the potential to cause long-term reductions in global emissions of carbon dioxide to the atmosphere. Safe and effective application of carbon sequestration technology requires an understanding of the potential risks to the quality of underground sources of drinking water. In particular, concern is warranted regarding the potential for CO(2) leakage through geological features and abandoned wells that may result in detrimental perturbations to subsurface geochemistry. Reaction path and kinetic models indicate that geochemical shifts caused by CO(2) leakage are closely linked to mineralogical properties of the receiving aquifer. CO(2) gas dissolution into groundwater and subsequent reaction with aquifer minerals will control the evolution of pH-bicarbonate envelopes. These parameters provide geochemical context for predicting how regulated contaminants associated with aquifer solids will respond via various mineral-water reaction processes. The distribution and abundance of carbonate, silicate, oxide, and phyllosilicate minerals are identified as key variables in controlling changes in groundwater geochemistry. Site-specific risk assessments may require characterization of aquifer geology, mineralogy, and groundwater chemistry prior to CO(2) injection. Model results also provide a frame of reference for developing indicative measurement, monitoring, and verification (MMV) protocols for groundwater protection.