Groundwater quality near the Montebello Oil Field, Los Angeles County, California

• New and historical water-sample data from existing groundwater wells did not provide conclusive evidence that oil and gas development has affected groundwater quality in the Montebello Oil Field. Some wells had trace detections of hydrocarbons or other potential oil-field fluid indicators, but most of these wells also had detections of manufactured volatile organic compounds (VOCs) and geochemical tracers indicating the water has been affected by other urban or industrial land-use sources at the surface.
• Many surface sources of hydrocarbons have been documented in the area.
• Two samples contained low concentrations of hydrocarbons and other potential oil-field fluid indicators in relatively deep, old groundwater, indicating no influence from modern anthropogenic sources at the land surface. However, available information indicates that these detections could result from either manmade oil-development or natural geologic processes.
• Definitive analysis of human and natural sources and mechanisms affecting groundwater quality was not possible because of the low density of data in the areas with the largest relative risks to groundwater from oil-field sources. Additional understanding of risks of oil-field development to groundwater could be developed from (1) expanded groundwater monitoring at different depths near and in active oil wells, (2) a focus on areas downgradient of known historical surface ponds and sumps, where high net injection of recycled oil-field water into oil-producing formations may cause higher pressures and upward fluid movement toward groundwater, and (3) a focus on zones with oil well integrity issues like uncemented boreholes that may provide vertical conduits.