24 October 2019
USGS study reveals impact of produced water on California groundwater
A study by the USGS shows some of the impacts of years of produced water disposal into underground injection wells in California’s Kern County
Last month, the US Geological Survey (USGS) published the results of a study showing that disposed produced water from California’s North and South Belridge and Lost Hills oilfields has led to higher than normal salinity levels in nearby groundwater.
The report considered archived data from northwestern Kern County injection wells and, for the first time, demonstrated the groundwater impact of produced water migration from injection wells. It also confirmed the migration of produced water plumes from disposal ponds.
USGS research scientist and lead study author Janice Gillespie used resistivity logs from wells drilled in the late 1960s through to 2016 to reach her conclusions. The logs measure electrical conductivity in formations. Because salty water conducts electricity more efficiently than fresh water, salinity can be determined based on how well electricity is conducted.
“As we watched the salinity increase with depth and started mapping the depth to the base of protected water, we started noticing that we were getting very odd diversions in some areas where we would see fairly high salinities occurring much shallower in depth than they should have,” Gillespie told Water in Oil. “In many of the newer wells in disposal areas, we're seeing places where the natural gradient has been disturbed.”
The disturbances were identified at 1,300 feet or less and produced water plumes were able to be tracked about 1,800 feet from the wells. Tracking beyond that distance was difficult due to the large number of injection wells in the study area.
John Borkovich, supervising engineering geologist for the Groundwater Monitoring Section of the California State Water Resources Control Board (CSWRCB), told Water in Oil that, “In some cases such as Belridge, underground injection disposal activities are in the process of being phased out to help prevent these issues in the future.”
Gillespie said that the groundwater to the west of the San Joaquin Valley – where the fields are located –was already of very poor quality and far from suitable to drink even before being affected by produced water. Borkovich said the water is used by some farmers for irrigation.
The study is part of the larger Regional Monitoring Program that began in 2014 at the request of the CSWRCB. The program was created to try and determine how oil & gas activities might be affecting untapped groundwater. Salinity mapping in areas adjacent to the Belridge and Lost Hills fields began in 2016 as those fields ranked highly according to the study’s criteria including density of oil & gas activity and fluid injection, as well as nearby water use.
“There are probably 30 oilfields in which we've got some sort of work going on right now,” Kim Taylor, program officer for the USGS California Water Science Center, told Water in Oil. “Salinity mapping and data collection from resistivity logs are some of the first things we do. It's a three- to four-year process for each oilfield.”
A new USGS study based on aerial electromagnetic surveys of the Belridge and Lost Hills fields will be released in 2020. That investigation entails the creation of 3D maps of resistivity to determine salinity levels over a wide area.