Groundwater and Surface Water Monitoring

Groundwater and surface water monitoring activities has continued since 1997 as part of the Resource Conservation and Recovery Act (RCRA) RCRA Facility Investigation/Remedial Investigation (RFI/RI). Installation and monitoring of additional wells have occurred throughout the years. Prior to Phase 1 groundwater remedy construction in October 2018, there were approximately 140 groundwater monitoring wells sampled routinely. Five wells installed in 2007 are "slant" or angled wells, extending underneath the Colorado River, and allow for water samples to be collected below the river. Five vertical wells and three slant wells were also installed on the Arizona shoreline in March and April 2008. In addition, five additional monitoring wells were installed in Arizona as part of the expanded monitoring program under the groundwater remedy. Based on sampling results collected over the years, the agencies and the Pacific Gas and Electric Company (PG&E) have been able to determine the approximate location and depth of the groundwater plume for cleanup purposes. Furthermore, the monitoring results show that the quality of the Colorado River is not adversely impacted by the hexavalent chromium plume from the Topock Project Site (Site).

An additional 74 monitoring wells were installed during Phase 1 groundwater remedy construction. Together, these new and previous monitoring wells have been used to collect groundwater data to assess the performance of the groundwater remedy since November 2021. There is no change to the surface water sampling frequency or locations.

Under the current Remedy Monitoring Program, hexavalent chromium, chromium, in-situ byproducts (arsenic, manganese, and iron), and COPCs (molybdenum, selenium, and nitrate) are sampled at the following frequencies (more information can be found in the quarterly reports located in the Document Library):

Monthly in 2 extraction wells
Monthly in 24 wells
Quarterly in 89 wells, 4 river shoreline locations, 2 other surface monitoring locations, and 10 in-channel river locations
Semiannually in 108 wells
Annually in 17 wells
Biennially in 7 wells

As part of the Remedy Monitoring Program, PG&E is also conducting sampling and analysis for other constituents and water chemistry parameters in select wells.

Currently, monitoring results are submitted by PG&E to the DTSCThe department within the California Environmental Protection Agency in charge of the regulation of hazardous waste from generation to final disposal. DTSC oversees the investigation and cleanup of hazardous waste sites. and the U.S. Department of the InteriorThe United States department charged with conservation and development of natural resources. The U.S. Department of the Interior uses sound science to manage and sustain America’s lands, water, wildlife, and energy resources, honors our nation’s responsibilities to tribal nations, and advocates for America’s island communities. in quarterly progress reports. These reports are also shared with the Consultative Work GroupA focused stakeholder subgroup of the Consultative Work Group (CWG) where various stakeholders and their consultants discuss technical project related issues in greater detail which are then reported back to the CWG. and are placed in the information repositoriesDesignated locations that provide public access to site-related documents as required by the DTSC. in the surrounding communities and in this website’s Document Library. Monitoring reports can be found here.

To view figures showing current groundwater and river sampling locations, or the most recent plume maps based on quarterly groundwater data, please click on the figures below.

Groundwater monitoring locations map and surface (river) water monitoring locations map:

(Click on images below for larger view)

Plume maps, based on quarterly groundwater monitoring data show various depths on Layers 1 through 5: