Selenium Mitigation Achieved in Cooling Water Discharge

By finding a solution deep underground, a Southern California electric utility will be able to address environmental concerns regarding its cooling water discharge.

Part of the century-old Imperial Irrigation District (IID) on the Mexican border, the El Centro Generating Station discharges spent cooling water into the Salton Sea, California’s largest inland body of water, covering some 376 sq miles.

The Salton Sea’s below sea level setting in the Colorado Desert allows for minimal inflow from rain; 90% of the entire inflow is nutrient-rich agricultural runoff from the Imperial, Coachella and Mexicali valleys. The inflow is also highly saline and the future of the shrinking lake is a statewide concern.

Tests found that the discharge from the El Centro plant exceeded the Maximum Contaminant Level for selenium, a semi-metallic, naturally occurring trace element that aquatic plants and invertebrates absorb.

An expert was hired by IID to evaluate its options, including RO treatment with evaporation ponds for the brine waste. Additionally, IID personnel learned that nearby geothermal energy plants had amassed a long, successful history with underground injection technology, making it a feasible option for consideration. 

“Although other options offered a lower capital cost, we were not confident that they would ensure a viable solution,” said Henryk Olstowski, IID assistant manager for energy. “Sinking the deep injection wells was considered a calculated risk because you don’t know if the well is going to comply with the permit requirements until you’ve drilled it. But we were confident that this option provided us with the lowest lifecycle cost and, more importantly, a long-term, viable solution to mitigating the selenium.” 

An RFP was issued and Layne Christensen Co. was awarded the project chiefly due to its nationwide experience with Under Injection Control (UIC) technology.

Where the geology is suitable, UIC technology provides industrial and municipal operations with an option that safely removes wastes from the surface environment by isolating them far below the land surface and away from drinking water resources, according to the U.S. Environmental Protection Agency (EPA).

According to USEPA, there are over 550 Class I deep injection wells in the United States. (Among six classes of UIC wells defined by EPA, Class I wells are characterized by wide diameters with multiple casings to protect underground drinking water sources.) The practice of underground injection of wastewater dates to the 1930s, when oil companies began disposing of oil field brines and other waste products into depleted reservoirs.

“Deep injection technology serves a diverse cross section of American industry,” said Edward McCullers, Layne Christensen’s general manager of injection well operations. “For several decades, deep injection wells primarily served water utilities in Florida and chemical-industrial concerns in the Midwest. In recent years, large-scale water users, including power companies, have been increasingly turning to UIC technology for discharge of blow-down water from cooling units.”

During a rigorous, two-year permitting process for the El Centro well, injection formation tests had to prove specific porosity, permeability, static formation pressure and effective thickness of the injection zone, in this case the Palm Spring Formation (sands and fine gravel).

The brackish, native water in the deep formation also had to be analyzed, and tests had to demonstrate the presence of a suitable formation below the lowermost Underground Source of Drinking Water (USDW). The USDW is characterized as containing total dissolved solids of 10,000 ppl or less (as defined in 40 CFR §§144.3 and 146.3).

“The well development was very tricky,” said Doug Watson, project manager for Layne Christensen.

He explained that in the course of drilling exploratory wells, the crew had to block and seal off formations, then demonstrate that they had located the injection zone below the lowermost USDW and that the injection zone met all the permeability characteristics that were required. Performance tests were conducted to define the flow and pressure that the IID will be held to under the operating permit.

“Naturally, we were concerned whether or not the wells would comply with the permit requirements, but Layne did a great job working with us, our consultant expert and the EPA,” Olstowski said. “In fact, the daily communication with the regulatory agency for this project was the most intensive oversight I’ve ever experienced on a project; but, at the end of the day, we were able to satisfy all requirements.”

The turnkey project performed by Layne Christensen has provided two deep injection wells with a design capacity of 850 gpm each at depths from 2,200 to 2,700 ft. The above-ground wastewater collection and injection system is currently being constructed from the El Centro Generating Station to the well field, where the IID expects to begin injecting in early 2013.

Edward McCullers is general manager, injection wells, water resources, Layne Christensen Co. McCullers can be reached at [email protected].