Enterprise Zone Well Operations and Treatment Alternatives Analysis Project,
City of Redding, CA
The City of Redding owns and operates thirteen groundwater wells in the Enterprise area, producing approximately 18 mgd, and plans to add several more in the coming years to meet increasing water demands. Several of the wells have manganese concentrations approaching or exceeding the secondary maximum contaminant level (MCL) of 50 micrograms per liter (ug/L), one well has periodic taste and odor issues believed to be caused by hydrogen sulfide, and one well contains detectable levels of arsenic approaching or exceeding the federal MCL of 10 ug/L. In recent years, the number of complaints related to iron and manganese has increased as the population served by the wells has increased, along with the number of wells and quantity of groundwater use. Given the planned development of additional wells to meet increased water demands, the water quality regulations for the groundwater constituents, and the increasing number of public complaints due to the water quality in the Enterprise Zone, the City hired Water Works Engineers to evaluate operational and well-head treatment measures to improve the quality of the water in the affected areas.
Water Works Engineers collected, compiled in a database, and analyzed over ten years of water quality and well production data, from various electronic and hardcopy data sources. It was determined that six of the wells regularly had manganese concentrations in excess of the MCL, one well regularly exceeded the MCL for arsenic, the most productive wells were run most often but also had the highest manganese concentrations, and that the ortho\polyphosphate dose was approximately 8 times lower than it should be for effective sequestering. An alternatives analysis was conducted to evaluate available treatment technologies for manganese removal, and present worth costs were developed for two types of oxidation-filtration treatment systems. Blending of raw water from two adjacent wells prior to distribution was recommended to reduce arsenic concentrations from one well to below the MCL while simultaneously reducing manganese concentrations from the other well to below the MCL, at a cost savings of more than $2.5 million. A final report was prepared with recommendations for immediate operational changes, for sequestering and well operations, and capital improvements for blending and treatment.