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Extremely impaired water sources represent a significant resource to Southern California. Withtreatment, these sources can produce potable water that is equal or superior to other availablesupplies. Water currently pumped from the I-105 dewatering wells near the Southern limits ofDowney (City) are an example of a source that may be put to beneficial use. Steps taken toachieve this begin with extensive analysis of water quality, treatment options and risk assessmentin compliance with Department of Health Service (DHS) requirements.The I-105 freeway was constructed by Caltrans and placed into service in 1993. A portion of thefreeway system between the Los Angeles and San Gabriel Rivers was constructed approximately35 to 40 feet below existing grade to mitigate noise concerns. However, groundwater rechargeactivity to the north, resulted in increasing groundwater levels negatively impacting the freeway.To resolve the high groundwater problems, Caltrans implemented a groundwater control anddewatering system in 1996. The system includes a series of wells installed to depths rangingfrom 80 to 110 feet. The wells were constructed for the purposes of dewatering and weretherefore not constructed to potable water standards. The water removed by the dewateringwells is pumped through granular activated carbon (GAC) filters prior to discharge to the LosAngeles River. The system has performed adequately and has lowered the groundwater tablepreventing damage to the freeway.There are several issues associated with the existing Caltrans groundwater dewatering systemthat make an alternative solution desirable:the system operates under a National Pollutant Discharge Elimination System (NPDES)permit allowing discharge into the Los Angeles or San Gabriel Rivers;high total dissolved solids (TDS) content, mineral constituents, and levels of volatileorganic compounds (VOCs) require GAC treatment prior to River discharge;a pump tax is imposed for water extracted from the Central Basin; and,negative public perception is associated with groundwater discharge without beneficialuse.Caltrans partnered with the City of Downey to investigate utilizing the groundwater for potablewater use, as a win-win solution for both agencies. The proposed plan consists of 5 newdrinking water wells (in addition to the test well 1D6A), a 3.6 mile pipeline, and a 6.5 MGDtreatment plant.To meet DHS requirements and public expectations, the water quality needs to meet or exceedthe quality of existing sources of supply. To meet these expectations, some of the treatmentalternatives under consideration include reverse osmosis, packed tower aeration and granularactivated carbon.In addition, an extensive source water assessment, including groundwater modeling(MODFLOW), was undertaken to delineate capture zones and determine well capacity to keepthe freeway dry, identify the sources of contamination, and develop a protection plan. This paperoutlines the considerations necessary to evaluate the innovative project to expand beneficialuse of local water resources by replacing dewatering wells with drinking water wells. Includes tables, figures.

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Edition: Vol. - No. Published: 06/15/2003 Number of Pages: 13File Size: 1 file , 3.5 MB