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A water district in Southern California supplies drinking water with a combination oflocal shallow well groundwater and imported raw and treated water. The well is located120 ft away from a seasonal creek. Based on limited temperature data, the CaliforniaDepartment of Health Services (DHS) determined that the groundwater was under directinfluence from the creek. In 2000, DHS required the district to install a water filtrationplant. Before expending upwards of $1 million on a new treatment plant, the districtelected to set up a sampling and testing protocol to determine more conclusively whetheror not groundwater was under direct influence of surface water, and if so, what treatmentcredit could be given to river bank filtration. This paper reports a natural filtration studyconducted in 2003 and 2005 for the district. The study, supported on well and creekmonitoring, was devised to establish how rapidly well water quality responds to surfacewater quality changes, and to quantify river bank effectiveness in removing turbidity,total and fecal coliform (TC and FC), Giardia and Cryptosporidium.Devising a meaningful well and creek monitoring program presented challenges becauseof the seasonal nature of creek flow and well production (i.e. the wells start productionearly in the rainy season before creek flow starts, and well production continues for aperiod after the creek stops flowing). The monitoring program included differentsampling intervals, tests and procedures for the various flow conditions. The monitoringprogram captured several significant rain events that resulted in high creek flows andassociated increased creek water turbidity.Tracer test data showed a 30-hour groundwater travel time from the creek to theproduction wells. Based on creek and well sample data, natural filtration providesremovals as high as 3-log turbidity, 4.4-log TC, and 2-log FC. The highest creekCryptosporidium concentration during the sampling program was 0.4 organisms/L andGiardia 0.03 organisms/L. Well samples showed none detected for both organisms. Dataevaluated included turbidity, coliform bacteria, protozoa, temperature, pH, electro-conductivity, and particle counts.Based on the results, well water is under the influence of creek water; however, naturalfiltration provides removal of turbidity, bacteria and protozoa. Particle count data showthat 5 to 15 micron particles, the size of pathogenic protozoa, although partially removedin the creek bank, still appear in product water. Includes tables, figures.

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Edition: Vol. - No. Published: 06/17/2005 Number of Pages: 11File Size: 1 file , 1.1 MB