A field of seven wells, owned and operated by Ramona Municipal Water District, has been closed by the Department of Public Health, as unsuitable for human consumption owing to excessive quantities of selenium. The major portion of the selenium is apparently derived from the chemical weathering of molybdenum-selenium-arsenic-bearing aplite dikes in the grandioritic bedrock. Minor amounts of selenium may be contributed to the groundwater system by the application of selenium-containing fertilizers and pesticides. Other industrial and domestic discharges and the introduction through irrigation with imported water were systematically eliminated as a source of the high selenium contents. Correction of the high selenium levels in the water will be difficult. Possible solutions include reduction of the well water with water of lower selenium concentration so that the selenium level is below the Health Department limit, relocation of wells to non-seleniferous units, or re-evaluation of the limit set by the Public Health Department.
