Selenium water contamination level

Low levels of selenium can also be found in drinking water. Most of the water sources tested in the United States have very low levels of selenium compared with the levels found in food. Selenium levels were less than 10 ppb (10 parts of selenium in a billion parts of water) in 99.5% of drinking water sources tested. The 10-ppb concentration is lower than the 50-ppb Maximum Contaminant Level (MCL), which the EPA believes will protect against adverse health effects. Less than 1% of the daily intake of selenium is estimated to come from drinking water. 

Sodium selenite has also been incorporated into styrene—butadiene mbber and used in a pellet form which results in the slow release of selenium into water. These pellets have been placed in lakes in Sweden which have fish contaminated with mercury owing to high levels of that element in the water. The selenium released by the pellets reacts with mercury to form insoluble, heavy mercury selenide which setties to the lake bottom and removes mercury from the ecosystem (126). 

Different metals and metal compounds have been in use since the beginning of human civilization. Metals include aluminum, antimony, cobalt, copper, chromium, iron, nickel, manganese, molybdenum, selenium, tin, vanadium, and zinc. The list of toxic metals includes but is not limited to arsenic, beryllium, cadmium, hexavalent chromium, lead, and mercury. Contamination of food, water, and the air by metals, particularly lead and cadmium, has caused global concern. Several studies have shown elevated levels of lead, nickel, chromium, and manganese in children s hair. 

There are virtually no sources of drinking water on Earth that are not contaminated with xenobiotics. Rain water cleanses the atmosphere as it forms and falls. As a result, it contains dissolved acids, organic compounds, and heavy metals such as mercury and selenium in many areas. Surface collection basins from which potable water is drawn—rivers, streams, and lakes—accumulate ground level pollutants in addition to those carried in rain water. Underground water, which is somewhat filtered and generally contains lesser quantities of pollutants than surface water, may itself be contaminated by ground releases of toxicants and by contaminants produced by chemical reactions in the soil and water. 

The disposal of selenium contaminated waste water has resulted in elevated selenium levels in sediments of Lake Belews, North Carolina. The concentration of selenium in sediments ranged from 4 to 12 pg/g (pre-1986), but has dropped to 1 1 pg/g (1996) due to the discontinued release of selenium laden waste water from a local coal fired power plant (Lemly 1997). Selenium was measured in 445 surface soil samples from Florida with a concentration range of 0.01 1.62 pg/g and an arithmetic mean of 0.25 pg/g (Chen et al. 1999). Selenium was detected in soils and bed sediment from the South Platter River Basin at concentrations of 0.30-3.80 pg/g (Heiny and Tate 1997). The highest levels were observed in areas consisting of a high degree of Precambrian rock formation. 

Environmental pollution by selenium results from burning of fossil fuel and trash, mining, and also as a contaminant in the air with sulfur dioxide. Selenium concentration in river water in the United States is normally less than 0.5 pg L However, certain alkaline streams draining selenifer-ous lands in the western United States contain elevated levels of selenium. 

The extent of the selenium problem in petroleum refineries depends on the types of crude oil processed. For example, crude oil from the San Joaquin Valley contains higher levels of selenium while other sources of crude are essentially selenium free. Because San Joaquin crude is processed in several reBneries along the western coast of the US, several of these refineries have problems with selenium contamination of their process waters. Depending on the process waters (stripped sour water or final biotreated effluent) the volumetric flowrates range from 10... 

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