USGS - science for a changing world

Understanding Our Planet Through Chemistry

IIIc. Public Health and Safety: Element Maps of Soils

Maps of natural contamination

Recently, an environmental problem was solved by mapping the soil chemistry in the San Joaquin Valley of California. In the 1980's, wildlife managers noticed increasing reproductive failure among nesting water birds at the Kesterson Wildlife Refuge in the northern end of the San Joaquin Valley. Chemical analyses of tissues from both birds and fish indicated toxic levels of selenium. The question was not only why, but why at that particular time? What had suddenly changed?

Satellite image of San Joaquin Valley.Beginning in the 1870's, irrigation was used to turn the nonproductive desert land of the San Joaquin Valley into the patchwork quilt of fields shown in this 1985 satellite image. [83k]

Photo of irrigation drain in crop field. To avoid the loss of crops caused by the buildup of salinity in the soil, a drain was built to transport the used irrigation water away. Instead of completing the projected 290-mile drain to the sea, it was halted 205 miles short of its goal, forming a wetland and leaving the water to evaporate. [89k]

Photo of mutated baby duck.Wildlife moved into the wetlands and prospered until selenium leached from marine shales in the Coast Ranges built up in the food chain and resulted in terrible mutations in higher life forms such as this baby duck. [188k]

Irrigation of arid soils in the San Joaquin Valley began in the 1870's, accumulating salts in shallow ground water perched on impermeable clay layers. Within a decade, farmers recognized the need for drainage facilities to lower the level of salts in the ground water or risk permanent loss of agricultural capacity, but the problem persisted. Finally in 1960, California voters approved financing for the State Water Project that included an extensive drainage system. Between 1968 and 1975, 85 miles (of the projected 290 miles) of the San Luis drain facility had been completed with a temporary termination at Kesterson Wildlife Refuge, still many miles short of the Sacramento-San Joaquin Delta, its projected destination.

By 1978, drainage into Kesterson had increased significantly. Unseen, the selenium levels were also increasing and by 1982 had built to toxic levels in the food chain of the wildlife refuge. Fish were affected first followed by waterfowl. Ultimately, the ponds were closed and filled in as the quickest solution to an environmental disaster. One question remained where did the selenium come from? The eastern side of the San Joaquin Valley has a deficiency in selenium, and, in fact, livestock grazing in the area needed to have selenium added to their food as a supplement. So why did Kesterson have too much selenium?

Further chemical studies focused on the Panoche Fan, the source of most of the drain water. Through chemical analyses using hydride generation atomic absorption spectrometry (AAS), high-selenium soils were found and mapped near the mountain front on mud-flow debris derived from selenium-enriched marine shales in the Coast Ranges.

AAS uses a bright source of the element s characteristic light, usually from a lamp whose cathode contains a large amount of the element. This light is then passed through a cloud of non-excited, ground-state atoms from the sample where it is absorbed proportional to the amount of the element present in the cloud. Next, the light goes to a monochrometer, that separates the energy wavelength of interest. The light is then converted into an electrical current, amplified, and rectified. A computer calculates the quantity of the element in the samples.

With the selenium data generated by AAS, the source of the selenium in the wildlife refuge was studied. Is the source of the selenium natural or caused by humans? The answer is both. The occurrence of selenium in the soils and ground water of the Panoche Fan is perfectly natural. Humans, however, are interacting with one part of the natural hydrologic cycle in which elements are transported from minerals to the ultimate sink the ocean. Here the elements would have been naturally recycled by reprecipitation as minerals in marine shales. By increasing the amount of rainfall (via irrigation), human activity has sped up the leaching of selenium out of the Panoche fan sediments.

The temporary halt of the San Luis drain had left the project 205 miles short of the sea, and the drain water was instead contained in holding ponds. The extra water turned the holding ponds into wetlands where birds that used the flyway made nesting sites. The ultimate solution to the San Joaquin drainage may lie in finishing the drain and discharging the water directly to the ocean so that nature can recycle it into marine sediments again.

To some people, the Kesterson Wildlife Refuge has been considered an environmental disaster. Nevertheless, it has served as an environmental lesson. The holding ponds demonstrate the feasibility of creating wetlands to clean up some forms of metal pollution. They also prove, however, that if we create wetlands for bioremediation, they cannot be built and left untended without risk to wildlife. The levels of toxic elements being concentrated in the wetland will have to be monitored so that they do not build up to levels that are toxic to wildlife.

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