Putting the Pieces Together

  • CLUE #1: Selenium is an element that has similarities with sulfur. It is widely distributed in soils, rocks, and water. In small amounts it is a nutrient, in large amounts, a toxin. It might achieve its toxic effect through its ability to replace sulfur in proteins. Selenium poisoning in animals results when they drink contaminated water or ingest Se that has worked its way up the food chain.

  • CLUE #2: Naturally occurring Se became concentrated in the marsh because the agricultural runoff contained Se and other trace elements that had been concentrated in the soil by thousands of years of evaporative processes. Once in the marsh, the runoff water could only leave by evaporation or percolation into the groundwater, leaving most of the Se and other trace elements and salts behind.

  • CLUE #3: When Se first entered the marsh, it was dissolved in the water as selenate. In this form it is very mobile and enters the food chain through plants and insects. The shortage of oxygen in the marsh creates an environment in which elements like Se can act as electron acceptors for microorganisms involved in decomposing organic material. While a small amount of the selenate (with a +6 Se ion) is given off as dimethyl selenite gas, most becomes selenite (with a +4 Se ion) or elemental Se (0 charge). These insoluble, immobile forms of Se collect in the sediment at the bottom of the pool and are not taken up by plants or easily diffused into the groundwater.

  • CLUE #4: The disappearance of selenate from the water can be explained by the action of microorganisms, which reduce the selenate to the insoluble forms selenite and elemental Se. Study of actual sediment suggests that these insoluble forms of Se accumulate in microsites rather than as a uniform layer on the sediment. SXRFM mapping identifies decaying vegetation as a possible cause of these microsites. Since the microsites present less surface area to the water than a uniform layer of Se, conversion back to selenate is slow, accounting for the low levels of selenate in the pond water.
Dr. Tetsu Tokanaga, an earth scientist at Lawrence Berkeley National Laboratory, came up with the microsites idea and is continuing to conduct x-ray imaging experiments with selenium and other wetlands contaminants, including chromium. Using a technique called x-ray absorption spectroscopy, he has been able to observe the reduction of chromium ions as they move toward a microsite.

Studying the complex chemical reactions that take place in all soils is made possible by powerful scientific instruments like the ALS. Why do you think scientists are interested in the chemistry of soils? What kind of useful results could come from studying how microorganisms cause chemical changes in soil?

Wetlands Introduction

Clue #1

Clue #2

Clue #3

Clue #4



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