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.
- 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.
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?