The Free Press, Mankato, MN

December 13, 2011

Day 1: Radioactive particles used to track sediment

By Tim Krohn
Free Press Staff Writer

Call it “CSI Minnesota River.”

They’re not crime fighters, but top researchers.

Their job is to look for the sources of sediment that annually flows into the Minnesota River and then into the Mississippi. How much comes from the mil­lions of acres of farm land in the watershed and how much from streambanks and ravines?

In the recent past, quanti­fying where sediment was coming from was very diffi­cult, if not impossible.


But not long ago, scientists found the answer — “radiometric fingerprints.”

Shawn Schottler, senior scientist at the Science Museum of Minnesota, said the process allows researchers to collect sediment samples downstream and identify whether it came from farm fields or banks and ravines.

Schottler, however, plays down any CSI comparison.

“After all, we are looking at mud all day.”

There are several kinds of radioactive isotopes that are naturally occurring and fall to earth each time it rains.

There are also radioactive isotopes that fell to earth because of above ground nuclear bomb testing in the 1950s and early ’60s.

Tilled farm fields, that are directly exposed to rain, have more isotopes and the tracers in them are different than those buried deeper in the soil in ravines, bluffs and river banks.

So, when scientists took deep core samples of dirt in Lake Pepin — which is being filled in with Minnesota River sediment — they could study dirt that was deposited there going back many decades.

And by looking at the ratio of farm field versus non- farm radioisotopes, they could also tell where the sediment originally came from. Schottler said the process lets them quantify farm field and non- farm field sediment in Lake Pepin, but so far they can’t analyze how much of the non- farm sediment came from lower river banks, upper bluffs or nearby ravines.

“ Work is continuing on ways to separate those nonfield sources.”