Uran-Ano­ma­lie im Lyss­bach

Vom Kan­ton Bern wur­den wir 2012 mit der Auf­ga­be betraut, die Ursa­che der Uran-Ano­ma­lie im Lyss­bach abzu­klä­ren. Mit dem Gewäs­ser­schutz­la­bor des Kt. Bern, mit dem Phy­si­ker Heinz Sur­beck (nuc­film GmbH) und dem Droh­nen­pi­lo­ten der Fir­ma Bird­view pic­tu­re GmbH ist uns dies gelun­gen.

The water and soil pro­tec­tion labo­ra­to­ry of the Can­ton of Bern detec­ted ano­ma­lous high ura­ni­um con­cen­tra­ti­ons in the creek “Lyss­bach” at the loca­li­ty of Lät­ti bet­ween bet­ween Lyss and Schön­bühl (can­ton of Bern) as part of the stan­dard water moni­to­ring pro­gramm. Values up to 45 μg/​l of dis­sol­ved ura­ni­um in cree­k­wa­ter and at up to 400 μg/​l in ground­wa­ter sam­ples have been mea­su­red. The average con­cen­tra­ti­on of dis­sol­ved ura­ni­um in Swiss creeks is in the ran­ge of a few μg/​l (Baert­schi & Keil 1992).

Air­bor­ne radia­ti­on map­ping

Water sam­ples were collec­ted from the “Lyss­bach” and from ground­wa­ter inflows in the creek to explo­re the pos­si­ble ura­ni­um source. Two sus­pi­cious con­ta­mi­na­ted sites were eva­lua­ted using a radia­ti­on detec­tor as a pay­load of a octo­cop­ter (Figu­re 1). Air­bor­ne radia­ti­on map­ping tur­ned out to be a very effec­tiv and low bud­get instru­ment to detect ano­ma­lous radia­ti­on aere­as wit­hin short time.

Scan­ning of 100’000 squa­re­me­ters

It was pos­si­ble to scan several 100’000 m² wit­hin a day with a reso­lu­ti­on of appro­xi­mate­ly 10 m2 and a detec­tion limit of the fly­ing Na(Ti) detec­tor 50 cps. In order to increa­se the reso­lu­ti­on to 2 m2 wit­hin the resul­ting ano­ma­lous radia­ti­on, the area was mes­u­red with the same detec­tor by foot. Soil sam­ples, taken from core dril­lings, were ana­ly­sed by gam­ma­spec­tro­me­try and X‑ray fluo­re­scence spec­tro­sco­py, watersam­ples by alphas­pec­tro­me­try. In addi­ti­on, pie­zo­me­ter mea­su­rements were made. The resul­ting radia­ti­on map is shown in  Figu­re 2. The obtai­ned ura­ni­um con­cen­tra­ti­ons of the soil sam­ples show­ed a max. value of 220 ppm U at a depth from 0.60 m. The ura­ni­um con­tent of the watersam­ple from the pie­zo­me­ter (420 μg U/​l) show­ed the same value as ear­lier (Schmidt 2013) mea­su­rements from the drai­na­ge­wa­ter.

Eva­lua­ti­on of ura­ni­um sources

Several pos­si­ble ura­ni­um sources could been eva­lua­ted, based on a his­to­ri­cal inves­ti­ga­ti­on: dis­po­sal sites, fer­ti­li­sers / was­te of fer­ti­li­ser pro­duc­tion or geo­ge­ne ori­gin. The depo­si­ti­on age of tht solid ura­ni­um com­pounds in the soil could be esti­ma­ted to 10’000 years b.p., based on the ratio of Ra-226 and Th-234. The­re­fo­re any pre­sent-day anthro­po­ge­ne ori­gin of the ura­ni­um com­pounds can be exclu­ded. Howe­ver, during in the past 150 years the redox con­di­ti­ons may have chan­ged by drai­ning the swam­py fields to gain agri­cul­tu­ral land. This could have cau­sed the input of oxy­ge­ne into the soil to remo­bil­ze the ura­ni­um.

Autoren:
Simon Werth­mül­ler, Geo­lo­gi­sche Bera­tun­gen SCHEN­KER RICH­TER GRAF AG 
Büt­tenen­hal­de 42, 6006 Luzern (simon.werthmueller@fsgeolog.ch
Heinz Sur­beck, Nuc­film GmbH,
Fine­ta, 1792 Cordast 
Rico Ryser, Amt für Was­ser und Abfall des Kan­tons Bern (AWA), Gewäs­ser- und Boden­schutz­la­bor (GBL),
Rei­ter­stras­se 11, 3011 Bern