The article attributes cesium-137 to discharge from Mühleberg Nuclear Power Plant.
From AFP, via Yahoo Malaysia (7/14/2013):
'Radioactivity found in Swiss lake' near nuclear plant
Scientists have discovered a radioactive substance in sediment under a Swiss lake used for drinking water and situated near a nuclear plant, the Le Matin Dimanche weekly reported Sunday.
While scientists cited in the report stressed there was no danger to human health, the discovery raises concerns about safety practices and a lack of transparency at the Muehleberg nuclear plant in northwestern Switzerland.
The plant is believed to have caused a spike in cesium 137 found in the sediment of Lake Biel and dating back to 2000 through the discharge of contaminated waste water into the Aar river that feeds into the lake, about 20 kilometres (12 miles) downstream, the weekly reported.
Geologists from Geneva University happened upon the spike while working on an unrelated research project in 2010, and chemists in the northern canton of Basel recently verified the findings, it said.
The Muehleberg plant is permitted to discharge water with very low levels of radioactivity subject to strict controls several times a year, according to Le Matin Dimanche.
Politicians and environmentalists however expressed outrage Sunday that the plant and nuclear inspectors had provided no information about the higher levels of cesium 137 released more than a decade ago into a lake that provides 68 percent of the drinking water to the nearby town of Biel.
"No one ever told me that there were abnormally high concentrations in the lake," Hans Stoekli, who served as Biel mayor from 1990 to 2010, told the paper, insisting that in light of the use of the lake for drinking water "the plant should have alerted us even in the case of minimal risk."
Environmental group Greenpeace voiced dismay at the news, urging the public prosecutor in the canton of Bern, where Biel and the Muehleberg plant are located, to investigate.
The group, which has long called for the plant's closure, also questioned in a statement how the Swiss Federal Nuclear Safety Inspectorate could have either missed the higher radioactive levels or decided not to inform decision makers or the public about them.
The Muehleberg plant, which came online in 1972, is 17 kilometres (11 miles) west of the Swiss capital Bern.
In the wake of the 2011 Fukushima disaster in Japan, the Swiss parliament approved a phase-out for the country's five atomic power plants by 2034.
As the article does not talk in numbers, I looked for the paper and found it: "Human impact on the transport of terrigenous and anthropogenic elements to peri-alpine lakes (Switzerland) over the last decades", by Florian Thevenon, Stefanie B. Wirth, Marian Fujak, John Poté, Stéphanie Girardclos, Aquatic Science, July 2013 (open access at Springer.com). It turns out that a cesium-137 spike was not just one time, but three different occasions:
Terrigenous (Sc, Fe, K, Mg, Al, Ti) and anthropogenic (Pb and Cu) element fluxes were measured in a new sediment core from Lake Biel (Switzerland) and in previously well-documented cores from two upstream lakes (Lake Brienz and Lake Thun). These three large peri-alpine lakes are connected by the Aare River, which is the main tributary to the High Rhine River. Major and trace element analysis of the sediment cores by inductively coupled plasma mass spectrometry (ICP-MS) shows that the site of Lake Brienz receives three times more terrigenous elements than the two other studied sites, given by the role of Lake Brienz as the first major sediment sink located in the foothills of the Alps. Overall, the terrigenous fluxes reconstructed at the three studied sites suggest that the construction of sediment-trapping reservoirs during the twentieth century noticeably decreased the riverine suspended sediment load at a regional scale. In fact, the extensive river damming that occurred in the upstream watershed catchment (between ca. 1930 and 1950 and up to 2,300 m a.s.l.) and that significantly modified seasonal suspended sediment loads and riverine water discharge patterns to downstream lakes noticeably diminished the long-range transport of (fine) terrigenous particles by the Aare River. Concerning the transport of anthropogenic pollutants, the lowest lead enrichment factors (EFs Pb) were measured in the upstream course of the Aare River at the site of Lake Brienz, whereas the metal pollution was highest in downstream Lake Biel, with the maximum values measured between 1940 and 1970 (EF Pb > 3). The following recorded regional reduction in aquatic Pb pollution started about 15 years before the actual introduction of unleaded gasoline in 1985. Furthermore, the radiometric dating of the sediment core from Lake Biel identifies three events of hydrological transport of artificial radionuclides released by the nuclear reactor of Mühleberg located at more than 15 km upstream of Lake Biel for the time period 1970–2000.
The researchers took the sediment core sample from Lake Biel and measured the radioactivity over several decades. The chart below, from the paper, shows the initial bump in cesium-137 in 1951, then a high peak in 1963, both from the fallout from atmospheric nuclear testing. After Mueleberg Nuclear Power Plant came on line in 1972, cesium-137 peaked in 1976, then in 1986 (Chernobyl fallout), and again in 2000.
According to the researchers, cesium contamination attributable to Muehleberg Nuke Plant occurred in mid 1970s, early 1980s, and 2000:
Additionally to the three 137Cs markers generally used for dating (1951, 1963 and 1986) European sediment records, the Lake Biel sediment core (BIE10-3) revealed two additional 137Cs activity peaks at 8.5 (41 Bq/kg) and 30.5 cm core depth (94 Bq/kg) (Fig. 2). On the basis of the 210Pb age model, these peaks are dated to 2000 and 1975. The peak dated to 1975 has been reported in a previous study and detected in several Lake Biel sediment cores, of which the nearest is only 300 m away from our site (Albrecht et al. 1998). This increased 137Cs activity was caused by higher radionuclide discharges of the upstream Mühleberg NPP due to the use of low-quality fuel rods between 1976 and 1978 (Albrecht et al. 1998). In addition, a small 60Co activity peak detected at 22.5 cm core depth is dated to 1984 on the basis of our 210Pb activity age model (Fig. 2). Albrecht et al. (1998) suggested that this peak was related to higher wastewater discharges from the Mühleberg NPP documented in August of 1982. The age difference of 1–2 years between the age model and the date of the known nuclear events lies within the error range of our age model (see next paragraph). Our results therefore confirm the previous identification of increased radionuclide discharge by the Mühleberg NPP in 1976 (137Cs) and 1982 (60Co). However, regarding the 1982 event, our data is not as definite as Albrecht et al. (1998, 1999) because 60Co, which was the only released radionuclide during this event, shows at present only slightly elevated activity in our Lake Biel record.
In the upper part of our Lake Biel sediment core, a small 137Cs activity peak dated to 2000 is for the first time revealed by this study (Fig. 1). The 60Co as well as the 210Pb in excess (obtained after subtracting the 226Ra activity to the total 210Pb activity) profiles demonstrate that this 137Cs activity peak was not caused by other processes, such as reworking of sediment containing Chernobyl-linked activity or changes in sedimentation rates which would appear in the 210Pb in excess profile. Therefore, this 137Cs activity peak that is not accompanied by a coeval 60Co peak can be explained by two possible causes: (1) only 137Cs radionuclides were released to the environment, or (2) the 137Cs was better scavenged by the minerogenic fraction than 60Co. Indeed, only 30 to 55 % of the discharged 60Co is transferred to Lake Biel sediments and the scavenging efficiency of 60Co is higher during the winter period than during summer, in spite of higher particle fluxes during summer (Albrecht et al. 1999). Thus, the found moderate 137Cs activity peak likely points to an additional and so far undetected release of liquid radionuclides from the Mühleberg NPP to the Aare River in the year 2000 ± 2. This discharge event could coincide with an event reported at the Mühleberg NPP on the 6th of June 1998, when an accidental opening of a steam-relief valve led to an emergency shutdown of the reactor (Prêtre 1999). Yet, the Swiss government monitors radionuclide activity in the air and water near the Mühleberg NPP, but registered values for 1998 are similar to those of previous years and lie all below 3 % of the legal limit (Prêtre 1999).