Background levels of atmospheric deposition to ground and temporal variation of 129I, 127I, 137Cs and 7Be in a rural area of Germany

Abstract

To obtain information on the background levels of the atmospheric deposition to ground (wet plus dry) of 129I and 127I (stable iodine), the monthly means of these quantities were determined in 1988 and 1989 in a rural area of Germany. For comparison, 137Cs and 7Be were also determined in each deposition sample. The results show that the monthly deposition of 129I (in mBq m −2) varied in 1988 between 0·083 and 0·71 (mean 0·34) and in 1989 between 0·1 and 1·3 (mean 0·55). The corresponding values for 127I (in mg m −2) were in 1988: 0·031-0·18 (mean 0·093) and in 1989: 0·037-0·18 (mean 0·086). The mean atom ratio 129I/ 127I was in 1988: 6·2 × 10 −7, and in 1989: 9·6 × 10 −7. A seasonal pattern or a statistical significant trend in these data with time was not detectable. The deposition of 137Cs exhibited a continuous decrease with time according to an exponential law, and the deposition of 7Be showed a periodic pattern with maximum values in spring/summer. Correlation analysis between the amount of the monthly precipitation and the monthly deposition of the above isotopes suggests that 127I is similar to 7Be or weapons testing-derived 137Cs deposited to ground, mainly from higher levels in the troposphere, while the deposition of 129I occurs in a similar way to that of Chernobyl-derived 137Cs, mainly via resuspension processes from surface air layers.