Long-term investigations of post-Chernobyl radiocaesium in fallout and air in North Croatia

Abstract

The long-term behaviour of (137)Cs activity concentrations in air and fallout has been studied in the city of Zagreb for the post-Chernobyl period (1986-2006) as a part of an extended monitoring program of radioactive contamination of human environment in Croatia. Annual mean (137)Cs activity concentrations in air and annual total deposition fluxes (wet plus dry) decreased from 2.8 x 10(-4) Bq m(-3) in September 1986 to 3.0 x 10(-6) Bq m(-3) in last quarter of 2006 and from 6,410 Bq m(-2) year(-1) in 1986 to 2 Bq m(-2) year(-1) in 2006 respectively. By fitting the measured (137)Cs activity concentrations to the theoretical curve the ecological half-lives of (137)Cs in air and fallout were estimated with respective values of 0.46 and 0.54 years for immediate post-Chernobyl period, increasing to 5.52 and 3.97 years afterwards. Using the data on (137)Cs activity concentrations in air and fallout total caesium deposition velocity of (3.34 +/- 3.13) x 10(-2) ms(-1) was estimated with median value being 2.13 x 10(-2) ms(-1). Such relatively high (137)Cs deposition velocities compared with pre-Chernobyl ones, are characteristic for the post-Chernobyl period and, according to Stokes' settling law, indicate that the diameters of aerosol particles associated with (137)Cs originated from the Chernobyl accident are pretty large, i.e. >1 microm. (134)Cs/(137)Cs activity ratio in fallout and in air has been found to be similar to the theoretically predicted values, initial value being about 0.5 and decreasing according to differential radioactive decay. The similar ratio has been observed in most of the other environmental samples.

Air sampling by pumping through a filter: effects of air flow rate, concentration, and decay of airborne substances

“…[9] cover the combinations of three experimentally observed φ(t) dependences and two profiles of a(t) that can be linked to certain situations of practical relevance. Owing to the generality of Eq.…”

“…[2] to write Eq. [8] as [9] The integration limits α and β may differ from 0 and T S if a(t) is not non-zero during the entire sampling period. We emphasise that Eq.…”

“…We emphasise that Eq. [9] includes a time-dependent a(t), which, to our knowledge, has not been considered in similar approaches. So far, only a constant activity concentration has been assumed, together with either a constant (1) or exponentially decreasing (2) φ(t).…”

“…For our sampling system (a highvolume sampler operating with chlorinated polyvinylchloride filters), we mainly observed φ(t)=φ 0 -φ 1 t [13] (where φ 0 ,φ 1 >0), with typically φ 0 ≈750 m 3 h -1 , φ 1 ≈2 m 3 h -2 , and T S ≈150-350 h. By again assuming that a(t)=a 0 , α=0, β=T S in Eq. [9], and using (13) for φ(t), we obtain ( )…”

“…Note that the decay in the air [reflecting the decrease of n(t)] is treated separately from the decay on the filter, the latter being accounted for in Eqs. [8] and [9] via the function Y(t)=exp(-λt). Since T 0 is known, as well as λ of a given radionuclide, in order to reconstruct a(t), one should find a relation between A F (T S ) and a m .…”