Human and environmental factors affecting the activity of ¹³¹I and ¹³⁷Cs in urban wastewater: A case study

The analysis of radionuclides content in the waste product of urban wastewater treatment (i.e. sewage sludges) is of great concern for both the safety of workers and in a re-use perspective of the final product. The study of the behavior and the partitioning of radionuclides into sewage sludge samples can be useful not only to determine engineering issues in removal efficiency of radioactive pollutants, but also to determine the factors affecting the content and trends of radionuclides (natural and artificial) in urban wastewater systems. In this study we determine people-related and environmental factors that affect the activity of ¹³¹I (nuclear medicine) and ¹³⁷Cs (fallout) in urban wastewater in a large area of northern Italy. 624 sewage sludge samples coming from 17 wastewater treatment plants were collected between 2012 and 2017 and analyzed with high resolution gamma-spectrometry. In addition to ¹³¹I and ¹³⁷Cs, also the following radionuclides were analyzed for the periodic monitoring: ⁷Be, ⁴⁰K, ⁶⁰Co, ⁶⁷Ga, ¹¹¹In, ¹²⁵I, ¹³⁴Cs, ¹⁹²Ir, ²⁰¹Tl. ¹³¹I is the most frequently detected artificial radionuclide in sewage sludge samples (56%), showing a large variability (mean activity ± standard deviation 93 ± 184 Bq kg^-1). The detection frequency of this radionuclide is significantly correlated to population density (R = 0.75, p-value<0.01) and its activity concentration is significantly correlated to wastewater treatment plant size, expressed in population equivalents (R = -0.56, p-value<0.05). The detection frequency of ¹³¹I in sewage sludge samples is modeled as a function of population density, sewer length, inflow rates and population equivalents, accounting for 83% of the variability of observations. Despite the frequency of detection, the activity of ¹³¹I was relevant for workers safety only in less than 0.5% of the cases. On the other hand, ¹³⁷Cs is detected in 36% of samples and its activity shows high correlation with mean annual precipitation (R = 0.67, p-value<0.01) and the activity of natural radionuclides (R = 0.58 and 0.63, for ⁷Be and ⁴⁰K, respectively, p-values <0.05). With a hybrid geostatistical model, we demonstrate that the spatial pattern of ¹³⁷Cs activity in wastewater estimated from sewage sludge samples is consistent with soil contamination from Chernobyl fallout. Moreover, we hypothesize a possible horizontal transport of ¹³⁷Cs in the study area highlighted by different trends of ¹³⁷Cs activity concentration in sludges. The results of this study depict the sensitivity of sewage sludges to the persistent contamination of ¹³⁷Cs, after 30 years from Chernobyl accident, and can be used as a database for future monitoring activities after any accident that includes the spreading of radioactive materials into the environment.