| Summary: | 碩士 === 國立成功大學 === 地球科學系專班 === 96 === In the past, researches on the natural purification and biogeochemical cycling of contaminants in the environment that have been carried out based on 210Pb-210Po radioactive disequilibrium, all stressed on the ocean. Relatively few have been done on rivers. In this study, I investigated the 210Pb-210Po radioactive disequilibrium in river waters and based on this disequilibrium, to deduce the biogeochemical transport, dissolution, and cycling parameters of trace elements or contaminants such as residence time, scavenging rate and time, etc. The results helped understand the impacts and implications of 210Pb-210Po radioactive disequilibrium on the environmental studies.
This research is composed of three parts: (1) the 210Pb-210Po disequilibrium in rain water, (2) the geochemical behavior of 210Pb and 210Po in river water, and (3) the use of 210Po as a tracer to study the migration mechanisms of the particle-reactive pollutants in rivers.
Results showed that the activity of 210Po in rainwater ranges from 3.5~42.0 dpm/100kg, averaging ~10.4 dpm/100kg and showing insignificant seasonal variability. In contrast, the activity of 210Pb shows greater temporal changes, ranging from 10.0~1181.0 dpm/100kg, with an average of ~192.0 dmp/100kg. The activity of 210Pb increases progressively during the period of typhoon, causing the 210Po/210Pb ratio to decrease as the typhoon progresses. This research shows that the 210Po/210Pb ratio varies from 0.04~0.48. It is estimated that the residence time of particle-reactive pollutants in the atmosphere is about 12~200 days, positively correlated with the 210Po/210Pb ratios in rain water.
The measurements on river waters indicated that the activity of dissolved 210Po in Erren River averages 1.53 dpm/100kg, much smaller than in rainwater. It is suggested that 210Po was easily absorbed and removed from the solution by the riverine particles. The activity of particulate 210Po in Erren River was 3~90 times that of dissolved one, or averaging about 77% of total 210Po activity in river water. In comparison, the activity of particulate 210Pb was 2.2 ~102.3 times that of dissolved 210Pb, or ~68 % of the total 210Pb. Obviously, 210Pb and 210Po in Erren river waters were mainly carried by particulates, in contrast with those in the open oceans (Nozaki et al., 1976).
This reflected that the particulates exert an important control on the transport of the particle-active pollutants. The research also indicated that the ratio of 210Po to 210Pb in the dissolved phases ranges from 0.45~4.87, averaging 1.44, proving that a significant excess of 210Po existed in Erren River waters. Because the 210Po/210Pb ratio in rainwater is in the range of 0.04~0.48, the atomsphere is obviously not the main source of the excess 210Po in Erren River. It is inferred that this unknown source of 210Po may be related to the organic pollution in Erren River.
During the rainy season (spring and summer), the dissolved and particulate phases in the Erren river waters both contain excess 210Po. However, In the dry season (fall and winter), their 210Po/210Pb ratio in stations adjacent to the estuary changes to < 1, a ratio close to that in seawater. It is likely that during the dry season, seawater may flow into the river and mix with river waters, causing the 210Po/210Pb ratio to decrease. It is estimated from the measured 210Po and 210Pb activities in the particulate and dissolved phases that the distribution coefficient (Kd) of particle-reactive pollutant is about 2.4×10^4 ~6.03×10^5 for 210Po and 4×10^3~4.52×10^5 for 210Pb. The calculated Kd values of 210Pb and 210Po both show negative correlations with the concentration of suspended particulate matter (SPM, mg/kg). This indicated that a significant amount of particle-active pollutant may exist in river waters in the form of colloid, decreasing the Kd values and increasing the residence time of pollutants in river waters.
This study proposed for the first time a scavenging model based on the observed 210Po-210Pb disequilibria of the dissolved and particulate phases in Erren river waters. Model calculations indicated that the water resident time in the Erren River basin varies temporally, ranging from 0.26~28.8 years and averaging 12.8. The scavenging rate constant of 210Po particles was between 0.003~ 0.067 day-1, averaging 0.025 day-1 and the scavenging time were 14.8~345.7 days, averaging 84.6 days. Except for stations influenced by seawater intrusion during the dry season or some stations upstream of the Namyung Bridge, where this model is not applicable because the 210Po/210Pb ratio is smaller than or approaches to the equilibrium value, our scavenging model can reasonably explain the scavenging and transport behavior of particle-reactive pollutants in the rivers.
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