A simulation study of the fate of radiocesium-137 in soil-plant systems around Ongkarak Nuclear Research Center, Nakhon Nayok, Thailand | |
| Author | Jantanee Jamsangtong |
| Call Number | AIT Diss. no.NR-05-01 |
| Subject(s) | Plant-soil relationships--Thailand--Nakhon Nayok Soils, Radioactive substances in--Thailand--Nakhon Nayok Soils--Environmental aspects--Thailand--Nakhon Nayok |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctoral of Technical Science, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. NR-05-01 |
| Abstract | This research study was designed to better understand the environmental risk might be derived from the operation of a small nuclear research reactor with the power capacity of 10 MW, located at Ongkarak district, Nakhon Nayok province, Thailand. Radiocesium-137 (137Cs), one of the products of nuclear fission was selected as a tracer. Its transformation in agricultural soil-flant based system was carefully investigated using lysimeter technique. In addition, the 137Cs adsorption-desorption from soils as well as potential leachability was estimated by a series of laboratory experiments. Adsorption and desorption of 137Cs by acid sulphate topsoil and subsoils collected near by the Ongkharak Nuclear Research Center (ONRC) were investigated using a batch equilibration technique. The influence of limestone (12 and 18 tons/ha) added into topsoils on 137Cs adsorption-desorption was also studied. Based on Freundlich isoterms, both adsorption and desorption of 137Cs were nonlinear. A large portion (98.26 - 99.97 %) of added 137Cs (3.7 x 103 - 7.03 x 105 Bq/L) was sorbed by the soils with lime and without lime added. The higher quantity of lime treatments, however, favoured stronger adsorption of 137Cs as compared with soil with no lime, which was supported by higher Kads values. By addition of lime, the CEC and pH of the soils increased and hence favoured the stronger adsorption of 137Cs. Acid sulphate soils with a high clay content, high CEC, and predominant clay types consisting of a mixture of illite, kaolinite, and montmorillonite were the main soil factors contributing to the high 137Cs adsorption. Competing cations, NH/, K+, Na+, Ca2+ and Mg2+ had less influence on 137Cs adsorption. The mes adsorption-desorption characteristics of the acid sulphate soils with lime and without lime added exhibited a very strong irreversible sorption pattern. Only a small portion (0.09- 0.58%) of 137Cs adsorbed at the highest added initial 137Cs concentration was desorbed by four successive soil extraction. Results clearly demonstrated that Nakhon Nayok province acid sulphate soils have a high 137CS adsorption capacity, which limits the 137Cs bioavailability and mobility. Therefore, risk of mes via plant uptake and groundwater contamination by this radionuclide is unlikely to occur. Results from three months of leaching experiments indicated that the acid sulphate topsoils (without and with lime at 12 and 18 tons/ha added) contaminated with 137Cs activity concentrations of 1500, 3000, 6000 and 9000 Bq/g provide a strong protection against 137Cs leachability. No 137Cs was detected in the effluent. The 137Cs vertical mobility in all soil columns revealed that the high amount of 137Cs (81.40 - 95.60 % of total application) of all soil columns was retained at the topsoil surfaces or contaminated layers (0 - 0.5 cm), while the rest penetrated to a depth of 0.5-3.5 cm. Only very slight downward movement of 137Cs occurred in all soil columns. Restricted mobilization of 137Cs could be explained by a strong adsorption capacity of the acid sulphate soil close to 100 % of added 137Cs was retained nearby the soil surface. The presence of lime caused 137Cs to be less mobile and the higher the rate of lime applied to the studied soils the lower migration of 137Cs to deeper than contaminated layer was found. Vertical mobility, potential leachability, speciation or binding form of 137Cs and 137Cs plant uptake from acid sulphate soils (with lime at 12 tons/ha + NPK fertilizer, with lime at 18 tons/ha + NPK fertilizer and with lime at 18 tons/ha + no fertilizer) contaminated with 137Cs concentrations of 1500 and 3000 Bq/g, for 0.5 cm soil surface, were observed by lysimeter experiments. Two types of vegetables commonly consumed, Water Convulvurus (terrestrial) and Chinese Cabbage (Early) were selected as indicators for 137Cs uptake for first and second cropping respectively. It was found that the vertical distribution of 137Cs in studied soils was limited at the top 5.5 and 10.5 cm depth for first and second cropping respectively. Mobility of 137Cs in a lysimeter was far better in extent when compared to the study done in a laboratory soil column leaching, because of the agricultural practice by hand plowing within the depth of 0-5 cm before second cropping. No 137Cs was detected in leachate coming out from all lysimeters. Chemical fractionation experiments of studied soils after plant harvesting indicated that 137Cs was mainly presented in residual form or non-bioavailable fraction. Because of this, total removal of 137Cs by studied vegetables confirmed that ≤ 0.0002 % of the originally supplied was removed from studied soils. The 137Cs activity concentrations in edible parts (stem + leaf) of first and second crop are 0.0160 - 0.1481 and 0.1240 - 0.4167 Bq/g, respectively. The first crop seems to slightly more uptake of 137Cs than second crop. However, those concentrations are lower than activity level of 137Cs allowed for international food transport for nuclear accidental or emergency situation (1 Bq/g, IAEA, 2002). The trend of the mes uptake per unit area (Bq/m2 ) for first and second crops was in order as following: stem>leaf>root. Limited transport of 137Cs from the contaminated soils to the plants were calculated and TFsp values of first and second crops are 0.00043 - 0.00550 and 0.00431 - 0.01517, respectively. As expected the higher the contaminated concentration of 137Cs applied to studied soils, the higher uptake of 137Cs to plants, because at higher concentration, sorption sites become increasingly saturated and increasingly higher proportions of the total added 137Cs remain available for plant uptake. The 137Cs uptake from studied soils can be arranged in the following order: limed soil with 18 tons/ha (without fertilizer applied) > limed soil with 12 tons/ha (with fertilizer applied) > limed soil with 18 tons/ha (with fertilizer applied). However, the higher the plant dry weight yield, the lower 137Cs contamination was observed. The higher the lime levels applied to studied soils the lower 137Cs uptake was observed. The reduction of 137Cs uptake with increasing lime level could be explained in terms of three factors: a rise in pH, the increasing of cation competition and CEC of the studied soils. The transfer of 137Cs from soils to plants in fertilized soils was lower than in non-fertilized soils because the non-fertilized soils become depleted in NPK availability. The influence of K+ and NH4+ on competition with 137Cs for root absorption might be more importance than the influence of K+ and NH/ on 137Cs for sorption sites. Soil treatment with limestone and NPK fertilizer application as normal agricultural practice can reduce 137Cs uptake by plants and can be used as a remediation measure avoiding both 137Cs bioavailability and downward movement. The 137Cs is strongly held to cation exchange sites of acid sulphate soils as indicated by irreversible sorption pattern of the acid sulphate soils, thus, physical processes, such as erosion and tillage, are expected to be the major causes of redistribution of 137Cs from soils to water. In the real field condition, macro cracking of studied soils with high clay content may enhance 137Cs deeper movement in the soil profiles. The other reasons may be from microorganism activities, burrowing of earthworm, other small animals and other farm management. |
| Year | 2005 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. NR-05-01 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Development and Sustainability (DDS) |
| Academic Program/FoS | Natural Resources Management (NRM) |
| Chairperson(s) | Preeda Parkpian; |
| Examination Committee(s) | Shrestha, Rajendra Prasad;Nguyen Cong Thanh;Apisit Eiumnoh;Pathom Yamkate;Carpenter, Roy; |
| Scholarship Donor(s) | National Science and Technology Development Agency (NSTDA), Thailand; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2005 |