My PhD research is in its second year at the University of Edinburgh studying the Geochemical Behaviour of Cd in a Contaminated Watershed, Tak Province, Western Thailand.
During the 1960-70s, major projects to develop chemically supported agriculture, infrastructure, industrial estates and mining were launched in order to enhance the Thai economy. Unfortunately, in the areas where these projects took place, this has led to ecosystem and, in some cases, human health deterioration. Over the last decade in particular, there has been an increasing number of biogeochemical studies which aim to provide a better understanding of the changes occurring within the ecosystem, especially in lowland areas which have been contaminated to a high level with potentially toxic heavy elements such as cadmium (Cd).
One example of such a contaminated area is Mae Tao, a small watershed in Tak province, western Thailand. In 2003, the International Water Management Institute in collaboration with the Department of Agriculture, Thailand, observed that the concentration of Cd in paddy fields and rice was more than a hundred times the common background values of 0.14 mg kg-1 and 0.2 mg kg-1 for soil and rice, respectively (Simmons, 2005). This caused great concern amongst farmers and local people not only due to the tangible health problems but also to the economic effects, e.g. decreased rice demand and a government policy of not supporting rice growing in these areas. This was particularly hard-hitting since Mae Tao watershed is one of the most suitable areas of land in Thailand (and the wider region) for rice farming, having fertile soils, a good water supply due to high annual rainfall and an appropriate irrigation system with plenty of small creeks and irrigation canals. The high quality of Mae Tao Jasmine rice had also previously been recognised by its attainment of premium grade runner-up in the national rice product awards in 2002. Following confirmation of the Cd contamination in this area, many government departments and university researchers were asked to characterise the nature and extent of heavy metals contamination and to suggest remedial action. To date, only one plan has been accepted by the Thai government; that is to replace paddy fields with bio-energy crops such as sugarcane and oil palm. Many local farmers are, however, reluctant to grow such crops for various reasons including uneasiness about being disconnected from food production.
Consequently, the question arises; is it possible to maintain the Mao Tao paddy fields without causing any impact on human health? In this project, one set of check-dams, canals and a group of 18 paddy fields near the Mao Tao creek were selected to study the geochemical behavior of Cd as it is transported from the creek to fields. Our initial results have confirmed that high concentrations of Cd are present in some of the paddy field soils with the highest values (up to ~88 mg kg-1) being obtained for fields furthest from the main creek. These fields were also the most low-lying and it was established from interviews with the field owners that the 0-25 cm topsoil had been transported from the fields nearest the irrigation creek to the lowest lying fields for water management reasons. Consequently, this may account for the high concentrations of Cd now present in the fields furthest from the creek.
Having established the extent and location of the contamination, it is important to assess its availability to the rice plants as this will have a major influence on the potential impact on human health. Initial experiments have involved the use of sequential chemical extractions to estimate bioavailable Cd in these neutral-basic soils. In moderately contaminated fields, Cd in the exchangeable fraction (extracted by MgCl2 at pH 7) ranged from ~22-45% and that bound to the carbonate fraction (extracted by CH3COONa at pH 4.5) ranged from ~45-72%. In the most heavily Cd-contaminated soils, the proportion in the exchangeable fraction was extremely high (over 70%) and it is predicted that rice grown in these fields will have Cd concentrations in excess of the safe level of 0.2 mg kg‑1.
Following quantification of Cd concentrations in the roots, shoots, husks and rice grain from each of the 18 paddy fields, further work will investigate the key factors controlling uptake of Cd by rice. The overall aim is to find a remedial approach that will minimise transfer of Cd from the soil to the rice grain and that can be readily implemented in Thailand.
The Mao Tao region is only one example of an ecosystem affected by development activities; it should be viewed, however, as an important case study for developers and regulators to learn from in terms of potential impacts on ecosystems, community food sources and human health.
Peerapat Kosolsaksakul, Margaret C. Graham and John G. Farmer, School of GeoSciences,, University of Edinburgh, EH9 3JJ, UK.
Simmons, R.W., Pongsakul, P., Saiyasitpanich, D., Klinhoklap. S. Elevated levels of cadmium and zinc in paddy soils and elevated levels of cadmium in rice downstream of a zinc mineralized area in Thailand: implications for public health. Environmental Geochemistry and Health (2005) 27: 501-511.