Analysis of the benefit of afforestation on heavy-metal polluted farm land
Started from September 28, 2010, this project investigated the benefits of phytoremediation by the afforestation for remediation of heavy metal polluted farmlands in Chung-Fu area, Lu-Chu township, Tao-Yuan county. Heavy metal contents of soils of polluted farmlands and woody biomass on the planted trees were measured, as well as overall woody biomass. Five afforestation plots (nos. 99-13, 103, 151-1, 152-2, 162, with total area of 2.09 ha) and five grass plots (nos. 116-1, 158-1, 158-2, 158-6, 102-5, with total area of 0.746 ha) have been analyzed for heavy metal contents of top (0-15 cm) and inner layer (15-30) soils, as well as their fundamental properties. 35 soil samples were analyzed for their heavy metals contents including arsenic, cadmium, chromium, copper, mercury, nickel, lead and zinc. Total woody biomass, mainly composed by camphor, was also estimated. The economic benefits for afforestation and the scenario of rice production after remediation were also estimated. Cadmium (Cd) was found to be the major pollutant among the heavy metals in Chung-Fu area. The averaged concentration was 3.86 mg/kg, with the range of 0.14~15.5 mg/kg. There were 11 samples found at 5.29~15.5 mg/kg in the soil, exceeding the control standard. Cd concentrations of 9 soil samples were found at 2.54~4.8 mg/kg, exceeding the monitoring standard. The averaged Cd concentration of forested soils was reduced from 6.08 mg/kg in 2003 to 2.98 mg/kg in this study. The averaged Cd concentration of soils of grasslands was reduced from 8.45 mg/kg in 2003 to 5.54 mg/kg in this study. The Cd contents of soil samples measured by Taoyuan county government in 2003 were 0.53~25.8 mg/kg. However, the difference the Cd contents between the results of this study and of the aforementioned 2003 study were found insignificant by statistical methods. For plant biomass analysis, Cd and Cr were the two major contaminants. 0.79~8.97, 1.28~15.4, 0.64~10.6 and 0.53~3.08 mg Cd/kg dried biomass were found in the leave, branch, sapwood and heartwood samples. Averaged Cd contents of the branches in the individuals were found larger than the values in the leaves, sapwood and heartwood. Initial analysis revealed that the wood produced by the studied sites could serve as wood trestles, trail steps and furniture materials after furnishing. In addition, there was no leaching for heavy metals after TCLP tests. Hence, the wood also could serve as general fuels. There are not a lot of differences of fundamental properties for soils in grass plots and forested plots. Based on their better soil adaptabilities, the acacia, the white chanterelle, Indian rosewood and thorn bamboos are recommended for plantation in the environments of the studied sites. This study estimated that the remediation cost for the 2.089 acre heavy metal polluted farmlands in Chung-Fu area was 4.85 million NTD. Averaged net income for 20 years of rice plantation was only 0.92 million NTD. 376 million NTD was expected for landowners enrolled for the plain afforestation plain of Taiwan Forest Bureau after 20 years. 3.76 million NTD of subsidy was expected for landowners enrolled for the plain afforestation plain of Taiwan Forest Bureau after 20 years. 1.65 million NTD of subsidy per acre was expected for landowners of polluted farmland enrolled for the plain afforestation plain of Taiwan EPA after 10 years. Only farmlands declared as the polluted control sites by the environmental agencies were eligible for Taiwan EPA subsidy. Since the tree growth is a long tern affair, the coordination of above two subsidy programs needs more discussion. After 20 years of afforestation, this study found that the income of wood harvested as sawn lumber is only 0.3 million NTD for the studied sites. If whole trees were sold for landscape plantation usages, the overall income would be 3.55 million NTD. However, the policies for more incentives for sales of landscape trees are required to increase the willingness of farmers’ participation. Three promotion meetings were held to present the principle of subsidy for ceased farming of polluted farmlands as soil control sites declared by Taiwan EPA, as well as the standard operation principles for pollution incidents of farmlands.
heavy metal pollution, phytoremediation, campor tree