Abstract
Abstract: Sediment, the major reservoir of contaminants in a lake, contains not only hazardous matter, but also nutrients like nitrogen, phosphorus, and organic matter. So the detoxification, recycling and reuse of sediment has been widely studied. Agricultural land use of sediment is a potentially feasible approach for contaminated site remediation and can achieve good economic and environmental benefits. The key concept is to control the pollutant content and soil burden according to the related environmental standards. Heavy metals reserved in agricultural soil would greatly affect the grain yield and quality, as well as food safety, and are one of the major environmental pollution sources threatening human health. In this article, the sediment content in Xinpo Pond of the Nandu River Valley was planned to be used as the substitute soil of plough horizon in farmland remediation engineering to develop vegetable farming. To investigate the nutrient content and the degree of heavy metal contamination of the pond sediment, the shape and dimension of the pond, characteristics of the cross-section, flow rate, and the amount of sediment were comprehensively explored. Typical cross-section and grid sampling methods were chosen to collect samples within the 0~40 cm depth of the sediment. Based on (a) farmland environmental quality evaluation standards for edible agricultural products as the heavy metals evaluation criteria and (b) the second national soil survey classification criteria as the nutrient evaluation criteria, adopting sample testing, statistical analysis, nutrients gray correlation analysis, Nemerow pollution index assessment and GIS spatial analysis, the nutrient characteristics, soil fertility, heavy metal content, pollution degree and spatial distribution of the pond sediment were analyzed. The results showed: 1) Total nitrogen, total phosphorus, total potassium and organic matter content of the sediment presented spatial heterogeneity. Sediment total nitrogen content was in the range of 0.1300 ~ 6.7600 g / kg with an average of 2.8717 g / kg; total phosphorus content was in the range of 0.1097 to 2.3108 g / kg with a mean of 1.1824 g/kg; total potassium content fell in the range of 2.3000 ~ 16.8000g/ kg with a mean of 9.3000 g/kg; and the organic matter content fell in the range of 22.7195 to 167.8769 g / kg with a mean of 92.9408 g/kg. Mean values of total nitrogen and total phosphorus at the lower reach outweighed those at the upper reach by 32.45% and 11.88%, respectively, while total potassium and organic matter content at the lower reach were lower than those at the upper reach by 14.64% and 14.62%, respectively. The variation coefficient of nutrient data from the upper reach sediment were generally higher than those from the lower reach sediment, resulting from the flow rate, ambient natural environment and farming activities. Total nitrogen, total phosphorus, and organic matter reached level 1 (abundant), and the total potassium content was in level 5 (scarce). From evaluation of sediment nutrient based on the analysis of grey correlation, the average fertility indices of Ponds sediment and control processing were 0.4703 and 0.4685, respectively. The nutrients in the sediment were higher than that of the control point, which supported agricultural application of the sediment. 2) The mean contents of Cr, Ni, Cu, Zn, Cd, Pb, As, and Hg in sediments of the study area were 135.5983, 51.0098, 78.5999, 113.3576, 0.4245, 30.5595, 5.3038, and 0.1739 mg/kg, respectively. Heavy metal contamination was mainly attributed to Cd and Cu, followed by Cr and Ni, whose exceeding rates were 76.92%, 47.44%, 56.41%, and 23.07%, respectively; their exceeding magnitudes were 58.45%, 104.58%, 6.06%, and 68.48%. Heavy metal contamination resulted from the fact that the soil parent materials are mafic volcanic rocks, which caused high background values of the metals in the soil. Heavy metals in the soil were flushed by acid rain into the pond through surface runoff. From the Nemerow Index evaluation, the sediment was mainly slightly polluted (46.15% of the samples) to moderately polluted (28.21% of the samples). Contamination of the sediment was also spatially distributed, among which 62.05% of the site was at the slightly contaminated level (level 3) and 25.80% of the site was at the moderately contaminated level (level 4). The watercourse at the lower reach was wider and the flow rate was slow, increasing the rate of sediment deposition, the deterioration of sediment contamination, and the heterogeneity of spatial distribution. Therefore, the patterns of agricultural land use should be critically determined according to the contamination levels and nutrient conditions of the sediment.
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