Evaluation of the Accumulation Characteristics, Pollution Status, and Ecological Risk of Heavy Metals in Sediments of Higher-Shrimp Ponds

Abstract

To study the environmental impact caused by higher-place shrimp pond aquaculture, Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Agilent 7700x) was used to detect the contents of Cr, Ni, Cu, Zn, As, Cd, and Pb in the sediments of three types of shrimp ponds (the bottom of the shrimp ponds was covered with sand, cement, and mulch, respectively) and their surrounding ditches in Dongying Town, Haikou City. The Vario TOC analyzer was used to detect the Total Nitrogen (TN) and Total Organic Carbon (TOC) contents in the sediments. The detected values were analyzed and evaluated using various parameters, such as the single factor pollution index, geo-accumulation index, and comprehensive potential ecological risk index, to reveal the spatial enrichment, pollution status, and ecological risk of heavy metal contents in the sediments of higher-place shrimp ponds and their surrounding ditches. The major results of this study were as follows: 1) The contents of heavy metals in the sediments of the unwashed shrimp pond increased significantly from the L1 layer to the L4 layer. 2) In the sediments of the surrounding ditches, the contents of all heavy metals (except for those of Ni and Cd, which were the highest in the L1 layer) were the highest in the L4 layer and were 1.29-15.51 times higher than those in the surface layer; 3) In the L1 layer of the sediment of the shrimp pond, Cd showed a significant accumulation (pollution); its content were 5.47-35.18 times higher than the soil background value in the Hainan Province . 4) The evaluation of Igeo indicated that there are two or more heavy metals in all types of sediments that reached a mild-medium pollution degree and above. The most serious pollutant in each sample pond was Cd, ranging from a level of 1.87 to 4.55; the highest level, 5, indicates strong-extremely severe pollution, while the lowest level, 2, indicates a medium pollution level. 5) The single-factor potential risk coefficient (Eri) of all heavy metals, except Cd, in the surface layer sediments of each sample pond was ≤40, which indicates a slight pollution degree. 6) The Comprehensive Potential Ecological Risk Index (RI) of the sediments was 176.97 to 1090.87, and the RIs of all plots showed moderate or higher ecological risks, for which Cd was the main contributing factor. The Cd pollution load sharing rate (the proportion of Cd in RI) in the seven sample plots exceeded 90%, mainly because the biological toxicity response coefficient (Ti =30) of Cd was the highest and the reference value (=0.05) was the lowest; this increased the RI values and the ecological risk index levels. 7) Pearson's correlation analysis showed that the Cr, Ni, Cu, As, Cd, and TOC contents in the bait residues and prawn excreta of shrimp pond bottoms had extremely significant correlations with each other, and the correlation coefficient was as high as 0.9 or more, indicating that the bait used contained the above heavy metals at varying concentrations and that organic carbon is an important factor that affects the enrichment of heavy metals in the substrate.

Keywords

• higher-place shrimp ponds
• sediment
• heavy metal
• pollution status
• ecological risk