Temporal and Spatial Trends in Livestock Manure Discharge and Water Pollution Risk in Chaohu Lake Basin

By inergency On Mar 14, 2024

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1. Introduction

Sizeable and intensive livestock industries have been emerging throughout the world to meet the increasing population’s demand for animal-based foods [1,2], resulting in massive manure production [3,4]. However, inadequate animal manure collection and storage has led to the extremely low utilization efficiency of manure in cropland and contributed to high losses of manure nutrients to water [1,5,6]. Chinese data showed that animals excreted 2.3 × 10⁷ t N nationally; however, only one-third of the total amount was recycled to cropland in 2010 [5], with losses via leaching, runoff, and erosion amounting to 3.4 × 10⁶ t of N [7]. In addition, the direct discharge of manure to water bodies or landfills contributed 78% to the total manure P losses and 61% to the total manure K losses [7]. A large proportion of the rivers, lakes, and coastal waters in China suffer from severe eutrophication, and approximately 46% of the rivers in China have been classified as harmful to direct human contact; a major contributor of nutrients to watercourses, which causes eutrophication, is industrialized animal production systems [5]. Hence, it is essential to predict the spatiotemporal distribution of livestock manure and assess its risk for watersheds in order to prevent its loss causing water eutrophication.

Currently, the spatiotemporal variation tendency of livestock manure has been studied to prevent regional environmental pollution [8,9,10]. Yan et al. [10] estimated the spatiotemporal distribution of livestock and poultry farms in the Shangjie region, China, using ArcGIS technology, and the loss rate of livestock manure was considered. Jia et al. [9] estimated manure production and its geographical distribution in China and strengthened the potential for manure reuse using scientific composting for sustainable and environmental aims. Gallengo et al. [8] offered GIS and multicriteria tools to assess livestock farms with animal waste for sectorial, environmental, and social risks, particularly the relationship between the high density of livestock in an area and the levels of eutrophication of surface water and groundwater.

In China, lakes and reservoirs are important strategic water resources, but the frequent eutrophication and water blooms of lakes and reservoirs have caused harm to aquatic ecosystem health and drinking water safety [11,12]. Chaohu lake has been listed as one of the “Three Rivers and Lakes” under the national water pollution prevention and control [13]. It is one of the largest freshwater lakes in China [13,14,15]. With the rapid development of the social economy in the catchments, it has become one of the most hypereutrophic lakes since the 1980s [15,16,17]. There was a wide range of blue-green algae bloom outbreaks in Chaohu lake in 2018 [12,18,19]. The eutrophication and blue-green algae bloom in the western part of Chaohu lake were more serious than those in the eastern part due to the main pollution occurring in the west of the Chaohu lake basin [20,21]. Currently, most research has concentrated on the harm to lakes caused by hazardous materials for lakes based on the hydrodynamic model [11], evaluation of transient storage, and nutrient retention, such as N and P, in lakes [12,22]. Nevertheless, the risk effect of animal manure on lakes or reservoirs has rarely been investigated in China.

Therefore, in order to effectively prevent animal manure loss from including water eutrophication, the spatiotemporal distribution of animal manure and its risk effect on water pollution were investigated and evaluated via a case study of the Chaohu lake basin. The goals of this work were to (1) quantify the amount of animal manure, including pig dung equivalent and its associated pollutants (COD, BOD₅, NH₄₊-N, TN, and TP) in different years (from 2009 to 2019), and obtain the spatial distribution at the county scale of Chaohu lake basin; and (2) evaluate the risk for Chaohu lake basin via perspective analysis of the diffuse concentration of pollutants and the water pollution risk index of animal manure utilizing ArcGIS technology. The study provides a scientific basis for decision makers to reasonably restructure animal farms and optimize their spatial layout to improve the sustainable development of animal husbandry and protect the water environment.

4. Discussion

On account of the high water pollution risk of animal manure in the Chaohu lake basin, animal manure poses a threat to the whole basin [37,38]. Previous studies [20,22,39] indicated that eutrophication was worse in the western part of Chaohu lake than in the eastern part due to the nitrogen and phosphorus discharge of anthropogenic activities, including the built livestock farms. The major pollution sources, such as animal manure, are mainly distributed in the western basin (Figure 6). However, there are animal manure losses via leaching and runoff [5,23]. It was reported that a large proportion of manure N from pig production is lost via direct discharge into water bodies or landfilling [7].

Table 2 shows the influence of every 1% decrease in the loss rate for COD, BOD₅, NH₄⁺-N, TN, and TP from animal manure in the Chaohu lake basin in 2019. Table 3 shows the influence of the reduction in the loss rate on the different water pollution risk levels (I) in the basin in 2019. Table 4 displays the influence of the reduction in the loss rate from animal manure for different water pollution risk levels (I) in the basin in 2030. The results showed that the water pollution induced by the pollutants COD, BOD₅, NH₄⁺-N, TN, and TP from livestock manure would be alleviated annually if the loss rate decreased by 1%. The water risk index would decline at the same time. The risk index (I) of animal manure for Chaohu lake would be less than 5 if the loss rate of animal manure was reduced by 10.65%. These results revealed that animal manure losses have a strong influence on water pollution. Adequate animal manure collection and storage in animal farms should be prioritized to prevent manure losses.

Environmental pollution (water, soil, and air) from livestock and poultry farms could be prevented and controlled by incorporating indicators (diffusion concentration, loss rate, and risk index) for the environmental pollution potential of livestock manure to evaluate the suitable spatial distribution of livestock and poultry farms [8,9,10]. The results of this study provide scientific and technical support for livestock and poultry management and decision making.

5. Conclusions

The spatiotemporal distribution of livestock manure discharge and its water pollution risk was estimated in the Chaohu lake basin based on the excretion coefficient method and ArcGIS technology. The amount of livestock manure was 1.04 × 10⁷ t in the Chaohu lake basin from 2009 to 2019, with an increasing and then decreasing trend. The pollutants COD, BOD₅, NH₄⁺-N, TN, and TP from livestock manure in Feixi and Feidong accounted for 54.26% and 54.40% of the total in the basin. The results show the potential risk for the entire basin. Further, the diffusion concentrations of COD, BOD₅, NH₄⁺-N, TN, and TP in the entire lake basin from 2009 to 2019 were in the following order: Feixi > Feidong > Chaohu > Lujiang > Wuwei > Shucheng > Hefei. The water pollution risk index in Feixi and Feidong was higher than 20, indicating that these regions suffered serious pollution from animal manure. In 2030, the water pollution risk index is predicted to be approximately 18 for the Chaohu lake basin, revealing that the Chaohu lake watershed will have moderate pollution from animal manure. These results provide a scientific basis for policymakers to undertake some effective measures, including coupling planting and breeding, changing manure into organic fertilizer, and reducing the manure loss rate to improve the manure utilization efficiency and prevent manure’s direct discharge into water bodies in the Chaohu lake basin.

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