According to the subsidence areas formed in Huainan mining area in different periods, Datong pond (the subsidence age is nearly a hundred years), Xieertang pond (the subsidence age is about half a century) and Pansantang pond (the subsidence age is more than ten years) are selected as the research objects, and the nutritional status of the three coal mine subsidence ponds is evaluated by the above grey situation decision-making method.

The monitoring data (Table 9. 1 1) of three pond collapses in the same period are substituted into the constructed membership functions (9.16) ~ (9.20), and the impact measures are obtained respectively. These impact measures constitute the decision matrix of evaluation parameters as follows:

Study on environmental ecology of coal mine collapse pond

Table 9. 1 1 Test data of water quality evaluation parameters

Similarly, the single-objective decision matrix of each index can be obtained. As mentioned above, when the situation has multiple objectives, the decision matrix is integrated, where n is the number of objectives (where N = 6).

According to the single-objective decision matrix, the comprehensive decision objective matrix is as follows:

Study on environmental ecology of coal mine collapse pond

According to the best posture method of rows and columns listed in Table 9.8, the following best posture and effect measures are obtained by synthesizing the best posture of rows and columns:

Study on environmental ecology of coal mine collapse pond

Therefore, the evaluation results of the nutritional status of the three collapsed ponds are as follows: (1) Datong pond belongs to a water body with poor nutrition; Xieertang is an eutrophic water body; Pansantang is an eutrophic water body.

Because Datong pond was formed earlier, the mine has been scrapped, and there is no large amount of industrial wastewater discharged from mines. The terrestrial biological residues originally buried at the bottom of the water have rotted, and the organic matter contained in the water will be reduced due to the self purification of the water. In addition, there is little silt, so the organic matter content in the water is poor, and the N and P are also poor, which makes the water body poor in nutrition, good in water quality and slightly polluted or basically unpolluted.

During the development of the mining area, the collapsed water bodies such as Xieertang and Pansantang are rich in aquatic organisms and high in organic nutrients, which are eutrophic-eutrophic, with moderate water quality and moderate pollution. The main reasons for the eutrophication of Xieertang are as follows: before the formation of Xieertang, there was a loose plough layer under the farmland vegetation, and the carcasses of plants and animals buried in the water rotted after the collapse, which increased the organic nutrients in the water and shortened the eutrophication time; Secondly, the discharge of domestic sewage from surrounding residents, mine sewage, industrial wastewater from hemp spinning factory, artificial fish farming and bait feeding all increase the nutritional components in the water. Since 1976, the fish yield in Xieertang waters has increased from only 10.45 kg per mu to over 150 kg in/984. During the monitoring period, Pan Santang also released fry, but the breeding time was short. It can be seen that the nutritional status of the collapse pond in the mining area has a certain relationship with the formation time of the collapsed water body, that is, it has a certain relationship with the collapse formed during the development of the mining area or the final collapse formed after the mine is scrapped. In a word, the eutrophication degree of water body in coal mine subsidence area is different from that in general lakes. Therefore, corresponding measures should be taken to comprehensively utilize the water body in the subsidence area and improve the ecological environment of the mining area according to the nutritional status of the water body in the subsidence pond.