Research On Eco-exergy Evaluation Of Newly-created Wetlands Caused By Coal Mine Subsidence Based On System Dynamics

With the advancement of China's industrialization process,coal production is increasing year by year.As the basic energy,coal's position is unshakable in the short term and an important foundation for the long-term stable development of the national economy.However,the large-scale coal mining also brings many negative effects on the regional sustainable development,among which the problem of coal mining subsidence is particularly prominent.The North China Plain has the longest history of coal mining and its landforms are mostly fluvial plain.Due to the rich surface water system and high underground diving level,the subsidence area gradually accumulates water.Its soil becomes gleying,forming a wetland.Therefore,it is important to achieve sustainable development of newly-created wetlands caused by coal mine subsidence.This is of great significance to understand its ecosystem structure,function and the main ecological processes,to explore the evolution trend,to understand the interaction between various elements of water,soil,and biology and to predict its development trend.In this study,Yanzhou Coalfield's newly-created wetlands caused by coal mine subsidence was taken as the research object,field investigation,indoor analysis and model construction were carried out.The formation and development process of newly-created wetlands were systematically expounded.And through the particularity and structure analysis of its ecosystem,the system dynamics model of newly-created wetland was constructed to simulate the state of the newly-created wetland ecosystem under different scenarios.According to the Eco-exergy evaluation analysis,the ecological restoration regulation approaches of newly-created wetlands caused by coal mine subsidence were proposed.The main contents and research results are summarized as follows:(1)The particularity of newly-created wetlands is mainly manifested in three aspects.First,in dynamic mining activities,the expansion rate of newly-created wetlands is significantly faster than that of other wetlands.In the initial stage of subsidence,the depth and area of subsidence are in a stage of rapid growth,so the capacity of the wetland expands rapidly.With the extension of the subsidence time,the growth of the subsidence depth and area has gradually slowed,and the wetland capacity has stabilized.It shows that the dynamic change of newly-created wetlands caused by coal mine subsidence is more dramatic in the early stage of formation,and as the time of subsidence increases,the dynamic change gradually slows down,and finally a relatively stable wetland type is formed.Secondly,the sediments of newly-created wetlands are transformed from fertile farmland,and the land use type around the wetland is also mainly farmland,so the material basis of ecosystem in newly-created wetlands includes endogenous input and exogenous input two aspects.In terms of exogenous input,under the effect of surface runoff,the surface soil nutrients in the newly-created wetlands are most severely lost in the middle of the subsidence slope.In terms of exogenous input,the sediments in the water mainly play a"sink"role for nitrogen and a"source"for phosphorus.Thirdly,the biodiversity level of plants and birds in the newly-created wetlands is significantly higher than that of traditional farming areas.There are differences in their species composition and community structure.The reasons for the high level of biodiversity in the newly-created wetlands mainly include:the collapse lead to an increase of spatial heterogeneity;subside seeper provides conditions for the habitat and reproduction of animals and plants;the water improves the local climate;the water adds new ecological niches.(2)Newly-created wetlands caused by coal mine subsidence affect the structure and function of the entire ecosystem through the interaction between internal environmental factors and biological factors.The characteristics of the environmental factors of the newly-created wetlands were as follows.There was no significant difference in water temperature,and there was a significant difference between different seasons.The water body was weakly alkaline overall,and there was no significant difference in p H seasonal dynamics.The electrical conductivity was higher than that of natural water bodies,and the seasonal dynamic showed that spring was significantly higher than other seasons.The salinity season dynamics showed that spring was significantly higher than other seasons.The contents of TN and TP were significantly different,and the seasonal dynamics were also significantly different,which was higher in summer and autumn,and lower in winter and spring.The characteristics of the biological factors of the newly-created wetlands were as follows.Phytoplankton biomass was significantly higher than other waters,and winter biomass was significantly lower than other seasons.Zooplankton biomass was less different from other waters and coal mine subsidence areas in Huainan,and spring was significantly lower in other seasons.The biomass of aquatic plants was larger than that in other waters,and was significantly higher in winter than in other seasons.The variation range of organic matter content in sediments was small,and the seasonal dynamics had significant differences.The leading factors of ecosystem in newly-created wetlands are phytoplankton biomass,salinity,p H and sediment organic matter content,of which TP has the most significant driving force for the leading factors,while the biological factors of phytoplankton,zooplankton,aquatic plants and sediment fluctuate less and are more stable.(3)The ecosystem dynamic model of the newly-created wetlands caused by coal mine subsidence was constructed using the system dynamics method,and through parameter calibration and verification,the simulated operation was carried out for one year.The sensitivity analysis results show that some parameters affected most of the state variables,while some parameters affected only one of them.The results showed that phytoplankton biomass increased with the increase of TP loading,while the ratio of zooplankton to phytoplankton biomass decreased with the increase of TP loading.By setting three scenarios,drastic change(Scenario 1),moderate change(Scenario 2)and mild change(Scenario 3),the different sampling points of the model were simulated.The results showed that nutrient loading has a large negative effect on the health of the newly-created wetlands ecosystem;the depth of collapse in newly-created wetlands has a more significant impact on phytoplankton biomass;there is a negative correlation between aquatic plant biomass and phytoplankton biomass,and the negative correlation was more significant under scenario 1(i.e.drastic change).(4)Based on the detailed explanation of the theoretical basis of Eco-exergy evaluation,the Eco-exergy Evaluation Index System(EEI)of newly-created wetlands caused by coal mine subsidence ecosystem was proposed.Then,the results of eco-exergy evaluation of newly-created wetlands in different subsidence years were compared and analyzed.The results showed that phytoplankton biomass EEI,zooplankton biomass EEI,Eco-exergy EEI,and Structural Eco-exergy EEI(Ex_st)all fluctuated significantly with the increase of subsidence years.According to the time of subsidence,the newly-created wetlands are divided into three stages:early subsidence,middle subsidence and later subsidence.The results show that the state of the newly-created wetlands ecosystem are gradually getting better after the later subsidence.(5)The practices of ecological restoration have a better role in improving the overall ecosystem service function of newly-created wetlands caused by coal mine subsidence.The water purification effects of the ecological restoration practice activities in the wetland park were obvious.The contents of electrical conductivity,chlorophyll,salinity,total nitrogen,and total phosphorus were significantly lower than the control area(newly-created wetlands outside the wetland park).Compared with the control area,the reduction rate of TN and TP reached 70%and 80%respectively.Diversity indexes of plant and bird in the wetland park,including Shanon-wiener index and Pielon index,were higher than those in the control area.Through the application of eco-exergy evaluation index system,the scores of each index and total scores in the wetland park were calculated,and compared with the results of different subsidence stages of the newly-created wetlands caused by coal mine subsidence.The results showed that the EEI index of the wetland park were higher than that of the newly-created wetlands at different subsidence stages,indicating that ecological restoration activities can better exert the overall service function of the wetland ecosystem.The ecological restoration and control methods mainly include the design of ecological buffer zone,dike-pond,littoral-forest,habitat island.The research on newly-created wetlands caused by coal mine subsidence is still in the exploratory stage.At home and abroad,the researches mainly focus on individual ecosystem elements,such as water,soil and biodiversity.Less attention has been paid to the ecosystem as a whole,and a systematic research system is lacking.This paper elaborates on the ecosystem characteristics of the newly-created wetland.Combined with the analysis of ecosystem elements and the methods of system dynamics and Eco-exergy,the regulation mechanism and dynamic development trend of the newly-created wetland ecosystem are clarified.In addition,the research provides a scientific basis and technical paradigm reference for the optimization of ecological restoration technology of newly-created wetland.