| Due to the specificity of geographic location and geological conditions,coastal groundwater environment is considered to be fragile,and its resistance to external environmental interference and recovery ability are relatively poor.With the increasing intensity of groundwater resources development and utilization,coastal areas are generally facing groundwater environmental problems such as groundwater level decline,seawater intrusion and groundwater quality deterioration.Vegetation is a sensitive indicator of environmental change,and groundwater environment change in coastal area will trigger ecological response of surface vegetation.For example,groundwater level decline,and seawater intrusion will lead to vegetation degradation,loss of diversity and ecological service function decline,which eventually affects human living environment and sustainable development of economy and society.Therefore,there is an urgent need to conduct a comprehensive and systematic study on the evolution of groundwater environment and vegetation response under the influence of groundwater development and utilization in coastal areas.In this study,the evolution of groundwater environment in the coastal area was systematically studied by applying hydrogeochemical theory,environmental isotopes,remote sensing interpretation,geostatistics and multivariate statistics,etc.Based on sufficient data collection,field investigation and laboratory analysis,this study systematically investigates the process and mechanism of groundwater environmental evolution driven by human factors,the magnitude of impact of natural and human factors on groundwater environment and response of surface vegetation by means of hydro-geochemical techniques,geo-statistics and multivariate statistics,environmental isotopes method and remote sensing interpretation.Finally,the resilience of the groundwater environment is evaluated.The main research methods and conclusions are as follows.(1)The effects of groundwater development and utilization on groundwater level dynamics and groundwater quality on the east coast area of Laizhou Bay were studied.To address the complexity and nonlinearity of groundwater level response to external disturbances,a BP neural network model was constructed for the response of annual groundwater level change to the effective irrigation area,water-saving irrigation area and other influencing factors.Further,the default factor method was used to quantify the sensitivity of groundwater level to the influencing factors,confirming that agricultural irrigation with groundwater as the main water source is the main driving force of groundwater level.153 groundwater quality samples were collected in the study area to analyze the spatial distribution pattern of groundwater chemical components.Based on the differences of nutrients,heavy metals and microorganisms in groundwater,the fuzzy C-mean clustering method was used to classify the groundwater quality samples into four spatially contiguous categories,quantify the affiliation of water quality samples to multiple water quality categories,and solve the problem of gradual change and boundary mutation of groundwater quality in geographic space.The causes of the differences were discussed based on the natural background of the groundwater environment and the types and intensities of human activities,and it was found that anthropogenic pollution sources including agricultural,industrial,domestic and mining pollution sources as well as seawater intrusion were important causes of regional groundwater pollution.(2)Taking the lower basin of Wang River as an example,the impact of combined surface water and groundwater use on groundwater environment in the coastal area was analyzed.The changes of groundwater flow fields in Wanghe underground reservoir and downstream before and after the implementation of the project indicated that the groundwater cut-off wall blocked the hydraulic connection between upstream and downstream,deprived the downstream aquifer of the underground runoff received in its natural state,and caused the groundwater level in the downstream to drop continuously.3H isotope was used to indicate the movement and exchange characteristics of groundwater,it was found that the 3H in reservoir water was homogeneous in horizontal and vertical directions.This indicated that the closed storage space formed by the construction of the groundwater reservoir,the large-scale artificial recharge and exploitation of groundwater,and the good connectivity between aquifers promote the internal circulation of groundwater in the reservoir area.NO3--N is the main form of inorganic nitrogen in groundwater in the reservoir area,and its concentration is spatially distributed evenly and significantly higher than that in the downstream area.The internal circulation pattern of groundwater in the reservoir area,combined with the high intensity of agricultural fertilization jointly caused the accumulation and uniform distribution of NO3--N.(3)Based on Modis MOD13Q1 NDVI long series data,the spatial and temporal characteristics and evolutionary trends of NDVI in the east coast of Laizhou Bay were analyzed.Factor analysis was used to extract five integrated influencing factors,including land use and agricultural cultivation structure,agricultural production level,groundwater environment,climatic conditions,socio-economic development level and environmental protection investment,and to establish a multiple linear regression model of NDVI response to the above integrated factors to quantitatively analyze the degree of influence of groundwater environment on NDVI in the east coast of Laizhou Bay.Further,based on the results of groundwater and soil environment surveys,the mechanism of the influence of groundwater depth of burial and TDS on NDVI in different land use types was analyzed.The results showed that the decrease of groundwater level led to the decrease of guaranteed rate of irrigation water,and the longterm use of highly mineralized groundwater for irrigation led to the accumulation of salts in the surface soil of cultivated land,which was unfavorable to crop growth.The NDVI and yields of winter wheat and summer corn rotations in the lower Wang River were investigated,and the effects of combined use of surface and groundwater on crop productivity were analyzed.The results showed that,after the operation of the project,due to the improvement of groundwater environment and irrigation capacity,the NDVI of the farmland adjacent to the upstream and downstream of the dam site was significantly different,and the NDVI of the farmland upstream was 15.1%higher than that of the farmland adjacent to the downstream.The NDVI was 15.1%higher in the upstream cropland than in the downstream cropland.After the operation of the project,the yield of the cropland upstream of the dam site increased by an average of 22.0%,while there was no significant change in the cropland far from the dam site.(4)The resilience theory is applied to the groundwater environment to study the resilience of the damaged groundwater environment in the eastern shore of Laizhou Bay.Based on sufficient field investigation and data collection,the natural conditions and socio-economic indicators that affect the resilience of groundwater in the eastern shore of Laizhou Bay were selected,and a hierarchical analysis and evaluation index system of groundwater environmental resilience was constructed.Using the variable fuzzy identification model to quantitatively evaluate the groundwater resilience and analyze its temporal changes,it is found that the groundwater resilience has gone down first and then up with the changes of natural conditions,socio-economic development and the improvement of environmental protection capacity.Using the obstacle factor diagnostic model to diagnose and analyze the obstacle factors that restrict groundwater environmental resilience,it was found that the main obstacle factors influencing the improvement of resilience are different from period to period,and the main obstacle factors at this stage are excessive application of chemical fertilizers and industrial and domestic wastewater discharge.On this basis,the targeted measures for groundwater environmental protection were proposed. |
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