| Jianghan Plain is an important part of the Yangtze River Economic Belt and a key ecological functional area at the national level.It undertakes the dual tasks of maintaining ecological security and economic development.However,long-term industrial and agricultural activities,and major water conservancy projects have seriously affected the rational development and utilization of groundwater resources.Hydraulic conductivity is a necessary parameter in groundwater resource evaluation,and its spatial variability affects the characteristics of regional groundwater flow field and determines the shape of pollutants in aquifers.Accurately obtaining the hydraulic conductivity and exploring its spatial variability have always been important research topics in hydrogeology.Therefore,it has an important theoretical basis and practical significance for the sustainable development and utilization of groundwater resources in Jianghan Plain to carry out research on the inversion of hydrogeological parameters,the spatial variability of hydraulic conductivity,and the causes of the spatial variability of regional hydraulic conductivity.The results of this study can not only provide data support for groundwater resource evaluation in Jianghan Plain,but also supply methodological guidance for the inversion of hydraulic conductivity and the estimation of its spatial variability at other similar fields.The main contents and conclusions are as follows:（1）Based on different types of pumping tests,the hydrogeological parameters of Qp2j（Jianghan strata）and Qp3s（Shahu strata）since the middle and late Pleistocene in the Han River watershed of Jianghan Plain were obtained by a variety of inversion methods.The pumping test data from 55 boreholes along the Han River watershed and perpendicular to the Han River were systematically analyzed,including 24 field pumping tests and 31 collected pumping tests（from the Hydrogeology and Engineering Geology Institute of Hubei Geological Bureau）.It includes transient pumping test with constant rate by multi-wells,transient pumping test with variable rate by single well,transient pumping test with constant rate by single well as well as steady pumping test with constant rate by single well.For different types of pumping tests,the Theis model and Cooper&Jacob model,intelligent optimization algorithms（particle swarm optimization algorithm,PSO;differential evolution algorithm,DE）,the numerical method,and the Dupuit model were used to obtain the aquifer parameters.Results showed that for transient pumping test with constant rate by multi-wells,PSO and DE were faster and more precise,especially DE.For transient pumping test with variable rate as well as transient pumping test with constant rate by single well,the numerical method fitted the drawdown better,while the PSO and DE had large deviations at the early stage of pumping.The Theis model and Cooper&Jacob model need to adjust the fitting results artificially on the basis of the system fitting.The results of hydrogeological parameter inversions showed that the hydraulic conductivity of the Qp3s strata ranges from 0.19m/d to 6.07m/d in the periphery of the Han River watershed,0.044m/d to 10.64m/d in the central area,5m/d to 7m/d along the Yangtze River.The hydraulic conductivity of Qp2j strata ranges from 0.33m/d to 2.01m/d in the periphery of the Han River watershed,and0.65m/d to 30.24m/d in the central area.（2）The two-dimensional spatial variability of hydraulic conductivity（K）was evaluated along the Han River watershed by using different geostatistical methods,including ordinary kriging,co-kriging with electrical resistivity and co-kriging with groundwater head distribution characteristics.Based on the K values by pumping tests,the spatial variability of K in Qp3s strata was analyzed by ordinary kriging.The mean estimation error（ME）,standard mean estimation error（MSE）,and standardized root mean square error（SRMSE）all deviated from the criterion.The residual‘Δσ*’map between observations and estimations also showed that the smoothing effect was obvious.In order to reduce the estimation error and improve the utilization rate of spatial multivariate information,a collaborative regionalized variable data set with K values as the main factor and the electrical resistivity and groundwater flow process（water head）at the same location as the auxiliary variables was constructed.The spatial variability of K in Qp3s strata was identified again by co-kriging（COK）method.The estimation results showed that compared with ordinary kriging,the ME,MSE and SRMSE of COK with electrical resistivity were lower 4.77×10-3m/d,lower 3.37×10-4m/d and higher 6.57×10-2,and the ME,MSE and SRMSE of COK with groundwater head distribution characteristics were lower 5.12×10-3m/d,lower 3.04×10-3m/d and higher 9.55×10-2,respectively.Obviously,the estimation error of COK is closer to the error criterion.In addition,as the interference of natural and man-made factors and the uneven distribution of geophysical exploration points,the two-dimensional distributions ofΔσ*also showed that COK with groundwater head distribution characteristics was better than COK with electrical resistivity.The spatial variability structure of K shows that S80E is the principal direction of longer correlation lengths,which corresponds to the flow direction of Han River.Perpendicular to the direction of the Han River flow,along the direction of N20E,the degree of variation is highest,having the shortest correlation length.The results indicate that the highest K values are in the western part of Shahu town and near the Yangtze River.The trend continues to diminish across western and northern Xiantao town and north-west of Xingou town,as well as across the transition zone from highland to lowland（from Shayang town to Zhanggang town）.The low K values occur around Hanchuan town,and are scattered around the high K values,with the lowest K values distributed in the north-west of the study area（from the catchment margin to the north of Jiukou town）,Yanglinwei town,and east of Shahu town.（3）Based on the drill core data,the three-dimensional spatial variability of hydraulic conductivity was characterized by the sequential indicator simulation method with the help of discrete geological variables.Considering that the above research results were limited to the two-dimensional spatial variability of hydraulic conductivity,this thesis will further reveal the vertical spatial variability of hydraulic conductivity.Firstly,2080cores from 48 boreholes were extracted（all with depth intervals of 1m）,and the 11lithologies in the Holocene（Qhg）（Guohe strata）and late Pleistocene（Qp3s）（Shahu strata）were simplified into three discrete hydrogeological units（HFU）according to the order of magnitude range of hydraulic conductivity:HFU-3(low hydraulic conductivity,K<10-7m/s),HFU-2(medium hydraulic conductivity,10-7m/s<K<10-5m/s)and HFU-1(high hydraulic conductivity,K>10-5m/s).After 50 random simulations,compared with the original core sampling input model,the ratios of HFU-1,HFU-2 and HFU-3 in the total output of the model are-2.33%,0.72%and 1.61%,respectively,which are all within the allowable error range.The results showed that in the area from the western periphery of the study area to Gaoshibei town,HFU-1 dominates the lower part,HFU-2 dominates the middle part and mainly forms local weak permeable lenses,and HFU-3 dominates the upper part and constitutes the local aquiclude.In the area from Zhanggang town to Hanchuan town,the lower part is dominated by HFU-1 with large thickness and good connectivity,while the upper part is dominated by HFU-3 and interconnected to form a stable aquiclude.HFU-2 is scattered in the aquifer and aquiclude.In the area near the Yangtze River,HFU-1 with large thickness and good continuity is widely distributed.HFU-2 is mostly sandwiched between HFU-1 to form continuous weakly permeable layers,while HFU-3 occupies a small proportion and mostly forms incoherent local weak permeable lenses.The hydraulic conductivity of the study area presents a four-layer spatial structure,which is divided into:from top to bottom,layered continuous aquiclude（HFU-3）and local weak permeable layers（HFU-2）,layered shallow aquifer（mainly HFU-1 and a small amount of HFU-2）,continuous stable aquiclude（HFU-3）,and layered continuous confined aquifer（a small amount of HFU-2 and a large amount of HFU-1）.The simulation results are basically consistent with the regional conceptual model.（4）Based on the sedimentary environment evolution during the late Pleistocene in Jianghan Plain,the causes of spatial variability of hydraulic conductivity in the Han River watershed were analyzed.In the area flowing along the Yangtze River and Han River,the accumulation of fluvial facies with alluvial facies developed during the late Pleistocene.The northwestern part of the study area（from the northwestern periphery to Jiukou town）is a hilly and mountainous area with structural denudation,which resulted in fine grain size of sediments and low values of hydraulic conductivity.The area from Shayang town to Zhanggang town is the transition from the tectonically denudative hills to the undulating plain.Because the low hills hinder the flow of surface water and groundwater,the capacity of the river to carry sediment was decreased,and the coarse-grained materials were gradually deposited and the fine-grained materials were continuously eroded and transported until the downstream accumulation.Therefore,the hydraulic conductivity presents a trend of gradually increasing and then decreasing,especially near the Gaoshibei town.The area from the east of Zhanggang town to the west of Hanchuan town is a low-accumulation plain area.Along the direction of the Han River,the grain size of the sediments changed from medium sand and coarse sand to fine sand with thick layers,and the hydraulic conductivity decreased gradually.Closer to the Yangtze River,stronger sediments were affected by fluvial fluctuation significantly,thereby reducing the sediment size as the hydraulic conductivity also revealed a decreasing trend from the Yangtze River to the center.In the center of the study area（east of Zhanggang town to west of Maiwang town）,shallow lacustrine facies,floodplain facies and marsh facies were developed during the late Pleistocene.The evolution of sedimentary environment in different periods has resulted in the alternate accumulation of different sedimentary facies,which made the distribution of hydraulic conductivity relatively scattered.In parts of Shahu town and Yanglinwei town,human activities have changed the original sedimentary characteristics of the strata,which has leaded to the point or massive maximum of the hydraulic conductivity. |
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