Study on the variation characteristics of backwater levels of the Pijiazhai karst spring in Luxi county,Yunnan

ZHOU Cuiqiong; CHAI Jinlong; ZHANG Gui; WANG Yu; HE Raosheng; WANG Jin; LI Jihong

doi:10.11932/karst20240606

Abstract：The Pijiazhai karst spring is located at the edge of the upstream Luxi basin,where the terrain shifts from steep to gentle slopes.The spring's recharge area covers 115 km2,where the recharge runoff area is the white water karst trough valley in the upper reaches of the basin and the peak-cluster depression area at the edge of the basin.There are more springs in the white water karst trough valley but the dynamic is unstable.The main aquifer formations in the spring are dolomite and limestone of the Triassic Gejiu Formation.Although karst development is strong,it is uneven,leading to significant variability in hydraulic properties.Besides precipitation recharge,leakage from the Baishuitang reservoir is a crucial source of recharge.The hydraulic gradient,from the Baishuitang reservoir to the spring outlet,ranges from 1.43%to 1.62%,with groundwater flow velocities between 144.82 and 176.45 m·h-1.The spring discharge varies from 1,072 to 1,957 L·s-1 and remains relatively stable.By means of drilling,geophysical exploration and backwater test,the water levels of the Pijiazhai karst spring are studied.A total of 16 boreholes were drilled near the spring outlet,with depths ranging from 15.1 to 45.1 meters,and the maximum rise in backwater level reached 0.66 meter. Results indicate that,despite the intense karst development near the Pijiazhai karst spring,the distribution of this development is spatially uneven.Horizontally,the analysis of joint fissures reveals four major karst development zones composed of densely distributed caves and solution fissures.Additionally,at varying depths,these dense zones of caves and solution fissures are also present vertically.To the north of the spring outlet,karst development is intense but uneven,predominantly featuring solution fissures and caves,with some caves diameters reaching 4.7 meters.Downstream of the spring,karst development is relatively weaker and more uniform,primarily consisting of solution fissures,with fewer and smaller caves,the largest being 0.6 meters in diameter.At the eastern side of the spring outlet,a significant karst development zone is dominated by solution fissures,which generally exceed 70 meters in thickness,and are widely distributed.Overall,at the area near the spring outlet,a complex and interconnected network of fissures and conduits have been developed.The Pijiazhai karst spring exhibits a certain degree of pressure resistance.Prior to the backwater at the spring outlet,the water levels in 15 nearby observation boreholes,with the exception of the upstream Borehole zk1,are consistently 0.3 to 3.0 meters lower than the water levels at the spring outlet.After backwater,the water levels at the spring outlet remains elevated compared to the levels in the other observation boreholes.The increase in water levels around the spring outlet during the backwater process,as well as the response time and distance,do not exhibit a direct relationship;rather,they are influenced by the development and connectivity of the karst system,demonstrating significant anisotropy.Notably,the spring outlet experiences the most substantial rise in water levels,while the northern upstream area shows a relatively larger increase.In contrast,the southern downstream area exhibits a rapid decline in the amplitude of the rise,and the eastern region experiences a smaller and slower increase.Upstream of the spring outlet,the aquifer structure is mainly characterized by fissure-conduit systems,which show substantial and rapid water level rises during backwater.This often results in a 2-3 hours of groundwater"surge"phenomenon,initially marked by a rapid rise to a peak,and then a brief decline,followed by a gradual rise to a stable level,forming a"double-peak"pattern.Downstream,the aquifer structure is primarily a fissure network,leading to smaller and slower rises of water levels during backwater,typically showing a more uniform"linear"rise,with occasional short-term anomalies of water level decrease before rising.When the backwater height at the spring outlet maintains within a specific range,short-term fluctuations in groundwater levels may cause the spring water to become turbid;however,this does not result in a significant decrease in discharge.Engineering practice confirms that a rise of 4.4 meters in spring head does not lead to a noticeable reduction in discharge.