| The distribution area of carbonate rock is around 3.44 million km2 in China.Karst watershed is widely distributed and the groundwater resources from it account for 1/4 of the total.In general,southern karst water resources outclass that in north.As the main place of karst groundwater’s storage and migration in south,underground river is the important research object in the field of karst geo-hydrology and environment.However,the karst aquifer is difficult to study owing to the coexistence of conduit,fissures and pores,and it is extremely vulnerable to pollution because of the strong water conductivity,so karst geo-hydrology is the front direction of groundwater science.Aiming at the groundwater resources management and pollution control in southwest China,it is of great practical significance for us to master the laws of groundwater movement in the karst area and characterize the karst aquifer.To characterize the karst aquifer,some field trials are carried out to Maocun underground system in Guilin,Guangxi City in this paper.First of all,constant rate pumping tests was carried out at the borehole ZK1 in 2016.10.We calculated the geo-hydrological parameters to analyze characteristics of karst aquifer through these tests.The aquifer anisotropy is calculated based on the rectangular coordinate system with ZK1 as the origin and this paper concludes that the dominant direction is around S71.41 ° E,and the transmissivity of dominant direction and direction perpendicular to the dominant is Tα=1274.4 m2/d,Tβ=402 m2/d,respectively,where ZK1-1 and ZK1-2 are about on the vertical direction to the dominant and ZK1-3 is on the dominant direction.The transmissivity in the different directions(ZK1-1,ZK1-2 and ZK1-3)is about582.77 m2/d、506.9 m2/d and 1257.14 m2/d,respectively.To characterize karst aquifer medium in the test field,the tracer test under a forced gradient is carried out,where ZK1 is the pumped borehole and rhodamine B,fluorescent brightener,uranine put into injected holes ZK1-1,ZK1-2 and ZK1-3.From the shape of tracer breakthrough curves,this paper shows that every injected hole and pumped hole in aquifer is connected by the fractured dominant flow passway.Due to widespread fractured media and porous media,the aquifer has good water storage capacity and has a strong retention effect on solute.The recovery rate of three kinds of tracer is low the value of 45.3%(uranine),11.32%(rhodamine B)and 2.6%(fluorescent brightener).This paper considers that the anisotropy of aquifer is a very important factor.The recovery rate in the dominant direction is obviously higher than in the vertical direction.In addition,flow porosity of the rock in the test layer is relatively larger for it is broken,which causes tracer produces strong dispersion and enters into rock medium,but the hydraulic difference was caused by pumping couldn’t release the solute fully.Besides,tracers sinking in the hole and being adsorbed by clay and rock also results in the lower recovery rate. |
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