The Hydrochemistry Characteristics And Causes Of Karst Groundwater In The Peak Forest Plain Of Eastern Guilin

Water resource is the high unification of quality and quantity. Once suffers from pollution, it will aggravate the situation of water shortage.Karst underground water is one of the major water resources of karst area. Although karst groundwater resource is abundant in the southwest of China. The distribution of groundwater is anisotropic, due to intense dissolution and karstification. Meanwhile, bedrock is exposed in most area, so natural protective function of soil has almost lost. Therefore, the pollution on the surface of ground is easy permeate into underground along with karst mediums like precipitation or surface water, and contaminates the groundwater. The environment of groundwater is worrying. Then shortage of water has seriously hindered local economic development, so protecting local water resources is urgent.This paper, taking peak forest plain in Eastern Guilin as study area, collected and analyzed several samples of different types of water（surface water, karst spring, well water, foot cave）.By examining the temporal and spatial variation of major ions and comparing present hydrochemical indices with those of 30 years ago, the trend of groundwater hydrochemical changes was revealed. Hydro-geochemical methods and numerical simulation methods were used to detect the sources of pollution and influence factors of water chemistry. The study has found the following results:（1）The groundwater chemical compositions were mainly HCO₃-Ca, HCO₃-Cl-Ca, Cl-HCO₃-Ca, HCO₃-Cl-Ca-Na, HCO₃-Ca-Na and Cl-HCO₃-Ca-Na type also existed. In the 30 years of urbanization, the groundwater quality has been deteriorating. Domestic refuse and industrial waste water had a profound impact on groundwater quality, while the influence of agricultural pollution on groundwater quality was relatively small.（2） The Nitrate pollution is obvious in the study area. In the past 3 decades of speed development, the concentration of NO₃^- is 20.56mg/L compared with the background value. The pollution ions concentration characteristics were NO₃^->SO₄^2->Cl^-. The Cl^-, SO₄^2-and NO₃^- concentration were higher in the south and lower in the north with little exception. The chemical characteristics of groundwater were decided by the aqueous medium. For instance, the aqueous medium of karst spring was mainly karst conduit, and the larger catchment area, so the water circulation was faster. In contrast, aqueous medium of well was karst fissure, which had small catchment area, so the water circulation was relatively slow and the transport of contaminant was slow. The finding indicated the ion concentration of karst spring was much smaller than the wells in the study area.（3）Atmospheric precipitation rapidly recharged by foot holes and by piston type infiltration though the soil covering layer differentiated chemical indicators, i.e. shortcut rapid recharge was manifested dilution of precipitation, while piston type infiltration recharge was displayed the action of rainfall leaching.（4） Numerical simulation of the flow field was compared with the actual flow field in 1980 s.The author found that the simulated flow field was the same as the actual flow field;and the main runoff zone had the control effect on groundwater chemical field. The dynamic changes of ion concentration the water samples which were in the same main runoff zone were basically consistent.