| The karst ecosystem is an environmental system as fragile as that of a desert. The main component of the karst environmental system is its hydrologic system, which though also includes various forms of rivers, lakes, sea and groundwater, notably differs from non-karst areas because of its underground drainage system.This article was based on precious data obtained from massive filed hydrogeological survey on the southern karst valley of Qingmuguan depression in Chongqing, and tried to investigate and summarize the geological structure's influences on karst development and the interactions amidst different types of water sources. We chose the karst valley of Tianchi-Dingjiawan section, and six water sources of varied geological structures in the valley as our subject. And by means of data collection and collation, field geological investigation and sampling, indoor contrastive analysis etc.; from the perspective of earth system science and combined with results from previous researches, preliminary studies were performed on traits and mechanism of water quality spatiotemporal variation in the upper, middle and lower reaches of the karst valley. Water resources protection of Tianchi karst lake in the high-altitude depression of the selected area was listed as a major objective of the study.First of all, preparatory data and the latest geologic drawings were collected. A series of field geological investigations were carried out on the selected area between December 2009 and April 2010, in order to further clarify the fundamental geological structure and fault zone distribution, and choose sampling sites of different hydrogeological structures.Secondly, consecutive field sampling, site observation and lab analysis were performed on all six locations along the southern Qingmuguan karst valley. The observing and sampling course lasted from May to December in 2010, with a procedural frequency of "once or twice a month". Site observation indexes include:water temperature (T), electrical conductivity (Ec), pH, Ca2+and HCO3- etc.; lab analysis indexes consist of major cations (K+,Na+,Ca2+,Sr2+,Mg2+) and major anions (HCO3-,NO3-,SO42-,CL-). Based on measurements of the above, determine physical and chemical characteristics of karst waters from upper, middle and lower reaches, as well as analyze interactions amid them.Thirdly, physical and chemical contrastive analyses, and ionic milligram equivalent proportion analyses on waters from different observing and sampling sites indicated that:high-altitude lake in karst valley possessed powerful short-term water accumulating capability, and great flood discharging ratio. Karst lake water quality deterioration's impact on middle and lower reaches was maximized in the rainy season. Lake water's karstification wasn't so strong as the underground fracture water, with sufficient water supply from the outside in the wet season, and slight rainfall replacement during the dry season.In the end, the results of this study were concluded as below:Water quality of each sampling site along the subject valley suggested prominent, seasonal variation. The quality of surface waters from Tianchi karst lake and Gantianchi reservoir was badly-deteriorated due to the lack of water supply in the dry season, and water of Gantianchi was too alkaline with pH reaching to 9.5. The overexploitation of surface karst water in the upper reach had resulted in postponed draining and early drying-up at the middle reach exit and lower reach underground debouchure during the wet season. And the source region pollution in the upper reach also caused environmental quality decrease in the karst lake tourism area, tourist quantity dropped and water quality in both middle and lower reaches was affected to a certain extent as well. Synthetically, to deal with the present circumstances mentioned above, we brought forward a few problems to be noted on future karst lake environmental improvement, as well as advices and ideas to protect the source region of karst water for reference.
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