| Red beds are widely distributed over the southwest of China, in which the red beds of Sichuan basin are the most remarkable. Over the past few years, with the promotion of ecological tourism, the red beds of mountain area in the rim of basin have gotten more and more attentions, thus the impact of human activities on it has been increased. On the basis of understanding the water-quality characteristics of the red beds of mountain area, it is urgent to reasonably develop, utilize and protect water resources in the red beds. Under the regional hydrological and geological background, this paper studies hydro-geochemical characteristics of surface water and groundwater as well as the temporal and spatial variation of hydrochemistry from the perspective of hydro-geochemistry, and gives an analysis and evaluation on the water quality with Principal component analysis(PCA), so as to master the characteristics of the water quality in the red beds and the reasons for the change of water quality.This paper takes the red beds of mountain area in the south rim of Sichuan Basin as an object of study, and takes Xishui- Jiangjin red beds of mountain area where is a common boundary of Guizhou and Chongqing as a typical area of research. With taking a complete hydrological year as the criterion, this paper makes a systematic sampling on groundwater and surface water in the area of research; with means of field monitoring, indoor experiment analysis, mathematical analysis and so on, this paper gives a systematic analysis on the hydro-geochemical characteristics, rules of temporal and spatial variation, as well as water quality characteristics of the surface water and groundwater in the area of research. The main results obtained are as follows:(1) In the area of research, the hydrochemical types are mainly including: HCO3·SO4-Ca, SO4·Cl-Ca, SO4·Cl-Ca·Na and HCO3·SO4-Ca·Na total four types, and dominated by the type of HCO3·SO4-Ca. The surface water and groundwater in Da Maopo is dominated by the type of SO4·Cl-Ca. Through Gibbs chart analysis, the hydro-geochemical component is distributed in the control area of rock differentiation, which shows that the surface water and groundwater in the area of research are mainly affected by rock weathering. The pH value of groundwater in the area represents a overall acidic value. For the anion composition, the concentration of SO42- is the most significant, and it is reputed that it is mainly relative to the weathering and dissolution of sulfide minerals and lower-part gypsum clumps in water-bearing level of red beds in the area of research; for positive ion composition, the concentration of Ca2+ is the most significant, which mainly comes from the weathering and dissolution of carbonate and other calcareous cements in sandstone and shale. For hydro-chemical type, mineralization degree, as well as pH value, the impact of the chemical composition of surrounding rock on them is the largest.(2) The variation range of pH value of surface water is basically between 6.36~ 7.34 and the amplitude with seasonal variation is small. Due to the impact of the oxidation of sulfide minerals in stratum on two reservoirs and groundwater in mountaintop of Da Maopo, PH values are mostly acidic, and the amplitude is large with seasonal variation. The conductivity of surface water and groundwater of farming area in Hou Caoding is affected by agricultural activities, the amplitude of variation is obviously larger than the sample points in Da Maopo and Ta Wodang, and the seasonal variation trend of groundwater conductivity and pH value is similar(increase in rainy season and decrease in dry season), which may be related to the diluting effect of rainwater on PH value and conductivity in rainy season. The change trend of positive ion concentration in surface water with different water bodies basically is the same. The amplitude of irrigation water and gully streams is comparatively large with the change of seasons, and the amplitude in wet season is higher than the dry season. The amplitude of cation and Cl-, NO3- concentration in the surface water of mountaintop of Ta Wodang and Da Maopo is small with the change of seasons, which is relative to some factors, such as the high degree of coverage of forest land, little human activities, etc. Na + in groundwater of mountaintop is mainly from the differentiation and dissolution of sulfurous strata, and the amplitude of SO42- concentration is the largest with the change of seasons, that is, the concentration is low during the wet season, while the concentration changes little in dry season and normal river-flow season.(3) The types of groundwater in the area of research mainly include the sandstone in red beds as well as the crevice water in the weathering zone of shallow layer of mudstone, of which burying is shallow. The limitation of mountainous landform on the conditions of recharge and flow-off and draining is very obvious, which has characteristics of local recharge and discharge. The groundwater is mainly recharged by rainfall and irrigation total two ways. With the decrease of altitude and slope position, the land use system is changed from forest land to farmland. Except for SO42-, the overall ion concentration in groundwater shows a trend of gradually increasing, which means that the impact of agricultural activities on groundwater in the farming area is significantly.(4) Analysis and evaluation of surface water and groundwater quality in the area of research with the method of Principal Component Analysis(PCA). The size load of the first principal component of surface water shows that, the impact of geologic background on the chemical composition of the water surface is larger than human activities, while the second principal component shows that, Fe and Mn concentrations in red beds were generally high, which mainly results from the dissolution and migration of sulfide minerals in red beds. Under the action of acidic water, Fe3+ is reduced into soluble Fe2+. The scores of comprehensive principal component of surface water shows that, with agricultural activities and environment changes leading to the increase of material input, the surface water quality of Hou Caoding is a little poor than Ta Wodang and Da Maopo, and the analysis results are consistent with the actual situation of groundwater; the first and second principal components of the groundwater reflect that, the water quality is mainly affected by water-rock interaction, Ca2+ and Na+ concentration respectively affected by the weathering and dissolution of calcareous cements and salt-bearing rock in red beds, and the main reason for the generally higher content of SO42- and Fe is the impact of the geological background of geologic background, on the contrary, the impact of human activities is very small.(5) As groundwater resources in the red beds of mountain area is taken as villagers’ drinking water, effective measures of in-taking water should be taken according to the buried distribution condition of crevice water. At the same time, try to avoid open wells in the stratum with high content of Fe and Mn, as well as control the distance of discharge sources between well water and daily sewage source, so as to avoid affecting the quality of drinking water. |
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