In this paper, first, we would like to mention and analyse main factors which effect the karst activity in limestone constituting Co Bai Formation (D2-3 cd), and then quantitatively estimate the alteration in the karstification intensity to this limestone as they are under the influence of the tropical monsoon climate in Cam Lo region, Quang Tri province.
JOURNAL OF SCIENCE, Hue University, Vol 69, No 6, 2011 THE EFFECT OF TROPICAL MONSOON CLIMATE ON THE KARST ACTIVITY IN CAM LO DISTRICT–QUANG TRI PROVINCE Nguyen Van Canh, Nguyen Thanh, Tran Thanh Nhan College of Sciences, Hue University Abstract In this paper, first, we would like to mention and analyse main factors which effect the karst activity in limestone constituting Co Bai Formation (D2-3 cd), and then quantitatively estimate the alteration in the karstification intensity to this limestone as they are under the influence of the tropical monsoon climate in Cam Lo region, Quang Tri province Keywords: Karstification, karst activity, Calcium carbonate dissolving product, calcium carbonate activity product, calcium carbonate saturation deficiency, average coefficient of calcium carbonate activity Introduction Karstification is one of the exogenic natural geodynamic processes that have developed most widespread in several nations all around the world as well as in Vietnam and Cam Lo district Therefore, what is karstification? In our opinion “karstification is the aggregation of geological processes occurring both in the crust and on the earth surface, being generated by dissolution (chemical erosion), corrasion (mechanical erosion) of the surface water and the underground water accompanied by the formation of various shape- and size-differed spaces in the crust, creation of special movement and behaviour of ground water as well as specific morphological topography and regional hydrological system” Karst territory may create both positive and negative effects on the environment, the local life, economy and national security These are the most sensitive places in terms of causing many nature and human-induced geohazards In Cam Lo plain-Quang Tri, karst has induced vertical land subsidences and indeed occurred frequently in recent days and has induced the loss of useland, damaged constructions and threatened the local life In order to minimize such damage, and re-settle residence area, the local government of Quang Tri province invested in the project of “Assessment of Karst activity creating geological environment change in Cam Lo area and proposal of countermeasures” We have synthesized the obtained results from this project and finished this paper Major factors influence the karst activity of limestone making up Co Bai formation The real investigation of karst in Cam Lo area showed that in this region, the karstification process is not only generated by dissolving, corrasing activity of surface water, ground water but also suffered from local natural and man-made factors’ effects However, in terms of this paper, we only mention the major natural factors such as the existence of dissoluble rocks, tectonic and neotectonic characteristics, dissoluble rocks with good water-bearing capacity and permeability, consecutive motion of water, appropriate solvency of water, topographical feature and regional meteorological, hydrological condition [1] Within Cam Lo plain area, there is no doubt that carbonate rock with certain solvency of Co Bai Formation (D2-3 cb) has the largest distribution Chemical analysing data on grams of this cacbonate rock gives chemical composition (%) as follows: CaO = 28.79-52.4%, MgO = 1.03-21.51%, Na2O = 0.01-0.45%, K2O = 0.03-0.59%, Fe2O3 = 0.06-1.32%, Al2O3 = 0.27-4.75%, SiO2 = 0.44-31.48% This carbonate rock was generally splitted by faulty systems (Fig 1), sustained subsequent weathering (factors) and the karstification process which distributes in the rock’s further fracture, and of course increases its permeability The permeability of this carbonate rock fluctuates from low level (K = 2.32 m/day) to high level (K = 88.06 m/day) Moreover, due to Hieu River and the affluent system cutting over carbonate rock constituted kame-and coteau-topography, the previous movement condition of fissured-karst water was very expedient and water-exchange process took place consecutively; these factors thus positively distributed more frequent and intensive dissolving and aggressive activities of water on carbonate rock than those of today Though there is still insufficient confirmation of starting and peak time of the karstification process, it seems to be possible to consider that the stable neotectonic period [4] rolonging between Miocen and Pliocen (from to 10 million years to present) is the most convenient environment for karstification development as the area under tropical monsoon climate condition and as a result, this had planeted mountain masses of limestone into current open-coteau topography and karst paleoplain Many underground caves with at 2-3m of size and with 35-50m of distribition depth were perhaps created during this stable tectonic duration However, because of blocky differential subsidence operation in PliocenQuantenary time and depositing and infilling processes related Flandrian transgression of Cam Lo paleoplain, the initially essential conditions of karst development can no longer be in advantage and therefore, current karstification’s intensity is much weaker than that that used to be in Miocen-Pliocen period Quantitative assessment of alteration in karstification intensity under the effect of tropical monsoon climate condition In order to clarify whether the karstification operation in Cam Lo plain has been either stopped or still in process and to determine the fluctuation of karstification intensity between dry and rainy season, we applied the comparison method using calcium carbonate dissolving product with calcium carbonate activity product (KCaCO3 = 9.9 109 ) and calculation method using calcium carbonate saturation deficiency [1] with input data obtained from chemical analysis of several fissured-karst water samples (Table 1) Table Physical property, chemical composition of fissured-karst water Dry season (July, August, 2008) Composition mg/l Rainy season (October, 2008) Me/l Composition pH = 6.98 3.1 mg/l me/l pH = 6.80 Na+ + K+ 19.10 0.83 Na+ + K+ 4.82 0.20 Ca++ 68.14 3.41 Ca++ 8.15 0.41 Mg++ 14.49 1.19 Mg++ 1.07 0.09 Fe+++ 0.002 0.0001 Fe+++ - - Cl- 24.85 0.70 Cl- 6.07 0.17 NO3- 0.05 0.0008 NO3- - - SO4- - 15.50 0.32 SO4- - 5.76 0.12 HCO3- 270.28 4.43 HCO3- 18.99 0.31 Quantitative assessment of karst activity in dry season - The comparison method using calcium carbonate dissolving product with calcium carbonate activity product Firstly, it is necessary to determine calcium carbonate activity product (aCa++ a CO3 ) via the calculation of element parameters including ionic force M , ionic activity coefficient f , content of CO3 and average coefficient of calcium carbonate activity yCaCO3 + Ionic force M Ionic force is determined by the following formula: M 10 3 x1 z1 x2 z x3 z3 xnzn (1) Where [xi], zi-ionic content and valence respectively By substituting the data in Table into equation (1), the result of M = 0.0079 is obtained + Determination of f value With M = 0.0079, search from specialty table we will receive the value of f = 0.54 + Determination of CO3 content The content of [CO3 ] is calculated by the following equation: CO3 3.77 x 10 11 HCO3 f 10 pH (2) By substitution of the data in Table into equation (2), we receive CO3 = 0.00077me/l + Determination of yCaCO3 The value of yCaCO3 is determined via equation as follows: log yCaCO3 (3) 1.98 M 1.62 M Substitution of M value into (3) we will receive the value of yCaCO3 = 0.7033 And activity product aCa++ a CO3 can be determined by the following formular: aCa aCO 10 6 yCaCO3 Ca CO3 (4) The substitution of the necessary data into (4), the result of calcium carbonate activity product will be of 0.462x10-9 + Comparison between calcium carbonate activity product and dissolve product It is evident that activity product aCa++ a CO3 = 0.462x10-9 is lower than dissolve product K CaCO3 = 9.9x10-9, and therefore karst activity still happens and so karst still develop during dry season - The calcium carbonate saturation deficiency As already known, saturation deficiency amount xdCa , xdCO can be determined by solving the following basic function: yCaCO3 Ca xd CO3 xd 0.038 (5a) By substituting the obtained data into this function and solving it, we will determine the calcium carbonate saturation deficiency in dry season of xd = 0.15 me/l (>0) From this positive and very small value of calcium carbonate saturation deficiency, it is clear that during dry season, karst still on process but with less intensity 3.2 Quantitative assessment of karst operation in rainy season - The comparison method between calcium carbonate dissolving product and calcium carbonate activity product: By taking the same calculating route and equations as in 2.1 and the chemical analysed data of fissured-karst water in rainy season (Table 1), we will obtain the necessary results as follows: M = 0.0096, f = 0.52, CO3 = 0.00004me/l, yCaCO3 = 0.6801, and finally the corresponding result of activity product of aCa++ a 2.79x10-12 will be determined CO3 = This result of aCa++ a CO3 = 2.79x10-12 is at remarkable lower than the calcium carbonate dissolving product of K CaCO3 = 9.9x10-9, and also indicates that in rainy season, karst operation occurs with righ stronger intensity than that in dry season - The calcium carbonate saturation deficiency: The amount of calcium carbonate saturation deficiency in rainy season can be calculated by an equation as follows: 0.68010.41 xr 0.00004 xr 0.038 (5b) By substituting the above obtained data and solving function (5b), the solution of xr = 0.1078me/l (>0) will be acquired for the calcium carbonate saturation deficiency in fissured-karst water in rainy season 3.3 Evaluation of the trend of and reason for the variation in karst acitivity intensity in limestone corresponding with climate season From the above quantitative assessment of karst activity under climate season effect using the comparison method between calcium carbonate activity product and dissolving product as well as calcium carbonate saturation deficiency method, we may define the alteration in increasing trend of karst activity intensity from dry season to rainy one Indeed, the comparison of fissured-karst water’s calcium carbonate activity product between both seasons demonstrates that activity product of fissured-karst water in rainy season (2.79x10-12) is sixty times smaller than that in dry season (4.62x10-10), meanwhile fissured-karst water in the former season is seven times as much in the calcium carbonate saturation deficiency ( xr = 0.1078 me/l) as in the latter ( xd = 0.1078 me/l) The reality of less intensive karstification in dry season is undeniably related to the disadvantageous environment for karst development during this time comparing with that in rainy season In which, limited rainfall is considered the essential factor reducing the amount of water supply for fissured-karst water, restricting water movement and exchange condition, especially water dissolving ability as its total mineralization increases Conclusion From the above mentions, it is possible to propose following conclusions: - Over the previous geological evolution history, particularly the stable period of neotectonics in Miocen-Pliocen, the advantageous condition of karst development had intensified karst activity with the formation of considerable cave numbers at 2-3m of size distributing at 30-50m of depth over the research area - Blocky differential subsidence and depositing, infilling processes related to Quaternary transgression in Cam Lo graben-shaped valley were the major contributions to the change in karst developing environment, from previous advantage to current disadvantage - Tropical monsoon climate determinant factors regulated and controlled seasonal karstification intensity In which, the larger rainfall, more expedient movement-exchange condition and better dissolving possibility of water consequently results in the more powerful karstification operation in rainy season References Lomtadze V.D., Engineering geodynamics, Nedra (in Russian), 1977 Popov I.V., Engineering geology, MGU (in Russian), 1959 Socolov D.S., Essential conditions of karst development, Bull MOIP, Division Geol., v.126, N02, 1951 Yem, T.N., On the circumstance of neotectonics in Souther Central Vietnam, Geology., v.202-203 (in Vietnamses with abstract), 1991 10 ... under the effect of tropical monsoon climate condition In order to clarify whether the karstification operation in Cam Lo plain has been either stopped or still in process and to determine the. .. factors influence the karst activity of limestone making up Co Bai formation The real investigation of karst in Cam Lo area showed that in this region, the karstification process is not only generated... season 3.3 Evaluation of the trend of and reason for the variation in karst acitivity intensity in limestone corresponding with climate season From the above quantitative assessment of karst activity