engleski

Groundwater behaviour in the Kostanjek landslide, western part of Zagreb, Croatia: geochemical constraints from water and rock samples

Groundwater behaviour, in many cases, is an important factor for the occurrence and activity of landslides. Hydrochemical characteristics of groundwaters reflect compositional properties of solid phases of the parent rocks through which groundwater flows and also provide signatures as natural tracers to better understand groundwater migration. During the September 2011 and June 2012, one sample of spring water, seven samples of stream water, 11 samples from the mining tunnel (entrance of the tunnel and inner part of tunnel) and 74 ground water samples from private wells in the Kostajek area were collected for hydrochemical and isotope analyses. The pH, EC and temperature of all samples were determined in the field, while Na, NH4, K, Mg, Ca, Sr, HCO3 and CO3 as alkalinity, Cl, NO3 and SO4, were analysed in the laboratory of the Research Institute for Natural Hazards and Disaster Recovery at Niigata University. On the basis of hydrochemical data we have classified into four water types by cluster analysis as follows ; (1) Type-A is typical Ca-HCO3 type water, (2) Type-B is weaker water in Ca-HCO3 component than type-A, (3) Type-C is Mg-rich Ca-HCO3 type water, (4) Type-D is Mg-Ca-HCO3 type water. Both type-A and type-B waters are predominately distributed over the research area and are closely related to the shallow aquifer lithologies. Type-A waters are derived Ca and HCO3 from marl aquifers. Type-B waters are formed by the mixing of Type-A water with dilute subsurface water from soil zones where soluble solids were removed during chemical weathering processes. Type-C waters are limitedly distributed around the eastern margin of the Kostanjek landslide and Type-D waters gush out from fissures in the dolomite outcrop in the inner part of the tunnel. In particular, Type-D waters are more enriched in Mg and depleted in Sr than type-C waters. It means that the dolomite is major source of Mg in waters from this area. On the other hand, isotopic compositions of δ18O and δ2H of all waters are meteoric water of origin even if type-D waters are depleted in δ18O and δ2H comparing with others. The depleted type-D waters in isotope compositions suggest that these are recharged in the higher area of the northern mountain, western part of Mt. Medvednica, and migrate through dolomite aquifer to the depths of the landslide mass. Type-C waters from shallow aquifers in the landslide also include Mg to some extent even though there is no dolomite layer in the landslide mass and the massive dolomite is distributed in more than 1, 000m north away from this area and also underlies in more than 200m depth beneath the landslide mass. Then type-C waters show a tendency to be slightly depleted in δ18O and δ2H comparing with type-A and type-B waters. It seems that type-C water is influenced by isotopically depleted type-D water from the dolomite aquifer. Ortolan (1996) suggested that artesian aquifer related to Badenian limestone and Triassic dolomite, underlays Sarmatian-Panonian marly sediments and Kostanjek landslide. According to hydrochemical characteristics, e.g. graphical plots of Mg/Ca vs. Sr/Ca ratios of waters, and isotopic compositions, it is clear that type-C waters are originally formed by mixing of common type-A and type-B waters with type-D waters derived from the dolomite aquifer. Therefore, it is most likely that type-D artesian waters from the deep dolomite aquifer continuously inject into shallow aquifers around the eastern margin of the landslide. Such injection of deep artesian waters has an impact upon the groundwater behaviour in the landslide mass and is also one of the key factors controlling the landslide susceptibility. In planning the counter-measures by groundwater drainages for the effective reduction of pore water pressure in the Kostanjek landslide, it is useful to identify the source of groundwaters based on hydrochemical characteristics, especially Mg-rich waters from the artesian aquifer.

Kostanjek Landslide; groundwater behaviour; geochemical constraints

nije evidentirano