To identify any phenomenon, it is necessary to know the constituents of the system the phenomenon is located in. The identification of faults and their effect on the incidence of sinkhole and gully in the north of Hamedan is not an exception. As sinkholes and karstic gully threaten engineering structures, especially power plant of Mofateh and residents in the area, the identification of faults based on sinkhole dispersion is as follows:
Data collection
It includes collection and analysis of maps and aerial photos, collection of data about sinkholes, region bedrock and alluvial characteristics, collection of information from hydrology, hydrochemistry and hydrogeology of the region (including chemical analysis of some deep and semi deep wells and qanats, type of aquifers, piezometric water level in long term).
Site investigation
In this stage, comprehensive field visits were carried out to evaluate the geological and hydro geological characteristics of the region including
Identification of location, quantity and geometrical features of sinkholes (diameter and depth).
Identification of other karstic geomorphologic effects in the region (cave, solution cavities, carens, gaseous springs, and gullies being formed around sinkholes).
Determination of geological condition
(deposit coverage type, overload thickness, bedrock type and depth in some parts).
Location of region springs and sampling spring water for chemical analysis and their acidity.
Evaluation of tectonic structures such as faults and fracture systems (data collection of 150 fracture systems in region rocks).
Data analysis based on data available, sinkhole dispersion map, water table lowering, isopotential map in terms of HCO3 and SO4 in underground water (water of springs and wells) were depicted.
Finally, two faults of NE-SW and NW-SE trend were determined in the region that played a role in the formation and dispersion of sinkholes and spread of karst.
Results
The presence of karstified geomorphologic effects such as sinkholes, caves, gaseous mineral springs like Gavergoli (Kaboudar-Ahang), gaseous spring of Ghinergeh (south of Razan) and hybrid abnormalities in well and spring water, we sought to identify faults, based on the understanding of structural geology and lithology and hydrology of region. The identification of sinkhole formation mechanism in the study area, provision of sinkhole dispersion map and chemical analysis of water with control of HCO3 and SO4 in underground water because of CO2 and H2S gases helped identify fault.
4.1 Specific structural and geological situation
As mentioned, the plains under study are located in structural Sirjan-Sanandaj. Field investigations and geological log of drilling wells show the presence of fine limestone of Qom formation crystals of oligomioceneage under alluvial deposits (sand, silt, clay and gravel). This lime bed rock has Protrusions in the village of Hamekassi but it is covered with a sequence of alluvial deposits. The thickness of plain alluvial deposits is about 70 to 100 meters. The information obtained from digging (drilling) shows that surface layers are made up of clay and silt under which sand and gravel deposit is located. Geological logs of wells in the region show that the depth of this limestone bed rock (Qom formation) fluctuates in different depths (51-93m of limestone depth). Fluctuates can be due to synclinorium influencing the region or the faults and old fractures displacing them.
Bed rock characteristics are one of the important factors in sinkhole distribution. Qom formation limestones are of higher purity in chemical composition and are seen in light grey to brownish grey in digging logs and site investigations. They are placed in packstone bioclastic class. The sample analysis showed that the presence of unstable bioclastic materials is a factor for porosity increase up to 20–25% in these stones providing the ground for karst development (Fig. 3).
Regarding the coverage of region by Quaternary deposits, most structures in the region are covered with these deposits, such as faults, and only a few of faulted structures are seen on the ground. To investigate the tectonic structures in rocks, the cracks available in limestones of Qom formation were measured around Village of Hamekassi and their situation is shown in Fig. 4. The rose diagram of joints shows three common types of the joint sets. The joint opening is about 10 mm in the area. The slow syncline and anticline development in limestones, crack systems and the presence of empty spaces have made Qom formation limestones susceptible to dissolution and formation of internal karsts and of sinkholes. In fact, carbonate layers in the study area are heavily influenced by tectonic structures in the region with numerous cracks and fractures.
4.2 Sinkhole development and mechanism
Sinkholes are generally seen in recharge areas and remove a great amount of lands along with death of people. As calcite solubility (CaCo3) at 25◦c in distilled water is only 14 mg/l, the formation of features such as sinkholes is coming from CO2 available in atmosphere reacting with water and forming Acid carbonic. In the case of unconsolidated deposits above limestone, this weak acid can flow downward and lead to the dissolution of limestone and formation of karstic conducts (Tucker and wright 1990). With the development of karstification and rock dissolution, these conducts develop and rock resistance decreases leading to land settlement and collapse of alluvial coverage (White 1988; Ford and Williams 1989). With CO2 in recharge zones, the dissolution of carbonate materials take place in the following reaction (Ford and Williams 2007).
CO2 + H2O→H2CO3
CaCO3 + H2CO3→Ca2+ +2HCO3−
From 1988 to 2010, over 50 sinkholes occurred in north plains of Hamedan (Kabudar-Ahang, Razan and Famenin), (Table 2), the diameter of which were sometimes 50m (Figs. 5a, 5b). There are some smaller sinkholes being formed (Fig. 6). The concentration of most sinkholes is around Mofateh power plant. In the study area, the development of these sinkholes and other karst features such as gullies is in discharge zones of underground water near karst limestone of oligomiocen age.
The occurrence of such sinkholes within a discharge zone is difficult to reconcile with the traditional conceptual model for two reasons. First, there is no downward moving water to carry away the regolith. Second, the upwelling water has been in contact with limestone for long periods, and should be near-saturation with respect to calcite. Because of these issues, generation of subsurface voids within the bedrock is uncommon. Groundwater circulation, thick overburden and a close proximity to deep-source carbon dioxide gas are hypothetical mechanisms that could generate the subsurface voids that are needed to allow collapse Sinkhole Formation in discharge areas. Chemical analysis of spring water and wells showed that the amount of CO2 and H2S is high in underground water. Gaseous springs in the region, such as Gavergoli, show high amounts of CO2 in underground water (Fig. 7). The acidic water dissolves limestone in fractures and joints leading to sinkhole formation. Acidity and corrosion of underground water on piezometric tubes result in corrosion of tubes (Fig. 8).
Table 2
Characteristics of sinkhole occurred in north of Hamedan (Razan, Famenine and Kabudar-Ahang plain)
No. | Location | UTM_X | UTM_Y | Diameter (m) | Depth (m) |
1 | Kerdabad | 299452 | 3888739 | 25 | 20 |
2 | Hamekasi | 314552 | 3877880 | 10 | 20 |
3 | Hassar | 279627 | 3901716 | 3 | 1 |
4 | Amirabad | 289590 | 3901017 | Unknown | Unknown |
5 | Gondejin | 290945 | 3893667 | 10 | 2 |
6 | Khanabad | 295344 | 3894429 | 7 | 12 |
7 | Kerdabad | 299567 | 3888821 | 22 | 8 |
8 | Kerdabad | 298819 | 3888249 | 31 | 50 |
9 | Noabad | 296997 | 3889035 | 24 | 60 |
10 | Negar khatoon | 310917 | 3890981 | 4 | 4 |
11 | Famanin | 315290 | 3887200 | 20 | 30 |
12 | Jahanabad | 315319 | 3883707 | 23 | 17 |
13 | Jahanabad | 315512 | 3883472 | 19/6 | 1/5 |
14 | Hamekasi | 313345 | 3879084 | 6/6 | 4 |
15 | Hamekasi | 313254 | 3879076 | 11/8 | 3 |
16 | Hamekasi | 313211 | 3879101 | 23 | 8 |
17 | Bizinjerd | 313024 | 3879519 | 20 | 3 |
18 | Hamekasi | 313857 | 3877387 | 2/5 | 1/5 |
19 | Hamekasi | 313833 | 3877306 | 35 | 5 |
20 | Hamekasi | 313832 | 3873060 | 6 | 3 |
21 | Hamekasi | 313832 | 3873060 | 100 | 4 |
22 | Hamekasi | 313832 | 3873060 | 50 | 5 |
23 | Hamekasi | 313891 | 3876783 | 2/5 | 1 |
24 | Hamekasi | 314376 | 3877351 | 30 | 2 |
25 | Hamekasi | 313748 | 3877647 | 2/5 | 0/5 |
26 | Hamekasi | 314089 | 3877762 | 10 | 1 |
27 | Bizinjerd | 311513 | 3877458 | 3 | 3 |
28 | Bizinjerd | 312268 | 3875934 | 3 | 6 |
29 | Bizinjerd | 312370 | 3876038 | 4 | 2 |
30 | Hamekasi | 313832 | 3873060 | 11 | 15 |
31 | Saritapeh | 328147 | 3876434 | 2 | 5 |
32 | Saritapeh | 328155 | 3876443 | 1/5 | 1/5 |
33 | Hamekasi | 312885 | 3875532 | 3 | 1/5 |
34 | Hamekasi | 312841 | 3875380 | 5 | 3 |
35 | Hamekasi | 312849 | 3875387 | 3 | 1/5 |
36 | Baban | 295573 | 3899806 | 25/6 | 17 |
37 | Baban | 295496 | 3899763 | 4 | 4 |
38 | Kerdabad | 299197 | 3888479 | 27/7 | 10 |
39 | Kerdabad | 298872 | 3888384 | 29 | 10 |
40 | Kerdabad | 298927 | 3888384 | 38 | 14 |
41 | Kerdabad | 299049 | 3888449 | 18 | 7 |
42 | Hamekasi | 316067 | 3878112 | 6 | 7 |
43 | Hamekasi | 314121 | 3877020 | Unknown | Unknown |
44 | Hamekasi | 314135 | 3877028 | Unknown | Unknown |
Table 2
No. | Location | UTM_X | UTM_Y | Diameter (m) | Depth (m) |
45 | Hamekasi | 314439 | 3877234 | Unknown | Unknown |
46 | Hamekasi | 313833 | 3877177 | Unknown | Unknown |
47 | Hamekasi | 313827 | 3,877,209 | Unknown | Unknown |
48 | Hamekasi | 314209 | 3876811 | Unknown | Unknown |
49 | Agh tapeh | 289526 | 3901848 | 40 | 1 |
50 | Tamchi | 337103 | 3873256 | Unknown | 2 |
51 | tamchi | 337095 | 3873176 | Unknown | 3 |
To identify the faults based on sinkhole array, the situation of sinkholes on the geological map of the region was determined using GIS. As seen in Fig. 9, two faults with NW-SE, and NE-SW trend in the region created dispersion and array of sinkholes in the plain. Regarding the concentration of sinkholes around power plant, it is concluded that, in addition to undue uptake (Indiscriminate harvesting) of underground water and drought, another factor in generation of sinkholes is presence and intersection of two faults providing the ground for gas exhaust from gaseous deep resources (hydrothermal and pneumatic resources) so that the sinkholes are concentrated around Mofateh power plant.
In Kaboudar-Ahang region (west south of Gharelar and Gholiabad mountains), the first fault, the one at the NW-SE direction, has placed Qom formation limestone on schist Shemshak formation in a higher altitude than alluvial aquifer limestone of Kabudarahang plain, which can spread the karst in region and great loss of underground water level in Kabudarahang plain.
The presence of karstified morphological phenomena at NW-SE direction, such as karstic gullies (Fig. 10) and gaseous springs of Gavergoli (Kaboudar-Ahang), gaseous spring of Babagorgor (Kurdistan) and gaseous springs of Ghinergeh (south of Razan), the presence of lime caves of Alisadr, Sarab and the caves observed in limestones of Qom formation in Hamekasi region of Famenine all confirm a fault of NW-SE direction (Figs. 11, 12).
Figure 11. A scheme of cave opening in limestone of Qom formation of oligomiocen age (Famenin)
4.3 Hydrogeology
The hydrogeological situation beneath the plain is complex and containing some small aquifer near the surface in Quaternery alluvial (Heidari et al. 2011). There is a deep aquifer in karstified limestone bed rock. The study area is a part of Gharechai basin and is heavily used as an underground water table. Due to the reduction of water, there have been generated semi deep and deep well in lime bedrock followed by great loss in underground water. This lowering of the water table become 50 m in some regions. The presence of springs and gaseous wells show acidic water which is sore (burning) or has a bad odor. CO2 with atmospheric origin combines with rain influencing limestones and making external karst systems such as karren (Fig. 13). But this water has a little effect on limestone buried under the deposits and interkarst system is created by internal gases of the earth (Moore 1939; Standard method 1990). The origin of CO2 and H2S in underground water is deep gaseous hydrothermal and pneumatolite resources rising up through fracture systems and faults and making the acidic water and karst development. In Latitum of Italy, it was reported that high amount of CO2 is the reason to form sinkholes in discharge zones of underground water (Boni et al. 1980; Ciotoli et al. 2002; Dall and Campanile 1996). Chemical analysis of water in 64 springs, Qanats and wells located in the study area was carried out. The presence of CO2 and H2S in this water was realized which were high in some parts such as Gavergoli and around Hamekassi village, respectively. To determine the faults and their role in sinkhole array, chemical analysis of underground water was used, in that HCO3 and SO4 isopotential maps of underground water analysis were drawn with GIS in spline (tention) and spline (regularized) methods.
Isopotential maps of HCO3 show two similar trends with sinkholes. In the direction of the first fault, there is seen an increasing trend of HCO3 from NW to SE and another one in direction of the second fault from NE to SW (Figs. 14, 15). Faults may act as preferential pathways for ascending deep- sourced fluids that may enhance groundwater aggressiveness by the incorporation of gasses like CO2 or H2S. Isopotential maps of SO4 show the sinkhole dispersion (Figs. 16, 17) in that amount of so4 and hco3 is high around power plant and in the concentration point of sinkholes. This indicates that tectonic structures of rock mass (faults and fracture systems) are a factor of gas rise from gaseous deep resources and can play a role in karst formation, rock dissolution and sinkhole formation. Moreover, effervescence in the water extracted from some wells suggests input of dissolved gases from deep sources. Acidic waters can be very aggressive and acidic (pH value less than 3) with high concentrations of dissolved and free CO2 and H2S. These acidic fluids increase the aggressiveness of the waters, enhancing their chemical dissolution of the carbonated fraction of the sediments and rocks. Finally, in this process sinkholes can occur in north of Hamedan.