A sinkhole is a depression in the ground that has no natural external surface drainage [1
]. They have very localized structural impacts but they may have far reaching effects on groundwater resources. Sinkholes can also have an impact on hydrologic systems, lakes, streams, and wet lands by changing water chemistry and rates of recharge or run-off [2
]. Since the Earth’s surface is constantly changing, sinkholes and other subsidence features will continue to occur with regards to both natural and human induced changes. Specific conditions can affect the type and frequency of sinkholes, including a general lowering of groundwater-levels, reduced runoff, and increased recharge or significant surface loading [4
]. Recognition of these conditions is the first step in minimizing the impact of sinkholes. In areas underlain by cavernous limestone with thin to moderate thickness of overburden, increased sinkhole development and property loss are strongly correlated to human activity [5
]. Land use changes in rapidly developing areas are often controlled and include drainage, new impoundments for surface water, and new construction in sinkhole prone areas. Finally, the changing land use is often associated with population increases and increasing demands for water supply, which may lead to an increase in ground water pumping capacity and the decreasing of local and regional groundwater levels [6
shows the mechanism of occurrence of cover collapse sinkholes.
A cover-collapse sinkhole occurs when upper sediment is composed of soft materials, such as is clay. An underground cavity is created under the bedrock, so that some of the upper sediment falls into the empty space created by the cavity thereby weakening the topsoil gradually. The weakening upper sediment begins sinking and finally a sudden collapse occurs. Although the occurrence of a large size sinkhole can take a long period of time, the collapse occurs suddenly, so the damage caused can be serious.
Part A at Figure 1
presents existence of soil or non-continuous materials (units PlQc, PlQcm) or members with adhesive characteristics over the Oligo-Miocene limestones. There are several cracks and fractures in these limestones that can transport the sediments into deeper parts of the karst. The water movement is vertical in this section. Part B shows water flow washes soils over the karstic bedrock and transports it through big fractures into the underground parts. The karstic caves make the transportation of water and soil more easily. Water movement is horizontal in this section. In part C it could be seen that eventually the hole gets so big that the roof cave cannot bear the weight and collapse.
shows the mechanism of occurrence of cover-subsidence sinkholes.
Part (A) shows existence of soil or non-continuous materials (units PlQc, PlQcm) or over the Oligo-Miocene limestones. There are several cracks and fractures in these limestones that can transport the sediments into deeper parts of the karst. The water movement is vertical in this section. Low adhesion of surface makes the motion vector more horizontal compared with the previous case. From part (B) it could be seen that water flow washes soils over the karstic bedrock and transports it through big fractures into the underground parts. The karstic caves make the transportation of water and soil more easily. There is a continuous transportation of sediments through cracks and fractures, as well as the occurrence of the sinkhole at Part (C).
A cover-subsistence sinkhole occurs in the form of small to mid-size holes. Many sinkholes of this type are generated in sandy soil where no viscosity is present in the surface layer, and it occurs mainly at a place where underground water is close to the ground. The subsidence occurs gradually rather than suddenly so sinking occurs gradually as well.
In last three decades, by growing agricultural activities, and accordingly increasing groundwater abstraction, land subsidence was reported from several parts of Iran. Frequent triggering factors in the development of land subsidence and sinkholes are overexploitation from groundwater aquifers, together with groundwater level fluctuations. Moreover, the existence of fine grain alluvium, tectonic features especially faults and dissolution of karst formations was reported as effective factors in the formation of sinkholes in Iran [7
]. Authors in [8
] have done research of the relationship between the declining groundwater level, land subsidence and sinkhole collapse in the Hamedan central plain in western Iran. Within this, the presence of fine-grained materials (for land subsidence) and karstic limestone bedrock (for sinkhole formation), increase of effective stress, and generation of turbulent groundwater flow at the soil–bedrock interface were investigated. The final result was a map of the hazardous regions. In paper [9
], there was presented research of the impact of overexploitation of groundwater on occurrence of the sinkholes.
In general, underground water resources located in karst arid and semi-arid areas and issues about their vulnerability and protection were discussed and analyzed in the literature. Authors in [10
] have conducted hydrogeological and hydrogeochemical study of a semi-arid karst aquifer in Tezbent plateau, Tebessa region, which is located northeast of Algeria. It has been shown that interaction between groundwater and surrounding host rocks is believed to be the main process responsible for the observed chemical characteristics of groundwater in the study area. Research provided by high resolution γ-spectroscopy, at the location of Nullarbor Plain in Australia by [11
] has been shown that in undisturbed sites, little sediment movement has occurred over the time scale of Cesium-137 from the year of 1960, comparing with the year of the provided research. Although, lands have been well sought over a much longer time scale. In other words, such change is explained by the anthropogenic impact.
Hydrogeochemical researching is very common in the analysis of the underground water resources in karst arid and semi-arid areas. Authors in [12
] provided ‘’in situ’’ measurement of electrical conductivity (EC), pH, water temperature, bicarbonates and anions in laboratory for the samples collected in the central part of the Cuddapah Basin of Southern India, which is karst arid area bigger than 100 km2
. It has been shown that land use and anthropogenic effects are factors that likely lead to the variability in water chemistry of the underground water resources within karst semi-arid areas. Same conclusion was highlighted in paper [13
] for the case study of the karst arid areas in Northern China.
Methodology very similar to the methodology (procedure) presented in this paper is an integrated approach to investigate the karst aquifer and clarify the factors, which affect the occurrence and quality of water contained therein, provided by [14
]. This researching has been done in the northwestern arid coastal zone of Egypt, which is an accessible area attaining promising lands for agricultural expansion beyond the Nile Valley and Delta, by using isotopes, remote sensing and GIS applications. The main hypothesis of the study was based on conjecture that a portion of rainwater is thought to be infiltrated to the groundwater through joints and fractures, which can recharge the karst aquifer. Although most of the water levels in the drilled wells are under the sea level, the isotope analyses indicate that no contribution of seawater intrusion affects the groundwater, which was very unexpected, but a promising conclusion regarding usage of the underground water resources.
It is important to point out new research in arid areas of central Zagros Mountains [15
], also very similar with researching presented in this paper. Researchers used isotopes data for determining the sources and elevations of the recharge area of the aquifer. Temporal variations of the isotopic data were compared with variations of electrical conductivity (EC). Unexpectedly, high EC was associated with a relative increase of discharge and depletion of δ18
O. The final conclusion is that recession of the discharge of karst springs after the rains is usually marked by increasing concentration of the solutes, which may negatively affect quality of the underground water resources. Authors in [16
] made physical, chemical and hydrogeochemistry analysis of groundwater samples in Ardestan basin in the central Iran. All samples were analyzed for conductivity, dissolved oxygen, pH, total dissolved solids (TDS), major cations, major anions and trace metals. Unfortunately, analysis conducted on the basis of major ion and trace element composition indicate that groundwater mostly falls under the status ‘’unsuitable’’ not only for the drinking water supply, but also for the irrigation purposes.
Researchers proposed defining of the protection zones in their researching work [17
], which was carried out for a large-scale managed aquifer recharge site in a semi-arid karst region in Jordan. The results divulge an extreme contamination risk resulting from livestock farming, arable agriculture and human occupation wide along the Wala dam surface catchment area of about 1770 km2
, which is a 50 km south of Jordan’s capital Amman. Such an approach is very effective, but it should be taken into the account that such measures imply a limitation of the human activity within protection zone areas. Interdisciplinary approach to the analyzed problem could be seen, for example, due to applying of the analytical hierarchical process (AHP) in [18
], machine learning algorithm in [19
], multicriteria methods in [20
], etc. It can be concluded that the scarcity of water in karst arid and semi-arid areas as well as exacerbation presents serious problems in such areas. This is a motivation for creating and presenting of a new procedure and researching methods for solving a problem of the quality and quantity of the groundwater resources in arid and semi-arid karst areas.
5. Vulnerable Areas for Future Sinkholes
For predicting of the vulnerable area for future sinkholes, which could appear, important factors should be prioritized according to the current conditions. These are the existence of limestone bedrock, as well as concentration points, where water from rivers and surface waters could enter into underground structures. Bedrock fractures and development of karst phenomena are also included as an important factor.
The bedrock in northern part of Abarkooh plain is made of Oligo-Miocene formations. River located in the northern part of the Abarkooh plain has an effective role in the forming of floods in the area. Therefore, in these areas sinkholes are likely to occur if there is an infiltration of the surface water. The operation of faults in bedrock, in addition of the development of karst and dissolution in limestones, and also with lifting of bedrock causes reducing the thickness of surface covering. The northern part of the Abarkooh plain and Abarkooh City have more deep fractures (critical area), therefore the possibility of occurrence of sinkholes are higher in mentioned areas.
To determine the quality of groundwater and sinkholes role, 20 wells were sampled by the bailer. The collection of water samples from groundwater wells occurs in five steps: sampling preparations, accessing the well before sampling and securing the well after sampling, measuring the water level, purging the well, and collecting with delivering of the water sample, in June 2010 in the Abarkooh Plain. Based on the experimental results, the most important type of groundwaters were Chloro-sodic, except well No 6, because of the existence of salty layers. The type of groundwater in well 6 is sulfate-sodic. The ionic frequency in cations are Na + K > Mg > Ca, Na + K > Ca > Mg, and in anions are Cl > SO4
, Cl > HCO3
> SO. According to the statistical characteristics of the different chemical elements—in terms of meqv/gL, which is the range of total sodium and potassium cations, as well as of the chloride anions are higher than other ions. The range of electrical conductivity is in the range until 4659 μS/cm. Water quality for agriculture has fluctuated from “salty—usable for agriculture” to “very salty—unsuitable for agriculture”. In the classification of water quality based on the hardness, samples are located in the range of “hard” and “very hard”. The electrical conductivity of the plain varies in the range between 1300 and 5000 μS/cm, Figure 12
In the central and south eastern area of the plain, an increase in groundwater electrical conductivity was observed, while in the north east region (District Feizabad) there was a significant decrease in electrical conductivity. It seems that the role of sinkholes in the aquifer charge prevents the increase in electrical conductivity from the west to the east. By comparing the electrical conductivity maps in 2000 with recent maps, it appears that the role of the sinkholes in the aquifer recharge in the area was more prominent. In other words, the creation of embankments and development of the sinkholes, in order to aquifer recharge, led to changes of the trend of increasing groundwater electric conductivity in the region.
Lines of chloride, nitrate and nitrite (Figure 13
, Figure 14
and Figure 15
) show that there is an increase of water quality in Feizabad, due to the recharge from sinkholes. In accordance with the standard by EPA [25
], the rate of nitrate in drinking water can be 10 mg/L, which is an acceptable standard, and can be a maximum of 50 mg/L. According to this, the standard maximum level for nitrite in drinking water is 1 mg/L. Thus, the nitrite in the water of the Abarkooh plain is in the standard range, but the level of nitrate is higher than the standard value. It should be noted that nitrate and nitrite ions arise from anthropogenic activities.
and Table 2
, show the chemical analysis of all other chemical parameters for the selected wells in the Abarkooh plain [23