Karst Geomorphology effects on the environmental hazard intrinsic vulnerability of groundwater resources (Case study: the Aleshtar and Nourabad basins)



Introduction

Karst is the result of the dissolution (physical and chemical) in carbonate (limestone and dolomite) and evaporate rocks. Karst developing is affected by climatological and geological factors. In the other words Karst landscapes and karst aquifers are formed by the dissolution of carbonate rocks by water rich in carbon dioxide waters. Karst aquifers include valuable freshwater resources, but are sometimes difficult to exploit and are almost always vulnerable to contamination, due to their specific hydrogeologic properties, therefore, karst aquifers require increased protection and application of specific hydrogeologic methods for their investigation. The groundwater protection in karst aquifer has a special importance, because the transit time for unsaturated and saturated zone is so quickly that the attenuation of the pollutant. Karst groundwater vulnerability mapping should form the basis for protection zoning and land use planning. A conceptual framework was devised for vulnerability mapping based on this European approach.

Social and economic life of cities such as Nourabad, Alashtar, and numerous rural societies is connected to the Gareen anticline springs. In this paper we used PaPRIKa method for vulnerability assessment in the Aleshtar and Nourabad basins.



Material and Methods

The Gareen anticline in the Zagros Mountain range is located in the active deforming Zagros fold-thrust belt and Sanandaj-Sirjan zon. Alashtar and Nourabad karst aquifers are located in the north of Lorestan province. There are several thrust faults with northwest–southeast strike such as Gareen-Gamasiab and Gareen-Kahman Faults. Nourabad unit is composed mainly by gray limestone rocks, embedded marl limestone, recrystallized limestone and pyroclastic rocks. One of the most important features of the structural geology of the Alashtar unit, is abundance of the sedimentary rocks and scarcity of igneous rocks in this area. In other words In the Study basins the main geological formations incloud: Bakhtiarian conglomerate, carbonates of Sormeh, Taleh Zang, Pabdeh and Kashkan Formations.

The groundwater vulnerability assessment methods (PaPRIKa) applied at the test sites were designed specifically for karst aquifers. They are based on various types of information concerning the physical characteristics of the unsaturated and saturated zones, the aquifer structure and its hydrological behavior.

The PaPRIKa method takes into consideration criteria for both structure and functioning of the aquifer. Based on EPIK and RISK resource methods, PaPRIKa method was developed as a resource and source vulnerability mapping method, allowing assessing vulnerability with four criteria: Protection, Rock type, Infiltration and Karstification. The P map (Protection) considers the protection provided to the aquifers by layers above the aquifers: the S (soil texture, structure and thickness), Ca (permeability formations), the Uz (thickness, lithology and fracture degree of unsaturated zone) and E (Epikarst aquifer). Moreover, including the catchments of water losses where the vulnerability is higher. R map (Rock type) considers the lithology and the degree of fracturing of the sutured zone. I map (Infiltration) distinguishes concentrated from diffuse infiltration. Ka map (Karstification development) assesses the drainage capacity and the organization of the karst conduits network.

To calculate the vulnerability index, the four mentioned maps(P. R. I. Ka) have been combined using the following equation coefficients (eq.1):



PaPRIKa Index= 0.2 P + 0.2 R + 0.4 I + 0.2 Ka (1) eq



Due to the fact that karst geomorphology has a great impact on the quantitative and qualitative characteristics of water resources and the vulnerability assessment of these resources, fuzzy logic has been used to zonation of the Karst development in the Aleshtar and Nourabad basins. In the fuzzy method used a gamma operator (eq.2):

µ _Combination= ((Fuzzy Algebraic Sum) (Fuzzy Algebraic Product)) ^1-γ (2) eq

The vulnerability map for aquifers was prepared using the software Arc GIS_10.4.



Discussion and Results

In the Gareen Antarctic region, due to the availability of suitable Karstification, includes: Lithology, Active Tectonics, Mediterranean climate (with average rainfall of between 454-448 mm and average temperature of 13 C˚) features are formed by various forms of karst such as closed pits (Doline, Swallow Hole, Aven, Polyeh (Peljee), several types of Karrens, dissolution Cavities, small and large Caves and Springs. The most important karst features in this area including Dolines (Solutional, Collapse, Subsidence and Dropout) which are known the Karst Nival. Based on the Karst development zoning map by using the fuzzy logic, 15% of the study area has been developed. Due to the vulnerability based on PaPRIKa method, the Aleshtar and Nourabad basins divided into 5 categories. Resuls show that the vulnerability of the study area is mainly classified as High or Very High, due to the highly developed Epikarst, which minimizes the protective function of the unsaturated zone. There are many karst landforms such as dolines and Swallow Holes that are highly vulnerable.



Conclusions

The final evaluation of the vulnerability ground waters in the Aleshtar and Nourabad basins using the PaPRIKa method shows that the study area is divided into five vulnerable (very high, high, moderate, low and very low). So that areas with a very low, low and moderate vulnerability are 27.3%, 22.3% and 20.6% of the basin area respectively. Also that areas with a high and very high vulnerability are 17% and 12.8% of the study area cover, respectively. Due to the lack of soil and plant cover, heavy snowfall and the formation of Karst-Nival (including Dolines) highlands of the Gareen Anticline have a very high vulnerability potential. Validation of the results of the karstic aquifers vulnerability to Electrical Conductivity (EC) data and monthly discharge of springs shows that the Zaz and Ahangaran springs are in a high vulnerability zone. In the aquifer of this springs, Rapid reductions in EC are detected after each recharge period. Also in contrast Rapid increases in EC with reductions in recharge. This situation shows the High developed of this aquifers, as a result, the potential for vulnerability in these aquifers is high.

But in the springs of Niaz and Abdolhosseini in the Nourabad basin, the EC chart has not changed much compared to recharge. Therefore, the aquifer of these springs is less undeveloped or low developed and also less vulnerable.



Key Words: Gareen Anticline, Geomorphology, Karst, Lorestan, Pa