Investigating Karst Geomorphology and the Effects of Drought on Quantitative and Qualitative Characteristics of Water Resources in Gareen Mountains (Lorestan Province)

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. Various processes and factors such as topography, lithology, tectonic, geomorphological and hydrological processes and climatic factors have an important role in the evolution of Karst. Karst aquifers are a precious resource because they provide drinking water for about 25% of the world’s population. Karst Aquifers are typically very productive due to their ability to convey water through a network of interconnected conduits. The analysis of hydrograph recession curves is an often-used method in hydrological studies, providing the interpretation of the characteristics and flow attributes of the aquifer.
The study of hydrodynamic and hydro-chemical properties of springs can be an indicative of the Karst development in Karst media. Social and economic life of cities such as Nourabad, Alashtar, and numerous rural societies is connected to the Gareen Mountains springs. Also the origin of Kashkan and Seymareh rivers is located in the Gareen Mountains. This shows the importance and necessity of studying Karstic springs in this area. The aim of this study is to investigate and evaluate the effect of drought occurrence and Karst geomorphology on the quantitative and qualitative features of Green Antarctic Springs in Lorestan Province.

The Gareen Mountains in the Zagros Mountain Range is located in the active deforming Zagros fold-thrust belt and Sanandaj-Sirjan zones. The Gareen Mountains is located in the west of Iran. The most important mountains in the area are Velash, Chehel-nabaleghan, Seh-kozan, Higdah-yal and Mishparvar. 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 Aquifer 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 Aquifer is the abundance of the sedimentary rocks and scarcity of igneous rocks in this area. Doline, Sinkhole, Cave, Ponor, and several types of Karrens and Karstic Window are the main Karstic features in the area.

The Standardized Precipitation Index (SPI) is the most commonly used indicator worldwide for detecting and characterizing meteorological droughts. The SPI indicator, which was developed by McKee et al. (1993) and described in detail by Edwards and McKee (1997), measures precipitation anomalies at a given location, based on a comparison of observed total precipitation amounts for an accumulation period of interest (e.g. 1, 3, 12, 48 months) with the long-term historic rainfall record for that period. The historic record is fitted to a probability distribution (the “gamma” distribution), which is then transformed into a normal distribution such that the mean SPI value for that location and period is zero due to increasingly severe rainfall deficits. The Standard Precipitation Index (SPI) is calculated using the following equation: 
SPI=Pi -) P/σ)                                              (Eq.1)
To achieve the goals of this research, 5 main draining springs were chosen in the Gareen Mountains, including Amir, Ahangaran, Chenareh, Laghary and Teymour Springs. After the survey of geomorphological, climatic, hydrodynamic, hydro chemical, physiographic properties and several field studies, the karstification degree in each aquifer was determined by using the hydrodynamic and hydro chemical properties. 
In this study we used a mathematical formula to describe the falling limb of hydrographs and the base flow proposed by Maillet (1905) (Eq.2):                                                          (Eq.2)
The dimensionless parameter α in Eq.1 is the coefficient of discharge (or recession coefficient) which depends on the aquifer’s transmissivity and specific yield, Qt is discharge in” t” time (m3/s), Q0 is beginning of discharge in t0, t is time elapsed between Qt, and Q0, e (mathematical constant) and α is the recession coefficient.
In addition, water samples in Karst springs in a period of 15 years (2001-2015) were used for hydro-chemical analysis. Hydro chemical analysis including total anions (HCO3, SO4 and Cl), Cations (Ca, Mg, Na, and K), Electrical conductivity (EC), Ca/Mg, total dissolved solids (TDS) and total hardness (TH) were used to determine the origin of the Karstic spring's water.

The calculation of α coefficient is one of the most important parameters in hydrogeological spring’s studies. Higher α coefficient represents springs much influenced by precipitation. This situation reflects the high porosity and high Karst development in the region. Conversely, the low α coefficient indicates the less karstification. Along with other factors, higher volume of dynamic storage in the basin of a spring indicates the high degree of karstification. In other words, less karst development leads to slow movement of water in the spring basin. This factor will increase the volume of dynamic storage. For identifying the karstification degree in the territory of the Gareen Mountains, we use the recession curve of spring's hydrographs and Malik and Vojtkova method.

According to the Hydrodynamic Study, Amir, and Chenareh springs have the karstification degree 2.7 and 2.5, respectively. Furthermore, the groundwater flow sub-regimes type is the combination of two or more sub-regimes with merely laminar flow characterized by different discharge. Ahangaran Spring has the karstification degree 4.3 and the discharge hydrogram is composed of a sub-regime with turbulent flow and a sub-regime with laminar flow. Laghari and Teymour springs have the karstification degree 5.5 and complex discharge regime, a combination of one sub-regime with turbulent flow and two sub-regimes with laminar groundwater flow. In other words, based on hydrodynamic results, Laghari and Teymour springs have the highest karstification degree in the Gareen Mountains. The results show that Amir and spring have the highest and Ahangaran and Laghari springs have the lowest volume of dynamic storage. According to the SPI method of years 84-88, the highest and most severe damage occurred within the scope of the study. Thus, during these years the springs of the study have dropped dramatically. The results of the comparison of SPI with discharge show that hydrological drought occurred due to the development of karst with different time delay has affected the spring’s aquifer. Thus, the fluctuations of precipitation with a shorter time lag appear in the aquifer of Ahangaran, Teymour and Laghari springs. However, in the aquifer of the Amir and Chenareh springs, due to the low karstification degree, the drought has been delayed for about 24 months .According to hydro chemical results, the highest and lowest total dissolved solids (TDS) values were found in the Chenareh and Ahangaran springs, respectively. Also, Ahangaran and Teymour springs have the highest and lowest t, otal hardness (TH) values respectively. Reducing precipitation and drainage have also led to an increase in TDS and TH in these springs.