Statistics parameters of chemical analysis results of elective resource in the dry season .
The karstic aquifers were the best source of potable water in the northeast of Rudbar, Iran. The present study was carried out to appraise the hydrogeological attributes of karst aquifer in this area. For this objective, saturation indices (SI values) are acquired through using the geochemical PHREEQC software for a number of chemical compounds existing in the karstic aquifer. Moreover, the sources of the ions and hydrogeochemical plots were obtained by using AqQA-RockWare software packages. The water type of all springs is the Ca-HCO3 type which is determined by plotting of Piper diagram and Durov diagram for spring water samples, that is confirming the calcareous effects of the region on the quality of groundwater and representing a single source for the springs. The degree of Karstification of the recharge area of the karst aquifer was determined to be 5.5 from an analysis of the hydrograph Sefidab Spring. The microbial contamination can be observed in all samples due to the intense development of karst, lack of self-purification in the karst system, and lack of an adequate cover layer on carbonate formation.
- saturation indices
- chloro-alkaline index
- cross diagram
- recession curve
Iran as a dry country with a population of 80 million has faced a water deficit in recent decades. The unorganized use of groundwater resources is created by subsidence of many plains in Iran. Guilan, a province situated in the north of Iran, has wet weather and thus is less subjected to water shortage problems compared to the southern and central provinces of Iran. The Dorfak karstic area is situated in the northeast part of the Rudbar city (Figure 1). The highest point of the area is a Dorfak peak with a height of 2733 m.a.s.l (meters above sea level). The karstic aquifers (excluding of the evaporating formation) are suitable groundwater resources for potable water. Dorfak karstic area is the initial well-known karstic region of Guilan province. The first study in this area was conducted by the Water Resources Management Company of Iran. In the following, we investigate the study area in terms of geology, tectonic structures, climate, and quantitative and qualitative properties of the aquifer by the analysis of water samples from springs in the region. For this purpose, information about the discharge rates of these springs and their chemical analysis was provided by the Regional Water Company of Guilan province. Information about various lithologies in the region was obtained using the 1:100,000-scale geological map published by Geological Survey of Iran. Additionally, a hydrograph analysis method was also used to estimate the degree of karstification in recharge areas.
2. Topography, climate, and vegetation
The Dorfak karst area, one of the largest karst regions of the north-east of Rudbar city, is situated in the east of the Guilan province of Iran. The region area ranges from about 1000 m elevation up to the Dorfak summit at 2733 m.a.s.l. The whole mountain range covers an area of about 429.59 km2. The study area is affected by two climates: Csa and Bsk. The majority of the study area is situated in the Csa climate. The monthly average minimum and maximum temperature is 0.40°C in February and 18.5°C in August. The mean annual rainfall has been 748 mm in the last 20 years (1993–2013). The most rainfall occurs in November, December, March, and February, in the order of their appearance. The Csa Mediterranean climate can be observed in the western continents between latitude 30–45°. The rainy weather and hot and dry summers are special characteristics of this climate. The climate depends on the high-pressure semitropic system, except that coastal areas have a cooler summer because of the cold ocean flow. This cool ocean flow often creates fog and prevents rainfall. Up to an elevation of approximately 2000 m, vegetation is covered by forests with beech trees in the west and north-west parts. Above the timberline, grassy vegetation persists up to the summit, where soil coverage has not been washed out due to karstification or erosion.
3. Stratigraphy and tectonic structures
The Dorfak karst area is a part of the West Alborz that is situated on the east of the Sefidrud Valley. The trend of the mountains was east-west (EW). In the southern slope of the Dorfak summit, one can observe the outcrop of the volcanic rocks with the Paleogene age. The limestone units in the Dorfak karstic area contain gray thick to massive bedded fossiliferous limestone, which is locally chert-bearing (JKL), gray, medium-bedded limestone including Orbitolina fossils (KL1); alternating beds of yellow argillaceous-sandy limestone, marl and sandstone (JKms); alternating beds of thin- to medium-bedded limestone, marl, sandstone and sandy limestone (Km2); and thin and well-bedded limestone and argillaceous limestone, locally with intercalation of andesitic lava (KL2) (Figure 2) . The effects of the Arabian Plate’s pressure on the Eurasian Plate can be observed in the reported kinetic axes for the West Alborz that is directed from the northeast to the north. In the view of structural geology, the Dorfak karstic area was situated in Lahijan fault shear zone. Safari et al.  were reported that the main compression axis for the Lahijan shear zone is N10E. This result is confirmed due to the former measured pressure axis in the area and left-lateral movement of Lahijan fault. The pop-up structure of the Dorfak peak was created by the enclosure between two thrust faults, which are the Dorfak thrust fault with a slope to the south and the Deylaman thrust fault with a slope to the north (Figure 3). This structure provides a sui basis for the karst development through creating of the large fractures in rocks of the area . The trend of these faults was almost perpendicular to in this region. The strike of faults was N90s–110 and has reverse movement with a left-lateral component .
4. Karst features
One of the main properties of karstification is the existence of karst features. In order to determine the karst extent in the study area, karst features were identified. The critical features of karstic regions such as the presence of sinkholes, karrens, shafts, caves, poljes, and springs can be observed in the Dorfak karst area. The different types of karrens such as grikes, clints, kamenitzas, rinnenkarren, meandering runnels, wall karren, hummocky karren, trittkarren, rainpits, rillenkarren, rundkarren, and cavernous karren are observed in the study area. Sinkholes are prevailing features in the karst region. The types of sinkholes are solution and collapsible. Shafts are prevailing features due to the high abundance of joints with an upright dip in the Dorfak peak. The Dorfak bowl is a polje that its water is supplied through melting snow and drainage of rainfall into the polje. Figure 4 shows some karst features.
5. Hydrogeology of the spring
There are several springs in the Dorfak karstic area. The spring’s situation is shown in Figure 5. In this section, we will investigate Sefidab spring properties as the most important spring of the Dorfak karst area. Sefidab spring operates as the permanent drain of the Dorfak karst aquifer. The Sefidab spring is situated in the west side of the Dorfak peak, in the Sefidab valley, 387 m.a.s.l (Figure 6). The annual average discharge of Sefidab spring is 500 l/s that the watershed area of spring was inappreciable against discharge content and it is about 2 km2. The runoff content of Kharashk watershed is considerably higher than the total content of precipitation because of the high development of karst in this subbasin; the water transition through shafts and sinkholes is situated in the upstream subbasin. The Sefidab spring is a typical fault-bound and limestone karst spring with a dynamic outflow. Since exact measurements done in June 2011, the highest discharge was recorded on December 21, 2013, with more than 2534 l/s. The lowest discharge was recorded on July 14, 2012 with 30 l/s. The mean discharge in this period (2011–2014) reached 500 l/s. The present study investigates the hydrogeochemical attributes of the karst aquifer through the analysis of water samples from karst springs in the area. For achieving this goal, database of the Regional Water Company of Guilan province about the discharge rates of these springs and their chemical analysis was used. Information about various lithologies in the region was obtained using the 1:100,000-scale geological map published by Geological Survey of Iran. A hydrograph analysis method was also used to estimate the degree of karstification in recharge areas.
6. Geochemistry investigation
By plotting a Piper diagram  of spring water samples, the water type of all sources of the region was discovered to be the Ca-HCO3 type, demonstrating that all springs in the region were discharged from a karst aquifer (Figure 7). Plotting of the Durov diagram of the samples is done to attain an accurate inspection of the chemical attributes of groundwater (Figure 8). Compared to Piper diagram, the Durov diagram distinguishes in terms of its ability to showing different types of water and hydrochemical processes such as water mixing of different qualities and ion exchange . The chemical composition of springs is bicarbonate type with the Ca cation being the dominant by using the Durov diagram, which has also confirmed the effects of limestone formations on the quality of groundwater in the region. Also, using the Durov diagram has proven that the springs recharge usually from a single origin. The anionic evolution cycle has more effect on the ion evolutionary cycle of groundwater than the cationic cycle, which has been obviously proven due to the increasing total dissolved solids (TDS) in water sample of springs. The groundwaters of the area are situated into the domain of alkaline springs due to the plotting of the Durov diagram. The statistical parameters of chemical analysis results of elective resource can be observed in Tables 1 and 2. A Cl-HCO3-SO4 ternary diagram (Figure 9) showed distinct zones of alkaline, acidic, and chloride-rich water . As shown in Figure 9, all water samples are located in the soda spring zone.
|Parameters||Ec (μs/cm)||TDS (mg/l)||pH||meq/l|
|Number of samples||29||29||29||29||29||29||29||29||29||29||29||29|
|Parameters||Ec (μs/cm)||TDS (mg/l)||pH||meq/l|
|Number of samples||30||30||30||30||30||30||30||30||30||30||30||30|
In order to evaluate the controlling mechanisms of water chemistry, the effect of host rock lithology on water quality is estimated, and the mechanism of groundwater flow is determined, and the composition of the main ions in groundwater is applied as per the Gibbs chart (1970) . The effective mechanisms of water chemistry and its evolution are separated into three classes by Gibbs: atmospheric precipitation, rock weathering, and evaporation . Also, he proposed two diagrams of
In samples with a high ratio of
7. Saturation indices
To estimate the extent to which groundwater in the karstic aquifer could dissolve of precipitate carbonate minerals, saturation indices (SI) of calcite, dolomite, and CO2 gas are determined to applying the samples the chemical analysis and PHREEQC software. The water was saturated due to the specified mineral if the saturation indices are zero, and water is undersaturated with mineral if it was negative; thus, the more content of mineral can be dissolved in water. When the water is supersaturated due to the specified mineral, the saturation indices will be positive. Coetsiers and Walraevens  represented that the more positive SI shows a higher mineral content in the water and appropriate status to precipitate in this SI. The saturation indices are provided by Eq. (1) as follows :
where the ion activity product of the dissociated chemical species in solution with IAP is showed and the equilibrium solubility product of the chemical involved at the sample temperature with Kt is expressed . The SI values due to the calcite, dolomite, and CO2 gas determined by applying PHREEQC software are shown in Table 3.
In accordance with the saturation index values, most water samples are undersaturated due to calcite, dolomite, and CO2 in the region; hence, carbonate minerals dissolve to a greater extent. White  represented that the springs with negative saturation index values (in the range from −0.2 to −0.4 or lower) have been transmitted through open conduit systems in the temperate and northern climates. A negative saturation index value of CO2 indicates that they have no potential for travertine formation.
8. Chloro-alkaline indices
The extent to which ion occurs between groundwater and the aquifer matrix is illustrated through the chloro-alkaline indices (CAI) as suggested by Schoeller . When there is an exchange between Ca and Mg in aquifer rocks with Na and K in the water (so-called “normal” ion exchange), the CAI value will be positive. When the CAI value is negative, the ion exchange is a reverse (so-called “reverse” ion exchange) .
The CAI value can be used as a diagnostic tool for the source of bicarbonate ion in groundwater. If the weathering of silicate minerals is the source of bicarbonate ions in water, the CAI value will have a positive amount. Conversely, if carbonate minerals were the source of the bicarbonate ions, a negative value of CAI is visible. Also, a negative CAI value shows that the source of bicarbonate can be created from human activities . CAI values are calculated through Eqs. (2) and (3) as below :
The CAI value of spring’s water samples was calculated by Eqs. (2) and (3) (Figure 11a and b). By determining CAI value, the ion exchange in the samples and the source of bicarbonate were investigated. The results show that the four springs (t1, t2, t6, and t8) in this investigation have positive CAI values, suggesting the normal ion exchange and a bicarbonate ion source of silicate minerals. However, in other springs, the source of bicarbonate ions was carbonate minerals.
9. Sources of ionic constituents in groundwater
By plotting the parameters of the water sample analysis on a bivariate diagram (known as a cross diagram), it is possible to interpret the processes that have impressed the chemical quality of groundwater. By plotting cations against anions, it is possible to detect the source in this method [18, 19, 20, 21, 22].
The common origin of the ions has been proven by the high determination coefficient between two ions. Diagrams of Ca:TDI (total dissolved ions) and Ca:HCO3, and diagrams of Mg:HCO3 and Mg:TDI are made to achieve this goal (Figure 12a–d). In order to evaluate the importance of ion exchange and different weathering process, the binary diagrams Ca + Mg:HCO3 + SO4 and Ca + Mg:TDI were also plotted (Figure 12e and f). If the dissolution of calcite, dolomite, and gypsum has provided Ca, Mg, HCO3, and SO4, there would be a 1:1 stoichiometric relationship available between Ca + Mg and HCO3 + SO4 . The displacement of points to the right side and below the 1:1 line indicates that the ion exchange has occurred. Conversely, the displacement of points to the left side and above the 1:1 line shows the reverse ion exchange .
Information about the stoichiometric process existence in groundwater provides the evaluation of Ca + Mg against HCO3. The evaluation of the water quality data represents that the main source of Ca and Mg in groundwater was the dissolution of carbonate minerals, while the source of HCO3 is likely to be the weathering of both silicate and carbonate minerals . The simple dissolution was the very common weathering process for carbonates. Zhang et al.  presented a 1:1 ratio of Ca + Mg:HCO3 or 1:2 molar ratio of Ca:HCO3. As a result, through the ion exchange process of calcium and magnesium in water by the sodium in the clay, or by the cationic exchange or HCO3 enrichment from weathering, the silicates create the low molar ratio of Ca:HCO3 (<0.5) in groundwater , whereas the high ratios (>0.5) indicate the other sources of calcium and magnesium, such as reverse ion exchange, which is also created in hard rock formations with increasing salinity.
A high determination coefficient in plotting the Ca against the HCO3 and TDI suggests that the calcium is originated from the dissolution of calcite (Figure 12a and b). Moreover, the moderate determination coefficient (classification of R2: 0–0.3: low, 0.3–0.6: moderate, 0.6–0.9: high, and 0.9–1: very high) due to plotting the Mg against HCO3 and TDI implies that magnesium ions may have been derived from the dissolution of dolomite (Figure 12c and d). Besides, the binary plot of Ca + Mg versus HCO3 + SO4 represented that 48.27% of groundwater samples shows a reverse ion exchange and 51.72% shows the ion exchange process (Figure 12e). The high determination coefficient due to the plotting of composite diagrams of Ca + Mg against HCO3 indicates that these two ions have a common source, i.e., dissolution of carbonates and silicate weathering (Figure 12g).
10. Evaluation of the degree of karstification in recharge areas
The multiple hydrodynamic behaviors of karst aquifers were created by the development of epikarst geomorphology and the existence of sinkholes and abundant joints in limestones. Factors such as the flow hierarchy, the setting of the output, and drainage network organization in an unsaturated area have impressed the flow condition in karst systems, which are determined by the karst system structure . The hydrographs of springs represent all the physical processes that have impressed groundwater flow in a karst aquifer . The important method for determining the hydrological characteristics of karst aquifers is done by the analysis of hydrograph recession curves . The period between peak discharge and next peak discharge at the end of the period forms the recession curve .
The perpetual drain of the Dorfak karst aquifer is done through the Sefidab spring, while the temporal spring is the Norcheshme spring. In the western part of the Dorfak karst area, the Sefidab spring is presented as a fault-bound spring that is situated in a fracturing zone created due to the faulting of bedded limestone. That way, the hydrograph plot was made to investigate the characteristics of the springs that include the extent of the reservoir, discharge rates, and discharge variability over time. Accordingly, the recession coefficient (α) and dynamic storage volume (as a determinant agent of aquifer development) were obtained. The discharge capacity of Sefidab spring was very variable that is related to the magnitude and duration of a precipitation event. The dynamic storage volume and recession coefficient (α) of Sefidab spring are determined through the following equations [32, 33] (Table 4).
where α is the recession coefficient, Q0 (m3/s) is a previous discharge at t = 0, Qt (m3/s) is discharged at t, and t is the period of time between primary and secondary discharge (day).
where V is the dynamic storage volume (m3), Q0n is the primary discharge at n part of hydrograph plot, and αn is the recession coefficient at n part of hydrograph plot.
The karstification degree in the recharge areas of Sefidab spring is determined through its 1-year hydrograph (due to the data loss in the monthly discharge hydrographs) and information from Malík and Vojtková . Therefore, the degree of karstification of the spring is estimated to be 5.5 due to the methodology described above.
The results of the investigation are represented that Sefidab spring has a complex discharge regime, with a combination of one subregime with turbulent flow and two subregimes with the laminar groundwater flow (Figure 13). The impact rate of the turbulent flow subregime was short time in comparison with overall groundwater discharge. Usually, such a situation is created when there are many open medium-sized fissures (both karstified and nonkarstic) in the phreatic zone of a fissure karst aquifer (according to α1) and with a smaller influence of connected conduits (groundwater of large karstic channels, according to β1).
The hydrograph of Sefidab spring (the most important spring in terms of discharge) is compared with a histogram of precipitation over a 4-year period from 2011 to 2014 to investigate the precipitation effects on the karst aquifer in the study area (Figures 14 and 15). Three peaks can be observed in the hydrograph that the initial and tertiary peaks have occurred after two maximum rainfall events, while the second peak occurred after a period of continuous rainfall.
11. Chemical and microbial spring water quality
Chemical parameters and microbial agents existing on the groundwater can have a natural and human origin. The nitrate ion (NO3) is the most important chemical parameter that shows the effect of human activity on the groundwater quality. The nitrate ion in the groundwater originates from urban wastewater influx, industrial, fertilizer, and animal excreta. Based on our measurements, the highest value of the nitrate ion was 1.392 meq/l for Soltankhani spring that is higher for world hygiene organization’s limit. The high nitrate value was due to the occurrence of some cottage and pasture in the upstream of the Soltankhani spring due to the animal and human excreta influx. Except for the Soltankhani spring, all appointive samples of the nitrate values have a favorable value in both wet and dry seasons. The water microbial contamination is usually determined based on the number and frequency of the special type of bacteria. The coliforms are the dominant index of microbial quality of the water resource that is frequently found in the human excreta and other endotherm animals. The most prevalent bacteria in the coliform group are Escherichia coli that are excreta contamination index of the water. The microbial contamination can be observed in all samples due to the intense development of karst, lack of self-purification in the karst system, and lack of an adequate cover layer on carbonate formation. Although the major elective samples have the better status of microbial contamination in the wet season than the dry season, the accomplishment of sanitary preparation and infiltration before consumption (among boiling of water and surcharge of chlorine) is of great necessity.
12. Contamination hazards in the spring catchment and vulnerability mapping
Potential hazards to the karst aquifer were created mainly from forestry, pasture, tourism, and an unnaturally high game population (mainly, chamois and deer) except airborne pollution. The input from tourism, pasture, and the game population was mainly feces with their microbial contaminations . Karst systems with joints and clefts and without a self-purification inexistence pose a high vulnerability to human activities. The high hydraulic conductivity, transmissivity, and intense heterogeneity and anisotropy of karst aquifers enhance their vulnerability to dopants. Various methods can be applied to the vulnerability analysis of karst aquifers. Kardan Moghadam et al.  applied the EPIK vulnerability index that consists of four parameters: epikarst, the preservative cover layer, permeation condition, and developed karst system. Preparation, weighing operation, and incorporation of the suitable parameters map was accomplished in the ArcGIS environment . The Dorfak peak Polje can enhance the vulnerability level of karst aquifer due to livestock aggregation in the spring and summer seasons. The risk map is provided through incorporating this information with vulnerability index . In general, carbonates in the study area were very well karstified throughout and covering layers were mostly absent, not protective, and sometimes even increased the point recharge. Vulnerability, according to the EPIK method (Figure 16), is high in 20% of the area (20% high vulnerability, 35% moderate vulnerabilities, and 45% low vulnerabilities) .
We would like to show our gratitude to the management of the Regional Water Company of Guilan province and Mr. Fatehi, the head of the Office of Water Resources Studies, for the support of this research.