GEP  Vol.5 No.6 , June 2017
Groundwater Quality Assessment Based on WQI and Its Vulnerability to Saltwater Intrusion in a Coastal City, Iran
Abstract
The purpose of this paper was to assess the quality of groundwater in Behshahr (Iran) based on water quality index (WQI). Sixteen water samples from this coastal aquifer were collected and analyzed to study physico-chemical parameters such as pH, hardness, chloride (Cl), electrical conductivity (EC) and total dissolved solids (TDS). The results showed that the annual average of TDS value increased by 343 mg/l between 1999 and 2015 due to anthropogenic activities. According to WQI, it was concluded that the groundwater quality degraded over the last sixteen-year period so that the percentage of samples identified as “good water” decreased by 18.5% and on the other hand, alas, the percentage of “poor water” quality soared by nearly 12.5%. Correspondence of WQI distribution diagram with that of chloride-bicarbonate ratio attests to the significant contribution of saltwater intrusion to groundwater quality deterioration in this area.

1. Introduction

As demand for freshwater soars and its supplies dwindle, the world water crisis is becoming an enormous challenge. If remaining unsolved, this problem may have catastrophic consequences for humans and wildlife alike. Meanwhile, groundwater which plays a vital role in water supply in innumerable areas has been exploited during last decades. Groundwater pollution, thereupon, has become an acute problem that if thrust aside will escalate to a point of no return. A precise assessment of groundwater quality is required to devise a strategy to tackle the issue.

Processes governing groundwater quality are divided into two main categories: anthropogenic and natural factors each of which comprises their own distinctive effects. From anthropogenic perspective, fundamental causes of ground- water pollution are leaked sewage, seawater intrusion, industrial wastewater and fertilizer. Natural causes, on the other hand, encompass geology and geochemical processes, weathering, precipitation, temperature etc [1] [2] [3] .

One of the most prevalent threats of groundwater quality deterioration in coastal aquifers is chloride contamination which for the most part occurs due to saltwater intrusion. Pumping from underground resources is the predominant cause of saltwater intrusion in coastal aquifers. It can reduce freshwater flow toward coastal discharge areas and cause saltwater to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of supply wells [4] . Furthermore, high concentrations of chloride have adverse effects on human health, crops productivity, soil permeability, and durability of equipment used in water distribution systems [5] .

It is noteworthy that retrieving the water quality is burdensome and costly once it is polluted especially by saltwater. Regular monitoring of the quality of groundwater, hence, appears to be essential [6] .

The importance of groundwater monitoring and the complexity associated with numerous methods reveals the necessity of a simple yet accurate technique for evaluating groundwater quality. Traditional practices mostly include reports with a lot of details about variables and their compliance with official guidelines; however, in many cases, managers and the general public have no tendency to study accurate reports. Summarizing complex water quality data in an exoteric parameter is possible by means of Water quality index (WQI). WQI was first expressed by Horton and later developed in many papers on the water quality monitoring of all around the world [7] [8] [9] [10] [11] . This method has been recognized as a measure that indicates the complex effect of several variables on the overall water quality rating [12] .

Since no study has been conducted in this area for groundwater resources, the main objective of this paper is to find the baseline groundwater quality of this region based on WQI and its relevancy to chloride contamination.

2. Material and Method

Behshahr is a coastal city in the east of Mazandaran province, Iran. This city is Located on the south coast of the Gorgan gulf (Caspian Sea) at the foot of the Alborz Mountain. The average elevation of the city is about 20 m above mean sea level also its area is approximately 940 km2 and has a population of 31,875.

In order to assess groundwater quality, 16 groundwater samples were collected in this study area. For each sample, quality parameters such as pH, TDS, EC, Total hardness, major cations and major anions were measured through the APHA standard methods. The analytical results in 1999 and 2015 are repre- sented in Table 1 and Table 2 respectively. The location of sampling wells which are recorded in the Universal Transverse Mercator (UTM) coordinate system is shown in Figure 1 as well.

Being a convenient method to appraise and present water quality for drinking, WQI method was applied to combine and summarize different parameters mea- sured for each sample. This method considers the effect of various water quality parameters on human health. Following four simple steps describe the calculation of WQI [13] [14] :

1) Assigning weight (wi):

In the first step, weights are assigned to each parameter on a sliding scale of 5 to 1 based on their contribution to water quality from most important, 5, to the least, 1. Table 3 shows weights relating to each parameter [14] .

2) Relative weight (Wi):

Relative weight is calculated using the following Equation:

(1)

where the wi is the weight of each parameter and n is the number of parameters (Table 3).

3) Quality rating scale (qi):

Table 1. Physico-chemical parameters for selected water sample in the Behshar basin (1999).

Table 2. Physico-chemical parameters for selected water sample in the Behshar basin (2015).

Figure 1. Google image of study area with sampling locations.

Table 3. Relative weight of physico-chemical parameters and the WHO standard for drinking purpose.

* USGS allowable limits [16] .

The quality rating scale qi for each parameter in each sample is determined by dividing the concentration of each parameter by its respective standard (Si) given by WHO multiplied by 100 [15] .

(2)

where the Ci is the concentration of each parameter which is reported in mg/l except for pH which is dimensionless and the Si is maximum allowable limit for each parameter in mg/l (Table 3).

4) Sub-index (SIi) and Water WQI:

To determine the sub-index and the WQI following Equations are applied respectively:

(3)

(4)

where (SIi) is the sub index of each parameter, qi is the quality rating scale of each parameter and n is the number of parameters.

3. Result and Discussion

3.1. Water Quality

The water quality analysis of Behshahr showed that pH values were in the range of 7.4 - 8 in all the sixteen sampling wells. Total hardness, moreover, varied between 180 to 565 mg/l as CaCO3 indicating a high rate of hardness in the study area.

Variation of TDS and EC were in the range of 356 to 4557 mg/l and 512 to 7120 µS/cm respectively. As anticipated, by approaching downstream total dissolved solid noticed to be higher in samples. The annual average of TDS has been soared from 592.4 mg/l to 935.3 mg/l in a 16-year period of 1999 to 2015. Currently, this value is disturbingly close to maximum allowable limits of drin- king water proposed by WHO (Figure 2). It is mainly caused by anthropogenic activities such as infiltration of agricultural runoff and domestic wastewater or saltwater intrusion. The comparison between the percentage of water quality parameters in the most polluted sample (W7) and of those in seawater, repre- sents significant similarities. These similarities are most noticeable in sodium and chloride concentrations (Figure 3). This could be the result of salt water intrusion into this aquifer which is highly probable since the area is located in the vicinity of Caspian Sea.

The chloride-bicarbonate ratio is a well-known and decent index to identify saltwater intrusion in coastal areas [17] . According to Raghunath, the ratio of Cl/HCO3 = 2.8 is assumed to be the threshold of saltwater intrusion. The distribution of the mentioned ratio in this study area between 1999 and 2015 indicates that not only W7 is already contaminated by saltwater intrusion but also the threshold of saltwater intrusion has significantly moved landward during these years (Figure 4).

Effective and immediate policies, consequently, entailing prevention of over- exploitation of threatened wells along with injecting fresh water into these wells are highly recommended.

Figure 2. Annual average growth of TDS between 1999 and 2015.

Figure 3. Comparison between percentages of water quality parameters in seawater with that of W7.

Figure 4. Distribution of Chloride-bicarbonate ratio between 1999(a) and 2015(b).

3.2. Water Quality Index

Calculation WQI can entail as many qualitative parameters as researchers desire. However, its outcome is a single dimensionless figure which represents water quality in each sample [12] .

In this study eleven parameters including pH, TDS, TH, major cations and anions were considered to partake in WQI calculation. Taking into account the significance of each parameter on human health, weights were allocated to them. Results categorized samples into four different groups based on guidelines proposed by WHO and U.S. Geological Survey. Variation of WQI parameter was in the range of 45.48 to 428.07 in 2015 (Table 4). In order to achieve a general understanding of water quality in the area, distribution of WQI is shown in Figure 5.

According to Table 4 and Figure 5, it is found that in 2015 about 56.25% of the groundwater samples which covered a wide-range of this study area fitted into good category while in 1999 about 87.5% of samples fitted into this category. Since the water quality of those samples identified as “good water” is approximately close to natural condition, they are suitable for direct human consumption.

Table 4. Water quality index (WQI) legend.

Figure 5. WQI distribution of sampling wells between 1999(a) and 2015(b).

Figure 6. Correlation coefficient between WQI and chloride-bicarbonate ratio.

Regarding the excellent category, although an increase of 12.5% can be observed between 1999 and 2015, the improvements in the water quality occurring in W5 was fairly insignificant and probably reflect natural events, furthermore, the improvement in W12 might have occurred as a result of being located in the recharge area which could make it quite sensitive to the raw water quality.

As can be seen in Table 4 in 2015 nearly 25% of samples fitted into poor category. Comparing to 12.5% of those in this category in 1999, it showed a twofold increase. The quality of water in these samples mostly located in northern and western part of this basin are occasionally threatened or impaired. Consequently, they can be only used for irrigation, bathing and swimming, though for drinking, industrial or laundry purposes conventional treatment is highly recommen- ded.

Since the water quality of those samples fitted into poor category occasionally depart from natural state, it is suggested that the mentioned category be considered as the threshold of unnatural activities. This consideration helps policy makers to make optimized decisions based on priority and importance of each quality zones to prevent groundwater contamination.

Through approaching northern parts of the basin, water quality perceived to be impaired. This is especially pronounced in W7 which fitted into “Unsuitable for drinking” category in 2015 whereas in 1999 it was located in the zone that was just in progress to be impaired. Water quality in W7 has been drastically affected by high concentration of TDS and sodium chloride during this period of time.

The parallelism between distribution diagrams of WQI and chloride-bicar- bonate reveals the fact that threshold of saltwater intrusion fairly corresponds with the very poor quality zone. Assigning a high weight to chloride in WQI calculation is a contributing factor to this similarity. This makes the results highly sensitive to drastic changes in chloride concentration. In addition, the correlation coefficient between chloride-bicarbonate ratio and WQI confirms that saltwater intrusion plays leading role in water quality deterioration in this study area (Figure 6).

4. Conclusions

In this study area annual average of TDS increased nearly 343 mg/l and reached to 935 mg/l in 2015 due to anthropogenic activity. The WQI rating represented various water quality categories in this study area. In 1999 about 87.5% of groundwater samples were suitable for drinking purpose whereas in 2015 the number of potable samples decreases to 69% of all groundwater samples. In a related vein, number of samples belonging to poor category increased about 12.5% over this period of time. This means extracted water from these wells and that in the immediate vicinity is only to be used for irrigation, bathing or swimming unless conventional treatment is conducted.

The comparison between Percentage of seawater quality parameters and those of in W7 represented significant similarities especially in sodium-chloride concentration. Furthermore, comparing the chloride-bicarbonate distribution diagram in the last 16-year period indicated that saltwater/freshwater interface has significantly moved landward.

The correspondence between WQI distribution and chloride-bicarbonate ratio shows that the main reason of very poor water quality zone is chloride contamination and also the correlation coefficient between aforementioned parameters confirms that saltwater intrusion is the most possible consequence of anthropogenic activities in this coastal area.

Accordingly, to gain a true understanding of water quality in an area WQI is highly recommended. This simple yet precise method presents a comprehensive interpretation of water quality in a basin. Its sensitivity to chloride that has high weight makes it a good measure for seeking out the threshold of saltwater intrusion in this study area. Relying on this method’s results a basin can be divided into different zones with different water usages. Policy makers would achieve a great benefit utilizing this method.

Cite this paper
Tavassoli, S. and Mohammadi, F. (2017) Groundwater Quality Assessment Based on WQI and Its Vulnerability to Saltwater Intrusion in a Coastal City, Iran. Journal of Geoscience and Environment Protection, 5, 88-98. doi: 10.4236/gep.2017.56010.
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