Abstract: Today, the necessity of quantitative and qualitative monitoring of groundwater is inevitable. In this research, the quality changes of groundwater in Rask Plain, using Shuler and Wilcox methods was investigated, in the period of 2012 to 2017. The results showed that water quality decreases over time. Schuler's chart revealed that the water quality, at the Rusk station, had reduced to medium state at the end of the period, on drinking aspect. Furthermore, Wilcox's chart sed that it had reduced to class (C3_S3), on agricultural aspect. However, these changes are not significant for Firozabad, Cheraghan, Parod, and Hajiabad stations. In general, the worst and best water quality is related to Rask and Haji Abad stations, respectively. Despite of thatthe quantity of water quality parameters, except the TDS parameter, are in the range of the national standard of Iran, at all stations.
Keywords: Rask plain, Water quality, Wilcox chart, Schuler chart.
Introduction: Over-exploitation of groundwater has decreased the volume and quality of groundwater aquifers (Daneshvar Vousoughi and Dinpazhoh, 2013; Soleymani et. al., 2013; Ghomishoun et. al., 2013; Frid et. al., 2013; Moghadam et. al., 2013). As a result, groundwater quality has affected agricultural production (Ranjbar and Banakar, 2011؛ Salehi and Zeinivand 2014). Furthermore, in Iran, groundwater provides about 50% of the country's water needs ( Farid et al., 2015؛ Najmi, et al., 2012). Therefore, monitoring the quality of groundwater resources and determining their usability for various uses, such as agriculture and drinking water, is important.
Various methods have been developed for quantitative and qualitative monitoring of groundwater, including: Spearman test to analyze the concentration of groundwater quality parameters in the Ardabil plain (Daneshvar Vousoughi and Dinpazhoh, 2013), Schuler diagram to determine groundwater quality in the Bam and Barvat plains (Malkutian and Akrami, 2004), Wilcox diagram to classify groundwater for agricultural use (Malkutian and Akrami, 2004), and Piper curve method to investigate the changes in quality parameters of the water of the Yellow River in Khuzestan Province (Mousavian et al., 2014).
In recent years, the Rask region has faced serious challenges in the management and use of water resources, thus, in this study, the quality of groundwater in the Rask Plain was assessed using the Wilcox and Schuler methods, and the water potential in different areas of the plain for consumption in the drinking and agricultural sectors was determined.
Methodology: Rask County is in the south of Sistan and Baluchestan Province at 24°60′E and 14°26′N (Fig. 1) (Ismail Rahmani et al., 2018). This county includes four cities: Rask, Sarbaz, Pishin, and Parud. The average rainfall in this region is 90 mm per year, which has decreased to less than 15 mm in 2019 (Sistan and Baluchestan Province Portal, 2019).
In this research, we use the Schoeller Diagram and the Wilcox Diagram to classify the quality of Rask water sources for drinking water consumption and for agricultural use, respectively. The Schoeller Diagram is a semi-logarithmic diagram and displays the hydro-chemical classes of water in terms of the main ions dissolved in water (Baba et al., 2014). The threshold values of various parameters for classifying the quality of water sources for drinking water consumption are listed in Table 3.
The Wilcox Diagram classifies water quality, based on the percentage of sodium dissolved in water and the electrical conductivity of the water (Salehi and Zeinivand 2014), to determine the suitability of water resources for agricultural use (Kumar, 2005). This diagram divides hydro-chemical parameters into four classes (Tables 1 and 2), which provide a total of sixteen different classes for classifying water quality for agricultural use (Ebadati and Houshmandzadeh, 2014).
Results and discussion: Qualitative changes in the Rask Plain were assessed at the Rask, Parud, Cheraghan, Hajiabad, and Firoozabad stations from 2003 to 2018. The annual average of calcium (Ca), magnesium (Mg), sodium (Na), chlorine (Cl), sulfate (SO4), bicarbonate (HCO3), total hardness (TH), and total dissolved solids (TDS) were used in this assessment.
The Schuller diagram is drawn at each station to determine the drinkability of Rask groundwater, Figs 3-7. Fig. 3 shows that the TDS and TH levels are above the desirable threshold of the Iranian National Water Standard, which is 1500 and 500, respectively (source). Hence, it is not suitable for drinking. At the Firuzabad station, chlorine content and the amount of dissolved minerals in the water have not exceeded the acceptable range, and the total water hardness value has been in the good range in all years (Fig. 4). The SO4 value at this station generally fluctuated from 7 units to 150 units. At Cheraghan station, the TDS value is within the acceptable quality range in all years and the SO4 value in this station is 60 units and its maximum is about 200 units. At Paroud station, the value of SO4 is more than 250 units for some years; since the value of Mg is less than 30 units for these years, it does not create any restrictions on water use. At Hajiabad station, the TDS value exceeded the good quality threshold only in 2015. Overall, the water is in the good class based on all indicators.
In Figures 8 to 12, Wilcox plots show the quality of groundwater for agricultural use at each of the stations of Rask, Firuzabad, Cheraghan, Parud, and Hajiabad. The Wilcox plot (Fig. 8) shows that at the Rusk station, the water quality has changed in classes (C3_S1), (C3_S2), and (C3_S3), respectively, meaning that the water quality trend is decreasing, making it unsuitable for agriculture in 2018. At Firouzabad station, the groundwater quality class in 2006 was (C2-S2), in 2007 (C3-S1), and in other years (C3-S2), so it is usable for agriculture. The sodium absorption ratio of the water was less than 13 units, and the water quality classes were (C3-S1) or (C3-S2), meaning the maximum salinity at the Cheraghan station is moderate (Fig. 10). Therefore, this water can be used for a variety of agricultural products. The trend of salinity changes at Parud station is almost consistent with that at Cheraghan station, although it experiences greater dispersion. The maximum sodium absorption ratio at Parud station has increased to 16. The Wilcox plot (Fig. 12) shows that the water salinity at Hajiabad station has not changed significantly except in 2007 and is mainly in class (C2_S1), moderate salinity, which shows the best water quality among all stations.
Conclusion: In this study, we evaluate groundwater quality in the Rask Plain, at Rask, Firouzabad, Cheraghan, Parud, and Hajiabad stations, using Schuller and Wilcox diagrams. The results showed that water quality, from a drinking perspective, has not differed significantly at each station during the years studied, however, the water quality is the worst at Rask station, while it is the best at Hajiabad station.
From an agricultural perspective, the water quality varies from (C2_S1) at Hajiabad station to (C3_S3) at Rask station. The water quality was unsuitable for agriculture at Rask station; it was suitable for irrigation of lands with a drainage system at Firouzabad, Chiraghan, and Parod stations; and suitablez.and it was very suitable for agriculture at Hajiabad region.