ارزیابی کیفیت آب زیرزمینی دشت فسارود (شهرستان داراب) برای مصارف کشاورزی و شرب

Introduction
In recent years, the indiscriminate withdrawal of water from aquifers in the south of Iran and the unprincipled disposal of municipal sewage and agricultural wastewater in receiving water bodies have caused a decrease in the quality of groundwater in most of these plains. The limited nature of these underground resources and their excessive use has created problems in Iran. Therefore, it is necessary to determine the chemical, physical, and biological qualitative characteristics of groundwater to specify the appropriateness of water for the intended use (Sedaghat, 2008). Various studies have been conducted in the field of groundwater quality and the effect of different factors on it. In a research, the evaluation, modeling, and zoning of groundwater quality in the Kashan Plain region were conducted using geostatistics and deterministic models. Five water quality parameters to determine the irrigation index and drinking water quality were determined using the Wilcox diagram. The final map showed that the groundwater quality increased from north to south of the studied area (Feizi et al. 2019). According to the research conducted in Saveh county, Kashan aquifer, and Zahedan plain using different groundwater quality indices, the high concentration of ions and high EC values indicates the unsuitability or problematic use of groundwater resources. Therefore, the main objectives of this research include (1) the use of hydrogeochemical and graphical methods to determine the factors affecting the groundwater quality in Fasarud plain (Darab county), (2) the use of different parameters, diagrams, and indices such as electrical conductivity, sodium adsorption ratio, soluble sodium percentage, magnesium ratio, corrosiveness ratio, and permeability index for classifying water for irrigation, and (3) comparing the chemical parameters of water samples with the guidelines of the World Health Organization (WHO) and the use of Schoeller's diagram for the classification of drinking water for the study area.

Methodology
This research investigated the groundwater quality in the Fasarud plain of Darab County, where wells are mainly used for irrigation and in some places for drinking, using quality parameters and indices. Darab County is located in the southeast of Fars province and a has range of 54o 11’ to 54o 47’ east longitude and 28o 33’ to 28o 49’ north latitude at a distance of 250 km from Shiraz. The total area of the studied area is 6500 km2 with an altitude of 1180 m. According to the De Martonne index, the climate of this county is semi-arid, the average annual temperature is about 25 Co, the average annual rainfall is 350 mm, the average annual wind speed is about 1.2 m/s, the average annual potential evapotranspiration is about 1821 mm, and the average annual sunshine is about 9.4 hours per day (Bahrami et al., 2020). In this regard, the results of a period of chemical analysis related to 31 wells in March 2018 were used. Electrical conductivity, sodium absorption ratio, sodium percentage, magnesium ratio, corrosion ratio, and permeability index were used to classify water for irrigation.

Results and Discussion
A relatively wide range of changes in electrical conductivity was observed in the studied area with the lowest amount (394 μmho/cm) in well No. 22 and the highest amount (5544 μmho/cm) in well No. 24. The electrical conductivity in most of the wells is in the good and acceptable category, one well is in the cautious use class, and three wells are in the unacceptable category. Irrigation of soil with high electrical conductivity can increase the concentration of soil salts, and due to its cumulative property in the soil, it damages agricultural lands and agricultural products. Considering that the water quality in most samples is good and acceptable for agriculture in terms of salinity hazards, there is no need to amend the soil after using the water. The results showed that 39% of the groundwater samples were in the good range, 48% in the average range, and 13% in the unsuitable water range for irrigation. A comparison of the average values of qualitative parameters with the permissible values of standards 1053 of Iran and the World Health Organization showed that the average concentration of Ca, Mg, Na, SO4, and Cl ions are within the permissible range for drinking purposes, while the mean of K, HCO3, and NO3 was more than the allowed values of both standards, which indicates the entry of pollutants into the unground water of this plain through agricultural runoff and domestic sewage. Agricultural runoff has a great effect on degrading the groundwater quality, which is more effective in hot and dry areas. Also, evaporation from these solute-containing wastewater causes the solutes to be left in the upper layer of the soil during infiltration into the ground and transferred to the groundwater with subsequent irrigation or rainfall. The concentration of carbonates in natural waters is more dependent on the amount of dissolved carbon dioxide, temperature, pH, cations, and some soluble salts, which are more concentrated in groundwaters than in surface waters. Also, the comparison of each of the 31 wells' water quality studied in this plain with the Schoeller's standards showed that the water in 19 wells is acceptable or permissible for drinking, seven wells are relatively suitable, five wells are unsuitable, and the hardness in 30 wells are less than 50 mg/L. In general, the results reveal that the water in most of the wells in the study area is suitable for drinking and only the water of five wells is unsuitable due to the higher amount of some elements compared to the permissible limit of the national standard and WHO, which the contaminant has entered groundwater through agricultural wastewater.

Conclusion
The results showed that 58 percent of the samples are in the excellent, good, and acceptable ranges for agriculture, and in other wells, at least one of the indices is in unacceptable or cautious use classes. Also, the water of 26 wells can be used for drinking and the hardness of 30 wells is less than 50 mg/L. It is suggested to continuously monitor the groundwater resources in the study area, taking into account the existing potential of water resource pollution, including the excessive use of nitrogen fertilizers for the fertility of agricultural lands within the limits of these resources.