Document Type : Research Article
Authors
Department of Water Sciences and Engineering, Collage of Agriculture and Natural Resource, Ardakan University, Iran
Abstract
Groundwater is the main source of drinking water in the Yazd–Ardakan plain. As human activities expand, comprehensive quantitative and qualitative evaluations of the aquifer have assumed critical importance in water-resources studies. One of the indicators for evaluating groundwater quality is the Pollution Index of Groundwater (PIG). To investigate the temporal and spatial changes in groundwater quality in the Yazd-Ardakan plain, the PIG index was calculated for the years 2011 and 2021 using water quality data from 27 deep wells within the plain. The parameters included sodium, potassium, calcium, magnesium, chloride, bicarbonate, sulfate, total dissolved solids, salinity, and acidity. The results showed increases in anions, cations, acidity, and salinity, with the average of all chemical parameters except acidity and bicarbonate exceeding the permissible limits recommended by WHO. According to the PIG index in 2011, 51.43% of the total plain area had low and negligible pollution, 9.58% had moderate pollution, 10.83% had high pollution, and 28.16% had very high pollution. Only after ten years, groundwater quality declined so that 24.80% had low and negligible pollution, 23.94% moderate pollution, 9.12% high pollution, and 42.04% very high pollution. In other words, a larger area of the plain fell into the very high pollution category, and the area with low and negligible pollution decreased. According to the results, groundwater quality, especially in the northern and southern parts of the Yazd-Ardakan plain, is in a worse condition compared to other areas of the study region and requires special management and protective measures to improve the quality and quantity of groundwater resources.
Introduction
Assessing the health and contamination of groundwater for drinking purposes is of utmost importance because these water resources are a primary source of potable water in many regions worldwide. Groundwater contamination can stem from various factors, including the infiltration of chemicals, microbial pollution, the improper use of fertilizers and agricultural pesticides, and unsanitary wastewater disposal practices, which, if consumed, can lead to various diseases and serious health problems in humans. Therefore, continuous monitoring of groundwater quality and conducting precise tests to identify and control contamination are essential to ensure public health and prevent environmental and health hazards. The Yazd-Ardakan plain, located in the center of the Iranian plateau with a very arid climate and low rainfall, relies primarily on groundwater resources to supply the water needed for the cities located in this plain. Due to excessive extraction and declining water levels, the region's aquifers face challenges such as land subsidence and reduced water quality. The groundwater pollution index is an important indicator in assessing groundwater quality, developed to measure water pollution with the aim of simplifying and standardizing laboratory results. This index is designed to summarize multiple water quality parameters such as pH, anion and cation concentrations, and dissolved solids into a single number, allowing for easy comparison and analysis of pollution levels within a specific range. It was developed in response to the need for quantitative and understandable methods for environmental decision-making and groundwater resource management. The use of composite indices such as PIG has become prevalent in recent decades with the advancement of analytical technologies and the need for rapid assessment of water quality in environmental studies. This index was first proposed by Subba Rao (2012). He showed that potassium, magnesium, calcium, sulfate, sodium, chloride, bicarbonate, as well as total dissolved solids in water and acidity are effective in determining this index. Although this index has been used in many parts of the world to assess groundwater quality, there are few studies in Iran, and those that exist are in the Birjand plain. The aim of this research is to assess the quality of groundwater in the Yazd-Ardakan plain using the PIG index, which has not been evaluated in this region so far. As a result, to investigate the temporal and spatial changes in groundwater quality, zoning maps of each element were drawn using GIS software, the PIG index was calculated for 27 deep wells in the region in 2011 and 2021, and finally, based on the results of the PIG index, the groundwater of the study area was classified in terms of pollution. This comparative information can help to formulate monitoring and management programs for Yazd-Ardakan groundwater resources by considering similar experiences.
Material and Methods
The Yazd–Ardakan Plain is one of the largest plains in Yazd Province. It lies between 53°15′ and 54°50′ east longitudes and between 31°15′ and 32°15′ north latitudes. The area of this plain is equal to 11775 square kilometers and includes the cities of Yazd, Ardakan, Meybod, Taft, Askezar and Mehriz. This plain is located in the arid belt of the central plateau of Iran and has an average annual temperature between 12 to 19 degrees Celsius, low and relatively constant relative humidity in the range of 30 to 50 percent, very high evaporation between 220 to 3200 mm per year, and an average annual rainfall of about 107 mm. To conduct this research, registered statistics regarding the water quality of wells in the Yazd-Ardakan plain for the years 2011 and 2021 were initially collected from the Yazd Province Regional Water Company. In this study, to investigate the qualitative parameters of groundwater resources, data from 27 wells with long-term and complete statistical periods and a good distribution in the study area were used, and groundwater quality maps for the years 2011 and 2021 were drawn for each of the chemical variables in the GIS software environment using the IDW interpolation method. The parameters studied in this research include the concentration of anions and cations, calcium, potassium, magnesium, sodium, chloride, bicarbonate, sulfate, acidity, sodium adsorption ratio, total dissolved solids, and electrical conductivity of water.
Results and Discussion
The results indicate that over a 10-year period, the levels of anions and cations, acidity, and salinity have increased, while the quality of the region's groundwater has declined. The average values of all chemical parameters, except for acidity and bicarbonate, exceed the permissible limits recommended by the World Health Organization (WHO). As the zoning results show, the concentration of cations, anions, and salinity in the year 2011 is highest in the north and part of the south of the plain and decreases toward the center of the plain, such that the lowest values are observed in the east, west, and center of the plain. Except for bicarbonate, which shows its highest amount in the east and center of the plain. The highest amounts reported are calcium 478.91, magnesium 283.99, sodium 2600.33, potassium 10.94, chloride 3823.65, bicarbonate 464.82, and sulfate 1998.34 mg per liter, with salinity at 13317.5 micromhos per centimeter. The highest acidity (8.05) and sodium absorption ratio (29.17) are also found in the northern part of the plain. Over the 10-year period, the groundwater quality of Yazd-Ardakan plain shows significant changes, so that the chemical element concentration zoning maps in 2021 compared to 2011 indicate changes. This means that both the maximum values of these parameters have increased, and except for the northern parts and some of the southern plain, the concentration of some elements in the east and center of the basin has also risen. In other words, temporally the groundwater quality of the plain has deteriorated, and spatially, larger areas of the plain have experienced a decline in groundwater quality.
Conclusion
The results of the temporal trend analysis of the PIG index showed that this index has been increasing over time, with a more noticeable increase in the northern part of the plain. Specifically, the well located at Dom Kaftar in the northern area of the plain experienced a significant increase (66%), with the PIG index rising from 6.73 to 11.20. Furthermore, the results indicate that over time, the quality of groundwater in the Yazd-Ardakan plain has deteriorated from the north and south towards the center of the plain. It can be concluded that this decline in quality is attributed to several factors. These include the lowering of the water table due to land use changes and excessive groundwater extraction, alongside a decrease in rainfall and the repeated occurrence of droughts over the 21-year study period.
Acknowledgements
The authors of the article would like to thank the Yazd Regional Water Company for providing the data required for this study.
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