مطالعه تغييرات زماني و مكاني كيفيت آب زيرزميني دشت يزد- اردكان با استفاده از شاخص GQI

Investigation of Spatial-Temporal changes of Yazd-Ardakan Plain Groundwater Quality Using GQI

امروزه با گسترش فعاليت‌هاي انساني، ارزيابي كيفي و كمي منابع آبي به‌ ويژه در مناطق خشك و نيمه‌ خشك جايگاه ويژه‌اي در مطالعات منابع آب پيدا نموده است. يكي از شاخص­‌هاي ارزيابي كيفيت آب، اندازه‌گيري مقدار غلظت يون‌هاي اصلي موجود در آب است. به‌ منظور بررسي تغييرات مكاني، شاخص كيفيت آب زيرزميني (GQI) كه تلفيقي از پارامترهاي مؤثر بر كيفيت آب مي‌باشد، مورد استفاده قرار ميگيرد. در مطالعه حاضر به‌ منظور بررسي نحوه توزيع يون‌هاي اصلي شامل كلسيم، منيزيم، سديم،كلر،سولفات و كل مواد محلول (TDS) و پهنه‌بندي كيفي دشت يزد اردكان، شاخص GQI با استفاده از نرم افزارArcGIS برآورد شد. براي بررسي وضعيت تغييرات زماني و مكاني شاخص GQI در دشت يزد - اردكان از آمار 53 چاه‌ پيزومتري وابسته به وزارت نيرو (شركت آب منطقه‌اي استان يزد) استفاده و نقشه‌هاي پهنه‌بندي كيفي سال‌هاي 1382، 1385 و 1390 تهيه شد. نتايج نشان داد كه دشت يزد - اردكان در كلاس‌هاي متوسط و قابل قبول از نظر شاخص GQI قرار دارد. بيشترين و كمترين مقدار شاخص GQI به ترتيب در غرب و شمال دشت مشاهده شد. شاخص خود همبستگي فضايي موران1، شاخص GQI و همه متغيرهاي شيميايي مورد بررسي به جز منيزيم داراي الگوي توزيع مكاني خوشه‌اي هستند و منيزيم داراي الگوي توزيع مكاني تصادفي است.با توجه به نتايج به‌دست آمده از ميان شش پارامتر مورد بررسي سه پارامتر مواد جامد محلول (TDS)، سديم و كلر با داشتن بيشترين ضريب رتبه‌بندي به ترتيب داراي بيشترين تأثير در مقدار شاخص GQI و در نتيجه كيفيت آب زيرزميني هستند. در مجموع مي‌توان نتيجه گرفت كه روند شاخص GQI در منطقه مورد مطالعه نزولي است و كاربري اراضي نقش بسيار زيادي در كاهش ميزان شاخص GQI و در نتيجه كيفيت آب‌هاي زيرزميني دشت يزد - اردكان دارد.

Introduction: Water is the principal source of economic development, social security and poverty reduction. The value of water source leads to enhancement of management measures to maintain its quality and quantity by communities. Environmental changes and human activities effect on the quality and quantity of water. Urban growth, increasing industrial activities and overuse of chemical fertilizers in agriculture result in pollution of groundwater and surface water which have adverse effect on the health of human beings, animal and plants. Groundwater is the primary source to supply agriculture and drinking water hence recognition and awareness of groundwater quality and the water classification based on the number of various elements of them will assist us in making management decisions and decline groundwater pollution. Groundwater is particularly important in arid and semi-arid areas. On the other hand groundwater deterioration, both in quantitative and qualitative terms is important in water resources management of these areas. The concentration of common ions in water is one index for assessing water quality. Groundwater quality index (GQI), a combination of parameters for water quality, that constitute a reliable tool in defining aquifer vulnerability is used to investigate the spatial variability. GQI shows the data related to the water quality in an explicit manner. This index presents a way of summarizing the overall qualitative condition of water which is understandable for the audience. Materials & Methods: Yazd-Ardakan plain has been faced with significant reduction of groundwater level in recent years. So, it is expected that the studying groundwater quality index can be effective for aquifer management in this plain. In this research GQI was used in order to study the distribution of major water ions including Calcium, Magnesium, Sodium, Chloride, Sulfate and Total Dissolved Solids (TDS); and zoning groundwater quality using ArcGIS10.1. The data of 53 piezometric wells provided by Iran Water Resources Management Company were used to study the status of temporal and spatial changes of GQI in Yazd-Ardakan plain. Water quality sampling campaigns were conducted during most vulnerable periods of early and late summer to ensure the representativeness of the targeted GQI under worst case conditions. Quality zoning maps were provided for 2003, 2006 and 2011. For this purpose, data were evaluated in GS+ 5.1 software, after calculation, the best model with the lowest estimated error was selected for zoning water quality parameters. Because of the lowest estimation error, Kriging, Gaussian and Spherical variogram models were selected as appropriate interpolation method for zoning the quality parameters. WHO standards were used to compare and investigate the quality status of the water. The water qualitative groups in the GQI map were divided into 5 classes of good, acceptable, medium, inappropriate, and poor, scoring from 0 to 100. The class which is close to 100 shows better quality and the class which is close to 0 shows lower quality. Results & Discussion: The results showed that Yazd-Ardakan plain is located in average and acceptable classes according to GQI index. The highest and lowest amount of Groundwater Quality Index (GQI) were in the west and north of the study area, respectively. Moran's I spatial autocorrelation index, GQI and all chemical studied variables except for magnesium have cluster spatial distribution pattern, but Magnesium have random spatial distribution pattern. Three parameters of Total Dissolved Solids (TDS), Sodium and Chloride having the highest coefficient ranking have a highest impact on GQI, respectively. On the other hand, these parameters have a high weight and GQI is more sensitive to them. In fact, these components in Yazd-Ardakan plain groundwater have more impacts on GQI model and their removal will cause greater changes in GQI. Therefore, they should be carefully evaluated and monitored. Generally, it can be concluded that GQI has the descending trend in the study area and land use has the important role in reducing GQI index and water quality. It can also explain the overall quality of groundwater and its threats in various uses of water. Finally, the regions with poor groundwater quality can be targeted for detailed studies and monitoring programs. Conclusion: According to the results, due to the nature of natural phenomena such as drought, their complete removal is not possible. The only principled way to prevent dangerous consequences of the water table decline and reduction of groundwater resources quantity and quality is the correct and systematic use of water and avoidance of uncontrolled groundwater withdrawal.