بررسي تاثير تغييرات ميزان بارش و برداشت از آب هاي زيرزميني بر تغييرات كمي و كيفي آب آبخوان (مطالعه ي موردي: دشت رفسنجان)

Investigation the Effects of Precipitation Change and Groundwater Overextraction on Both Quantitative and Qualitative Changes of Groundwater (Rafsanjan Plain(

چكيده افزايش جمعيت و افزايش نياز آبي در بخش‌هاي مختلف كشاورزي، صنعتي، شرب و بهداشت باعث برداشت بيش از حد مجاز از منابع آب زيرزميني در دشت‌ها شده است. هدف از انجام اين مطالعه ي بررسي روند تغييرات كمي و كيفي آب‌هاي زير زميني و پهنه بندي اين تغييرات در دشت رفسنجان است. براي انجام اين مطالعه ي ابتدا آمار داده‌هاي سطح ايستابي در 80 پيزومتر به صورت كمي و در 50 چاه به صورت كيفي به عنوان چاه‌هاي منتخب تهيه شد. براي بررسي نوسانات ساليانه ي سطح آب زيرزميني، هيدروگراف دشت تهيه شد و تاثير بارش و ميزان برداشت برتغييرات سطح آب هاي زيرزميني در سال هاي مختلف از طريق رابطه ي همبستگي مورد تجزيه و تحليل قرار گرفت. براي بررسي تغييرات كيفي، نقشه هاي كيفيت با روش ويلكوكس در ابتدا و انتهاي دوره تهيه و مقايسه شد. بر اساس نتايج حاصل شده نقشه‌هاي كمي و كيفي آب‌هاي زيرزميني به روش ميانيابي كريجينگ معمولي و با استفاده از سيستم اطلاعات جغرافيايي (GIS) تهيه شد. نتايج حاصل از اين مطالعه نشان دهنده ي افت كيفيت و سطح آب‌هاي زيرزميني منطقه است به طوري كه متوسط افت سالانه سطح آب هاي زير زميني دشت حدود 8/0 متر در سال بوده است و اين افت در طول دوره ي مورد مطالعه (1381-1391) روند افزايشي داشته است به طوري كه متوسط افت سالانه در سال اول مطالعه از 99/8 درصد به 48/9 درصد در سال آخر رسيده است. براساس نتايج حاصل برداشت بيش از حد از منابع آب زيرزميني در اراضي كشاورزي عامل اصلي كاهش كيفيت و افزايش افت سطح آب‌هاي زيرزميني در منطقه مي‌باشد.

Introduction Groundwater is one of the most important resources of fresh water in the world, especially in arid and semi-arid areas. In these areas, the demand for groundwater has increased due to the decline of rainfall, population growth, and industrialization, while its quality has declined via industrial and urban contamination. The removal of the groundwater pollution is very costly and time-consuming. Consequently, the prevention of the groundwater contamination is the best way for groundwater protection. The main aim of this study was to investigate the trend of groundwater quality and quantity changes in the Rafsanjan plain in relation to the groundwater discharge and rainfall change. Methodology The study area is the Rafsanjan plain with an area of 5459.36 km2 (with an altitude of 45?, 30? to 56?, 30? and latitude of 29?, 59? to 29?, 15?). In this study, the essential maps including topography, drainage, piezometric wells location, and groundwater quality and quantity maps were created using GIS10.1. The groundwater level in 80 pizometric wells and the groundwater quality in 50 wells were investigated and analyzed for a period of 10 years (2002-2012). The groundwater unit hydrograph and rainfall pattern were compared to indicate the impacts of rainfall variability and the groundwater over-extraction on the groundwater level variation. Water quality maps were created using Vilcox method. Based on kriging interpolation method, the quantitative and qualitative maps of the study area were prepared using geographic information system (GIS). Results The groundwater hydrograph of the study plain indicated that the groundwater level declined continuously. As during the past 10 years, the groundwater decline was 8 m, so the annual groundwater decline in the study plain was 0.8 m. comparing the groundwater level of 2002 and 2012 via piezometric wells indicated a significant decline of the groundwater level. In 2002, for 81% of the study plain, the groundwater level was between 30-90 m, while it declined to 68% in 2012. The maximum groundwater decline was related to the area where groundwater level in 2002 was between 30 and 60 m. The area where the groundwater level was between 90 to 120 m, it increased from 683.8 km2 in 2002 to 999.7 km2 in 2012. Also the area where groundwater level was more than 120 m, it increased by 5.3%. A significant relationship was observed between the groundwater level and the volume of the groundwater extraction in 10 years of the study (R2 = 0.6). However, no significant relationship was observed between the groundwater level and the average rainfall between 2002 and 2012 (R2 = 0.04). These results indicated that the impact of the groundwater extraction on the groundwater level decline was more important than the rainfall change. In this study, Wilcox method was used for the investigation of the variability of the groundwater quality. Based on Electric conductivity (EC) and Sodium absorption rate (SAR) in Wilcox method, 16 classes of groundwater quality should be investigated. According to these results, in 10 years of the study period, the number of wells located in C3S2 and C4S2 classes of groundwater quality declined by 2 and 4% respectively. The number of wells located in C4S4 increased from 33% in 2002 to 38% in 2012. Cumulative discharge of all study wells decreased from 610 liter per second to 469 liter per second. The maximum decline was related to C4S3 and C4S2 groups. Discussion The results of this study indicated that the groundwater quality and level declined in the study area. According to the results of the water quality maps, the area of the aquifer with groundwater quality located in C3S2 and C4S2 respectively decreased by 6 and 1.4 %, while the area of the aquifer with groundwater quality located in C4S4 increased by 4.5 percent. The study of the piezometric wells with a depth of 30 m and less indicated that 15% of these wells dried between 2002 and 2012 due to groundwater level declination. The water quality of the profound wells (with a depth of 31 to 200m) decreased by 8.5%. These results indicate that the groundwater quality decreases with increasing of the groundwater level. Conclusion According to these results, the groundwater decline due to the rainfall decline, and the role of the groundwater abstraction in the agricultural area were more important than the rainfall deficits. The qualitative and quantitative maps of groundwater were also prepared via kriging interpolation method and GIS. Based on these results, it can be suggested that rainfall decline leads to the decline of groundwater, but excessive removal of groundwater resources in agricultural lands is a major factor that should reduce the quality of the groundwater in the study area.