تخريب كيفي و تغيير الگوي هيدروژئولوژيكي آبخوان بحراني گيسور، خراسان رضوي

Qualitative degradation and hydrological regime Changing of the Geysoor critical aquifer, Khorasan Razavi

در مناطق خشك و نيمه خشك به دليل كمي بارندگي و برداشت بي­رويه از منابع آب زيرزميني باعث تغيير الگوي هيدروژئولوژي، افزايش افت و در نهايت مرگ آبخوان­ها مي گردد. دشت گيسور در جنوب استان خراسان رضوي واقع شده است. جهت بررسي تغييرات حجم ذخيره آبخوان و همچنين اثر آن بر كيفيت آب زيرزميني، از اطلاعات سطح آب 18 پيزومتر موجود در دشت گيسور استفاده شده است. مقادير هدايت الكتريكي در صحرا با استفاده از دستگاه هدايت سنج HANA اندازه گيري گرديده است. با توجه به نقشه هم­پتانسيل، تغذيه آبخوان عمدتا از بخش­هاي جنوبي، جنوب شرقي، و شمال غربي صورت مي­گيرد. متوسط سطح آب پيزومترها داراي افت 5/5 متري در طول دوره 23 ساله مي­باشد. بررسي بيلان آب زيرزميني براي دوره يك ساله نشان از كسري مخزن به مقدار 4/12 ميليون متر مكعب بوده است. با توجه به نقشه­­ي پراكندگي هدايت الكتريكي، تغييرات هدايت الكتريكي تقريبا با جهت جريان آب زيرزميني همسو مي­باشد. مقدار شوري در جهت جريان تا بخش ­هاي مياني روند افزايشي را نشان مي­دهد كه نتيجه تخريب كيفي منابع آب زيرزميني ناشي از نفوذ آب شور و ايجاد گراديان زياد به سمت مركز در طي برداشت بيش از حد مي­باشد.

1. Introduction In arid and semi-arid regions, due to low rainfall and overexploitation of groundwater resources, and subsequent saltwater intrusion, the available fresh sources are reduced. Hydrological study and water budget are the first steps in identifying and assessing the region's water potential to provide management plans for the preservation of the fresh water aquifer. Recently Groundwater is becoming increasingly important as a source of drinking water, agriculture and industry activity. More than a hundred countries are facing water shortage of varying severity in the world. About 40% of the global population is situated in heavily water deficit basins. Estimating the terrestrial water balance is one of the main scopes of hydrology. Access to good quality of drinking water is the most basic of human needs, but these conditions are not always available in many of the arid and semi-arid regions. Arid regions are particularly sensitive to variability induced from climate changes. Investigation of water budget variables, such as precipitation (P), evapotranspiration (ET), stream flow and terrestrial water storage (TWS), provides improved understanding of water resources under a changing climate system. The Geysoor Plain is situated in the semi-arid region of Khorasan Razavi Province in the East of Iran. The area of the aquifer in Geysoor Plain is 11784 km2. The Salty plain exists in the northwest of the study area. Geysoor aquifer is located in Quaternary alluvial deposits and clay basins. The Geysoor plain is located in arid with hot and dry summers and cool winters, and has a low sporadic rainfall from 100 to 120 mm for each year, and this precipitation is the main recharge source of the groundwater in this area. This research investigates the hydrogeology and water budget of groundwater resources in this area. 2. Materials and methods To investigate the hydrological setting of the plain, the iso-potential map of the area was drawn using the data of 18 piezometer wells. Then, all of the water budget components including inputs (sub-surface flow, direct precipitation etc.) and outputs (groundwater flow, harvesting of wells, etc.) were calculated for a year. 3. Results and discussion The data from water level variations in 18 piezometer wells are used to study the hydrogeological properties. The aquifer is mainly recharged from south, southwest, and northwest and discharged toward the northeast of the plain. According to the iso potential map, the general groundwater flow direction is from west and south toward the northeast of the Geysoor plain. According to the EC map, the electrical conductivity is consistent with the direction of groundwater flow, so that in the southern and northwest parts of the area, the lowest EC value is measured at 989 µS/cm. The highest EC was observed in the central areas of the plain, with a value of about 16800 µS/cm, indicating an intrusion of salt water from salt plain. Based on long term drawdown of water level, the Geysoor plain is divided into three zones. Geysoor plain with negative groundwater balance, showed an average storage reduction of 4.12 Mm3 per year. This in result of a considerable groundwater level decrease of about 5.38 m during a 24-year period. Decreases in recharge and overexploitation could cause progressive depletion of the groundwater resources and more budget deficits. 4. Conclusion Hydrogeological investigation is as the first step in identifying and assessing the water volume storage to provide management plans for the protection of available fresh water. In the study area, piezometric water level data and chemical features of the groundwater samples from the alluvial aquifer in the Geysoor plain were measured to assess the hydrogeological condition, groundwater balance and hydrochemistry characteristics of the aquifer. The decline of the piezometric heads in the study area is indicative of an imbalance between recharge and extraction of groundwater and points to the necessity for restriction on groundwater extraction from these aquifer systems to avoid adverse effects. The Geysoor Plain with a negative groundwater balance showed a considerable reduction in the groundwater level. Therefore, the overexploitation of the wells and consequent change in hydrologic regime is the main etiologic factor of the groundwater quality degradation in the central part of the plain. Along the direction of groundwater flow towards the outlet of the plain, the salinity has been reduced due to recharging by carbonate formation in the eastern regions. This detailed investigation is of a particular importance for sustainable management of these groundwater resources, especially in arid zones coupled with increasing exploitation for irrigation and domestic water supply which requires immediate attention.