شبيه سازي جريان آب زيرزميني و نفوذ آب شور در آبخوان ساحلي دشت شبستر با استفاده از نرم افزار GMS

Modeling groundwater flow and salinity intrusion at Shabestar plain aquifer using GMS software model

دشت شبستر در سمت شمال شرقي درياچه اروميه؛ يكي از دشتهاي حاصلخيز كشور است كه ساليانه كشاورزي در آن به وسعت بسيار زياد انجام مي پذيرد. علاوه بر آبهاي سطحي كه بخش كمي از آبهاي مورد نياز كشاورزي منطقه را تامين مي نمايد؛ بطور كلي كشاورزي منطقه به آب زيرزميني دشت وابسته است و حجم زيادي از آب زيرزميني هر ساله در همين جهت استخراج مي شود. دشت شبستر طي ساليانه گذشته تحت تاثير نفوذ پيشرونده آب شور قرار گرفته است كه منبع و علت شوري آن شورابه هاي به دام افتاده در رسوبات انتهاي دشت مي باشد. مدل كمي و كيفي دشت با استفاده از نرم افزار GMS جهت مديريت صحيح برداشت از آب زيرزميني منطقه و جلوگيري از پيشرفت جبهه آب شور در اين پژوهش انجام شده است. تغييرات سطح آب با استفاده از آمار سطح ايستابي 24 حلقه چاه پيزومتري موجود در محدوده مورد مطالعه، مورد ارزيابي قرار گرفته و پس از تعيين پارامترهاي آبخوان در اثناي واسنجي مدل و اطمينان از منطقي بودن جواب ها و پس از طي مرحله صحت سنجي وضعيت هيدروليكي آبخوان بررسي و تحليل گرديد. در دو منطقه از دشت يكي در قسمت جنوبي و ديگري در قسمت شمالغربي دشت بعلت وجود تراكم چاه ها افت شديد تراز آب ملاحظه گرديد. درنهايت از كد عددي MT3DMS كه انتقال توده اي ذرات را شبيه سازي مي كند براي تخمين مسير حركت جبهه آب شور استفاده گرديد كه نشان داد نفوذ جبهه آب شور در قسمت جنوبي و در منطقه اي كه افت تراز آب ايجاد شده بود اتفاق مي افتد

In arid and semi-arid areas with limited water availability such as Iran، groundwater resources are a very important source of freshwater for domestic، agricultural، and industrial use. Regarding، the most of the plains have been salinized due to the consistent groundwater over-extraction. Since these reservoirs are located underground، monitoring of them is not easy as well as it is time-consuming and expensive. Finney et al. (1992) developed an optimization model to control the saline water infiltration in the Jakarta Basin، Indonesia. Western and Peralta (1994) introduced an optimization model for quantitative and qualitative management of complex and nonlinear aquifers in Salt Lake Valley in Utah، USA. Shabestar plain aquifer at north-east of the Urmia Lake is one of the fertile plains of Iran. Agriculture activities use the significant amount of groundwater in the area. Surface waters can only provide a small portion of the waters used for agricultural and industrial activities. Therefore، the life of local people is severely dependent on the groundwater whereas annually a huge amount of groundwater pumped by the farmers for irrigation peruses. Over the recent years، some parts of the aquifer located in the adjacent of the Urmia Lake is influenced by the progressive incursion of saltwater as a result of over-extraction of the groundwater. The main source of the salinity is the salt solutions which entrapped within fine-grained matrix at the southwest-end part of the plain (Immobile zone). In this research، we conducted a mathematical model to explore the qualitative and quantitative characteristics of the Shabestar aquifer using GMS (Groundwater Modeling System) processor. Material and methods GMS is a complete program for building and simulating groundwater models (Ref). Therefore، in this research، the GMS model was applied to simulate the groundwater flow in the area. This model was originally developed by the University of Brigham Young in the late 1980s. In addition، the MT3DMS (mass-transport 3D multi-species) numerical code، as a tool that simulates solute transport in ground water، was used to evaluate the saltwater intrusion in the aquifer. Results and Discussion To qualitatively and quantitatively investigate the Shabestar aquifer، the water-level data measured in twenty-four observation wells in 2012 were imported into GMS 7.1. The model covers the most of the Shabestar basin with 624 km2 in area. Each grid cell covered an area of 1 km2. The optimal parameter values of the aquifer were determined after model calibration settings and verification. Simulation results show that in two parts، located in the southern and the northwest of the plain، the water table consistently declines due to the over-extraction from the groundwater. Subsequently، the MT3DMS (mass-transport 3D multi-species) numerical code that simulates solute transport in ground water was used to evaluate the saltwater intrusion. The results of the model indicate that saltwater encroachment has occurred in the southern part of the plain where the groundwater depletion is primarily caused by sustained groundwater pumping. Conclusions To safely manage the groundwater of the area، the GMS model was conducted to quantitatively and qualitatively explore the Shabestar aquifer. The results indicated that the storage depletion is occurred in the two parts of the plain: one in the central part of the plain where the water samples were taken from the production wells show high values of electrical conductivity، and another in the northwestern part. In fact، salty solutions entrapped in the immobile zone migrate toward the production wells due to the over-extraction of groundwater. In the area، salt water intrusion can occur by two mechanisms including molecular diffusion and mechanical dispersion. The distribution of the contaminated (salinized) zone can be explored by drilling of exploration wells. In order to prevent the progressive incursion of salt solution into the aquifer، we suggest to determine the annual safe pumping rate in the aquifer