مطالعه عوامل كنترل‌كننده شوري در آبخوان آبرفتي دشت مند،استان بوشهر

Assessing the Controlling Factors of Groundwater Hydrochemistry in Mond alluvial Aquifer, Bushehr

در اين تحقيق عوامل موثر در شوري منابع آب دشت مند در استان بوشهر مورد مطالعه قرار گرفته است. در اين دشت هدايت الكتريكي چاههاي بهره‌برداري بين 1727 تا بيشتر از 6000 ميكرومهوس بر سانتيمتر در تغيير است. بهره‌برداري از اين آبخوان در دو زون مجزا از هم باعث برقراري بيلان منفي شده است. عوامل گوناگوني مانند، سازندهاي انحلال‌پذير در اطراف دشت، وجود گنبد نمكي، نفوذ آب شور دريا و برگشت آبهاي مصرفي از جمله عواملي هستند كه مي‌توانند منجر به كاهش كيفيت آب در اين دشت شوند. با استفاده از نتايج آزمايش‌ فيزيكي و شيميايي مربوط به 28 حلقه چاه بهره‌برداري، نقش هر كدام از اين عوامل در شوري منابع آب مورد تحليل قرار گرفته است. از روشهاي آماري چند متغيره در ارزيابي مشخصه‌هاي كيفي استفاده شده است. نتايج نشان داده‌است كه رخساره هيدروشيميايي غالب (Ca,Mg-SO4) و مشاهده شده در 84 درصد از نمونه‌هاست. تحليل خوشه‌اي، منابع آب را در 2 گروه مجزا قرار مي‌دهد كه منطبق با زون‌هاي بهره‌برداري در آبخوان است. نتايج تحليل عاملي نشان مي‌دهد كه 3/83 درصد از تغييرات كيفي آب با 3 عامل كنترل مي‌شود. عامل اوًل و مهم‌ترين عامل، انحلال كاني‌هاي سولفاته موجود در سازند‌هاي گروه فارس در اطراف دشت مند است. انحلال هاليت در گنبد نمكي موجود در شرق دشت و نفوذ آب دريا، به‌عنوان عامل دوم در نظر گرفته شده است. عامل سوم را ميتوان به انحلال گازكربنيك در آب نفوذي به آبخوان دانست كه باعث تغيير در ميزان بيكربنات شده است. در زون بهره‌برداري شمال دشت عامل دوم تاثيرگذار نيست.

In this research, controlling factors affecting salinity of groundwater in Mond aquifer, Bushehr province, was investigated. Hydrogeochemical modeling of the waters draining an aquifer not only can characterize their spatial and temporal variations, but also can be useful in understanding the interaction between groundwater and the aquifer matrix. These types of investigations have proved useful techniques and different methods of studying the chemical composition of groundwater that provide an understanding of the main factors which contribute to groundwater salinity. Composite diagrams have been used as a tool to interpret groundwater chemistry data in various parts of the world. The saturation indices (SI) for minerals most representative of the rock can serve as a useful tool in understanding the interactions between groundwater and host rock. The calculation of saturation indices has been performed by a number of researchers. Groundwater is a critical resource on the Mond Plain as it is the main source of water for agriculture. The electrical conductance of groundwater is more than 6000 JIS in some parts of the Mond Plain. This paper deals with characterization of the chemical composition of groundwater from supply wells in the Mond Plain aquifer. The factor analysis was applied to illustrate the dominant processes contributing to the chemical evolution and high salinity of the groundwater. Study area The Mond Plain lies in the southeast of Bushehr City, south of Iran, extending between 28°, 04-28°, 28TST and 51°, 14-51°, 46ʹE. The main population centres of the study area are Kaki and Bordkhon cities. Geologically the Plain area is sited over the Zagros tectonic zone forming between two parallel anticlines directed northwest to the southeast. Namak and Derang Anticlines are respectively located in the northeast and southwest of the Mond plain. Derang anticline is completely an outcrop of Aghajari formation (Marl, sandstone) with some outcrops of Bakhtiary Conglomerate in the margins of the anticline. Namak anticline mainly consists of older formations such as, Bangestan Group (Massive limestone, Dolomite and Shale), Pabdeh-Gurpi formation (Shale and Marl), Asmari-Jahrom formation, karstic carbonate, Gachsaran formation (Anhydrite, Gypsum), Mishan formation (marl with layers of limestone) and a salt dome in the east part. The climate is arid with an average annual precipitation of 221 mm. The monthly temperature ranges from 15.2°C to 38.6°C E-mail: m_rezaei@tmu.ac.ir The Mond Plain aquifer and its vadose zone are hosted by Quaternary sediments. These sediments are coarse grained in the hillsides, containing gravel and sands, and the grain size reduces towards the central parts of the plain, changing to very fine sand, silt and clay with salt, gypsum and marls in the centre ad main parts of the aquifer. Corresponding author: Tel; 09124108554 Groundwater is abstracted via two well fields. Well field 1 (Kaki Zone) is in the vicinity of Kaki city and well field 2 (Bordkhon Zone) is close to Bordkhon city. The bulk of abstraction is used for irrigation. Annually more than 23 MCM of water discharge from about 627 wells via the two well fields. The result of water balance calculation indicates that the outputs of the aquifer have been more than the inputs by 5.2 Mm3/year in the recent years. Methodology 28 water samples were collected from water wells. Measurements of temperature, pH and conductivity were carried out in the field. Major elements (i.e., Na+, Ca2+, Mg2+, K+, Cl~, S042", HC03", N03") were analyzed in laboratory using standard methods. By the utilization of composite diagrams, the relations between different ions were identified. The statistical technique of multivariate analysis was used to characterize hydrochemical processes through data reduction and classification. The factor analysis derived principal components from a correlation matrix and rotated axes with a quartimin rotation. Discussion of Result Most of the groundwater samples are highly saline with electrical conductivities ranging from 1727 to 6190|xs. There is a trend from Sulfate dominant to chloride dominant type water. The ranges, arithmetic means and standard deviations for cations, anions and physical parameters have been presented in Table 1. According to hydrochemical modeling using PHREEQC software, groundwater samples fall into three main hydrochemical groups: 84% in a Calcium-Sulfate group, 12% in a sodium-sulfate group and 4% in a sodium-chloride group. This range of graphically classified water type can then be taken to indicate a water-rock reaction history. Table 1: physico-chemical properties for groundwater samples and their statistical features EC PH T.D.S K Na Mg Ca so4 CI co3 HCO3 Mean 3495 7.54 2072.4 14.1 319.5 211.3 471.6 1997.3 412.1 5.1 156.8 Std Deviation 1080.2 0.34 662.1 12.7 226.2 74.1 172.2 725.3 357.7 20.4 62.8 Maximum 6190 8.29 3710 67.7 979.8 370.9 687.2 3300 1479 99.3 371.5 Minimum 1727 6.95 1036 5.1 104.9 84.8 127.6 799.7 70.6 0.0 92.7 Cluster analysis was carried out which resulted in two groups of water samples on the basis of variables (i.e., major elements). Group 1: low salinity samples (800