نوع مقاله : مقاله پژوهشی

نویسندگان

هیدروژئولوژی، دانشگاه صنعتی شاهرود

چکیده

بررسی رابطه الگوی جریان و کیفیت آب از عوامل مهم کمک‌کننده در مدیریت کمّی و کیفی آب‏های زیرزمینی است. تحقیق حاضر به منظور استفاده از شواهد هیدروشیمیایی و ایزوتوپی در تعیین الگوی جریان آب زیرزمینی در آبخوان شاهرود و بررسی آثار هیدروژئولوژی بر کیفیت آب منطقه انجام گرفت. جهت جریان کلی آب در آبخوان شاهرود از مناطق شمال‌شرقی به  جنوب‌غربی است. به وجود آمدن یک خط تقسیم آب در منطقه باعث ایجاد تغییراتی در جهت جریان کلی آب در برخی بخش‏ها و انحراف جریان به سمت شرق شده است. بررسی‏های هیدروشیمیایی 120 نمونه آب نشان داد در این آبخوان پیوستگی ژئوشیمیایی، به‌ویژه در بخش جنوب‌شرقی، وجود ندارد؛ به گونه‏ای که بخش شرقی و بخش جنوب‌شرقی اختلاف کیفیت فاحشی با بخش‏های شمالی، غربی، و مرکزی نشان می‏دهد. جریان ورودی قوی‌تر از مرز شمالی همراه خط تقسیم آب زیرزمینی باعث جدا افتادن و ساکن شدن آب‏های شور جنوب‌شرق منطقه شده‏ است. در نتیجه، به مرور زمان، تغییر تیپ آب از سولفاته به کلروره در این توده آب نسبتاً راکد رخ داده است. جدا افتادن جریان آب در بخش جنوب‌شرقی باعث شده آب شور آن تأثیر چندانی بر شیمی آبخوان نگذارد. آنالیز 3 نمونه تریتیوم وجود حداقل دو منبع تغذیه با سنین مختلف در آبخوان را اثبات می‏کند. نمونه‏های شور جنوب‌شرقی با میزان تریتیوم کمتر از TU 8/0 نشان‌دهنده آب نسبتاً قدیمی تغذیه‌شده قبل از سال 1952 است؛ درحالی‌که آب شیرین شمال منطقه با میزان تریتیوم TU 2/2 آبی است که به‌تازگی تغذیه شده است. سن بیشتر آب در بخش جنوب‌شرقی به دلیل عدم‌ارتباط هیدرولیکی با آب‏های جوان، سرعت کمتر جریان، و نیز عدم‌خروج آب قدیمی از این بخش از آبخوان به دلیل کم بودن میزان بهره‏برداری است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Hydrochemical and Tritium dating Evidence in Understanding the Hydrogeological Pattern of Shahrood Alluvial Aquifer

نویسندگان [English]

  • Zahra Buslik
  • Hadi Jafari

Hydrogeology, Shahrood University of Technology

چکیده [English]

Introduction
One of the main factors in quantitative and qualitative management of groundwater in different regions is investigation the relationship between flow pattern and water quality, in other words, studying the effects of hydrogeology on aquifers hydrochemistry. Meanwhile, the Shahrood aquifer is one of the most distinctive aquifers in showing hydrogeological effects on aquifers hydrochemistry. The Shahrood alluvial Aquifer, is a single-layered and unconfined aquifer which its main inputs are recharge from rainfall, sewage return flow from urban areas, agricultural return flow and underground water flow from adjacent karstic aquifers. The diversity of recharge sources of Shahrood aquifer and the distribution of exploitation wells have caused complexity in the pattern of groundwater flow in this aquifer. The general flow direction of groundwater in the Shahroud aquifer is from the northern, eastern, and southeast regions to the central, western and southwest areas. But the creation of a water distribution line in the eastern and southern parts of the region has caused changes in the overall flow of water in these parts. So that the flow takes direction to the east, around the villages of Royan and Hussein Abad. The more distance between potential lines in the southern part indicates lower hydraulic gradient and, consequently, less flow rate in this part of the aquifer. The purpose of this study is useing of hydrochemical and isotopic evidence in determining the pattern of groundwater flow in Shahrood alluvial aquifer and also to study the effects of hydrogeology on water quality and geochemical anamolies in the region.
 
Method and material
 For hydrochemical studies, 120 samples of groundwater were analyzed at Shahrood University of Technology. Also, 3 samples were analyzed for determining the amount of tritium isotopes at the University of Ottawa.
 
Discussion  and Result
Hydrochemical studies were showed that the EC value in shahrood aquifer varies from 671 μs / cm in the north of the aquifer to 12000 μs/cm in the east and southeast. There is no geochemical continuity in this aquifer, especially in the southeast. In other words, there are two different salinity anamolies in the Shahrud aquifer. These anamilies are related to the eastern-northeast and southeastern parts of the aquifer, that show a significant difference in the quality rather to the north, west and central parts. For example, the EC in sample W109 is 9000 μs / cm, but near and upstream this sample, in W59 sample, EC is about 1200 μs/cm. The groundwater type is sulfate, bicabonate and chlorine in the eastern entrance to the central northern, western parts and southern areas, respectively. Bicarbonate samples are related to karstic Formations in the north of the region. While the emergence of sulfate and chlorine types are associated with the dissolution of marl and gypsum outcrops in the east and south eastern parts. Given that Marl and Gypsum Formations are the same in eastern and southeast regions, it is expected that their water type to be similar and mainly sulfate-type. But the all waters are chlorine in the southern part of the aquifer. This can be due to the longer resident time of water in the southern part and the achievement of geochemical evolution due to reaction with aquifer rocks over time. In fact, the largest volume of inflow and recharg in the Shahroud plain comes from the north and Bastam Plain. The volume of this input flow is very impressive rather than the volume of the inflow from the south-east of the region, and it prevents the entering flow of southern saline water, and reduces the velocity or stagnation of the inflow water from the southeastern border. This strong inflow from northern boundary, along with the water distribution line in the southeastern part of aquifer, causes separation and isolation of saline flows from the south-east of the region. Consequently, over time, changing the type of water from sulfate to chlorine occurs in this stagnant water mass. The southern saline water has not much effect on the Shahrood aquifer chemistry due to the stagnant or slow water flow in the southeastern part. In fact, the hydrogeological control has an important role in changing the water type in the Shahrood aquifer. According to tritium analysis, there are at least two different recharge sources in the aquifer. The southern saline samples with a tritium content less than 0.8 TU indicate relatively old water fed before 1952, While the freshwater from the north of the area with a tritium content about 2/2 TU, is a mixture of relatively old water and modern water. The southeastern waters to become older, due to the lack of hydraulic communication with young water and the less velocity of water flow.

کلیدواژه‌ها [English]

  • Hydrogeochemical discontinuities
  • Hydrogeology
  • Hydrochemistry