عنوان مقاله [English]
Groundwater is one of the essential resources for supplying requirements of drinking water,
agriculture and industry. This resulted in establishment of civilization in plains and lands where
are far from rivers of freshwater.
Regarding low amount annual rainfall in Iran and loss of water as a result of evaporation,
volume of water production is about 128 billion cubic meters. From this volume about 83
billion cubic meters form surface water resources and 45 billion cubic meters join the
groundwater by penetration. In recent decades, the growth of population and the need for water
resources have led to an increase in exploitation to these essential sources hidden in the heart of
Study area of this research is located in southern coast of Caspian Sea in Guilan Province.
The research problem is that in some wells the coefficient of pressure drop shows negative
figures whereas in some other wells these figures are positive. It is not clear that what is the
effect of these changes on exploitation of water resources of the aquifer and how much this
effect is and in what field it may be. This research tries to recognize the factors that affect this
phenomenon besides recognizing the effective reason in negative coefficient of pressure drop in
aquifer and lining of wells and determining scientific results and its applied effects.
In this research, field method and using step back test has been applied to find the reason of the
negativeness of pressure coefficient in available wells in the plain. Thus, through providing
necessary conditions, digging operation and pumping of more than 600 deep wells have been
supervised and controlled. More than 300 regions were directly controlled. Geographical
features in some of sample wells in the region were taken and the selected points were specified
on the map by using ArcGIS. Statistics and information related to geographical and
geomorphologic conditions and geology of location of the wells and testing scope of pumping
in the selected wells were collected and was analyzed in the form of graphs by SPSS program.
Then, the results were analyzed and all wells were classified in separate groups and those with
similar situation in the view of pressure drop coefficient were assigned in the same groups.
Finally, the selected wells have been classified in three different groups.
In the next step, the features of wells in each level such as geographical and
geomorphological conditions, geology of constituting sediment layers in location of the wells,
texture of surface water and layers in location of them, technical and expert characteristics of
the company, the quality of digging and supervising system were all evaluated and compared.
Common aspects of well was distinguished in each group. Then, after recognition and regarding
the common aspects of the wells belonging to each group it had been concentrated on the wells
which had negative coefficient of pressure drop.. Being sure about the result, besides repeating
considerations on the ground, they have been controlled and evaluated and in some cases the
test has been performed again.
Results and Discussion
Doing pumping test for determination of hydraulic features of groundwater aquifer and well is
one of the usual methods. In this case, the pumping test has been done in two different methods:
one is pumping by fixed rate (shuttle test) for determining aquifer coefficient and pumping by
varying rate (step pumping) for determining well's coefficients. Therefore, the well's coefficient
specification has been possible just by using the result of pumping with varying rate (step
pumping). So, in this research by paying attention to the importance of the coefficients, the data
of step pumping is used for determining features and situations of well and aquifer. Before
starting test first the water level in the wells has been measured. Then, by doing the operations
of digging and tubing of a well, washing and exiting of the fine-grained materials remained in it
has been done by pure water and pumping method. After providing suitable condition for water
arrival from aquifer layers into the well, water pumping operations has been continued with
different engine turn until the water level became fixed. During this period water level was
measured based on a predetermined timetable. The process of water level changes and its
constancy was distinguished and from the results (result related to the first step of the test) the
coefficient related to aquifer has been computed. After being sure about the constancy of the
water level (reaching the water level to the dynamic level), well's rate and in another word
engine's turn has been increased and the act of measuring water depth from the surface up to the
constancy of water level in well has been continued as before. This plan has been implemented
at least in four steps with 4 different rates as 4 engine's turns. In the next step, the result of step
pumping was analyzed and after finishing field operations and recording figures it was
abstracted in related table.
After designing the mentioned table, the coefficient of pressure drop in aquifer (B) has been
obtained by using the curve of especial drop to the rate that the mentioned crossing point of the
curve connect with the widths axis with drawing the related curve. From obtaining pressure
drop in aquifer, the pressure drop in lining network of well (C) was obtained by using y2-y1 (x2-
x1) that is, in fact, the slope of especial drop curve to the rate. Finally, after calculating B and C,
the table of brief result of pumping is provided.
Considerations show that available problems in taking groundwater in Guilan Plain is related to
the nature of the region in terms of sediment texture and hydrologic characteristics of the plain,
humanistic factor, the performance of feeding, unsuitable quality of digging and also
exploitation methods. The groundwater feeding is through different sources such as rivers and
networks of irrigation channels, floodgate and pools of growing fish that are extensively in the
plains, and also free aquifers situated on semi-confined aquifer that have hydraulic balance
state. These sources were useful and can have positive function at increasing the age of
economic exploitation of a well. However, due to unfamiliarity of most experts with the
mentioned phenomenon and varying performance of this phenomenon in different seasons, the
wells affected by the mentioned elements will be faced with damages and sometimes it is so
severe that may destroy the well.
Considering the effective factors on the pressure drop of hydraulic data in Guilan's Plain by
pumping method indicates that the step back method is better for determining hydraulic
coefficient of well. It is suggested that to use this pumping method instead of step pressure.
Because for determining the features and real situation of wells and aquifers especially in plains
and coast region, implementing step pumping through step drop is suitable. The data gained
from pumping which has been done on more than 600 wells were tested by step drop and
approved these results. The lack of observing technical principles for digging leads to the
increase of pressure drop in well's lining network and causes a speed at the entrance of water to
the wells. Therefore, this will change the physical conditions of water. This may lead to
transformation of sodium bicarbonate soluble in water to insoluble carbonate which by
sedimentation on the well's lining and general pack of the back of lining's tube and tracks of
lining network (shell investing) will increase pressure drop of lining network. Moreover, it
decreases discharge of the wells and intensifies shell investing action which leads to a decrease
in economic life time of exploitation.