Introduction and background
The
Kathmandu Valley is one of the largest intermountain basin built in the
Himalayan range, which includes three ancient cities of the Kathmandu, Lalitpur
and Bhaktapur (see fig 1). The ancient spouts (Dhungedhara) and dug wells
spread in different part of the valley indicate that the inhabitant of the
valley were dependent on groundwater since historic time. So at present,
groundwater accounts for about 50% of the water supplied by the Nepal Water
Supply Corporation in the Kathmandu Valley (ENPHO, 1999). This dependent of
municipal water supply on groundwater shows the significance of potential
aquifer identification techniques and good well design practice.
Due
to the uncontrolled growth of population and unplanned urbanization; the daily
water consumption rate has been rapidly growing up. In this context the lack of
sufficient and suitable surface water resources to meet this increases demand
of water, the method of maximum possible utilization of groundwater should be
established. Due to urbanization, surface infiltration has been vastly reduced
while consumption of groundwater is ever rising. In addition, the over
exploitation through excessive abstraction of groundwater (both shallow and
deep) resource exceeding its replenishment capacity in the course of
socioeconomic development has resulted to experience severe water stress
particularly in Kathmandu Valley. All these facts show that the groundwater
should be used with utmost percussion not to degrade the situation further.
With
this aim the Department of Drinking Water and Sewerage conducted Pumping tests
on some of the selected wells as part of the study in order to make estimates
about the long-term sustainability of the wells. R. M. Engineering Consultancy P. Ltd conducted
pumping tests on the wells..
Present Status of
Groundwater Abstraction
The
increase in use of groundwater in Kathmandu has to be analyzed in relation to
the development of the water supply system. A modern piped water supply system
was introduced in Kathmandu in 1891 to serve the nobility and the elite. The
common people were served with public stand-posts. In the 1950s, the pace of
building new systems increased, and in 1960 water from the Bagmati River (the
tailrace of the Sundarijal hydropower plant) was used to meet the growing needs
of the valley population. This nucleus has expanded over the decades into a
large municipal supply system, currently operated and managed by the Nepal
Water Supply Corporation (NWSC)/ Kathmandu Upatyaka Khanepani Limited (KUKL) in
Kathmandu Valley. The system exploits water from rivers as well as from a
network of wells tapping the lower, confined or semi-confined, aquifers of the valley.
Municipal
water production from surface sources has always remained in short supply in
the valley. To meet domestic water needs, tubewells were installed at Balaju
and Bode in 1961. Since then, the rate of groundwater extraction has gradually
increased to meet domestic as well as commercial needs. Increasing extraction
has, however, proved insufficient to meet the demand.
To
meet the water demand, KUKL (then NWSC) has tapped all major surface sources
within the valley. In 2004 the demand
for water supply in the valley was estimated at 294 MLD while the production
was only 145 MLD while 2009 data shows that the demand at 221 MLD and
production at 109 MLD (Table No. 1). The deficit is met by private sources
including private drillings and supply by tankers.
Table No. 1: Status of Water Supply in the Kathmandu
Valley
|
Description
|
1999
|
2001
|
2002
|
2003
|
2004
|
2010
|
|
Production capacity million liters per day
(MLD)
|
125.0
|
132.0
|
141.0
|
144.0
|
165.0
|
125.0
|
|
Water demand (MLD)
|
160.0
|
177.0
|
281.0
|
290.0
|
294.0
|
221.0
|
|
Average daily production (MLD)
|
105.0
|
112.0
|
120.0
|
124.0
|
145.0
|
109.0
|
|
Water leakage (waste) in %
|
38.0
|
37.0
|
37.0
|
37.0
|
36.0
|
30.0
|
(Source:
NWSC 2004, 2010)
Data on water supply and demand in
Kathmandu valley presented (Table No. 5.1) indicate facilities at NWSC/KUKL
cannot meet the demand; the average daily production is 109 MLD against the
demand of 221 MLD. Large portions of
supplied water are being met by private groundwater sources
The
objective of present study is also to determine the hydrgeological properties
of the aquifer. The specific aim is to recommend the safe yield of the wells so
that the tubewlls can be operated sustainably.
In
hydrogeology, a pumping test is a controlled field experiment in which a well
is pumped at a controlled rate and water-level response (drawdown) is measured in
one or more surrounding observation wells and optionally in the pumped well
(control well) itself. Aquifer test and aquifer performance test are alternate
designations for a pumping test. Although pumping test and pump test are often
used interchangeably, pumping test is the preferred term (Woessner and Anderson 2002).
The
goal of a pumping test, as in any aquifer test, is to estimate
hydraulic properties of an aquifer system. For the pumped aquifer, one seeks to
determine transmissivity, hydraulic conductivity (horizontal and
vertical) and storativity (storage coefficient).
The
pumping test was performed in the five tube wells. The location of these tube
wells is shown in the figure 2. Test
pumping commenced on Oct-nov 2012 , continued over 13 days and was completed on
11.03.2012. The discharge rate was regulated and the measurement was done
through flow meter. The pumping test was
conducted on the basis of a step-test and recovery test. Water levels were
measured during the pumping and recovery phase of the test in the using water
level indicator.Test
pumping data were analyzed using standard techniques. For the analysis purpose
, computer software aqua test was used.
The
Kathmandu valley consists of two series of geological successions; one is
quaternary, which overlies the lower portion of the valley; the other is
Precambrian to Devonian, which forms the basement and surrounds the Kathmandu
Valley. Several low hills are confirmed in the southwestern part of the valley
bottom. These hills are on the line connecting Naikap, Kirtipur, Chobar,
Thanagau and Magargau from the northeast to southeast. Many other mountain
ridges extend to the valley bottom from the surrounding mountains, implying
there are many buried ridges. The depth to the Precambrian bedrocks from the
ground surface range from several tens of meters to more than 500 m as
confirmed by electrical prospecting carried out by JICA (1990) and existing
well logs. The maximum thickness of the sediment is in the Harisidhi area where
bed rock has not been found even at the depth of 457 m below ground surface
(Gautam and Rao, 1991). But in some areas like Soyambhu, Pashupati,
Shovabhagabati, and Balkhu; bedrock are exposed at the surface also. The
thickness of the sediment increases gradually towards south and attains the
maximum thickness in the central and southern part of the basin.
Lithologs
obtained from various sources suggests that the coarse sediments occupy the
northern part of the basin, while proportion of fine sediments increases
towards central and southern part of the valley. The central and southern part
of the valley is covered by lacustrine deposits. The aquifer in this area is
confined by about 200 m thick impermeable clay deposits. The gentle foothill
area of the valley is covered by the alluvial fan deposits (Figure No. 3).
The
bedrock forming the basement and surrounding the valley are mostly consists of
carbonate rocks like limestone; calcareous sandstone, siltstone, phyllite,
quartzite and granite. These rocks are generally fractured and weathered. These
rocks can also form good aquifer.
Groundwater
Condition of the Kathmandu Valley
Aquifers
are the geological formation containing water and that are permeable enough to
transmit water through them to yield sufficient quantity of water to the wells
and springs. The ground water system of the Kathmandu Valley is considered as a
closed and isolated ground water basin, with more or less interconnected
aquifers. Depending upon the nature of sediments, the Northern, North-Eastern,
deeper parts (>90m) of the Central and Southern provinces fall under good
aquifer zones (DMG/BGR, 1998). Geologically, the deep aquifer horizon is the
basal gravel bed overlying the basement rock in the Southern part of the Valley
and is more or less continuous laterally.
The
aquifers zones mostly confine to the northern, northeastern and central part of
the basin. In the northern part, the aquifer zones lie from near surface to
great depth and are more or less continuous from east to west direction. The
central part of the valley consists of two aquifer zones. The first aquifer
zone lies near the surface at shallow depth. Most of the dug well in the valley
are dependent on this aquifer. Other aquifer lies at great depth below the
thick impermeable lacustrine clay deposit.
Ramesh
Gautam and G. Krishna Rao (1991) classified Kathmandu valley into 4 zones as
unconfined aquifer zone, two aquifer zone, confined aquifer zone and no
groundwater zone.
These
types of aquifer zones lie at north of Maharajgunj and Boudha and west of Gorkarna
extending upto western and northern foot hills of the valley. The area between
the Manohara and Bishnumati Rivers has been classified as interbedded aquifers
and treated as an unconfined aquifer zone. Medium to coarse grained sand,
gravelly sand and silty sand consitute the major aquifer materials forming
unconfined aquifers. Unconfined aquifers on the terraces in other parts of the
valley, which may have limited potentiality by virtue of finer grain size of
sediments, are not considered here.
This
aquifer zone lies at South of Maharajganj and Boudha, and West of Bode and
extends up to the Western and Southern boundaries. The aquifer is characterized
by the presence of thick Kalimati Clay wich acts as the confining impervious bed.
Coarse to very coarse sand, pebble, cobble and gravel are the chief
constituents of the confined aquifers which form the main aquifer system within
the Valley. The piezometric surface in the confined aquifer area deceases
towards the central part of the valley indicating that the flow direction of
ground water form periphery towards the middle part of the basin conversing at
the center. The piezometric head increases with the depth of the aquifer which
is in conformity with the hydraulic principle.
The
central part of the basin consists of two aquifer zones: Shallow aquifer at the
top and deep aquifer at the bottom. These two aquifer horizons are separated by
thick column of impervious sticky black clay. The shallow aquifers at about
1380 masl in the north central part at Dhobi Khola well field have been tapped
for municipal water supply (BGR/DMG, 1998). These shallow perched aquifers are
generally composed of clayey sand, silt, gravelly sand with limited local
extension. The thickness of the top shallow aquifer increases towards north and
northeast up to 44m while it is only 5m thick in the central part. And the
thickness of the bottom deep aquifer increases towards central part from 17m to
108m (Gautam and Rao, 1991).
The
southern, southeastern, and the southwestern part of the valley are covered by
inter bedded limestone, sandstone, shale, and siltstone. These rocks are highly
jointed, fractured and porous (limestone terrain).When they undergo intense
weathering, they become favorable for the formation of groundwater reservoir
with the development of underground drainage system. Some industries and
institutions have developed the exploration tube wells in the rock aquifers. The
sites in Syuchatar, Saukhel, along the foot hills of Kapan and Tokha are
considered to fall within the potential zone of rock aquifers. The areas along
the foothills of the southern part of the valley like Pharping, Thapagaun can
also be considered as the rock aquifer zone.
Bhaimal 1
The deep well at Bhaimal 1 was pumped at a discharge
rate of 10lit/sec during the constant-rate test. Data for the tests are
presented as a plot of drawdown versus time.Rapid drawdown occurred during
the first minute and was largely attributed to well losses. The shape of the
early part of the drawdown curve suggests casing storage may have affected
drawdown in the bore for up to about 13 minutes. The rate of drawdown was then
effectively stable after 360 minutes. The drawdown of the tube well at the
discharge of 10lit/sec is 8 m bgl. The rate of drawdown was effectively stable after
360 minutes. Analysis of data using the method of Theis method indicated transmissivity
of the aquifer was 1.9x101 m2/day. It should be noted that this is
the transmissivity in the immediate area of the well.
Bhaimal II
The deep well at Bhaimal II was pumped at a discharge rate of 10lit/sec during the constant-rate test. Data for the tests are presented as a plot of drawdown versus time in Figure 5 and in tabular form in annexes.
Rapid drawdown occurred during
the first few minute was largely attributed to well losses. The shape of the
early part of the drawdown curve suggests casing storage may have affected
drawdown in the bore for up to about 15 minutes. The rate of drawdown was then
effectively stable after 160 minutes. The drawdown of the tubewell at the
discharge of 10lit/sec is 6.5mbgl.The
pumping test indicates that the tansmissivity of the aquifer is 19m2/day
Sanothali
The deep well at Sanothali DTW
was pumped at a discharge rate of 10lit/sec during the constant-rate test. Data
for the tests are presented as a plot of drawdown versus time in Figure 6 and
in tabular form in annexes.The rapid drawdown observed
during the first few minute was largely caused due to well losses. The shape of
the early part of the drawdown curve indicates casing storage may have affected
drawdown in the bore for up to about 10 minutes. The rate of drawdown was then
effectively stable after 270 minutes. The drawdown of the tubewell at the
discharge of 1.5lit/sec is 9.42mbgl.The well parameters calculation
shows that the Transmissivity of the aquifer is 19m2/day
Sirutar Tubewell
The deep well at Sirutar DTW was
pumped at a discharge rate of 10 lit/sec during the constant-rate test. Data
for the tests are presented as a plot of drawdown versus time in Figure 7 and
in tabular form in annexes.The well parameters calculation
shows that the Transmissivity of the aquifer is 19m2/day and
Ichangu,
Nacidhoka
The speedy drawdown observed
during the first few minute was largely caused due to well losses. The shape of
the early part of the drawdown curve indicates casing storage may have affected
drawdown in the bore for up to about 20 minutes. The rate of drawdown was then
effectively stable after 270 minutes. The drawdown of the tubewell at the
discharge of 1lit/sec The well parameters calculation
shows that the Transmissivity of the aquifer is 61.98m2/day and hydraulic conductivity
is 3.51 m/day. Similarly the storativity of the well is 3.21x10-2.
In a step-drawdown test the well is initially pumped
at a low constant rate until the drawdown within the well stabilizes, i.e.
until a steady state is reached. The pumping rate is then increased to a higher
constant rate and the well is pumped until the drawdown stabilizes once more
A stepped-rate which is also known as step drawdown
test was also performed on the all five DTW, to determine the well drawdown for
several pumping rates. Specific capacity is the well yield per unit of
drawdown. It varies with pumping time generally decreasing the longer a well is
pumped. It also generally decreases as the pumping rate of a well is increased.
The
step drawdown test was carried in two to three steps in which first step was
run for 360 minutes and rest two steps for 180 minutes. Step 3 was the maximum
rate available from the test pump. The drawdown data from the stepped-rate test
are presented in Appendix.
Based
on these data, it will be determined the best suitable discharge that would be
the pumping rate used for the extraction of groundwater. The discharge fixed in
this way will be the highest rate that the well would be able to sustain without
bringing the pumping water level down below the top of the screens.
The pumping rates and specific
capacities are calculated and presented in Tables 2 for all the dtw in which
the pumping test was performed.
Table
2: Pumping Rates and Specific Capacities in the Stepped-Rate Pumping Test
|
Well ID
|
Discharge Rate (LPS)
|
Duration (min)
|
Max Drawdown
(meter)
|
Specific Capacity
(lit/m)
|
|
Bhaimal, Gothatar
|
10
|
360
|
8.25
|
1.21
|
|
12
|
180
|
12.45
|
0.96
|
|
|
15
|
180
|
17.75
|
0.845
|
|
|
Bhaimal,ii Gothatar
|
10
|
360
|
6.5
|
1.538
|
|
15
|
180
|
15.5
|
0.96
|
|
|
Sanothali
|
10
|
360
|
5.1
|
1.96
|
|
15
|
180
|
7.54
|
1.99
|
|
|
20
|
180
|
8.81
|
2.27
|
|
|
Sirutar
|
10
|
360
|
5.1
|
1.96
|
|
12
|
180
|
8.0
|
1.5
|
|
|
15
|
180
|
10.1
|
1.48
|
|
|
Ichangu
|
12
|
360
|
2.55
|
4.7
|
|
|
15
|
180
|
3.89
|
3.85
|
|
|
|
|
|
|
Ideally, wells should not be pumped at a high enough
rate to draw the water level down into the screens. It would be prudent to
design pump equipment for a lower pumping rate, to allow for changing aquifer
conditions. These well screens multiple aquifers, which makes determination of parameters for
individual aquifers difficult.
The SWL of Bhaimal I, Bhaimal II and Sanothali DTW are very
deep in the range of above 50mbgl except to that of Ichngu and Sirutar which
has only 6.5m and 15m respectively below ground level.. The drawdown in tube
wells are not very high except at The transmissivity at the Ichangu is high
where others have within the range of 20 m2 /day.
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