Variations in some groundwater characteristics, Belawai Water

Geol. Soc. Malaysia, Bulletin 26, April 1990; pp. 109 - 132
Variations in some groundwater characteristics, Belawai
Water Supply, Sarikei Division, Sarawak.
MOHAMMED HATTA ABD. KARIM
Geological Survey of Malaysia
Kuching, Sarawak
Abstract: A systematic monitoring programme has been carried out on the water
supply in the Belawai catchment area since August 1987 to gather information and data
on the quality and quantity of the water as well as the trend of domestic water demand;
the information is essential for the proper development and management of the groundwater system of the Belawai water supply.
The aquifer in the Belawai Catchment Area is shallow and underlain by beach sand
and marine sand.
The groundwater in general flows from southern part of the catchment area to the
north. However, the water flow changes according to the recharge-discharge relationship
of the catchment area. Recharge of the groundwater in the catchment area is mainly
from precipitation and groundwater in flow. The response time is short where the
rainwater available reached the water table almost immediately. Discharge is mainly by
the abstraction of water from the production wells, evapotranspiration, groundwater
outflow and surface drainage. During the dry season, the total discharge is more than
the total recharge resulting in the water supply being supplemented by the aquifer
storage. The effect of water use in the catchment area aquifer changes according to the
pattern of abstraction and this groundwater table at the beginning.
The quality of water changes with every well and at different months. There
appears to be relationship between chloride contents with the total rainfall. From the
trilinear diagram, water quality changes becoming less satisfactory when the total
recharge decreases.
Abstrak: Kajian pemonitoran yang bersistematik. telah dijalankan semenjak Ogos
1987 keatas Kawasan Tadahan Punca Air Tanah Belawai untuk memperolehi dan
mengumpul data-data kuantiti dan kualiti air, juga jumlah permintaan penduduk;
maklumat ini sangat diperlukan untuk pembangunan dan pengurusan yang berkesan
kepada Punca Air Tanah Belawai.
Akuifer di Kawasan Tadahan Belawai adalah jenis terdedah yang cetek di mana
ianya terdiri dari pasir pantai dan pasir laut.
Air tanah pada amnya mengalir dari selatan ke utara Kawasan Tadahan. Walau
bagaimanapun aliran air akan berubah bergantung kepada pertalian antara imbuhan
dan luahan air kawasan tadahan. Punca imbuhan adalah dari penyerapan air hujan
dan larian air tanah. Masa sambutan adalah pendek dimana air hujan didapati sampai
ke paras air tanah dengan kadar yang segera. Punca luahan adalah dari pemampaan
perigi pengeluaran, evapotranpirasi, larian air tanah dan larian permukaan menerusi
parit. Semasa musim kemarau, jumlah luahan adalah melebihi jumlah imbuhan yang
mengakibatkan punca air terpaksa di ambil dari simpanan akuifer. Kesan kegunaan air
dalam akuifer Kawasan Tadahan didapati berubahan mengikut corak pemampaan dan
keadaan paras air tanah pada peringkat awal.
Presented at GSM Annual Geological Conference '89
110
MOHAMMED HATTA Aim.
KARIM
Kualiti air berubah pada setiap perigi dan pada bulan yang berlainan. Didapati ada
perta1ian antara kandungan klorida dengan jumlah hujan yang turun. Dari rajah
triliner, kualiti air berubah menjadi kurang memuaskan apabilajumlah imbuhan berkurangan.
INTRODUCTION
A systematic monitoring programme has been implemented at the Belawai
Water Supply since August 1987 to gather information on the seasonal variation in the groundwater quantity, quality and trend of domestic water demand; the information is essential for the proper development and management of the groundwater system of the Belawai Water Supply (Hatta, 1988;
Yogeswaran, 1984). It would allow for immediate action to be taken for problems, such as saline water encroachment, pollution of aquifer and reduction in
well performance, that may be encountered during the present and future
stages of the development of the groundwater resource. As the groundwater
source of the Belawai Supply is from a shallow coastal sand aquifer, this
monitoring programme will also help to improve the understanding of groundwater resources in shallow coastal sand aquifers elsewhere in Sarawak.
BACKGROUND OF THE BELAWAI WATER SUPPLY
The Belawai Water Supply is. located at Kampong Jerijeh Baru, and is
about 35 km west of Sarikei (Fig. 1). Three villages namely Kampong Belawai,
Kampong Jerijeh Baru and Kampong Rajang, having a total population of
8000 people, are depending on this water supply scheme for their domestic
water requirement.
Preliminary investigation of the Belawai groundwater potential was carried out from 1977 to 1979. This was followed by the construction of 12 arrays
of wells during 1979 and 1980. In 1982, 20 more wells were constructed. At
present there are. 12 arrays of production wells which are represented by a
seven - well array (PW1), a five - well array (PW2), and and 10 three - well
arrays (PW3 - PW12). Figure 2 shows the layout of these well arrays.
The Belawai Water Supply Scheme, which is fully treated by aeration,
sedimentation, flocculation, filtration and chlorination, was commissioned in
1982. The wells are all located within an area of 900,000 m2 which has been
assigned as the water catchment area for the scheme. The production wells
are located in the northern part of the catchment area. The general layout of
the wells and the ground contour of the northern part of the catchment area is
shown in Figure 3. The production wells are located approximately 200 m from
each other.
VARlATIONS IN SOME GROUNDWATER CHARACTERlSTICS, BELAWA I WATER SUPPLY
III
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111° E AST
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10
20
40 km
20 Kilomet r es
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CHINA
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LEGE ND
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Be l owo;
Water
Supply
11 1° EAST
SMI790/3/88
MHK / RC
Figure 1:
Location of the Belawai Water Supply.
MOHAMMED HATI'A AIm.
112
PW
PW
( 7 - well
2
(5 -
KAruM
array)
well array)
5'
5'
5'
,'""
~
0
,>~Q~e
<?~o"
,
0
Moin production well
Subsidiary well
~
5'
Scale I: 400
o
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PW3 -
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PWl.2 (3 - well array)
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....... ___ ....~_
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Main Production well
Subsidiary
production well
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11
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Tube for cleaning horizontal
well
Horizontal well
I
I
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o
Scale I: 250
4210/11/88
Figure 2: Schematic layout of production wells in Belawai Water Supply.
MHK/HC
b.ll.U!Q
PW
Production Well
MW
Monitoring Well
p
Temporary Peg
-L,-
...,
<::
Treatment
Plant
>
2!:!
~0
Z
Access Rood
en
Z
Surface ,Woler Outlet
en
Catchment Area
0
- 3·00- Ground level 3·QOm above mean sea level
rs::
tTl
Note: MW7 is 900m north of PW4
Cl
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150
PWI2
200
250 ME TRE S
~
SP4213/11/6a
I"~,
MW4
>
til
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CI:l
c:
:g
r
><:
W
Figure 3:
General layout and ground contour of the Belawai Water Supply Catchment Area.
114
MOHAMMED HATTA ABD.
KARIM
GEOLOGY AND HYDROGEOLOGY
. The Belawai area is underlain by Quaternary sediments which comprise of
beach sand and other shallow marine sands rich in shells and broken wood
fragments. These deposits, which form the unconfined aquifer system of the
area, are underlain by estuarine silty sand, clay and gravelly clay.
The beach sand and the shallow marine sands offered the most promising
prospect for the development of groundwater in the area.
Figure 4 shows the cross-section of the Belawai area and its general
groundwater movement. The detail lithology of the catchment area is shown in
isometric fence diagram, of Figure 5.
THE REGIONAL HYDROLOGICAL SYSTEM
The groundwater generally flows from the groundwater-mound at the
southern part ofthe catchment area to the north and also to the northeast into
the nearby Drainage and Irrigation Department (DID) drain (Fig. 6).
The flow directions of the groundwater in the catchment area very according to the height of the water-level. Variations of water-level are dependant on
the recharge - discharge relationship of the catchment area.
There ate basically three distinct patterns in the flow directions of the
groundwater in the catchment area. When a large groundwater - mound, with
a water-table of greater than 2.7 m at monitor well MW5, develops in the
southern part of the catchment area; then the groundwater flows to the north
and to the drain, northeast of the area (Fig. 6). As the water-level drops, a
localised groundwater - mound is found around production well PW9 and
groundwater flows away in all directions expect to the south (Fig. 7). During
drier periods, and when the abstraction of water from the aquifer is maintained, the groundwater flows from the surrounding area into the catchment
area (Fig. 8)
The distinct patterns described above illustrate the progressive change experienced from dry period in September 1987 to the wet period in May 1988,
and the intermediate situation in July 1988. From these observations, the
need to monitor the groundwater levels cannot be overemphasized to avoid
contaminated or poor quality water from outside the catchment area to flow
into the production well areas.
GROUNDWATER RECHARGE
Recharge of the groundwater in the catchment area is mainly from precipitation and groundwater inflow. Table 1 shows, besides other details, the total
available recharge throughout the monitoring period.
V ARIATIO NS IN SOME GRO UNDWATER CHARACTERISTICS, B ELA WAI W ATER S UPPLY
115
Table 1: Summary of the development of Groundwater Resources, Belawai
Water Supply.
Abstraction
m 3/d
1987
Sep.
483
Tot. Available
Recharge m3/d
1822
Total
Discharge m3/d
Ditt. In Aquifer
Storage m 3/d
3216
-1394
Oct.
471
7547
3615
3932
Nov.
505
5166
3762
1404
Dec.
400
4365
3308
6057
1988
Jan
405
9821
2790
7031
Feb
476
3148
3384
- 236
Mac
460
6833
3473
3360
Apr.
462
4512
3595
917
May.
495
6020
4761
1259
June
493
2629
3568
- 939
July
499
2999
4153
-1154
Aug.
473
5094
5106
- 12
The precipitation and water-level data were plotted to study the recharge discharge behaviour in the catchment area. The precipitation data were collected from the rain-gauge at the treatment plant and the water-level was
recorded by a Stevens Continuous Wat er Level Recorder installed at monitoring well MWl. From Figures 9 and 10, the magnitudes of the water-level
change is not similar for different rainfall intensities, but the response time of
the rise in the water-level due to the recharge from precipitation was generally
short. This suggests that the rainwater reached the water-table almost immediately.
Figure 9 shows the water-level and rainfall data during the monitoring
period of August and September 1987. There was occasional rain during this
month. As shown in the figure the water-level dropped continuously between
29th August to 11th September 1987. This fall is generally attributed to the
extraction of water from the production wells. The daily rise in the water-level
during this period was at night when there was no pumping, and the effect of
the inflow is reflected during such times. From the monitoring period of
October and November 1987 (Fig. 10), the catchment area experienced more
recharge mostly, from the rain. The rise in the water-level on the 18th and
19th of October, 1987 is due to contribution from groundwater inflow.
,....
Q\
RECHARGE
RECHARGE
o
BH42
~
BH35
PW6
PWI
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Boreholes used to
construct sections
E===~OC
· 2====O~'.~=30
. 6Km
Di rection of
groundwater flow
MHK/AB
SP4211/1I/88
Figure 4:
Cross - section of the Belawai Area with general movement of groundwater.
117
VARIATIONS IN SOME GROUNDWATER CHARACTER ISTI CS, BELAWAI WATER SUPPLY
LEGEND
~
~
Sand
~
Silt
~
Clay
..
177.7)';1
~
SCALE
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am
Sam
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Silty sand
MS L
PW3
Peat
MSL
~-. -. ""'~7-
Mean sea leve l (MSL)
Lithological boundary
.
MSL
PW7
MSL
PW8
MSL
MSL _ _- t -
MSL
SP4222/11/88
Figure 5:
MSL
MHK /A8
Isometric fence diagram showing the subsurface geology of the Belawai Water
Supply Catchment Area.
.......
00
Treatment
Plont
LEGEND
PW
Production Well
MW
Monito ring
Well
P
Temporary
Peg
c::::::::t:>
©
MWI
Direction of groundwater
flow
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,
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PW8
2 , 60
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0
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0
PWII
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--------
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.0
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SP4215/ 1I /88
,00
,.0
0
PW7
0
PWI Q
0
S CALE
0
~
PWI2
200
250 ME TR E 5
©
MW4
Figure 6: Belawai Water Supply static groundwater level as on 14.5.1988.
~
<:
»
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:;
;:j
o
Z
en
@
Z
MWI
en
LEGEND
PW
PrOduCtion Well
MW
Monitoring
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Well
Tempora ry Peg
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~ Direction of
flow
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z
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r
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MW4
MHK/R ·C .
......
\D
Figure 7: Belawai Water Supply static groundwater level as on 15.7.1988.
.-.
N
o
LEGEND
PW
Pfoduclton Well
MW
Monitoring
Well
Tempo(ary
Peg
c=t:>
/
Direction of groundwoter
flow
~
-:
0 /
PW2
7'
/
~
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o
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P3
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t:l
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i
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N
t
o
SC ALE
'0
'00
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PWI2
200
2~O METRES
3
SP4217/ II/BB
Figure 8: Belawai Water Supply static groundwater level as on 18.9.1987.
V ARI ATIO NS IN SOME GROUNDWATER CHARACTERI STICS, B ELAWA I WATER S UPPLY
2 ·300
2 ·2 5 0
;;;
>
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oJ
~
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z
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2 -000
1-950
1-900
1·850
1·800
20
25
AUG UST, 1987
SP420S / II / 88
30
10
15
SEPTEMBER , 1987
MHK/AB
Figure 9 : Relationship between rainfal l data and water-level record
during August-September 1987 monitoriJ;lg period.
121
122
MOHAMMED HATTA ABD. KARIM
2 -850
2 · 800
2 ' 7~O
..J
"'>
~ 2 ·700
<t
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"'<t
"''"
Z
~
>
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2 -600
0:
"'
i
2·550
2 · 450
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..J
<t... E
;:; E
<t -
o:
00
25
0
10
SP4205/11/88
15
20
OCTOBER. 1987
I
NOVEMBER,I9B7
MHK/AB
Figure 10: Relationshtp between rainfall data and water-level record during
October-November 1987 monitoring period.
V ARI ATIONS IN SOME GROUNDWATER CHARACTERISTICS, B ELAWAI W ATER S UPPLY
123
GROUNDWATER DISCHARGE
Groundwater in the catchment area is discharged mainly by the abstraction of water from the production wells, evapotranspiration, groundwater
outflow and surface drainage.
Table 1 shows the rate of abstraction of the groundwater during the
monitoring months. The abstraction rates were quite similar except during
drier months and during festive seasons when more water was needed. The
average abstraction rate was 469m3/d. The relationship between the monthly
total available recharge and the total discharge is shown in Figure 11. For five
months the amount of discharge was more than the available recharge, especially during the drier months of August 1987, January, May, June and July
1988.
The relationship between the water level measured at monitor well MW5
and the abstraction rate is shown in Figure 12. The water level showed a
prominent inverse relationship with the abstraction rate during the dry season, but this relation is masked by high recharge during the wet season.
VARIATION IN AQUIFER QUANTITY AND QUALITY
From the contours of the difference in the water-levels between two consecutive months, it was observed that there are some variations as pumping
pattern changes. These variations depend not only on the amount of water
being pumped from individual wells but also on the initial groundwater level of
the catchment area.
From the contours of the difference in water-level for June and May 1988
(Fig. 13), it is clear that substantial amount of water was taken out at the
eastern and western corner of the catchment area. This this was due to the
large volume of water being abstracted from wells PW2, PW8, PWll, PWI0
and PW7. By plotting the difference in water-levels, the management was able
to react immediately and wells PW6 and PW9 were pumped in order to
distribute the impact of pumping to the whole of the catchment area.
Due to the initial existing low water-level around PW9 and PWI0, the
difference in water-levels for April and March 1988 (Fig. 14) showed a depression around these two wells even though all wells were pumped uniformly.
Figure 15 shows the contours of the difference in water-level for January
1988 and December 1987, where only the wells on the northern half of this
sector of the catchment area had been utilised to extract large volume of water.
The effect ofthe DID drain can also be seen by the steep gradient ofthe watertable in that area of the catchment area.
From the contours of the difference in water-level for May and April 1988
(Fig. 16), it can be seen that the water held in storage has increased as there
124
MOHAMMED HATrA ABD. KARIM
10,000
o:
totol
discharge
R : totol availoble recharge
,
,
I
___ r--
-I
I
L ____ •
D
200
__ __ ,
___ J
,L ___ ,,
R
A
s
o
N
D
F
M
1987
A
A
M
1988
s
D
N
D
Figure 11 : Discharge - recharge relationship' of Be lawai Water Supply.
.;
WOfer level
3-0
Of
MW 5
..
~ 2-0
600 ~
Abstraction rote
400
'·0
200
1987
1988
Figure 12 : Abstraction rate and water-level at MW5 in Belawai Water Supply Scheme.
~
PW
P roducTi on Well
MW
Monito ring Well
;J>
Tempo rary Peg
:;;:
<
;<J
-;
(5
Z
en
Waler level difference (m)
o
<-0·15
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- 0 ·15 -
o
®
Z
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en
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s:
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Volume of wole r abst racted :
0- 500m'
•
A
•
;<J
0
c
PI
z
MW8
500-1000 m'
1000- 15 00 m'
1500- 2000m
•
m
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m
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."
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2~ O ME TRE 5
r
3
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@
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MW4
MHK/R
.c.1
>-'
N
Figure 13 : Belawai Water Supply: Wa ter-level difference June - May 1988.
Ul
LEG EN 0
PW
Prod ucti on Well
MW
Monitoring Well
P
Temporary Peg
Treatment
Ptant
Waler level diffe rence (m)
o
\
\
< - 0 ' 15
D
- 0 ' IS' - - 0 '20
D
- 0 ' 20 - - 0 ' 2S
@
MWI
...
>-0' 2 S
..
/
c;>PWJ
Volum e of wa ter abst rac ted·
+
o.
0-500m3
PW2·
•
SOO-IOOOm'
.. 1000 - ISOO m'
•
1500- 2000m '
•
> 2000 m'
I
PI
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t:4
.
. - /
..
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. PW6
N
t
.
o
PW I2
'0
I~ O
200
250 ME TRE S
- 0 ' /5
@
S P4219/66
MW4
Figure 14: Belawai Water Supply: Water-level difference April - March 1988.
LEGEN D
PW
Prod uct ion We ll
MW
MonilorinQ Well
Treatm ent
Plant
-<
;J>
;<l
Te m po rar y Pe g
:;
--l
Wat er leve l difference (m)
o
o
o
0
Z
<-0 ' 15
en
Z
- 0 '15 -' - 0 ·20
en
0
-0'20 - -0'25
5]
s:
m
0
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C
> -0,25
Vol ume of wole r abs t rac te d ;
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Z
..
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A 100 0- 1500 m'
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o
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r
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200
-C
-C
2 5 0 ME TR E S
r
3
S P42 21/ 1I / 88
n
Y'
~
SC AL E
'00
•
PW9
PWII
N
' 0
M W5
©
-<
M W4
......
Figure 15: Belawai Wa ter Supply: Wa ter-level difference December 87 - J a nuary 88.
N
-....l
.......
N
00
LEGEN D
PW
P rod uction We n
MW
Monitor ing Well
P
Tempo ra ry Peg
Treatment
Plant
Wote r level difference (m)
D
<- 0'15
@
D
- 0,15 _-0,20
o
-0 · 20 - - 0·25
I2SJ
MWI
> - 0·25
O·
Volume of wa ler abstrac ted;
~
00
0
~
/
+
a -500m3
•
50Q-IOOOm 3
..
1000-1500m'
•
1500- 2000m'
•
> 2000m'
P2
0
MW9
P3
0
MWIO
+
o
PW6
~
tJ
~
~
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~
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•
PWII
N
t
.
>0
o 10
•
o
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SP4220/ II/88
Figure 16: Belawai Water Supply: Water-level difference May - April 1988.
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VARI ATIO S IN SOME GROU NDWAT ER CHA RACTERI STICS, B EL A WA I W AT ER SUPPLY
129
was a rise in the water-levels. The only exception was the area surrounding
production wells PW1 and PW2. where more water was pumped. Similar
results would be obtained for those months which have rainfalls higher than
the rainfalls of the preceeding months.
The quality of water from the aquifer varies at every well as well as at
different months. Figure 17 shows the variation of the chloride content at well
PW2 with respect to the average amount ofthe rainfall for the period of August
1987 to July 1988. The value of the chloride content varied from 2 to 75 ppm. It
shows that there is an inverse relationship between rainfall and chloride
values of the groundwater from the aquifer. Generally the value of chloride
content varied from <1 ppm to 680 ppm. Well MW6 and occasionally wells PW1
and MW7 h ad chloride values greater than 200 ppm. The higher chloride
values are due to the presence of connate water around these wells .
The values of the total iron of all the wells vary from 3.1 to 26 ppm. Wells
PWll, PW12 and MW4 were having values between 22 to 26 ppm.
The range of total solids is from 62 to 1734 mg/ l. Wells MW6 and MW7 had
total solids greater than 1000mg/1, whereas wells PW1, PWll, MW4 and MW7
had value for total solids ranging from 600 to 1000 mg/ l.
Using a trilinear diagram plot, the variation of the overall quality of
groundwater in the Belawai Water Supply area can be demonstr ated quite
effectively. Generally, the quality drifted from 'not satisfactory' to 'good' as the
amount of available recharge was increased. Figure 18 shows the general
behaviour of the groundwater quality based on information obtained from two
months having a large difference of total available recharge. The total available recharge was the lowest in September 1987 (1822 m 3/d), and was the
highest in January 1988 (9821 m 3/d). The groundwater in September 1987 may
be classified as a calcium - sodium - chloride - bicarbonate water but, as
available recharge was increased as in January 1988, the groundwater becomes a calcium - sodium - bicarbonate water.
CONCLUSION
From th e systematic monitoring programme of the Belawai Water Supply
Scheme, variations of some groundwater characteristics were observed.
The shallow unconfined aquifer of the Belawai Water Supply catchment
area is sensitive to its recharge - discharge behaviour. During dry period,
continuous monitoring is necessary so as to avoid problems such as saline
water encroachment and pollution of the aquifer. The pumping pattern has to
be monitored to avoid excessive pumping in certain areas of the catchment
area.
The monthly abstraction rates are quite similar and are sufficient for the
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Figure 17: Relationship between rainfall and chloride content at PW2.
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VARIATIONS IN SOME GROUNDWATER CHARACTERISTICS, BELAWAI WATER SUPPLY
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Quality field from production wells - September 1987
(Catchment area total recharge : 1822 m3 / d )
Quality field from production wells - January 1988
(Catchment area total recharge : 9821 m3 /d)
Figure 18: Trilinear plot of hydrochemical data for two extreme conditions in Belawai Water
Supply Catchment Area
132
MOHAMMED HATIA ABO. KARIM
present domestic needs. Though the quality of the groundwater varies with the
seasons, it still is suitable for the domestic use.
ACKNOWLEDGEMENT
I would like to express my thanks to Pengarah Penyiasatan Kajibumi,
Sarawak, Mr. Chen Shick Pei, for giving permission to publish this paper, and
to Mr. Victor Hon and Mr. Yogeswaran Mailvaganam for their stimulating
discussions and critical reading of the manuscript.
REFERENCES
MOHAMMED HA'ITA A.K., 1988. Pemonitoran Punca Bekalan Air Belawai. Jabatan Penyiasatan
Kajibumi Malaysia. Lapuran bulanan dari Januari 1988 hingga Ogos 1988 (Unpublished).
YOGESWARAN M., 1984. Hydrogeological Report of the Belawai Area, Sixth Division, Sarawak.
Geological Survey of Malaysia (Unpublished).
Manuscript received 25 May 1989