Posted on April 25, 2001
Is Jakarta Free from Sea Water Intrusion?
By : Soetrisno
S.
On April 2nd, 2001, Kompas daily written that Jakarta is free from sea
water intrusion. Despite it's published a day after April's Fool,
hopefully the news is true. Hence, Jakartans may take a breath a while, in
the mid of hard living today, that their wells are threat free from sea
water intrusion and sustain its water supply. It's that really like this
the current groundwater condition in Jakarta?.
Sea water intrusion in Jakarta, particularly in the
northern part closed to coast line, it seem have been a something clasical
as a debate amongst the groundwater experts. Therefore, before it gets to
the answer of the question as the title of this paper, firstly will be
described the groundwater condition of Jakarta.
Geological Setting
As far as groundwater is concerned, groundwater experts oftenly referred
Jakarta as a sytem of groundwater basin. The base of the system is
underlain by impermeable rock of Miocene age (about 70 million years
before present), partly croped out in south of Jakarta. near Depok
and Ciseeng. Basin filling comprised of Quaternary deposits (about 3
million years ago), mixture of marine, deltaic, lagoon, fluviatile
deposits and volcanic fan consists of sand,clay,silt and tuff. During
Holocene on that Quaternary age Java sea level rose and flooded over
Jakarta plain, reached around Gambir and deposited shallow marine
sediment. Since it was deposited in marine environment, part of the
sediment has high salinity content. This content affected salinity content
of water therein.
The basin filling has thickness of 0 m in the south and more than 300m
in the north close to shore line. It means part of it is under basement
of present Java sea at Jakarta bay. Vertical cross section of this basin
filling is dominated by clayey layers, while sand layers occupied only 20
% of total filling. A single sand layer has a thickness of 1 m to 5 m
(Soefner, 1985).
Groundwater setting
The horizontal permeability of rock layers was found
to be between 0.1 m day-2 and 40 m day-2. The
vertical permeability is estimated as varying between 1/100 and 1/5000
of the horizontal permeability. It means horizontal groundwater flows
basically is not so fast, and vertical flows either. This will define
the distance and areal of sea water distribution from shore line.
Referring to its geological setting, it's
difficult to define a single system aquifer which can be trace
continously its distribution. Therefore several hydrogeology zones are
introduced, based on its permeability and
the depth. (Soefner,1985; Jabotabek Water Resources Management Study -
JWRMS,1994). Each zone consists of aquifer, aquitard, and aquiclude
intercalation.
Aquifers in upper 40 m zone known as a shallow
aquifer system. Groundwater occurred in unconfined aquifer, an
auifer where no impermeable boundary in the upper part. Groundwater
pressure in that aquifer is equal with air pressure. Since there is
no top impermeable layer, this type of aquifer is vulnerable to
contamination and easily suffer from sea water intrusion in coastal
area. Groundwater from this system is usualy utilized by mostly Jakartans
through dug wells and driven wells.
Aquifers in the zone of 40 to 300 m known as
deep aquifer system. Groundwater is occurred usually under confined
condition, where both top and base of aquifer is sealed by
impermeable or semi permeable layer. Therefore, groundwater
therein is underpressure and its pressure is higher than air
pressure outside. Consequently, if a drilling encountered the aquifer in
favourable location, groundwater will be flowing out freely without
pumped it out. Such conditions were common thing found in Jakarta
until the seventies. Due to its aquifer properties, groundwater from this
system is generally protected from contamination, including from
sea water intrusion. Groundwater is currently utilized for
industries, hotels, offices, apartments and real estates by
drilling deep wells equiped with submersible pump.
Under natural flow conditions, the recharge area of
the deep aquifer system was situated in the undulated area at
elevations between 25 and 200 m. Discharge from the confined aquifer
to the natural base level in the flat coastal area occurred mainly by
upward leakage, evapotranspiration and outflow to the surface water
system. Today, recharge to the deep aquifer system, other than horizontal
inflow from south of Depok (less than 1 million m3 year
-1 (Schmidt et al, 1985), may occurr throughout the city
area by downward leakage, as head levels of the confined aquifer
system have dropped regionally between 2 and 4.6 m year-1
below the water table of the unconfined shallow aquifer system.
It means that in the initial state shallow groundwater is supplied by
the deeper system. Therefore, until the seventies, water table is near
surface and wells never dried up even during long dry season. In the
contrary, after the increasing water abstraction correspond to
increasing population and industries, shallow water recharges to deep
groundwater system. It affected to shallow wells are currently deepened
its water table and oftenly dry up during dry season. Other negative
impacts are deep groundwater is being vulnerable to contamination from
shallow system and sea water intrusion in area close to the coast.
Based on depositional environment of rock forming
aquifer or aquitard, groundwater which may be occurred during the
sedimentation (connate water) or after the deposition of rock
water bearing, has a high salinity content. Result of water
analysis since 1920 up to now from various location in Jakarta shows
Cl- content of more than 600 mg l-1
(Soefner,1986). However, the analysis proved also an increasing
Cl- content correspond to time.
Therefore, groundwater is found salty or brackish in
various location in Jakarta since its origin is a true
statement. But is also true that salty or brackish groundwater just
happened after excessive groundwater abstraction. The problem is, what
is the main cause of increasing groundwater salinity. It will be
described below.
Salt/sea
water intrusion
Saline water in aquifers may be derived from any of
several sources:(Journal Hydraulics, ASCE, 1969)
Saline water intrusion is an event where
saline water enter and displaces or mixes freshwater in an
aquifer. Saline water intrusion may produce groundwater
unused and wells should be closed or abandoned.
Based on that definition and source of saline
water, sea water intrusion me define as salt water intrusion
coming from sea water, therefore it could only be occurred in
coastal areas. Whereas salt water intrusion may be occurred
anywhere, even in inland areas.
Basically, intrusion may only be happened where an
action is exist, which is a groundwater abstraction. Intrusion is a
reaction from those action and it changes natural hydrostatic balance of
interface between fresh and saline groundwater.
There were Badon Ghyben a Dutch researcher and
Herzberg a German researcher, around 1889 and 1901, working
independently along the North Sea coast. Both researchers found that salt
water occurred underground not at sea level but a depth below sea level of
about 40 times the height of the fresh water above sea level. This
distribution was attributed to a hydrostatic equilibrium existing between
the two fluids of different dencities. The equation derived to explain the
phenomenon is generally referred to as the Ghyben-Herzberg relation after
its originators. This natural equilibrium may change when the head
pressure of the fresh groundwater changed due to over pumpage, hence, the
new equlibrium is occurred, where saline water pushed interface inland,
and sea water intrusion phenomenon is started.
In Jakarta case, referring to the above
understanding, it is ensured that sea water intrusion is already
occurred. The evidences are :
Firstly, as mentioned above that the result of
groundwater analysis from both shallow and deep system since prior
to 1920 up to now, shows an increasing of Cl-
content. Cl- is generally found in low content in
groundwater under normal condition. Main sources of Cl-
are wastes, conate water and sea water. The fact is that
groundwater in various places in Jakarta, particularly in the
northern part, at least in the north of a virtual line connecting areas of
Kapuk - Grogol - Gambir- Cempaka Putih and Sunter, rise its
salinity and change from fresh to brackish or saline.
Second, intrusion is occurred as a reaction to
lowering of water table and piezometric head from shallow and deep
aquifers system of Jakarta. As it is mentioned, the declining of
piezometric head reached 2 to 4.6 m year-1. In several
places water level is more than 40 m below surface. Lowering
groundwater head has changed the hydrostatic balance of interface
between fresh and saline groundwater in the Jakarta basin. It can
not be denied, there was an evidence that groundwater abstraction in
Jakarta for many purposes is steadily increasing. In 1995, groundwater
contribution to Jakarta water supply was about 250 million
m3 year-1. This amount of water particularly was
abstracted from innumerable wells (80 %) and more than 3,000 deep wells
(20 %). Between 1900 and 1950 groundwater abstraction was below 10 million
m3 year-1 but since that time. mainly after 1970, it
has steadily increase. In the year 1994, deep groundwater abstraction was
estimated 53 million m3 year-1 which was about 50 %
higher than that of registered wells . It is difficult to get an exact
figure of groundwater abstraction in Jakarta, since not all wells are
registered by the responsible authority.
Back to the title of this paper, based on
mentioned descriptions, then the answers are :
CLosing
Remarks
However, the most important thing is that steadily
increase of salinity and Cl- content of groundwater in
Jakarta is an evidence. Lowering groundwater continously due to steadily
increase of water abstraction is also a fact. If such conditions
persist without any efforts to halt it, then within a short time
Jakarta environment may become worst than present. Groundwater
scarcity, contamination, land subsidence which may cause flood lasts
longer, might be happened, soon.
Change of recharge pattern where at present
shallow groundwater supplies to deep groundwater system, causing
deep groundwater is vulnerable to contamination.
Short, medium and longterm efforts to overcome
those problems may include:
Groundwater occurrence could not be separated with type,
structure and distribution of lithology of aquifer. Therefore,
to understand groundwater of Jakarta, it's required to understand
its geological setting which describe type, structure and
distribution of those rocks.
Basin filling formed aquifers, aquitards, and
aquicludes. An aquifer is most utilized its water by drilling
or digging wells to tap this layer. Jakarta basins comprised of
intercalations between aquifers,aquitards, and aquicludes. Therefore,
aquifers in Jakarta basin is offently referred as multi layer aquifers.
A sea water intrusion/encroachment terminology covers basically a
narrower meaning than salt water intrusion, sice saline water is
not merely coming from or as sea water. Saline water is all
water having a high salinity value. The degree of salinity is
commonly reflected by total dissolved solids (TDS). Free
groundwater generally has TDS less than 1000 mg
l-1, while saline or brackish groundwater its TDS more
than that value. Chloride (Cl-) occurrs to only limited extend
in groundwater under nornmal condition. High Cl- content is
generally found in saline water. Saline water is a common contaminant to
groundwater.
Author opines that the existing or non existing of sea water intrusion
in Jakarta is more heavy to the debate amongst groundwater experts
concerning the origin of groundwater salinity or Cl-
content, both in shallow and deep groundwater system. Both sides had
their strong arguments.