Posted on March 15, 1999
By: Soetrisno S.
ABSTRACT
Urban Development in North Bandung, where its part is main groundwater recharge area for the Bandung basin, directly affects to the groundwater recharge of the basin.
How many those influences are still need further investigation. However, referring to the result of recharge investigation in urban areas which is conducted in Mexico, Bolivia, and Thailand, proved that urban development was not always decreases its groundwater recharge.
Technical engineerings should be introduced as an intergated part of urban development in North Bandung, in order to maintain total amount of groundwater recharge of post-urbanisation equal or greater than pre-urbanisation recharge (SRp ³ SR0), as applied also to its quality (QRp ³ QR0), hence, groundwater resources are sustainable.
These technical engineerings include determining of North Bandung area for development ability, and its Building Coverage Ratio (BCR), recharge well constructions, retarding basins development, centralised drainage and sewerage system availability, recycling of water uses, gardening and greenery programs, and the existing of water supply system using source other than groundwater.
Supervisions and controls of urban development in North Bandung should be strictly conducted. However, since the negative impacts due to excessive groundwater abstraction in the Bandung basin are an evidence, therefore any controversial issues which appeared due to urban development in North Bandung does not necessarily neglect supervisions and controls of groundwater in the Bandung basin itself.
__________________ *)
1. BACKGROUND
Groundwater is one of renewable natural resources. However, its renewal time is depend on the rate of recharge, which might be occurred within seconds to milleniums. Many groundwater users have misunderstanding regarding to the renewable characteristic of groundwater, which may lead unproper conducts and tend to spoil it in using of groundwater resources.
It is happened also in using groundwater in the Bandung basin. Observation results show that degradation of groundwater is an evidence, which is caused by misunderstanding of renewable characteristic of groundwater and negative impacts of the developments and man activities on land surface.
It is a nature that any action will create a reaction. An action such as development on land surface will result reactions, one of it to the natural resources in subsurface, among other is groundwater resources. Therefore it is understandable that real estate developments in North Bandung will give impacts to the groundwater resources in the Bandung basin.
The problems are how the negative impacts which might be appeared could be minimised, hence, in one side urban development could be implemented (as long as the policy of the local goverment of the West Java province allowed), whereas groundwater resources in the Bandung basin are still sustainable its usage in other side.
This paper intends to discuss on groundwater resources in the Bandung basin and impact of urban developmment in North Bandung to the recharge and groundwater in the basin, in order to convey the proper understanding about a renewal characteristic of groundwater and efforts which should be conducted to maintain the sustainability of groundwater in the basin.
It's better in the beginning of this paper to be agreed upon which part of the Bandung is defined as North Bandung. Honestly, the author does not exactly know which area is included to the North Bandung. However, referring to the Decree of the Governor of West Java Province, No. 181/1982 and information from the West Java Development Planning Board, then area of North Bandung includes of Bandung Regency and City of Bandung which has elevation of 750 m above surface or higher, and lies in the north of 750 m contour line.
2. GROUNDWATER RECHARGE
Referring to Purbo Hadiwidjojo, et al, 1987, groundwater recharge may be defined as process, natural or artificial, by which water is added from outside to the zone of saturation of an aquifer, either directly into a formation, or indirectly by way of another formation.
Recharge is the result of those process. In case of groundwater in the Bandung basin, groundwater recharge may derived from rain fall, rivers, groundwater system itself, water channels, drainage system, another water bodies, and man made activies through recharge wells.
Recharge to the shallow aquifers is generally a direct process, either natural or artificial, which lasts immediately or at the latest within weeks. Therefore, it can be understood that after rain falls in the basin, hours or days later, dug or shallow wells which are tapped shallow aquifers will increase its water. Hence, recharge process last directly and immediately.
Recharge to the deep aquifers system occurs either direct or indirect. Direct recharge process takes place in the main recharge area, while indirect recharge process occurs almost in the entire of the basin. This condition due to actual stage of piezometric head which lies under phreatic head in almost of the whole basin. Its mean that firstly recharge occurs to the shallow aquifers, then due to head diferrence, groundwater from the shallow system will induce to the deeper system, through downward leakage. In the present situation of the Bandung basin, phreatic head lies between 10 and 20 m below surface, while piezometric head is between 40 and 75 m below surface.
To determine exactly groundwater recharge area is not a simple thing. Referring to the geometry and origin of aquifer lithology, then northern part area of the Bandung basin may be defined as main recharge area.
Analyses of stable isotope (d18O and d2H) of 44 groundwater samples from wells and springs in the Bandung basin, proved that the recharge area is located in the northern part of the basin at elevation of between 1050 and 1300 m above sea level (Geyh, 1990). Time rate of groundwater flows is in order of decades to 700 years, while result of isotope 14C analyses show that residence time of groundwater relatively becoming younger to the westward of the basin. In eastern part (Cicaheum) residence time is 10,670 years before present (BP), while in the western part (Cimahi - Cibereum) becoming younger, about 940 years BP (Geyh, 1990).
Those figures proved that even groundwater is renewable natural resources, however, the length of its replenishment time is dependable.
Sukrisno and Warsono, 1991, calculated the total amount of groundwater flows from its recharge areas at 1000 m contour line in North Bandung to the basin was 52,9 millions m3/year.
Haryadi and Schmidt, 1991, calculated the water balance for the entire basin :
groundwater flows from the whole recharge areas = 116,5 millions m3;
recharge from the shallow system = 77,9 millions m3/year;
recharge from river and surface run-off = 267,7 million m3/year.
If we go back to the agreement in the beginning of this paper, then part of North Bandung area lies in recharge area. Total recharge is strongly influenced by physical conditions of recharge area, which includes :
topography;
morphology (slope inclination, etc.);
geology (structure, lithology, etc.);
hydrogeology (permability, etc.);
soil coverage;
precipitation;
hydrology (river system, run-off, etc.).
Therefore it can be understood, that any changes in the physical conditions will affect to the groundwater recharge rate into aquifers. Even the human behavior is basically giving significant influences, not only to the groundwater recharge quantity, but to its quality, too.
3. IMPACT OF URBAN DEVELOPMENT IN NORTH BANDUNG
Urban development in North Bandung is mostly in the form of housing complex and tourist resort development. It's mean includes all activities which change physical conditions of respected areas, such as topography, landscape, soil cover, and surface run-off system.
Referring to the recharge influence factors above, then it can be understood that those urban development will bring an impact to the groundwater recharge into the Bandung basin.
The question which might be rose afterward is how big is those influences to the recharge rate. In efforts of sustainable groundwater usage in the Bandung basin, with the assumption that the development was allowed, then recharge rate pre and post development should be maintained equal or even higher,
where Rp = post development recharge;
R0 = pre development recharge (initial stage).
Back to the above question, does the impact decrease or increase the recharge rate?. It is often that we think the development on surface or in more specific meaning, urban development, decreases the recharge rate. It is caused of surface impermeability occured due to build with concrete, building coverage, and road paving.
However, in the contrary, it is also oftenly true, where recharge rate under urban areas is substantially higher than pre-urbanisation (Foster et al, 1993). Sources of additional recharge are a huge water supply imported from well fields at suburb or surface water, leaking water mains, on-site sanitation and drainage system, soaking road system, and channels (Morris, et al, 1994). Study on three different urban areas done by Foster et al (op cit) proved that post-urban recharge is higher than pre-urban recharge (Table 1). It should be noted that groundwater quality may be deteriorated due to quality of recharge sources.
Table 1. Urban Recharge for Three Cities Studied
|
City |
Population
(cap/ha) |
Rainfall (mm/a) |
Pre-urban
(mm/a) |
Urban Recharge (mm/a) |
Increase in
(%) |
Major Sources of Urban Recharge |
|
MERIDA (Mexico) |
35 |
1000 |
100 |
600 |
600 |
a) leaking water mains
b) on-site sanitation c) pluvial drainage |
|
SANTA CRUZ (Bolivia) |
45 |
1200 |
170 |
260-290 *) |
150-170
|
a) leaking water mains
b) on-site sanitation c) pluvial drainage |
|
HAT YAI (Thailand) |
140 |
1900 |
Ca. 180 |
240 *) |
130
|
a) leaking water mains
b) waste water and urban drainage disposal to ground |
*) does not included leakage from surface water courses
Sumber : Foster et al, 1993
In case of urban development in North Bandung, the conditons as the result of above study might be happened. In Indonesia as long as writer knows, there is no such study has been conducted. However, referring to the result of the above study, at least we have to think about with a cool head to respond North Bandung urban development case, where its main issue is substraction of infiltration rate will arise , and groundwater recharge rate to the Bandung basin decreases eventually. The most important thing is how efforts should be conducted, to maintain
SRp ³ SR0, if government policy allowed development in that area. Its mean that technical engineerings should be applied, in order to keep an area has recharge function and groundwater usage in the Bandung basin are sustainable.
The activities of the development in that area at least will include cut and fill, excavation, soil coverage through paving, building construction, build with concrete, asphalting, beside gardening and greenery.
From those activities, if we connected to the influence factors to the recharge rate, then there will be changes to the factors of topography, morphology, soil covers and surface run-off system, whereas precipitation is assumed to be relatively constant. Due to the factor of change above, therefore it can be understood that SR0 also will change.
From the soil covers factor only for instance, if the Building Coverage Ratio - BCR, ratio between area of soil which is covered by building to total of land area, is 30 %, its mean at least 30 % of precipitation on those land, which formerly has possibility to infiltrate into soil and partly recharges to groundwater, will be obstructed to infiltrate and will only flows as surface run-off.
If an average of 25 % precipitation in the Bandung basin will infiltrate into soils (Haryadi and Schmidt, 1991), then with above BCR value, there will be a possibility of recharge rate change of :
where :
Rp = recharge after soil coverage;
R0 = initial recharge;
BCR = Building Coverage Ratio;
P = precipitation.
Referring to the above equation, it is easy to understand that without technical engineerings, then SRp < SR0, and it will create an unsustainability of groundwater usage in the Bandung basin.
In other side, additional housing complexes and may be also tourist resorts will certainly increase waste deposits, either solid or fluid. If there was no available facility for treatment before discharges it to water courses, it will deteriorate groundwater recharge quality. Therefore it should be efforts to keep post-urban groundwater recharge quality equal or even better than pre-urban recharge.
In a simple way, it can be written as :
where :
QRp = post-urban recharge quality;
QR0 = pre-urban recharge quality.
4. EFFORTS
From the discussion above it is clear from the groundwater recharge point of view, that the problems of urban development in North Bandung is how to maintain:
QRp ³ QR0
For those purposes, technical engineering efforts should be applied, includes:
To determine development ability area of North Bandung based on its morphologic, geologic and hydrogeologic conditions;
To define BCR value of developmnet ability area as determined in point 1.;
Recharge wells construction as an obligation for any building, as long as hydrogeologic condition is favorable;
To create ponds or artificial lakes in every site of housing complexes or tourist resorts. It is an obligation by law, and should be conducted as far as hydrogeologic condition of that area is admissable. The magnitude of those ponds or artificial lakes is determined by existing land area, beside those above condition.
Ponds and lakes, apart of its function as retarding basin which govern surface run-off, have role also as source of recharge, as long as its hydrogeologic properties allowed.
Apart of that, ponds and lakes may also act also as source for raw water supply to fulfill water demand of housing complex/ tourist resort. Lakes may also can be used as water recreation facility.Waste water system and sewerage system should be centralised. Waste water and sewer water should be treated before discharge to water courses.
To apply recycling process and to introduce water efficiency in using of groundwater.
Gardening and greenery in every housing complexes and tourist resorts.
Source of raw water for water supply should not be used groundwater resources (incl. springs water), but surface water, as long as surface water sources are favorable.
Those technical engineerings should be integrated part of urban development in North Bandung. The consequences, in economic term, are the cost of that development increases , since part of that cost will be used for overcoming of any degradation of the environment. The cost is an investor expense, in order to maintain its development estate free of problems and environment friendly. However, the cost will be eventually overburdened to the consumers.
Those efforts should be accompanied with supervisions and controls by responsible institutions, and its implementations are truely obeyed by those who are obliged, to comply laws and regulations, and technical hints.
5. CLOSING REMARKS
Urban developmnet in North Bandung will certainly create an impact to the recharge and will eventually affect to the groundwater resources in The Bandung basin in general.
The controversies which are risen (if any) regards to groundwater, technically could be solved by introducing technical engineerings as integrated part of the whole development in North Bandung. By which, the sustainability of groundwater usage in the Bandung basin is ensured.
Those controversies should not neglect the existing groundwater problems which occurred in the Bandung basin itself, which is degradation of its quantity and quality, in both shallow and deep groundwater system, due to overexploitation as the consequences of the development in industrial sector and rising of population.
Those degradations are reflected in continuously declining of groundwater head and intrusion of contaminants into groundwater. These conditions should draw more attention and our efforts, rather than make a fuss about impacts of the development. The developmnet itself is still in planning stage or at least in early implementation stage, which is logically easy to be supervised and controlled.
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Acuan:
1. Foster, S.S.D., Morris, B.L.,and Lawrence, A.R., 1993, Effects of Urbanisation on Groundwater Recharge (in Groundwater Problems in Urban Areas), ICE International Conference, London.
2. Geyh, M.A., 1990, Isotopic Study in the Bandung Basin, Indonesia, Project Report No.10, Directorate of Environmental Geology - German Environmental Geology Advisory Team for Indonesia, Bandung.
3. Haryadi, T. and Schmidt, G., 1991, Groundwater Quantification for Four Urban Development Areas, Groundwater Flow Simulation Model, Bandung, Bundesanstalt fuer Geowissenschaften und Rohstoffe, Hannover.
4. Morris, B., Lawrence, A.R., and Stuart, M.E., 1994, The Impact of Urbanisation on Groundwater Quality, British Geological Survey, Nottingham.
5. Sukrisno dan Warsono, S., 1991, Laporan Pengumpulan Data, Evaluasi, dan Pengembangan Konsep Model Airtanah Daerah Bandung, Jawa Barat, Directorate of Environmental Geology, Bandung - Federal Insitute for Geosciences and Natural Resources, Hannover,
6. Purbo Hadiwidjojo, M.M., Guritno, I., Murdiyanto, D., dan Martodinomo, M., 1987, Kamus Hidrologi, Departemen Pendidikan dan Kebudayaan, Jakarta.
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