Posted on May 1st, 1999
- A commitment to sustainable development
By: Soetrisno S.
ABSTRACT
Environment and development, two words which are occasionally contradictory, as a result of two different interets. On one side development should be sustained for enhancement of the national welfare, whereas on the other a healthy environment is required for the benefit from the result of the development itself.
Mining of karstic limestone will certainly give impact to the environment. Since the karstic aquifers have already been explored for domestic and agricultural purposes, impact of mining is a great concern to those aquifers.
Karstic aquifers are prominent aquifers in Tuban area. In the Merakurak basin, these aquifers discharge 47.6 million m3/year and 12 million m3/year of those are naturally flowing somewhere to the Java Sea. A single well tapping these aquifers will yield 50 l/sec., with average spesific capacity of 167 l/sec. per m drawdown.
Understanding of the karstic limestone on their fissured system, karstified degree, hydrographic zones, and flow direction will give a significant contribution to the mining design, particularly to which certain depth excavation is still allowable. Karstification develope significantly on the surface as well as the sub-surface. The upper part of intensive karstification zones occupies elevation of 80 m above mean sea level (msl) in the sourthern part, declining to 10 m above msl in the northen part of the projected quarrying area. Zone of intensive karstification indicate the existing of transition zone and zone of permanent water flows of hydrographic zones in karstic limestone. These zones play basic roles in controlling karstic aquifers system and their groundwater flows.
Based on the hydrogeology characteristic of karstic limestones in the Merakurak area, quarrying of limestone should be conducted with single bench type and mining base level is not deeper than the zones which permanent water flows.
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1. BACKGROUND
The increasing demand for portland cement for construction purposes, requires the expansion of the production capacity of the existing cement factories or to build new cement plants.
In cement portland production, limestone is the major raw material. Million of tons of limestone are quarried by the cement industries. However, quarrying limestones certainly will harm the environment, unless environment oriented mining methods are applied, the goals of the development will not be attained, then the industry will not be beneficial to the welfare of the nation.
The Merakurak area in Tuban, East Java, yields the potential limestone resources for cement industry. It has to be noted however, that the limestones are karstic forming prominent aquifers which have already been exploited for domestic and agricultural purposes. Therefore, the mining of limestones will have a negative impact to the karstic groundwater resources in particular and to the environment in general.
In this respect, all effort has to be made to protect the environment while the construction of the new cement factory at Merakurak can be implemented for the social welfare, particularly for the people who are living surrounding the factory.
Understanding the behavior of karstic limestone aquifer of Merakurak will provide an insight to the mine planning. Therefore, hydrogeology plays an important role and put a valuable contribution in constructing proper mining design, a design which is taking the preservation of the environment into its considerations.
This paper discusses the hydrogeology of karstic limestones of the Merakurak area and its role in the design of limestone mining with the aim of attaining a sustainable economic development.
2. KARSTIFICATION
Situmorang, 1988, assigned the limestones in the Merakurak area to the Paciran Formation of Pliocene age. This formation consists of coral reef limestone, with abundant coral fragments, algae, and shells. The Miocene Tawun Formation which consists of claystones, underlies the limestones.
Karstification in various degrees exixts in the limestones which developed on the surface (exo-karst) as well as in the sub-surfeca (indo-karst).
Karst phenomena in this area includes underground streams, sinkholes, and open air caves. Drilling tests in the projected quarrying areas, disclosed the existence of sub-surface solution openings in the form of cavities at different depth. Water lost during permeability tests at several drillholes is also reported. which supported the evidences mentioned above. On the other hand, surface drainage is less dominant.
Fissures of various sizes are recorded at several locations which are thought to be associated with tectonism. Structure analysis using Rose diagram, Schmidt net, and photogeologic interpretation shows that the trend of the major fissure system in this area is approximately north to south and southwest to northestst directions. This trend apparently controls the groundwater flow in this area.
Referring to tCvijic's hydrographic zones in karst (in Mijatovic, 1984, three hydrographic zones (Fig. 1.) may be recognised in the Merakurak area, including:
Dry zone. The surface is charcterised by complete dryness; coves and widened fissyres of the zone conceal the flows only during the rainy periods. Appearance of ascending waters is rather absent here.
Transition zone, as Cvijic (op. cit.) points out, "shows two phenomenas, one of permanent another of periodic nature. In its fissures and caves the permanent water flow is noted, even the underground courses here and there. At the points where this zone comes in contact with relief surface, these groundwaters appear as springs whose yield varies considerably during the year". Here Cvijic allows occurence of ascending water courses, linking them with various karst depressions in which springs "work: only during floods.
The zone with permanent water flows is, in fact, a water saturated area in karst, and according to Cvijic's definition it is linked with Grund's "Grundwasser". Cvijic says that "water of this zone does not appear on the surface in the same proportion as water of higher zones. The major part drains towards the water body on impervious bed". Ascending flows appearing in this zone at the bottom of highest poljes, even in the deeper uvalas, are normal phenomenon.
In the karstic aquifer system, the three zones are closely interelated. Canges in the dry zone - either naturally or by human activities - will affect the groundwater setting in the zone with permanent water flows.
In the Merakurak area, each zone has various thicknesses. The dry zone is encountered at 25 m depth beloe the surface, and the thickness increases southward, reaching 100 m. The zone with permanent water flows in the projected quarrying area occurs at a depth of 100m in the sourthern part, and 50 - 15 m in the northern part approaching in the Merakurak plain.
It can be noticed, That the transition zone and the zone with permanent water flows correspond to the zone of intensive ckarstification, which can be distinguished from core drillings in the Merakurak area.
3. HYDROGEOLOGY
Since the Merakurak area is a karstic terrain, yhe occurrence and behavior of groundwater in this area is dominated by the karstic structure.
The deepest zone, e.g. the zone with permanent water flows, is the zone in which all groundwater flows will accumulate, since the karst water moves, predominantly vertically, through fissures and caves, observing the principle of gravitation, without regard to sea level. The decent of the karstic water will terminate at the impervious claystones of the Tawun Formation. The above three zones are not distinctly separated among themselves. One zone grades into another. The boundary between the zones is irregular forming a zig-zag line, most often of wavelike shape, depending on the degree of karstification.
The piezometric head of karstic groundwater in the Merakurak area varies in depth e.g. 20 m below the surface in the Merakurak plain, and deeper in the hilly area of the southern part. In reference to the sea level, the piezometric head is 8 m above msl in the hilly area and decreases to 2 m above msl in the northen part.
The karstic limestone aquifers apparently dip northward and are buried under alluvial clay deposit in the Merakurak plain,which is situated to the north of the projected limestone quarries. Since the limestones are overlain by clay deposits relatively thick (up to 20 m), the aquifers are considered to be confined.
The direction of groundwater flow correspond to the direction of the major structure which is approximately south - north. Isotope tracing studies would support this argument. It has been reported (Anonymous, 1988), based on the I-131 and Cr-51 isotope tracing results, that the projected quarries contribute to the groundwater in the discharge zone, but no correlation with the Merakurak vaucluse spring seems to exist.
The Merakurak area may be considered as a groundwater basin. The basin boundaries are controlled by the surface and sub-surface distribution of the impervious claystones of the Tawun Formation, which means that all geohydraulic events would take place within the boundaries.
The southern boundary of the basin coincides with the anticline axis exposing the Tawun Formation at Sawang village. The western and northern boundaries are represented by the outcrops of the Tawun Formation, whereas the Cringin river and the Java sea are the eastern and northeastern boundaries. There are no flows across the boundaries. The boundary between the recharge and discharge areas or the hinge line is defined by the 25 m contour line.
Recharge calculation in the basin gives 47.6 million m3/year groundwater across the hinge line to the discharge area. A part of this will be released naturally by springs and artificially through wells, with total amount of 35.5 million3/year. There is still approximately 12 million m3/year of groundwater discharge across the basin boundary to somewhere in the Java Sea. It seems to be a high figure which can be still applied for water purposes, however, it is important to be noted that those amount of groundwater flows are required to maintain the balance between the fresh and brackish groundwater interface.
Numerous wells have been drilled in the discharge area for irrigation purposes. A single well in average yield 50 l/sec, with an average specific capacity of 167 l/sec per m drawdown and high transmissivity of 5,000 to 7,000 m2/day (Anonymous, 1986).
4. CONTRIBUTION TO THE MINING DESIGNS
Based on our hydrogeological knowledge of the Merakurak area, some importance points can be noted that will be considered in defining pre-conditions in the mining of the karstic limestone in this area.
Karstic limestones are prominent aquifers which occurred in the zone with permanent water flows which is characterised by intensive karstification.
There is a close interrelationship between three hydrographic zones in the karst area. Change in the dry zone would almost certainly influence the groundwater balance in the deepest zone.
The upper part of the intensive karstification zone occupies elevations of 100 m above msl in the southern part, declining to 50 - 15 m above msl in the northern part.
The projected quarry areas are hydraulically related to the groundwater in the discharge zone.
Based on the hydrogeological characteristics as mentioned above, quarrying the limestones in the Merakurak area will exert a negative impact to the environment, such as the decline of the piezometric head, decrease of productivity of the aquifers, and the increase of groundwater turbidity.
In order to minimise the impact, it is recommended to the quarry of limestones as follows:
To maintain quarry base level above the zone of intensive kartification and at maximum depth of 78 m above msl;
To construct a drainage system and mud check dam in low lying areas;
In addition the following steps may be taken, in order to achieve the better results.
No mining activities should be carried out on the dry valleys;
To create artificial fracture system in the abandoned mining areas;
Replanting has to be done in unvegetated areas;
To use excavation sites for rain water collecting pools.
The detailed mining design has been elaborated by mining engineers , however, is not discussed it in this paper.
5. CONCLUSIONS
The hydrogeological factors have to be considered in the mining design. The hydrogeological balance can be maintained by means of the co-operation between hydrogeologists and mining engineers which will lead to a proper way in the management of karstic environment. On the one side, quarrying the karstic limestones is necessary to develop a cement industry, while on the other side, it is imperative to maintain the preservation of karstic water.
In general sense, hydrogeology contributes to mining design in its commitment to sustainable economic development, an effort for better living in a healthy environment.
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Reference:
1. Anonymous, 1986, East Java Groundwater Development Project, Volume 3, Ministry of Public Works, Jakarta.
2. Anonymous, 1988, Studi Karst Berair dan Tanah Liat/ Lempung di Kecamatan Kerek dan Kecamatan Merakurak, Kabupaten Tuban, Jawa Timur, Universitas Brawidjaja, Malang.
3. Mijatovic, B.F., 1984, Hydrogeology of Dinaric Karst, Heise Verlag, Hannover.
4. Situmorang, R.L., 1988, Geological Map of Jatirogo Sheet, scale 1 : 100,000, Geological Research and Development Center, Bandung.
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