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<Technology Needs for Lake Management in Indonesia - Investigation of Rawa Danau and Rawa Pening, Java> G. Catchment Management Affecting Water Quality Various physico-chemical and biological factors affect water quality in Rawa Pening. These include the area of critical land in the catchment, the extent of siltation in the lake, the amount of domestic waste coming into the lake, the amount of synthetic chemicals, toxic chemicals, and oil entering the lake, the accumulation of agricultural waste in the lake, the input of fertilisers from agriculture, and the accumulation of organic matter derived from water plants growing in the lake. These factors lead to conditions which can be recognised as eutrophic. In addition, seasonal changes in the volume of water coming into the lake and in the amount of evaporation and in the utilisation of water from the lake lead to changes in water quality. All of the above factors which affect water quality are dependent to various extents on events in the catchment. Such effects could be controlled by effective systems of total catchment management. Many of these factors are interactive and vary seasonally. For instance, the smaller volume of water in the lake during the dry season could result in a greater probability that the bottom deposits of the lake will be stirred and put into suspension by a particular wind strength on the surface which would not affect the deposits during the wet season. The higher water temperatures at some seasons will have an important effect on biological events at that time. Water temperature will not necessarily be higher during the months of December to March when the air temperature is higher, because at that season, rainfall is maximal and the incoming rain water during storms will be relatively cool. 1. Increase in the area of critical land Tanah Kritis or critical land refers to areas of land which are actively eroding or likely to erode. These are often areas of bare soil on hillsides or slopes. It is likely that throughout Indonesia the total area of tanah kritis is increasing each year, despite intensive efforts by the Government under the re-greening program. No data on the area of tanah kritis were actually available for the investigators in regard to the Rawa Pening catchment. Investigation of the catchment reveals that there has been, and still is, extensive development of hotels and tourist facilities in the catchment. Many of these are on steep slopes in areas at altitudes of 1,000m or higher. Such areas provide spectacular views and cool climates in hot weather. Little effort is made to control run-off carrying clay and soil from such building sites. Often such recently developed areas are associated with land areas which are no longer used for farming but for which the owner/developer has no immediate use. Such bare, unmanaged areas erode readily. Forest areas which have been logged or burned are areas of unstable soil and again these erode easily on the steep slopes. Land use and land capability maps show that much of the land surrounding Rawa Pening is rated as being very susceptible to erosion. The soils in the catchment are relatively recent volcanic derivatives, which are fertile but relatively unconsolidated and immature. They are therefore susceptible to erosion. The catchment includes many steep slopes on the sides of dormant volcanos such as Gunung Merbabu and Gunung Telomoyo. We can conclude that extensive erosion will occur in this catchment whenever the ground cover is destroyed. The extension of tanah kritis in this catchment is not directly due to the increase in population in Central Java but rather is it due in part to the increase in the living standard of the people. Greater numbers of people can now afford weekends on holiday. More people are able to go on bus tours for a day to mountain areas. The development of tanah kritis is directly related here to the increased socio-economic status of the population and cannot be isolated solely as a physico-chemical problem. Whether any soil erosion in the catchment can be attributed to poor management of forests is beyond the scope of this investigation. The causes of tanah kritis merit a full investigation on their own. 2. Siltation The input of silt and clay from erosion in the catchment is particularly high in this area, for reasons listed above. Inspection of streams near the lake shore in the Muncul area reveals large boulders, over one metre in diameter, which have been carried down the mountain side in floods. Floods which are powerful enough to move such boulders as bed-load will be able to carry large quantities of silt and clay into the lake. The sudden drop in velocity of the river water as it enters the lake from the steep slopes of the hillsides will lead to a large amount of sediment being deposited at once in the stream bed and on the banks of the stream. The rivers flowing into the lake are developing small estuaries as a result. These are very productive areas as the silt carries with it nutrients derived from the basaltic rocks on the hill sides. When the water from hill streams moves into the body of the lake, there will be a further drop in velocity as the water encounters the mats of water hyacinth. The roots of these plants hanging in the water will slow the flow of the water even further, and lead to an increase in the rate of sedimentation in the lake. It is recognised that the development of any extensive mat of water plants will lead to a tendency for localised sedimentation under the mat. This sediment will be mixed with dead organic matter from leaves, roots, and leaf fragments from the water hyacinth plants. It is noteworthy that quantities of the lake sediment are dug out of the lake and shovelled into boats, taken to the lake shore, dried, and then converted into fertiliser by the addition of limestone. This is sold as fertiliser for padi fields in less fertile areas. Some of the lake sediments removed in this way are probably sub-fossil organic remains of the original swamp forest, which was destroyed in an eruption and then submerged under the lake waters. 3. Domestic waste The input of a variety of types of domestic waste contributes to the problem of water quality in the lake. Values of the coliform bacterial content for some of the streams flowing into the lake are high, (such as in the stream from Ambarawa), and this shows that the streams and rivers are commonly used for disposal of completely untreated sewage. This is another source of organic matter, nitrogen and phosphate which will contribute to the potential for eutrophication in the lake. As mentioned above, most houses have inadequate sewage disposal systems, and there is little effort on the part of authorities to improve this situation. The rapid development of hotels and tourist facilities leads to increased amounts of domestic garbage and vegetable material being disposed of in gutters and roadsides. The traditional methods of garbage disposal (burial or burning in the garden) which worked perfectly well in villages in the past, is no longer effective in small towns where many houses do not have gardens or where the garden is small. Contemporary domestic waste in Salatiga includes much plastic material such as plastic bags and plastic containers for water. This plastic material does not decompose readily, nor does it burn easily. Examination of the rivers leading into the lake shows large quantities of plastic containers and plastic bags of all kinds are being carried into the lake. Domestic waste from Salatiga also includes quantities of synthetic organic chemicals, such as domestic pesticides, fly sprays, mosquito repellents, and air fresheners. Empty spray-cans which contained such toxic chemicals can also be seen in rivers entering the lake. Examination would show that such tins are seldom completely empty, and usually carry some residual material, including the biocide as well as propellant. 4. Inputs of industrial and synthetic chemicals Acid rain may not be a problem in this lake, but there should be some concern about the input of oil and petrol and similar synthetic organic compounds to this lake. With increasing tourism there is an automatic implication of increased travel, and increased travel facilities. Thus, during the last ten years, new petrol stations have developed on the roads round the lake. The large bus station at Bawen, with its re-fuelling facilities, is new, and the bus station in Ambarawa is already overcrowded and will need expansion in the near future. The road from Jogjakarta and Salatiga to Semarang passes along the east side of the lake. The road from Magelang to Semarang passes along the west side of the lake. These major roads meet at Bawen. These are the two major roads which connect the north coast of Java to the south coast in Central Java. They carry heavy traffic, with many trucks and buses as well as cars and motor bikes. There are clear signs of oil accumulation on the road surface, and the smell of petrol is noticeable at filling stations. Since petrol is cheap in Indonesia, and since Pertamina has a monopoly on sales, there is little incentive on the part of the managers of the service stations to be efficient in the handling of petrol or diesel fuel. It is well known that the effect of thunderstorms in hot, tropical, and sub-tropical conditions on tarred roads impregnated with oil, diesel fuel and petrol from traffic, is to release quantities of water soluble chemicals such as phenols into surface water in gutters. Phenols, along with many other synthetic organic compounds, are toxic, and will kill a wide variety of animals including stock and fish. In the case of Rawa Pening it is very noticeable that all the run-off from roads near the lake is guided directly to the lake or to streams flowing into the lake. The problems posed by the continuous flow of small amounts of oil, petrol, tar, and diesel fuel into the lake should be seen as problems which will assume more importance in the future as the volume of traffic increases. Methods of dealing with the problems are required now. 5. Agricultural waste and fertilisers A considerable area of the fertile flat land surrounding the lake consists of irrigated rice fields. These sawahs are highly productive and several crops of rice can normally be obtained from the land in the course of one year. Quantities of rice straw are burned on the land surface each year to minimise the risk of transfer of crop pests from one crop of rice to the next. Some of the rice straw escapes burning and, if there is heavy rain, this material along with some of the ash, is washed into the lake. The ash contains plant nutrients, particularly potassium, calcium, and phosphorus. These minerals are usually soluble and dissolve in the rain water and are washed into the lake in solution. There they add to the nutrient content of the water. Despite the high nutrient content of the soils round Rawa Pening, many farmers will add fertilisers to their rice crops. Some of this fertiliser is not incorporated in the soil and ends up in the lake after the first thunderstorm which floods the padi fields and equivalent fields on the surrounding hillsides. 6. Autochthonous organic matter The huge biomass of water plants growing in the lake produces equivalent large volumes of dead and dying organic matter. A mat of water hyacinth may have a mass of dead leaves below the surface, held in place by the roots of the living plants floating on the surface. This mass of dead organic matter is an ideal substrate for bacteria and microinvertebrates. Many micro-organisms may live in and on such mats, and the oxygen under the living plants may be consumed rapidly as a result. Inevitably much dead organic matter breaks off and falls to the bottom of the lake, adding to the organic content of the benthic deposits. It is noteworthy that leaf material from Eichornia and Salvinia contain fibres which are relatively resistant to decay. The fishing industry contributes another source of organic matter. Catches are not particularly good on the lake. Large fixed dip nets are used, operated from floating platforms, and lowered into the water using a long lever. The fish catch is augmented by baiting the surrounding area. Various kinds of organic material are used as bait for the small fish and shrimps which are caught. In general the bait has to be in the form of small particles to be attractive to the fish. Much of the bait sinks onto the bottom of the lake and is lost. This input of finely divided organic material is regarded as one of the major sources of oxygen uptake in the areas of the lake where fishing occurs. It also contains quantities of plant nutrients. 7. Decrease in the potential volume of water stored in the lake It seems likely that the total volume of water flowing into the lake in the course of one year has decreased in recent years. This may be due in part to the development of "Aqua" factories (factories producing bottled water for domestic consumption) using water from springs on the slopes of the catchment. Such spring water would previously have flowed into the lake. Other springs are now used for domestic consumption and water is gravity-fed from them to houses in the catchment. While some of this water may eventually return to the lake, much will be lost. What water does eventually reach the lake is likely to be of much poorer quality with a high content of nutrients and organic matter. It is likely that much greater quantities of water have been used in recent years for irrigation of rice-fields south and east of Ambarawa, where extensive irrigation canals have been developed. There is no record of the quantity of water actually used in each year in this system. If more water has been used in irrigation than in previous years then this would lead to a noticeable decrease in the amount of water seen to be stored in the lake at any one month during the year. The increase in sedimentation and siltation in the lake also leads to a decrease in the potential volume of water which can be held in the lake. The extraction of bottom deposits for conversion to fertiliser form an insignificant volume compared to the volume of silt carried into the lake each year. If the volume of water stored annually in the lake basin has been decreasing, then this will have led to changes in the effect of some other physico-chemical factors on the water quality of the lake. 8. Eutrophication The lake water may be considered to be eutrophic or hyper-eutrophic depending on the value of phosphate recorded and on the criteria used. It is certainly, by any standards, a highly productive lake. Much of the land in the catchment is farmed and some is being developed at present for recreational facilities. Silt and clay carried into the lake during floods bring nutrients from the fertile volcanic soils on the surrounding hills. Waste material from villages (kampungs) and towns also contains nutrients. Many villages have, at best, inadequate sewage disposal systems. Vegetable refuse from restaurants, local markets (pasars), and houses is often flung on the ground and left to rot. The decomposing remains are left in gutters and end up in streams flowing into the lake. A major source of nutrients is likely to be the numerous padi fields which surround the lake. Where these are irrigated, the water used in irrigation is usually drained off as the plants mature, and flows back to the irrigation canals. During floods the irrigation canals act as drains and lead water from the fields back to the lake. This nutrient-laden water will carry unused fertiliser from the rice fields (mainly nitrogen, phosphorus, and potassium) into the lake. The degree of eutrophication in Rawa Pening is such that any measures which could be taken to reduce the levels of phosphate and nitrogen should be considered. Eutrophication in this case is essentially cultural eutrophication, and results from the activities of people. It is unlikely that implementation of any one or even two measures to reduce the rate of eutrophication are likely to be successful. Any serious effort to reduce the nutrient levels in the lake will call for careful planning and implementation of a wide range of measures affecting all of the factors listed above. The longer the present situation is allowed to continue, the more difficult it will be in the long term to control the nutrient levels in this lake.
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