INTRODUCTION:

Much has been spoken and written about the importance of water reservoirs or "tanks". As cities and towns expand, these tanks are being converted into receptacles for urban sewage. Clean and wholesome tanks and lakes are being taken over for industrial and commercial development, including high-rise apartments and new "layouts". In designing interventions to rescue these tanks in urban areas, the usual treatment suggested has been desilting, deepening, development of recreation facilities like boulevards, picnic areas and boating, and stopping sewage inflow. A recent addition has been the concept of the "wetland system" of sewage treatment, quoting experiences in developed countries. This approach has been applied in a couple of externally aided projects in Bangalore, of which the Madivala tank is at an advanced stage of implementation.

The "wetland system" of sewage treatment is basically using the natural capacity of living organisms to absorb and transform nutrients from eutrophic waters and thereby reducing the organic load. Where such an aquatic ecosystem is maintained a number of other animals and plants will also be attracted. The sewage treatment system then doubles as a refuge for wildlife, especially for fish, amphibians and water birds. In this respect it will be similar to manmade wetland reserves like Bharatpur, except that the water used is sewage rather than rainwater runoff or irrigation runoff. The distinction from the usual technology of sewage treatment would lie in using vegetation (including naturally co-evolved micro-organisms) rather than chemical or physical methods of treatment like mechanical aeration, filtration, bleaching, etc.

The "wetland system" in Madivala is sought to be created by a series of ponds (much like the settlement ponds in a normal sewage treatment plant), but with aquatic plants like reeds or cattails, and inter-connecting pipes (most probably for sending the contents from one stage to the other). A primary treatment plant is also provided to filter out the solid matter. Other added features are a "jogging path" (complete with stone pitching) along the margins of the tank, and artificial "islands" in the center, ostensibly to increase areas for water birds to roost and nest (when the saplings have grown up) and increase access of the lay public (including birdwatchers!) to the birds.

When created de novo, such a system will probably add to the wildlife value of the locality, especially if the sewage is led into dry tracts (as apparently is the case in some of the examples quoted from USA). When an existing water body is used, the net gain is open to debate. Firstly, it is obvious that the water body (here a "tank") is already acting as a "wetland system", albeit an extended one as there is no "bunds" to divide the process into stages. The backwater reaches have always carried aquatic vegetation like reeds and cattails, as well as fodder plants in the seasonally inundated areas and as a result serve two purposes: natural treatment of sewage and as a refuge for wildlife (water birds, both local and migratory, predators and scavengers, insectivorous birds like swallows, wagtails and pipits, and others). The net result of building the bunds for the "wetland system" has been to reduce this area of shallow water, seasonal inundation and mudflats at the water margin. The "jogging path" has also in effect taken away a long and sizeable stretch of the margin, much of it consisting of reed-beds used by waterfowl or mudflats for waders. Conversely, piling the silt up to form the "islands" in the middle has probably reduced the open water area favoured by diving waterfowl (e.g. ducks) that "raft" on open waters. If the islands are left undisturbed (i.e. no connecting bridges and pathways to the mainland or the bunds), there is a possibility that they may be occupied by rookeries of herons and egrets (after the planted trees have grown up). This could however be achieved by planting trees around the margins (especially in the backwater area) without reducing the open water area.

Therefore, some issues have to be kept in mind when designing such interventions. On the face of it, many people would consider desilting and deepening as good interventions to improve water bodies. Since they are called "improvement" or "restoration" programmes, it can be accepted that they are well intentioned. The principle in any intervention should keep ecological formations in mind. One has to exercise constant vigil against interventions that would cause, or has the potential to cause, adverse changes. This is especially important in the case of these water bodies, as they are hosts to a multitude of migratory species that land here every winter after long and grueling journeys from other continents. The best treatment in ecologically sensitive interventions is to work with the existing conditions and vegetation, and introduce modifications slowly after studying the ecological implications. Large scale earth moving is not indicated in such sites.

The following questions become relevant, in this view:

Where to desilt?

Different animals and plants use different parts of the water body (wetland), and will therefore be affected in different ways. Specific attention must be paid to animals and plants that are adapted to the water-land inter-zone. If the "improvement" alters this zone, it will deprive a whole set of life forms, their ecological niche. Building of high banks with stone pitching, and leaving a deep and vertical cutting all along the margin instead of a gradual slope, are examples of such interventions. On this reasoning, desilting would best be done in the deep-water zone (it would not penalise the shallow water users).

Where to deposit the silt?

In net terms, excavation will not reduce the silt load unless it is transported out, nor add to the water-storing capacity of the lake if it is re-deposited in a different form (islands, roads, bunds) within the same area. To the extent that some of the silt is re-consolidated above the old water line, of course, one could argue that the volume of the reservoir has increased due to removing the silt. The net benefit, however, would be substantially less than the volume of silt handled. In the Madivala case, it appears that soil has to be transported in from outside to build up the "jogging paths" and islands.

Why desilt?

Whether or not desilting is done, the reservoir will still receive rainwater and runoff (so it would be specious to state that a lake is receiving the bounty of the rains just because of desilting). The question is whether it is important for the lake to hold more water. In a system of inter-linked reservoirs (tanks), overflow would appear to be a necessity, so increasing the water-storage capacity may not be a priority. With sewage laden water coming in, the whole series of inter-linked tanks would appear to be acting as a "wetland system", so holding more water may not be as valid an objective as smoothening the flow from one to the other.

Why build up soil structures?

Introducing more access through "jogging paths" may result in more disturbances to the wildlife rather than improving protection (the 'jogging paths" in Madivala are like roads). If access is to be improved, it should be limited and modest in reach and scale (size), without interfering the available habitat by "borrowing" on the lake margins. Comments on islands are already given; their damaging effects will increase if they are connected to the shore and are used for recreation.

Why create "wetland system"?

The "wetland system" depends on aquatic vegetation to reduce the organic load, and this is already in operation in the shallow backwater region. As mentioned above, the series of tanks is itself like an extended "wetland system". Then there may be no net gain in creating the series of shallow ponds that is the "wetland system" within the ecologically valuable backwater area of each tank or lake. Similarly, reintroducing the same type of vegetation after forming the "wetland system" results in no net gain, but may probably result in a net deterioration in the conditions for nesting, flocking and feeding behaviour.

How much "cleaning"?

One of the common complaints about urban lakes (tanks) is the growth of mats of aquatic vegetation (often water hyacinth). Cleaning them, amounts to removing this build-up, often to the extent of eliminating it, which would leave certain species without their favoured habitat (e.g. jacanas, called lily-trotters in America, grebes, coots, moorhen, etc.). Aquatic vegetation, needless to say, is an integral part of the wetland ecosystem. In fact, traditional use by local people who continuously remove a portion of the vegetation (as long as they leave the birds alone) coupled with grazing by buffaloes may well be a sustainable system (reminding us of the controversy about the role of grazing in maintaining the wetland status in the Bharatpur bird sanctuary).

How much water treatment?

Wetlands are aquatic ecosystems based on high nutrient levels, i.e. tending to eutrophic conditions; they are also usually a stage in the succession to less wet, land-based ecosystems. Wetlands are rich in life mainly because of the fortunate combination of circumstances-water, nutrients, light, and soil. In the tanks discussed here, eutrophication is the result of either sewage or other wastes, including agricultural wastes. In urban Bangalore, the general understanding is that sewage comes in through the storm water drains, but diverting this inflow will only lead to drying up of the tanks. It is a moot question whether the sewage flows can be segregated from the storm water drains (apart from the natural course of least resistance of using the tanks as the end-of-line receptacle for the official sewage line as well). Some people feel that without the sewage inflow, the tanks will shrink! Therefore, lake improvement will not take place unless the basic problem of sewage load is addressed. Given that wetlands need higher than normal nutrient content, how much should the sewage water be purified? Whether this is going to be achieved by the "primary" treatment plants set up as part of such a project (e.g. Madivala) needs to be discussed. For the "wetland system" as a whole, the external energy required to pump the water from pond to pond may also be a significant factor in deciding its effectiveness compared with leaving the whole tank as it is to take care of the sewage (in effect, an "extended wetland system").

The objective would be to maintain a reservoir in its intermediate state, so that it functions as an "extended wetland system" for purifying sewage, as well as a habitat for aquatic wildlife. A judicious balance has to be struck between too much intervention (like scraping off many feet of bottom sludge along with the vegetation) and too little (which will lead to building up of the rotting vegetation and deposits and progression to a land ecosystem). These reservoirs are important not just for sewage disposal, but also for maintaining local groundwater levels and as a refuge for local and migratory wildlife. Before understanding large interventions, it would be wise to examine their potential effect on these valuable components and make sure that minimal damage is caused to these ecosystem values.

Address:

Chief Conservator of Forests & Project Director,
Western Ghats Forestry Project,
Van Vikas,
near 18th Cross,
Bangalore-560003