Water: Resources, Uses & Pollution

Back To : The Environment In Mauritius

Introduction:

Over the last 20 years the increasing demand for water has exerted great pressure on the fresh water bodies of the island with the result that there has been a slight deterioration in both quality and quantity of inland fresh waters. Water is used for the production of electricity as well as the usual industrial, commercial and domestic usage.

Water Resources And the Environment


In Mauritius, the source of water is simply rain. The rainy season is from December to April and the dry season from September to November. In general February and March are the wettest months.

The average annual precipitation varies from 1300 mm on the east coast to 900 mm on the west coast and 4000 mm on the central plateau.

92 rivers, 232 rivulets, 5 man made lakes, 2 natural lakes and ground water distributed into 7 ground water basins constitute the main fresh water bodies of Mauritius.

Ground Water Potentialities:

During the fifties the Mauritius Chamber of Agriculture and the Government resorted to a massive programme of prospection for ground water resources to be used for irrigation purposes in the drier parts of the island. The industrialisation of the eighties led to further and intensive prospection and exploration for ground water. Mauritius has four main aquifers known as the Curepipe aquifer, the Northern, Southern and Eastern aquifers.

At present more than 40% of potable water is supplied by the system of aquifers and this is estimated to increase to about 50% by the year 2010 at which time around 90% of underground water resources will be exploited. Aquifers are very sensitive to over-exploitation, rainfall precipitation is vital in order to maintain their full potential. During the dry season, yields from boreholes tappings into the aquifers are lower by about 20%.

According to the National Physical Development Plan ( volume I P 134), pollution of the aquifers by poor sanitation (sewage, waste water and refuse disposal ) and by poor agricultural practices is becoming apparent and there is an urgent need for proper sanitation systems especially in the Plaines Wilhems towns which are above the Curepipe aquifer.

It appears that sea water intrusion in coastal aquifers is not yet a major problem in Mauritius. (The Geology and Water Resources Of Mauritius P 450 )

In 1990, the Central Water Authority (C.W.A.) established that 239 boreholes and wells were in operation in Mauritius. 70 were exploited by the C.W.A. for domestic, industrial and commercial purposes. The industrial sector exploited around 82 boreholes. Presumably the balance being used for agricultural purposes. On average, the diameter of a borehole is around 30 cms and 30 metres in depth.

Water Demand and Usage

The present domestic water consumption is between 180 to 220 litres per day per person and it is expected that by the year 2000 this domestic consumption will rise to around 250 litres per day per person.

In the tourist sector this consumption varies from 1000 litres per day per person in five star hotels to 180 litres per day per person in ordinary hotels. (This high water consumption of tourists in five star hotels may become an important factor in determining the maximum number of tourists the country can support per year given that it is government policy to encourage high class tourism which naturally entails high consumption patterns, water included.)

The global water demand for domestic, industrial, commercial and tourism purposes stands currently at around 342,000 cubic metres daily. This is set to increase to 418,000 cubic metres daily by the year 2010.

Currently the water losses in the supply system is estimated to be of the order of 55%, which indeed is a very high figure. Therefore the water requirement is roughly double the water demand figure. It is estimated that if losses are cut to 25% or even 35% of water requirement, then demand will be met in the year 2010. Hence controlling the unaccounted water or losses in the system is the single most important factor in managing efficiently the island's water resources and in determining the level of investment needed for the water supply systems up to 2010.

Daily Water Requirements And Demand Graph

Sources Of Water Pollution

Water pollution can be classified according to its source:

  1. Industrial waste water
  2. Agrochemical usage and water pollution
  3. Domestic Waste Water
  4. Sugar Mills And Waste Water
  1. Industrial Waste Water

It is estimated that there are around 100 industrial units which are engaged in activities causing some form of water pollution. Any liquid effluents, when discharged will eventually find its way into the hydrological cycles and thereby can have adverse effects on the ecosystems and eventually on the quality of water to the consumer. It is important to bear in mind that the small size of the island mean that very quickly water sources can be adversely affected by effluent pollution.

There are 3 main industrial zones in Mauritius which are the Plaine Lauzun, Coromandel and the Vacaoas-Phoenix industrial zones. Although most polluting industries are found in these zones, other polluting industries are also scattered all over the island.

Map Of Industrial Zones And Sewage Network

At the Plaine lauzun Industrial estate ( found at the entrance of Port Louis ) there are dye houses, soap,detergent and chemical manufacturers, galvanizing, food canning and ethanol distilleries among others. Theses industries consume about 5000 m3 of water daily and their effluents are discharged to the Fort Victoria sewerage treatment plant in Port Louis. Unfortunately only pre-treatment is carried out there and the effluent is then discharged 800 metres into the sea through a sewerage outfall. It appears that this effluent finds its way to a nearby beach called Bain Des Dames. Heavy pollution has been reported there with high fish mortality. In September 1996, the daily L'Express reported that a report published by the Central Environment Laboratory ( The Institute has not been able to obtain a copy) mentioned that pollution by industrial waste water from nearby factories had caused the fish mortality. The fish mortality could have been caused by hot industrial effluents and high BOD load (biological oxygen demand). In January 1996, fish mortality was reported at Tombeau Bay and the Ministry Of Fisheries Report of 1996 mentions eutrophication due to seepage from a nearby dumping ground and sewage out fall as the most probable causes.

In the Coromandel industrial zone, dye houses, soap and food processing industries contribute to the water pollution originating from there. The water consumption is around 3000 m3 per day and the effluent is discharged untreated through a 600 metres outfall into the sea at Pointe aux Sables close to Port Louis. This effluent has resulted in the total death of the reef opposite Pointe Aux Sables.

In the industrial zone of Vacoas-Phoenix there are around 30 industries amongst which are found 6 dye-houses and one edible oil refinery. The water consumption is about 2000 m3 per day. Liquid effluent is untreated and is discharged into the local sewerage network leading to the St Martin treatment plant. There, only a primary treatment is carried out before the effluent is discharged out to sea at Pointe Moyenne. Furthermore, during the rainfall season, overloading occurs resulting in a significant amount of effluent being discharged into river Du Mesnil that joins up with Grand River North West from which water is abstracted for domestic consumption. The potential for health problems is evident.

The other industries that contribute to water pollution are scattered all over the island and that includes leather tanning, galvanising, battery manufacture, dyeing and washing industries. Very few industries carry out on site treatment of effluents before discharge in surface waters or pits and caverns. Contamination of aquifers by such discharge methods is a distinct possibility.

At Poudre D'Or , in the north east, industrial effluent from the nearby Riviere Du Rempart industrial estate is discharged in the water ways, mostly untreated, as there is no sewerage system there. This effluent has resulted in significant pollution of the adjoining lagoon where heavy fish mortality has been reported. According to the local fishermen, several species of fish such as Cordonniers ( Siganus Sutor ) and Dames Berry are no longer to be found in the vicinity.

Waste Water From Sugar Mills

Up to half of the 19 sugar mills of the island are found close to the sea or to rivers. Cane crushing operations last from June to November of each year. In most cases, it seems that the liquid waste from these operations does not undergo any treatments prior to it being discharged into rivers or streams.

The state of the environment report 1991 mentions that this waste gives rise to damages to aquatic life and mass mortality of fish in lagoons receiving sugar industry waste has occasionally been reported. But the government has taken steps to encourage the use of decantation ponds for the primary treatment of effluents before discharge into the environment.

Deforestation And Water Resources

According to the State of the Environment report 1991, significant erosion of top soil by rainfall following deforestation has occurred in most regions of Mauritius. This top soil then reaches either the lagoon or water reservoirs. Part of it is deposited in natural river basins which are now filled with silt. Fresh water ecosystems have also been affected by top soil erosion, the report says. Many species of fishes and shrimps have disappeared and aquatic plants show signs of necrosis. The " La Nicolière" reservoir in the north has had its capacity reduced by siltation. The same report goes on in saying that major efforts must be done to prevent siltation of the reservoirs or otherwise that may result in water scarcity in the future. To avoid this dramatic situation, reforestation must become a priority.

Wetlands

Wetlands can be defined as areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres.

Wetlands are important for several reasons. They are reservoirs of biodiversity, for example for many species of fish and waterfowl they act as breeding grounds or staging areas in migration routes. Wetlands are amongst the most productive ecosystems in the world providing rich fishing grounds or important grazing areas for cattle and wildlife. They also act as buffers against floods, storing rainwater and then slowly releasing it. Vegetation of wetlands have a binding effect, stabilising banks and shores. Mangroves, for example, help built up sediments along coastlines providing protection against tidal surges. Another key function of wetlands is their ability to filter off waste and contaminants as water moves from the wetland to underground aquifers. For example, nitrates from fertiliser runoffs can be transformed into nitrogen gas by denitrification thus preventing nitrates from reaching aquifers or the lagoon. Thus wetlands can be efficient, low cost, water purification systems.

Unfortunately wetlands are very sensitive to interference by human activities like general building and settlement developments, irrigation projects, agricultural land conversion, reclamation projects, river and spring diversions or ground water extraction.

In Mauritius wetlands in Grand Baie and Flic en Flac have been to a great extent destroyed by settlement development projects. The state of the environment report 1991 states that the partial destruction of the wetland in Grand Baie has resulted in serious imbalances in its ecology. The wetland now no longer acts as a regulator of tides and floodwaters during heavy rains and so there is a grave risk to the local inhabitants of flooding during the rainy season.

The blockage of natural drains has led to a rapid rise in the water table during the wet season and analysis of samples collected at Grand Baie has indicated that the water table, which is only one metre deep, is heavily contaminated with faecal coliforms indicating a high degree of pollution.

Furthermore, It is suspected that the parcelling of wetlands for construction purposes at Flic en Flac, Tombeau Bay and Pereybere may lead to ecological and geological imbalances that could result in similar problems as in Grand Baie.

Bibliography
  1. Government Of Mauritius, State of the Environment report 1991
  2. Government Of Mauritius, National Physical Development Plan - Volume 1: Strategy and Policies 1994
  3. Government Of Mauritius, Ministry of Fisheries Report 1996
  4. V. Proag, The Geology and Water Resources of Mauritius - Mahatma Gandhi Institute 1995.

Date on the web: Thursday, January 22, 1998