THIS is apropos the news ‘Another source of Keenjhar Lake exposed’ (April 22).

The 2008 and 2011 editions of WHO’s drinking water guidelines give a value for mercury as 0.006 mg per litre, or 6 mg per litre, or 6 parts per billion and not as 1 ppb, as reported.

In the report, WHO guidelines for drinking water have been compared with  Keenjhar Lake water. How can one compare oranges with apples? All surface water bodies in Sindh are polluted. Drinking those raw waters would end up in massive
health complications. Water-treatment plants are used for treating polluted raw water. Only the quality of treated water, meant for drinking, should be compared with WHO’s drinking-water guidelines.

Water quality problems of a lake are not assessed by taking a few water samples, as is being done by some persons. Lakes are open systems and are integrated components of the watershed. Watershed influence the lake, as such, lakes’ problems cannot be handled in isolation. They must be co-related to the watershed and its environment. The operation of the lake is governed by what materials enter the lake, leaves the lake and remains in circulation.Lakes are characterised by a low, average current velocity of 0.001 to 0.01 meters per second at its surface. Water residence times in lake range from one month to several years. Currents within lakes are multidirectional and, many lakes have alternating periods of stratification and vertical mixing.

While causes of the water quality problem in a lake may relate to inputs, like discharge of municipal and industrial wastewater, diffused pollution from agricultural sources, discharge of toxic substances from industries and thermal discharges, a specific water quality problem may have its origin elsewhere. The following principle will illustrate the point:

Lakes have large buffering capacities and can withstand certain levels of pollutants. The buffering capacity is in the shape of the ability of sediments to accumulate pollutants.

Once the capacity has exhausted, further input of pollutants will be reflected in the water. This may give an impression that the problem has occurred recently (e.g., lead in Keenjhar Lake), but, in practice, the problem may have occurred many months ago.

Eutrophication causes oxygen depletion in the hypolimnion layer. Depletion of oxygen in the lake’s bottom results in the remobilisation of manganese, iron and phosphorus. Some trace elements are also released from bottom sediments.

Particulate matter plays a major role in water quality assessment as it regulates the uptake and release of pollutants. Thus, besides water sampling, the particulate matter and biological material are also analysed by limnologists for a complete water-quality assessment study.

The major problem of surface water bodies in Sindh pertains to the governance issue. Nobody knows who controls the water bodies in Sindh. Technically, the Sindh irrigation department should control the hydraulic aspect (water quantity, water supply, water flows) and the Sindh EPA should control the water quality aspect.

F. H. MUGHAL
Karachi