Dehiwala-Mt. Lavinia groundwater highly contaminated by faecal matter and chemicals

Groundwater in Dehiwala-Mt Lavinia is highly contaminated with faecal matter from domestic septic tanks and soakage pits as well as chemicals from industries, a report on a proposal to extend pipe-borne sewage disposal there says.

A comprehensive study in the project area identified a deterioration of surface water quality as well as “extremely high total and faecal coliform levels were detected in all water bodies”, the environmental impact assessment (EIA) for project observes. It is available for public comment on the Central Environmental Authority (CEA) website.

The report also says that Mt Lavinia’s near-shore waters, where people engage in recreational activities, show faecal contamination. During both Southwest and Northeast monsoons, a “couple of rainfall events” result in faecal coliform levels rising to more than 6,000 MPN (bacteria count) per 100 millilitres. The permissible level is just 150 MPN per 100ml for contact recreational activities such as swimming and bathing.

Analysis of shallow wells showed that there was “extensive faecal contamination”. And the nitrate-nitrogen levels detected in well-water samples were greater than those seen in surface water samples, indicating possible nitrate contamination in shallow groundwater sources, the report says.

Nitrogen is abundant naturally in the environment, the study accepts. But it can also be introduced through sewage and fertilisers. Excess levels can create conditions that make it difficult for aquatic insects or fish to survive.

The extension of pipe-borne sewage disposal will be carried out in four grama niladhari divisions in Ratmalana, three in Dehiwala and one in Kesbewa. The groundwater table is high throughout most of these areas: one to two metres below surface.

The report attributes poor water quality with high faecal content to discharge of pollutants to nearby water bodies from unsewered houses, businesses or industries. Low dissolved oxygen levels were seen at all locations as well as high ammonia levels, further confirming pollution due to such discharges.

The main water bodies in the study are the Attidiya Lake and Weras Ganga or Bolgoda Lake. These are connected with the Bellanvila-Attidiya marsh via natural canals that act as retention areas. Surface drains in the project area are “hardly cleaned”, the report says, and they discharge storm water, domestic sewage and industrial wastewater into these water bodies which are highly polluted.

The Attidiya Lake is threatened by eutrophication– when the water becomes overly enriched with minerals and nutrients that induce excessive growth of algae–due to inflows from Badovita and surrounding areas. Solid waste dump yards are in close proximity to low lying areas. This means leachate also ends up in the water bodies.

Most canals, especially flanked by low income settlements, are severely polluted. This also causes groundwater contamination, spread of water-borne disease and loss of natural resources like flora and fauna.

In March 2016, the University of Moratuwa selected eight sampling locations in the project area. Three are wastewater canals while one is the Attidiya Lake. “Extremely high total and faecal coliform levels were detected in all water bodies,” tests proved. There were also low dissolved oxygen levels and high levels of total ammonia.

Other tested sources included shallow wells that are still used for drinking, watering and bathing. These, too, showed “extensive faecal contamination” as well as all the other pollutants. The nitrate-nitrogen levels in well-water samples were greater than those in surface water samples, “indicating possible nitrate contamination in shallow groundwater sources”.

A detailed report was also prepared on coastal waters along the Dehiwala-Mt. Lavinia beach stretch. Currently, it said, the beach stretch from Wellawatte to Mt Lavinia is “severely polluted” due to untreated wastewater discharges.

“Any authority responsible has not dealt with this situation yet and, as a result, there has been a great risk of people getting sick due to the direct exposure to the waters that had undergone faecal pollution,” the EIA warns. “The proposed project will dispose of untreated wastewater through a long sea outfall, which might bring in back the pathogens to the shore, making the present situation worse. However, the real impact cannot be predicted, as the proper near-field and far-field modelling has not been carried out.”

With the implementation of the proposed project, wastewater generated in homes, offices and businesses will be collected and conveyed to the treatment plant (main pumping station) after which it will be disposed via a long sea outfall.

The report says this will reduce wastewater discharges into canals and prevent untreated wastewater contaminating and polluting the beach stretch.

At present, most residents of the highly-populated Dehiwala and Mt Lavinia urban centres discharge their domestic wastewater into roadside canals. The report says that some residents “directly discharge their sewage mixed wastewater to the nearby storm water canals”. Many have limited or small land plots and, therefore, limited space to build septic tanks of adequate capacity. There are places, too, where septic tanks overflow.

These have led to problems such as severe blockage of storm water, natural and man-made canals; unpleasant odour from blocked and contaminated canals; no gully bowsers for emptying and cleaning septic tanks; additional expenses to overcome issues arising from frequent spillage of toilets; and high vulnerability of shallow wells used for domestic purposes to faecal contamination. Some paddy fields in the area also receive polluted water, creating health hazards for farmers.

The EIA says that over 50 per cent of the population will receive a safe, efficient and appropriate wastewater disposal system with the implementation of the project, and the majority will have proper sewage disposal facilities.

“It can be concluded that the project is not expected to have any significant adverse impact on the physical, terrestrial and social environment,” it concludes. “Minor environmental issues have to be addressed during the construction and operational phases, and will be mitigated through the implementation of proposed measures and regular monitoring.”