Understanding coastal water quality in Cape Town
The Cape Town coastline stretches for approximately 307 km from near Silwerstroom on the west coast, around the Cape Peninsula and beyond False Bay to the Kogelberg coastal area in the east. One of Cape Town’s most significant assets in terms of marine and coastal biodiversity, the coastline attracts high levels of tourism and recreational activities. Cape Town’s beaches are world renowned for their beautiful landscapes, offering opportunities to use an accessible natural environment. Ensuring the protection of important ecosystems on the city’s coastline and public health in the case of coastal water pollution requires effective water quality monitoring.
Coastal water quality
Coastal water quality is impacted by numerous sources of bacterial pollution, with the three main sources being overflows from the sewage reticulation network, wastewater discharge from waste water treatment works (WWTWs), and storm water run-off. Final treated effluent from the 26 WWTWs in Cape Town is discharged into rivers flowing into the coastal environment or is released directly into the ocean after initial preliminary treatment (maceration and screening) via deep-sea marine outfall pipes. While this wastewater is treated in a manner which reduces contaminants in accordance with licenses issued by the Department of Water and Sanitation (DWS), overflows from the sewage reticulation network are serious, since the sewage flowing in this pipe network has not yet been treated at a WWTW, and thus very high levels of E. coli enter the environment as a result of such spills. If not addressed, these spills ultimately flow into the storm-water network and, from there, into rivers or coastal areas.
Water systems are interconnected. Efforts to improve the water quality of freshwater systems have significant impacts on the quality of coastal water systems.
Storm water that is contaminated with pollutants may also significantly impact the water quality of the coastal waters. Storm water flowing through inadequately serviced informal settlements often contains both grey water and untreated sewage as a result of residents using the storm-water network to informally dispose of household washing water and sewage. This form of domestic wastewater disposal is often caused by residents being unaware that storm water and sewage reticulation networks are not intended to be connected. Waste from domestic pets and livestock is an additional source of faecal contamination of storm water.
Coastal water quality is significantly influenced by rainfall patterns. Increased storm-water flow in the winter washes significant quantities of pollutants deposited within the catchment during the preceding dry period into rivers and the coastal environment. In the summer months when rainfall is minimal, lower levels of pollutants are found in coastal waters with spikes only occurring after occasional ‘unseasonal’ rainfall events.
Effects of pollution
Polluted coastal water may detrimentally affect the health of humans swimming, surfing and diving in coastal waters and near-shore marine ecosystems. Human contact with bacteria and other pathogenic organisms present in the water may cause gastrointestinal illnesses and dermatological problems. Water quality may also have a higher range of other potentially harmful pollutants that have the ability to detrimentally affect delicate near-shore coastal ecosystems. The DWS has recommended coastal recreational guidelines aimed at safeguarding human health, which have been in place for a number of years. However, the Department of Environment Affairs (DEA) has taken over the mandate to manage coastal and marine waters and has recommended a new set of recreational guidelines which coastal municipalities are gradually switching over to.
Although the City has commenced monitoring coastal water quality in accordance with the new DEA guidelines, for the purpose of this report and to facilitate comparison with previous State of Environment reports, coastal water quality results are reported in terms of the previous DWS South African Water Quality Guidelines for Coastal and Marine Waters (Volume 2: Recreational use). These guidelines set standards for the maximum number of indicator organisms (such as faecal coliforms including E. coli) that can be present in water used for full-contact recreational activities, such as surfing and swimming.
Guideline (strict): 80% of samples must contain no more than 100 indicator organisms per 100 ml.
Guideline (relaxed): 95% of samples must contain no more than 2 000 indicator organisms per 100 ml.
The target for faecal coliform (including E. coli) counts is based on 80th and 95th percentiles. These are calculated on long-term data sets typically covering a 12-month period. For public health reasons, in order for a beach to fully meet the guidelines, it must meet both targets.