The primary sources of bacteria and pathogens in waters used for recreation include sewage, urban (stormwater) and agricultural run-off, livestock and wildlife. The effect of these contaminant sources on bathing water quality is largely dependent on weather conditions (rainfall, wind direction and intensity), the nature of the receiving waters in terms of available dilution and mixing, and, in marine areas, tidal conditions. 

In the Wellington region, there is a moderate to high risk of microbiological contamination present at most freshwater bathing sites as the majority of our popular river swimming spots are located in agricultural catchments or receive discharges of urban stormwater.  The risk of contamination at these sites is highest during and shortly after rainfall when rural and urban run-off is greatest.  Care must also be taken during dry weather when stock access upstream, faecal contamination from wildlife, or poor water quality in tributary streams may also influence water quality

The majority of the coastal bathing areas in Wellington have a low to moderate risk of microbiological contamination.  Urban stormwater discharges are the most common source of contamination, as well as rivers, streams or drains with agricultural or urban influence, or supporting large numbers of waterfowl. In some areas, sediment re-suspension as a result of high wave energies and/or strong winds, also affect water quality at times.

Sewage (wastewater) from many communities is collected via a network of pipes and pumped to a central facility for treatment prior to being discharged into the environment. In the Wellington region, treated wastewater is discharged into fresh and marine waters at a number of locations.

At times sewage can enter the environment before completing its journey through the treatment process. Some examples include:

• Broken or leaky pipes - usually as a result of ageing pipes, construction activities or road works.
• Overflows, during rainfall - the treatment facility may not be able to cope with the volumes of water and sewage entering the system, and raw or partially treated sewage is discharged directly into the environment. This situation can be made worse where households have stormwater from roofs and other hard surfaces illegally connected to the sewerage system. Also, some older sewerage systems do not have completely separated sewage and stormwater pipes. During the high rainfall, stormwater can enter the sewerage system and cause sewage to overflow into the stormwater pipes and, subsequently, directly into surface waters.
• Emergency overflows - these can occur periodically during maintenance of sewerage systems.

There are also other sources of sewage. For example:

•  Some older properties, especially holiday homes, are still connected to septic tanks.  Where these tanks have deteriorated, they may leak contaminated water into the groundwater which, in turn, ends up in surface waters.  
• Some boat owners discharge wastes directly into the sea, without treatment.