Bathing water quality in lakes

 
Image/logo for showing indicator 'Steady'.
 
Escheria coli (E.coli) counts at bathing sites
 
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Purpose of indicator

Monitoring bathing water quality gives an indication of whether or not water is safe to swim in and enjoy. Run-off from surrounding land can contain bacteria such as E.coli which at elevated levels is a health risk to humans. Vegetated riparian margins and exclusion of stock help to reduce this risk.

Current information and trend

Popular recreational bathing waters are surveyed annually during warmer months from October to March by Bay of Plenty Regional Council. The surveys monitor and identify risks to public health from pathogens indicated by E.coli, which is an indicator of faecal matter.  This information is used by public health services and local authorities to assess the risk to human health and to inform the public whether it is safe to use these waters. Table 1 shows the Ministry for the Environment and Ministry of Health ‘traffic light’ system guidelines, using sample counts to determine recommended action or management response.

Figure 1 shows monitoring sites of both lakes and streams in the Bay of Plenty part of the district. Environment Waikato also monitors swimming water quality which will be reported here in future. For more information on swimming water quality in the Environment Waikato part of the Rotorua district.

 
Table image for Current information and trend 
Table 1
Source: Ministry for the Environment and Ministry of Health, 2011
 

Monitoring results from 2003/04 to 2010/11 show that most lakes are relatively stable, with a slightly decreasing E.coli count trend (figure 2). This is shown in trends for lakes Rotorua, Rotoiti, Tarawera, Rotoma and Okaro. Lake Rerewhakaaitu shows a slightly increasing trend. The trend for Lake Tikitapu shows a decline in E.coli counts from 2003/04 to 2008/09 after which the trend shows an increase to levels similar to 2003/04. The Bay of Plenty Regional Council report, Bathing and Shellfish Surveillance Report 2010/2011, states monitoring sites of the same year (2010/11) had a median of 15cfu/100ml (that is, 15 colony forming units/ cfu found in 100ml of sample), indicating that on average all lake monitoring sites had a low level of contamination.

 
Map of Westen Bay
Figure 1.
Source: Bathing and Shellfish Surveillance Report 2010/2011, Bay of Plenty Regional Council
 
Line graph of Average E.coli counts
Figure 2
Source: Bay of Plenty Regional Council, 2011
 
Chart/table showing the yearly red and orange alert
Figure 3.
Source: Bay of Plenty Regional Council 2011
 

Figure 3 shows the number of yearly red and orange alerts (as defined in table 1) for each lake from 2003/04 to 2010/2011. During this period Lakes Okaro and Tikitapu have not shown E.coli levels to warrant an orange or red alert. Lake Rerewhakaaitu had one orange alert in 2008/09 while lakes Okareka and Rotoma had one red alert in years 2006/07 and 2007/08 respectively. Lake Tarawera had 7 orange alerts from 2003/04 to 2010/11 while Lake Rotoiti had 10 orange alerts and 2 red alerts in the same period. Lake Rotorua had the most number of orange alerts (16) and red alerts (8). Lake Okataina has previously been included for monitoring of swimming sites, however E.coli levels were found to be consistently low and monitoring was considered no longer necessary.

Levels of faecal coliforms in lakes can be widely variable and are more likely to be elevated after periods of rainfall. Land use in a lake’s catchment directly affects what is likely to be washed into a lake via watershed. Vegetation, particularly in valleys and riparian areas help to buffer faecal coliforms and other water contaminants before they reach the lake. In the last 15-20 years considerable effort has been made by landowners, regional and district councils to fence lakes to exclude stock, including jointly funded environmental programmes with a focus on lake water quality.

In summary

  • Five of the 7 lakes monitored show a generally stable, slightly declining trend in E.coli counts in swimming areas
  • In 2010/2011 sampling period all lake sites had a median of 15 cfu/100ml, meaning there was overall low levels of contamination in swimming areas
  • Lakes Okaro and Tikitapu have not had levels of E.coli at the threshold that triggers an orange or red alert for swimmers, since 2003/04
  • Lake Rotorua had the highest number of orange and red alerts of all the lakes in the period 2003/04 to 2011/12
  • Monitoring in Environment Waikato part of the Rotorua district will be reported here in future.
Page reviewed: 03 Jul 2019 10:11am