This indicator is based on data collected on the West Coast in 2008-2009 and on the Coromandel Peninsula in 2007-2008.
Sites are grouped into three zones:
We monitored a range of sites at weekly intervals during the summer - usually during the months of December to February. In the 2008-2009 West Coast survey 12 samples were collected at each of the 9 sites. In the 2008 Coromandel Peninsula survey 12 samples were collected at each of the 16 sites.
On the West Coast, data were collected weekly over the summers of 1995, 1996, 1997, 2001, 2003, 2005, 2007 and 2009. Around the Coromandel Peninsula data were collected weekly over the summers of 1991, 1995, 1996, 1999, 2000, 2002, 2004, 2006 and 2008.
We collected samples by wading into the sea to about thigh depth and collecting water from just below the surface. Samples were analysed for enterococci levels (APHA method: membrane filter/mE agar/EIA substrate). We measured water temperature and salinity on site, and record the numbers of people swimming and on the beach.
We developed three categories of coastal water quality for swimming: excellent, satisfactory, and unsatisfactory. These were based on the ‘critical values’ for levels of enterococci.
Click to see the critical values for the three categories that were derived from national guidelines and from expert opinion.
At each monitoring site, we determined the proportion of samples collected during the most recent summer survey (2009 for West Coast beaches, 2008 for Coromandel Peninsula beaches) which met our guidelines for excellent water quality. Similarly the proportions which met our guidelines for satisfactory and unsatisfactory water quality were determined.
The results for the individual sites were then compiled according to site location. Results from the 9 West Coast sites were amalgamated into a West Coast result; and those for the 6 Hauraki Gulf and 10 Coromandel East Coast sites were amalgamated into Hauraki Gulf and East Coast results, respectively. Check out the individual results for the sites we monitored.
Water quality for swimming in the region’s coastal waters is usually at least satisfactory and is often excellent. Occasionally, some beaches have high bacteria levels.
The region’s coastal waters generally receive less bacterial contaminants than inland waters. Also, on the coast any contaminants are often diluted quickly and dispersed by tidal flushing and waves. Because of this, the levels of contaminants in the region’s coastal waters are often much lower than in rivers. However after heavy rain, contaminant levels from run off are likely to be higher.
|Water quality variable (units)||Relevance||Categories|
|Enterococci, single sample (no./100 mL)||Human health||<28||28 – 280||>280|
All the coastal water quality data from different sites within a zone have been amalgamated. This obscures any differences between the sites in that zone.
The coastal swimming guidelines differ from the river swimming guidelines. Water clarity is not included in the coastal indicator, although it is included in the river indicator.
The Ministry for the Environment/Ministry of Health 1998 guidelines note that in “surf break areas, fine bubbles and suspended sand greatly reduce the visual clarity”. This means that the areas where we collect our water samples have low water clarity because of the turbulence of the surf. As a result, we have decided it would not be useful to include water clarity in this indicator.
The Ministry for the Environment and the Ministry of Health also state that the clarity of the water beyond the surf zone could be considered, but we have few data available on this. Casual observation and expert opinion suggest that water clarity beyond the surf zone is usually at least satisfactory for swimming (>1.6 m horizontal visibility), and is often excellent (>4 m horizontal visibility).
The national guidelines recommend using 20 samples collected at weekly intervals. We collected 12 samples. Therefore, the median value is less precise.
We undertake quality control measures in accordance with WaikatoRegional Council's ISO 9002 standards. These include procedures for the collection, transport, and storage of samples, and methods for data verification and quality assurance.