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Water quality and monitoring FAQs

http://www.gw.govt.nz/water-quality-and-monitoring-faqs

Water quality and monitoring FAQs

Updated 10 December 2019 4:47pm

Quick links

Land, Air, Water Aotearoa (LAWA) has a comprehensive guide to help you know where it is safe for swimming. It is now showing the daily swimming risk model for the Wellington region that we introduced in December 2018. We display this model, rather than the latest test results, as it provides more timely information about the suitability for swimming at popular swimming sites.

The latest test results are only updated once a week or fortnight, but the overall swimming risk rating is updated twice daily.

We are still undertaking monitoring, but using this information to assess and refine the model. This is done for the information about toxic algae as well as E. coli (at our rivers/streams) and enterococci (at our beaches). When we know about any sewage spills and infrastructure issues we add these warnings to the interactive map as well.

 Summer water quality FAQs

Why shouldn't I swim after heavy rain?

Water quality in our rivers and at our beaches is generally pretty good over summer, except in poor weather conditions. Heavy rain flushes contaminants from urban and rural land into waterways and we strongly advise you not to swim for at least two days (48hrs) after heavy or prolonged rainfall – even if a site generally has good water quality.

Another good rule of thumb is: can I see my feet when I am knee-deep in the water?

My local swimming spot is graded D (or poor) – does that mean I can't swim there?

Not necessarily. It means that there is an increased risk of getting sick compared to a site that has a better grade. Water quality at sites graded poor will not always exceed trigger levels for safe swimming but will do so more frequently than other sites. As a rule water quality is most likely to be affected during and up to 48 hours after rainfall, but sites graded poor are also more likely to be unsafe for swimming during dry weather. Greater Wellington and local councils are currently investigating the sources of faecal contamination at sites graded D (or poor) so that water quality can be improved.

Where are the best places to swim?

The best sites are generally in the upper reaches of rivers that are in forested catchments, and at beaches that are away from streams or stormwater pipes draining urban or agricultural catchments.

What causes poor water quality?

The cause of poor water quality depends on the land use in the surrounding area or upstream catchment. In rural areas the most common source of pollution is agricultural land use. In wet weather, excess rainwater flows over farms and into nearby streams and rivers, picking up manure and other contaminants along the way. In dry weather, stock defecating directly into streams or rivers can cause contamination of swimming spots. Septic tanks can also be a source of contamination in rural areas.

In urban areas stormwater and sewer leaks or overflows are the main sources of contamination. During wet weather, rainwater from roofs, roads, car parks and other surfaces is piped directly into rivers, streams and the coast. Along its travels this stormwater picks up sediment, rubbish, animal faeces and other contaminants. During very heavy or prolonged rainfall, sewage overflows can result in untreated sewage being discharged to rivers or the coast via the stormwater system.

Stormy weather conditions, heavy rain and wind can also churn up sediment from the bottom of the waterway or sea, releasing pathogens in the sediment back into the water.

During dry weather, cracked or blocked sewer pipes as well as illegal connections of private sewers to the stormwater system can also cause contamination of our waterways. Large numbers of birdlife such as ducks, geese and seagulls can also contribute to faecal contamination, especially in dry weather.

What effects do treated sewage discharges have on swimming water quality?

It is difficult to assess the effect of treated sewage discharges on water quality as the treatment process often removes faecal indicator bacteria such as E. coli but may not remove the pathogens that can make people sick (such as viruses). Unless tests have been undertaken to show how effective a sewage treatment plant is at removing all pathogens, we consider it appropriate to give potentially affected sites a more conservative grade. For example, sites on the Ruamahanga River that are downstream of discharges from the Masterton, Carterton, Greytown and Martinborough sewage treatment plants are graded C or D depending on how far away they are from the discharges. This is despite these sites often having low E. coli counts during dry weather.

 Summer water quality monitoring

Interactive water quality map

Greater Wellington coordinates water quality sampling at over 80 freshwater and coastal sites in the region during the summer months (from mid-November to 31 March).  This information is used to calculate the long term microbiological water quality grades, or the ‘microbiological history’ of each site. This is done in accordance with national guidelines. 

For toxic algae - high priority sites, known as ‘Tier one’ sites have been identified from historical toxic algae monitoring records, and these are now routinely checked for toxic algae, depending on local river conditions. Monitoring is extended to ‘Tier 2’ sites if toxic algae cover is on the rise, or if local conditions are known to exacerbate the risk of toxic algae blooms. 

The model on the interactive map is updated twice-daily, year round, and combines the information about recent and forecast weather, and the “microbiological history” of each site.  The structure of the model matrix is set out below. It also integrates the information about toxic algae where appropriate.

This assessment generates a rating on the map which should give you a good idea of water quality at your favourite swimming spot on any given day. It is also updated with special warnings if there is a pollution event such as a sewage overflow.  This approach is known as minimum operator – whereby the highest risk for a site is displayed. The structure of this approach is set out below.

If water quality at any location becomes a public health risk – for example a large toxic algal bloom or a sewage overflow in dry weather – Greater Wellington will issue a public health alert through the media and local authorities.

Risk criteria model matrix for Wellington bathing sites

Factor

Criteria

Score

Weighting

History of Faecal Indicator Bacteria

MAC Grade 

MAC A

0

2

MAC B

1

MAC C

2

MAC D

3

Rainfall

Observed rainfall 24h before prediction

0-1mm

0

3

1-5mm

2

5-10mm

3

10-15mm

4

>15mm

5

Observed rainfall 48h before prediciton

0-1mm

0

2

1-5mm

2

5-10mm

3

10-15mm

4

>15mm

5

Forecast rainfall 24h ahead of WQ prediction

0-1mm

0

1

1-5mm

2

5-10mm

3

10-15mm

4

>15mm

5

Predicted score = sum of weighted scores

 

WQ = Water Quality

MAC = Microbiological Assessment Category in the 2003 MFE/MOH Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas

 Risk message and the minimum operator approach

Symbol on map

Rank FIB

Rank Toxic Algae

Score

Rating

Advice

Green

1

1

<10

Good

Good water quality & suitable for swimming

Amber

2

 

10 to 16

Fair

Fair water quality, may not be suitable for swimming, check for signs of pollution

Red

3

3

>16

Poor

Poor water quality & unsuitable for swimming

FIB = Faecal Indicator Bacteria (E. coli or enterococci)