Aroaro Wetland in healthy condition

A check of the Aroaro Wetland on Wallis St, Raglan in March found that the wetland was in a healthy condition. Carried out by Kessels Ecology, the check found that the work undertaken to date has considerably reduced weed infestation and improved indigenous vegetation cover. Provided that restoration works are maintained it is expected that the site will be restored in accordance with the restoration plan within 5 years.

Several mosquitofish were observed in the main wetland channel, and a school of yellow-eye mullet was observed near the culvert under the road near the wetland’s entrance. One dead common smelt was observed on the bank of the main channel. During the spotlighting survey, 15 shortfin eels were observed, with an estimated length range of 25 – 80 cm. Eels were widely distributed and common within the wetland. Two estuarine triplefins were also seen in the inland areas of the wetland, estimated to be 10 and 12 cm long. Two yellow-eye mullet were observed near the wetland outlet. 

One note of caution was a finding on the water quality with heavy metals — Arsenic Cadmium and Nickel being often within or close to ANZECC guidelines. Chromium copper lead and zinc results were generally outside the guidelines and in the case of zinc excessively so. However the use of a first flush measurement means that these are worst case results. ANZECC guidelines are for receiving waters rather than discharges. The Raglan Harbour greatly dilutes any discharge received.

So all in all the wetland is in a healthy condition.

Full report follows: >>>>>>>>>>>>>>>>>>>

KESSELS ECOLOGY
SURVEY, ASSESSMENT AND RESTORATION

17 March 2014

Our ref: WKDC.00216
Colin Hart
Waikato District Council
Private Bag 544
Ngaruawahia 3742

Dear Colin

Field Survey of Aroaro Wetland – Whaingaroa Harbour

1 . Introduction
Following your email of 2 December 2014 myself and my team have undertaken a brief survey of the Aroaro Wetland. We focused our survey on an evening spotlight of fish as well as a walk through to assess the restoration efforts which have been conducted over the last eight years or so. The vegetation survey is descriptive only but does allow for some comparisons with the quantitative data collected for the resource consents by Kessels Ecology over the last ten years.

2. Fish Survey Results
Methods
The wetland’s aquatic habitats were surveyed by a combination of a site walkover and spotlighting survey on 17 February 2014. The survey took place near low tide, which was at 6.34 pm. The wetland’s floodgates were open for the duration of the survey. Spotlighting was carried out between approximately 9 pm and 9.30 pm and consisted of walking alongside the main wetland channels searching for fish with a bright headlamp. Sunset was at 8.17 pm. Water quality was measured at the wetland outlet using a YSl Proplus hand-held meter.

Results
The marine green alga Ulva sp. was observed in the wetland channel, as were several crab burrows, indicating the brackish nature of the wetland. Green filamentous algae was also present. The estuarine species mudsnail Amphibola crenata and cockle Austrovenus stutchburyi (dead shells observed on surface) were also abundant.

Water quality was measured at 6.25 pm and was as follows: temperature: 23.7 °C; pH: 7.32, salinity: 27.62 PSU, conductivity: 41 mS/cm, and dissolved oxygen: 153.4% and 11.40 mg/L. The high salinity and conductivity readings indicate a brackish environment (for comparison, seawater has a salinity or approximately 35PSU). High dissolved oxygen concentrations are common in the late afternoon in environments with large amounts of aquatic vegetation due to plant photosynthetic activity.

During the daylight site walkover, several mosquitofish Gambusia affinis were observed in the main wetland channel, and a school of yellow-eye mullet Aldrichetta forsteri was observed near the culvert under the road near the wetland’s entrance. One dead common smelt Retropinna retropinna was observed on the bank of the main channel. During the spotlighting survey, 15 shortfin eels Anguilla australis were observed, with an estimated length range of 25 – 80 cm. Eels were widely distributed and common within the wetland. Two estuarine triplefins Forsterygion nigripenne were also seen in the inland areas of the wetland, estimated to be 10 and 12 cm long. Two yellow-eye mullet were observed near the wetland outlet.

3. Habitat Walkthrough Survey
During the fish survey a walk-through of the wetland vegetation was also undertaken.  It is evident that considerable weed control and replanting has been undertaken since the last formal review report (Floyd, 2010). The estuarine portions of the wetland have recovered well and are now virtually devoid of any weeds. The margins are largely restored although some weed control still needs to be undertaken. The plants on the south east side of the wetland on the mound will still need releasing for a year or two — they have obviously suffered from two dry summers. However, indigenous plant cover is good and once full canopy closure is achieved, these areas will be dominated by indigenous vegetation.

4. Conclusion
The aquatic species present within the wetland were all indigenous except for the mosquitofish. The species assemblage present is typical for a brackish environment and indicates that the wetland is providing a functioning habitat for several indigenous species.

The restoration work undertaken to date has considerably reduced weed infestation and improved indigenous vegetation cover. Provided that restoration works are maintained it is expected that the site will be restored in accordance with the restoration plan within the next 5 years.

Yours sincerely,

Gerry Kessels
Senior Ecologist & Managing Director
Kessels Ecology

Kessels Ecology PO Box 4225. Hamilton East. Hamilton 3247, New Zealand

Test Results
Raglan Stormwater discharge consent conditions require ‘first flush’ sampling and testing at selected sites around the Raglan business area. Monthly testing from a manhole at the harbour end of Bow St and 3-monthly testing at 3 other sites – 2 of these are catchpits on Bow St and the other is a manhole on Wainui Rd near Stewart St.

Summary
pH 6.0 to 7.2
Turbidity 34 to 600 NTU
Suspended Solids 65 to I980 g/m3
Arsenic 0.0025 to 0.22 g/m3
Cadmium Chromium 000! IS to 0.83 g/m3
Copper 0.0113 to 1.47g/m3
Lead 0.0091 to 3.6g/m3
Nickel 0.0036 to 0.53g/m3
Zinc 0.185 to 19.3g/m3
Petrol Hydrocarbons mostly at limits of detection
PAH mostly at limits of detection.

Comment on results

Suspended solids- tests yielded some very high results. WRC regional plan rule 3.2.4.6 sets a high limit of 100gm/m3 for stormwater discharges. Most sample results were greater than 100g/m3 and several results were greatly in excess of this figure. First flush test means that these were worst case rather than typical stormwater suspended solids concentrations.

Heavy metals — Arsenic Cadmium and Nickel tests were often within or close to ANZECC guidelines. Chromium copper lead and zinc results were generally outside the guidelines and in the case of zinc excessively so.

Again first flush measurement means that these are worst case results. ANZECC guidelines are for receiving waters rather than discharges. The Raglan Harbour greatly dilutes any discharge received.

Petro! Hydrocarbons and PAH levels are nearly all at the limits of detection i.e. not able to be detected.

Conclusion
This sampling regime gives an indication of what is going into the harbour from the Raglan business area. It does not show the state of the Raglan harbour.

This job is carried out by Waikato Regional Council in its sediment surveys of various points in the harbour. Surveys are carried out annually and reports were published in 2003 and 2008. Another report is due.

That some of the sample values are near to the ANZECC trigger values is encouraging considering that samples are first flush and therefore more concentrated than typical stormwater. Samples are of the contributing discharge rather than the receiving waters.

Suspended solids levels are a concern as are zinc levels.

To date Waikato Regional Council have considered Raglan harbour to have a low likelihood of damage to harbour organisms from trace elements and organic compounds in stormwater runoff.