2. Mangatawa catchment consents for earthworks, storm water ...

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
Concentrations of total nitrogen and total phosphorus were analysed in each of the sediment samples collected
from Rangataua Bay. Total nitrogen concentrations were somewhat variable with a slightly elevated
concentration recorded from Site 2 in the main channel as the Mangatawa Drain discharged across the
intertidal flats. The total phosphorus concentrations in the sediment samples suggest an attenuation effect
with a reducing concentration of total phosphorus with an increasing distance from the Mangatawa Drain
across the intertidal flats of Rangataua Bay. Together these nutrient results suggest that the Mangatawa Drain
may provide slightly elevated concentrations of total nitrogen and phosphorus to Rangataua Bay, however, the
variation in concentration is not high and these results are not indicative of serious nutrient input to the upper
reaches of Tauranga Harbour. No sediment quality guidelines for total nitrogen or phosphorus are listed in the
ANZECC guidelines. A wide range of haloethers, nitrogenous organic compounds, organochlorine pesticides,
polycyclic aromatic hydrocarbons, phenols, plasticisers and other halogenated organic compounds were
analysed for in the sediment quality samples. In none of the samples were any semi-volatile organic
compounds found in concentrations above the minimum detection limit for the analytical methods used.
The Mangatawa Drain, the stormwater management area and their respective catchments do not contribute
significant quantities of metallic or persistent organic contaminants to the intertidal area of Rangataua Bay and
the upper reaches of the Tauranga Harbour. There is some suggestion of slightly increased nutrient input into
this part of the harbour, however, any such nutrient inputs are not currently significantly impacting upon the
ecological function of either Rangataua Bay or the upper Tauranga Harbour.
5.13.6 Conclusion
The stormwater treatment wetland is of moderate significance but only on a local basis, as it contains a
relatively large area of flax – cabbage tree – shrubland habitat, a type that does not appear to occur elsewhere
in the Te Maunga to Mangatawa area. Nevertheless it only supports species that would also occur in other
types of adjacent habitats around the upper part of Rangataua Bay. Calls of spotless crake (Porzana tabuensis
plumbea; puweto) were played at four locations adjacent to the existing meanders but no response was
elicited. The presence of crakes (sparse; at risk) is unlikely given the lack of any significant riparian rush, Carex
or raupo fringe that would provide cover and nesting habitat. Most of the fringing aquatic vegetation consists
of Myriophyllum only.
In summary, the stormwater treatment wetland provides habitat that is of moderate value but only in a local
context and no species of national conservation concern was recorded. The rushmarsh – mangrove habitat of
SES No.13 is considered to have high ecological values in a regional context and contains fernbird, a species
considered to be at risk nationally, while its use by a second at risk species, banded rail, is also likely. This
area should be avoided by any major works associated with modification to the existing stormwater
management area.
The Mangatawa drain and Maranui waterway (swale, wetland and significant ecological site) are both typically
open waterways, with cool to warm water and variable levels of dissolved oxygen, clarity and conductivity. The
variable levels of conductivity are considered to reflect the presence of saline waters within the lower reaches
of both the Mangatawa drain and the Maranui waterway. Elevated counts of E. coli bacteria were also recorded
in both waterways and reflects the agricultural land use. While overall the water quality of the Mangatawa drain
was reasonably good, with the water for the most part being cool and well oxygenated, although discoloured
on occasions, that of the Maranui swale was poor, with low levels of dissolved oxygen and poor visual clarity
(0.1m).
Status: Final Page 59 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE