Lake Te Koo Utu Ecology, stormwater management and restoration options Prepared for Waipa District Council 3 April 2020
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Lake Te Koo Utu
Ecology, stormwater management and restoration options
Prepared for Waipa District Council
3 April 2020
Document Set ID: 10372718
Version: 3, Version Date: 17/04/2020
Document Quality Assurance
Bibliographic reference for citation:
Boffa Miskell Limited 2020. Lake Te Koo Utu: Ecology, stormwater management and
restoration options. Report prepared by Boffa Miskell Limited for Waipa District Council.
Prepared by: Andrew Blayney
Ecologist – Associate Principal
Boffa Miskell Limited
Mike Chapman
Water Resources Specialist
Te Miro Water Consultants
Ecology Inputs Kieran Miller
Reviewed by: Ecologist – Associate Principal
Boffa Miskell Limited
Status: Final Revision / version: [1.1] Issue date: 3 April 2020
Use and Reliance
This report has been prepared by Boffa Miskell Limited on the specific instructions of our Client. It is solely for our Client’s use for
the purpose for which it is intended in accordance with the agreed scope of work. Boffa Miskell does not accept any liability or
responsibility in relation to the use of this report contrary to the above, or to any person other than the Client. Any use or reliance
by a third party is at that party's own risk. Where information has been supplied by the Client or obtained from other external
sources, it has been assumed that it is accurate, without independent verification, unless otherwise indicated. No liability or
responsibility is accepted by Boffa Miskell Limited for any errors or omissions to the extent that they arise from inaccurate
information provided by the Client or any external source.
Template revision: 20180621 0000
File ref: BM19451_Lake_Te_Koo_Utu_report_draft_20200304_Final.docx
Cover photograph: [Lake Tee Koo Utu, © Boffa Miskell 2019]
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Version: 3, Version Date: 17/04/2020
CONTENTS
1.0 Introduction 1
1.1 Ecological context 1
2.0 Lake Te Koo Utu’s Catchment 2
2.1 Catchment Geology 2
2.2 Land use 2
3.0 Habitat values 3
3.1 Vegetation 3
3.2 Fauna 4
4.0 Discharges/outlets to the Lake 7
4.1 Discharges 7
4.2 Lake outlet 7
5.0 Current water and sediment quality 12
5.1 Water quality 12
5.2 Sediment quality 12
5.3 Botulism 14
5.4 Lake health implications 15
6.0 Ecological restoration approach 16
6.1 Short-term approach 17
6.2 Medium-term approach 18
6.3 Long-term approach 18
6.4 Lake Te Koo Utu Reserve ecological enhancement 19
7.0 Conclusion 20
8.0 References 21
Appendices
Appendix 1: Lake Te Koo Utu catchment map
Appendix 2: Lake Te Koo Utu water quality data
Appendix 3: Lake Te Koo Utu sediment quality data
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ort\BM19451_Lake_Te_Koo_Utu_report_draft_20200417_final.docx
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Appendix 4: Sediment sample – Certificate of Analysis – Hills
Laboratories.
Appendix 5: GHD option analysis review and Boffa Miskell and Te
Miro Water Consultants comments.
Figures
Figure 1: Lake Te Koo Utu reserve aerial and location. .................................1
Figure 2: Lake Te Koo Utu riparian edge showing sprayed grass to the
lake edge (foreground) exotic tree cover (background) and
aquatic emergent vegetation raupo (right background) and
bamboo spike sedge (right foreground). .......................................4
Figure 3: Outlet for commercial catchment on south-western edge of
Lake Te Koo Utu............................................................................ 8
Figure 4: Outlet for commercial catchment on south-western edge of
Lake Te Koo Utu – Looking along the flow path. ..........................8
Figure 5: Outlet for large residential catchment on western edge of
Lake Te Koo Utu............................................................................ 9
Figure 6: Outlet for commercial catchment on southern edge of Lake
Te Koo Utu – Looking along the flow path. ...................................9
Figure 7: Outlet for small residential catchment on north-eastern edge
of Lake Te Koo Utu......................................................................10
Figure 8: Lake Te Koo Utu lake outlet. .........................................................10
Figures 9 a-d: Time series of aerial images of Lake Te Koo Utu
showing the expansion of Raupo in the western end of the
lake since 2006. Indicative of sediment accumulation from
Large residential and commercial catchments. ...........................11
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1.0 Introduction
Boffa Miskell and Te Miro Water Consultants have been engaged by Waipa District Council to
produce a concept plan for the Lake Te Koo Utu Reserve (Figure 1). As part of this project it
was identified that the water quality, stormwater management and ecology of Lake Te Koo Utu
should be a primary driver to guide reserve management objectives and actions. This report
provides a summary of the ecology of Lake Te Koo Utu with an emphasis on the aquatic
environment. The report also explores the water quality issues of the lake, the sources and
contributors to these issues, and provides recommendations to begin to restore the Lake’s
water quality from both an ecological, hydrological, and engineering perspective.
There have been several iterations of recommendations for improving the water quality of Lake
Te Koo Utu over the last 20 years ranging from “soft treatment” options such as wetlands to
“hard treatment” options such as filters, desludging, and “nanobubble” technology. We intend to
focus our recommendations on options that provide the most sustainable outcomes over both a
short and long-term period with emphasis on solutions that act to reduce the long-term
contaminant inputs to the lake.
Figure 1: Lake Te Koo Utu reserve aerial and location.
1.1 Ecological context
Lake Te Koo Utu is a shallow lake, with an approximate depth of 2.4m, within the Cambridge
township and receives stormwater from the northern half of the town which is then discharged
east to the Karapiro stream. It is located in the Hamilton Ecological District and the area
surrounding the lake would have once been dominated by podocarp forest (primarily kahikatea
in the lower lying areas and a mixed conifer-broadleaf forest on the elevated landforms
(Deichmann & Kessels, 2013; Leathwick et al., 1995). It’s estimated that, since 1840, there is
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now less than 1% of wetlands, forest, and scrub remaining in the ecological district (Leathwick
et al., 1995). The area surrounding the lake itself has a threatened environment classification of
1, which means there is less than 10% of indigenous cover left in the area (Walker et al., 2015).
2.0 Lake Te Koo Utu’s Catchment
2.1 Catchment Geology
Deposition of the Hinuera Formation underpins the geology of the Lake Te Koo Utu catchment.
The Hinuera formation is characterised by alluvium deposits of alternating layers of sand, silt
and gravels which were deposited by the historical flow path of the Waikato River during the fan
forming phase. This sand, silt, gravel formation is influenced by regular changes in the channel
position as the Waikato River migrated across this surface. The Lake itself was once part of the
main river channel but was then ‘cut off’ from the main flow by mass deposits from the Taupo
eruption. The lake catchment soils range from highly permeable sandy loams to less permeable
silty deposits in depression areas.
The lake catchment (~177Ha including the lake reserve) includes a broad flat area of free
draining soil which provides a significant groundwater recharge zone for the lake. There is low
runoff from pervious garden and reserve areas and maximum rainfall infiltration can be
expected. Groundwater discharge was observed around the southern and western perimeter of
the lake during the site visits. It is expected a rapid increase in groundwater flow will occur
around the base due to the increased piezometric gradient created by the steep lake sides.
Tritium results for the Waikato show that waters from the shallow unconfined aquifers of the
Hinuera Surface are of recent origin, originating as precipitation within the last 5 years. Water is
discharged rapidly from these shallow aquifers. Similar age, well filtered groundwater is
expected to be discharging into the lake.
2.2 Land use
A map of the Lake Te Koo Utu catchment is provided in Appendix 1.
2.2.1 Residential Areas
The overall catchment is dominated by residential land use. The houses within the catchment
are typically low-density large lot residential with the majority of runoff going to private soakage
systems some of which will overflow to the road via kerb outlets. These kerb outlets then
connect to the reticulation network draining to the lake. Some runoff from residential areas is
therefore expected to reach the lake although it is difficult to estimate the exact area as it
depends on how well the private soakage devices are maintained and whether their overflows
are connected to the road kerb.
2.2.2 Roads
There are several high use and minor residential roads (total length approximately 7.2km)
which connect to the reticulation network. Stormwater trunk lines underlay Victoria Street, Taylor
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Street, Clare Street and Bowen Street as well as Lake Street through the southern commercial
catchment. The road network will be a contaminant source for runoff to the lake with an
estimated total impervious area in the order of 5.70Ha (assuming on average 8m wide
carriageway). The typical contaminants for urban roads are suspended sediments, heavy
metals, hydrocarbons as well as elevated temperature during summer months.
2.2.3 Commercial
A relatively small commercial sub catchment (5.35Ha) is located to the south of the lake. A
commercial land use has higher impervious surfaces compared to low density residential. Aerial
photography suggests the impervious coverage of this sub catchment is upwards of 90%. The
commercial area contributes more runoff to the lake relative to the larger residential catchment
to the north. Generally, commercial areas will generate higher contaminant loads and water
temperature per square metre compared to residential areas.
3.0 Habitat values
Ecological values were assessed through a site walk over and desktop review of existing
information for the area including previous reports and national databases.
3.1 Vegetation
Lake Te Koo Utu is immediately surrounded by a complex patchwork of planted non-native and
native vegetation. Most of the southern lake edge is mown grass with a sprayed lake edge.
Most of the canopy cover in the reserve surrounding the lake is comprised of exotic trees (of
which many are large, older specimens) with native shrubs, tree ferns, and pest plant species,
such as Tradescantia, Agapanthus, ivy, and hydrangeas, common below the canopy. The
vegetation overall is highly modified, and it is unlikely any of the surrounding vegetation is
remnant native vegetation. Riparian vegetation is variable in cover and predominately exotic
deciduous trees with occasional kahikatea (Dacrycarpus dacrydioides) and swamp cypress
(Taxodium distichum) occurring directly on the lake edge.
Within the lake, emergent aquatic vegetation is primarily made up of raupo (Typha orientalis),
bamboo spike sedge (Eleocharis sphacelata), and white water-lily (Nymphaea alba) (Figure 2).
Submerged aquatics were limited but Potamogeton sp. and starwort (Callitriche stagnalis) were
noted during the sediment sampling. Native cover is the most dominant emergent vegetation in
the lake.
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Figure 2: Lake Te Koo Utu riparian edge showing sprayed grass to the lake edge (foreground) exotic tree cover
(background) and aquatic emergent vegetation raupo (right background) and bamboo spike sedge (right foreground).
3.2 Fauna
3.2.1 Fish
The predominant fish species present in Lake Te Koo utu are non-native pest fish such as
goldfish (Carassius auratus), gambusia (Gambusia affinis), and perch (Perca fluviatilis). There
have also been captures (at least 20 years ago) of shortfin eel (Anguilla australis) and common
bully (Gobiomorphus cotidianus) in the lake (Kessels Ecology, n.d.). Due to the significant fish
passage barrier between the lake and Karapiro stream (see Section 4.2) it is unlikely that it has
much importance or value for native fish species. However, a fish survey is needed to confirm
the value of the lake to native fish.
3.2.2 Bats
Long-tailed bats (Chalinolobus tuberculatus) which are “Threatened – Nationally Critical”
(O’Donnell et al., 2018) were recorded in 2014 at the Lake (Kessels Ecology, n.d.) and are
detected frequently in the gully systems and wider landscape surrounding Cambridge. Long-
tailed bats are highly mobile. Their home range is potentially very large (657-1589 ha in studied
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populations), with bats frequently changing and utilising a wide network of roosts across their
home range. They preferentially commute and forage along linear features and/or over water
bodies (Rockell et al., 2017), foraging for a wide selection of insect prey (Gurau, 2014).
Long-tailed bats typically roost in trees that are greater than15 cm diameter at breast height
(DBH), and have one or more of the following features:
• cracks, crevices, knot holes, cavities and/or fractured limbs large enough to support
roosting bat(s);
• sections of loose flaking bark large enough to support roosting bat(s);
• hollow trunks, stem or branches;
• deadwood in canopy or stem of sufficient size to support roost cavities or hollows; and
• epiphytes in the canopy.
Roost trees, particularly those used as maternity roosts (used for breeding), are a limiting
resource for all long-tailed bat populations due to the specific thermal requirements necessary
for high quality roosts (Sedgeley, 2001). Long-tailed bats generally select the oldest and largest
trees available as maternity roosts 1, in both natural and modified environments. Solitary roosts
often occur in less ideal roosts and bats can opportunistically use tree fern crowns and cabbage
trees.
The Lake Te Koo Utu reserve contains a large number of potential roost trees due to the wide
variety of exotic trees of varying ages. This resource should be carefully managed to ensure
large, old trees are not lost through reserve management activities and are actively conserved.
Where large trees with potential roost features need to be removed or pruned, a bat tree fell
protocol should be followed to ensure bats are not present in the trees/limbs being felled.
3.2.3 Birds
Lake Te Koo Utu has a bird assemblage reflective of its urban location, habitat type
(lake/wetland), and its proximity to the Waikato River. In terms of abundance, the avifauna
assemblage is likely dominated by naturalised introduced species which are common in the
agricultural and residential landscape surrounding Cambridge such as common starling, house
sparrow, and European goldfinch. Several At-Risk shag species are present and use the
surrounding trees for roosting and fish in the lake itself. Table 1 provides a list of bird species
seen during the site visits, detailed in previous reports, or otherwise expected in the area
Table 1: Avifauna species list and conservation status – confirmed and likely species.
Species Conservation
Status - (Robertson
Common name Scientific name et al., 2017)
Black shag Phalacrocorax carbo At Risk - Naturally
novaehollandiae uncommon
Common myna Acridotheres tristis Introduced and
naturalised
Common starling Sturnus vulgaris Introduced and
naturalised
Domestic duck Anas platyrhynchos domesticus Introduced
1Also referred to as a breeding roost or a maternity roost. Roosts where female bats
congregate to give birth and raise pups before they are old enough to fly.
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Species Conservation
Status - (Robertson
Common name Scientific name et al., 2017)
Eastern rosella Platycercus eximius Introduced and
naturalised
Eurasian blackbird Turdus merula Introduced and
naturalised
European goldfinch Carduelis carduelis Introduced and
naturalised
Grey warbler Gerygone igata Not Threatened
Greylag goose Anser anser Introduced and
naturalised
House sparrow Passer domesticus Introduced and
naturalised
Little black shag Phalacrocorax sulcirostris At Risk - Naturally
uncommon
Little shag Phalacrocorax melanoleucos
brevirostris Not Threatened
Little shag Phalacrocorax melanoleucos Not threatened
brevirostris
Malay spotted dove Streptopelia chinensis tigrina Introduced and
naturalised
Mallard duck Anas platyrhynchos Introduced and
naturalised
Mallard duck/grey duck hybrid Anas superciliosa x platyrhynchos Not Threatened
North Island fantail Rhipidura fuliginosa placabilis Not Threatened
Paradise shelduck Tadorna variegata Not Threatened
Pied Shag Phalacrocorax varius varius At Risk - Recovering
Pukeko Porphyrio melanotus Not Threatened
Sacred kingfisher Todiramphus sanctus vagans Not Threatened
Silvereye Zosterops lateralis Not Threatened
Song thrush Turdus philomelos Introduced and
naturalised
Spur-winged plover Vanellus miles novaehollandiae Not Threatened
Swamp harrier Circus approximans Not Threatened
Tui Prosthemadera novaeseelandiae
Not Threatened
Welcome swallow Hirundo neoxena Not Threatened
White-faced heron Egretta novaehollandiae Not Threatened
3.2.4 Lizards
Very few records exist for lizards in the Cambridge area. The native species most likely to occur
around Lake Te Koo Utu is copper skink (Oligosoma aeneum) which are Not Threatened
(Hitchmough et al., 2016). This species, in the Waikato, prefers damp, complex vegetation
within which it can seek refuge. The vegetation surrounding the lake could provide habitat for
this species. Potentially also present is the non-native plague skink (Lampropholis delicata)
which is a pest species and found throughout the Waikato.
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Version: 3, Version Date: 17/04/20204.0 Discharges/outlets to the Lake
4.1 Discharges
The three catchments that discharge to the lake discharge via separate outlets (Figure 3 to
Figure 7). There is no water quality treatment or effective mitigation of water velocity associated
with any of these outlets. The outlet fed by the large residential catchment is set back from the
edge of the lake and is fed into the main water body via a large concrete chute structure which
is unlikely to reduce velocity in increased flow periods (Figure 5). Both the large residential and
commercial catchment outlets show signs of scour and flattening of adjacent vegetation (Figure
4 & Figure 6).
Both of the large residential and commercial catchment outlets are located at the western end of
the lake and there is a large amount of soft sediment accumulated in this area which has been
colonised by raupo. Raupo would not have been the initial cause of sediment disposition as it
typically encroaches from the shallow edge of waterbodies. However, raupo may have
contributed to an accelerated rate of sediment deposition. The establishment of raupo and other
vegetation will exacerbate and increase the rate of sediment deposition by slowing water flows
and encouraging settlement of sediments. The accumulation of soft sediments described is
visible in aerial imagery indicated by the expansion of raupo colonisation from the western end
of the lake while the other plant species, such as water lilies and bamboo spike edge along the
southern lake end, have changed little over this time (Figures 9 a-d).
With reference to Table 5-5 contaminant loads for various daily traffic counts (NZTA Stormwater
Treatment Standard for Highways, May 2010) sediment loading at the lake outlets from the total
road system connected to the pipe network could be in the order of 170kg/year based onFigure 3: Outlet for commercial catchment on south-western edge of Lake Te Koo Utu
Figure 4: Outlet for commercial catchment on south-western edge of Lake Te Koo Utu – Looking along the flow path.
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Version: 3, Version Date: 17/04/2020Figure 5: Outlet for large residential catchment on western edge of Lake Te Koo Utu.
Figure 6: Outlet for commercial catchment on southern edge of Lake Te Koo Utu – Looking along the flow path.
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Version: 3, Version Date: 17/04/2020Figure 7: Outlet for small residential catchment on north-eastern edge of Lake Te Koo Utu.
Figure 8: Lake Te Koo Utu lake outlet.
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Version: 3, Version Date: 17/04/2020a: 2006 c: 2010 (winter so raupo and deciduous tree leaves have browned/dropped off)
b: 2016 (winter so raupo and deciduous tree leaves have browned/dropped off) d: 2019
Figures 9 a-d: Time series of aerial images of Lake Te Koo Utu showing the expansion of raupo in the western end of the lake since
2006. Indicative of sediment accumulation from Large residential and commercial catchments.
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Version: 3, Version Date: 17/04/20205.0 Current water and sediment quality
5.1 Water quality
Waikato Regional Council has been monitoring the water quality of Lake Te Koo Utu monthly
since July 2019. Appendix 2 details all the parameters tested during this monitoring. Throughout
this period, monitoring has revealed consistently elevated total suspended solids, electrical
conductivity, turbidity, and total nitrogen levels when compared to Australia and New Zealand
guidelines (Australian and New Zealand Governments, 2018) and Waikato Regional Council
(WRC) guidelines where applicable (Waikato Regional Council, n.d.) 2. There was also an
observed spike in E. coli in the December sampling, however this remained within the
“satisfactory” range provided by Waikato Regional Council. Phosphorus is very high compared
to all guideline values with the lowest result three times higher than the WRC unsatisfactory
limit. Total nitrogen is universally elevated above guideline values and WRC unsatisfactory limit.
Compared to the Draft National Policy Statement for Freshwater Management limits Chlorophyll
a, total phosphorus, and total nitrogen fit into the D attribute band (the worst health band and
below the proposed national bottom line). We consider, based on the data available, Lake Te
Koo Utu is, without significant intervention, likely to continually fail to have improved water
quality parameters that would move it into the C attribute band and therefore above the
proposed national bottom line.
WRC measurements at 0.2m intervals throughout the water column of the lake for dissolved
oxygen and temperature show no thermal stratification of the water body with consistent
temperatures throughout the water column. The elevated water temperatures in November and
December detected are a particular cause for concern for fish health.
Dissolved oxygen (DO), measured at 0.2m intervals, shows a high level of stratification in the
September sampling round with water near the bottom having DO levels of >40%. This
suggests that lake mixing during this period was limited. In November and December, DO levels
were low (>55%) throughout the water column with some evidence of stratification. DO levels
are below guideline values and in the WRC unsatisfactory category below 1.4m in September
and October (with DO only marginally higher above 1.4m deep in October). In November and
December DO levels are very low throughout the water column and well below WRC
unsatisfactory level.
Temperature profiles are as expected for a shallow lake with relatively little shading and an
estimated residence time (water flushing rate) of 26 days (Kessels Ecology, n.d.).
The full water quality data set is available in Appendix 2.
5.2 Sediment quality
Three sediment samples were collected on 11 December 2019 including;
2
These parameters are not Lake specific and therefore should only be used as an indicative benchmark for water
quality.
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Version: 3, Version Date: 17/04/2020• A composite sample of several areas of deposited sediment around the major
stormwater inflow pipe at the western end of the lake;
• A composite sample of multiple locations along the northern lake edge; and
• A composite sample on several points in the outlet are of the lake.
Contaminant concentrations from samples taken along the northern lake edge and in the
outlet, are below relevant guideline values. However, the deposited sediment around the
stormwater pipe shows elevated levels of iron, all heavy metals, and polycyclic aromatic
hydrocarbons (PAHs) compared to the other two samples. Of these lead, zinc, and PAHs
concentrations either exceed or are approaching the Australia and New Zealand (Australian
and New Zealand Governments, 2018) guideline values for sediment quality.
These elevated contaminant loads found in the western end are typical of urban stormwater
runoff, particularly lead, copper, zinc, and PAHs. Major sources of these contaminants from
urban catchments are likely to be (Auckland Regional Council, 2008):
• Copper; brake linings (break wear) and atmospheric deposition.
• Lead; garden soil, atmospheric deposition, and lead roof flashing and nail heads.
• Zinc; tyre wear (tyres contain up to 2% zinc by weight) and roofing materials,
PAHs can be derived from multiple sources, and this depends on the activity and land use of
the residential areas. PAHs are often attributed to motor vehicle emissions in urban catchments.
Identifying the sources of PAHs can be achieved through the comparison of the ratios of certain
PAHs as different sources contain different ratios of individual PAHs like a fingerprint. The PAH
ratios detected in Lake Te Koo Utu sediments ware not consistent with fossil fuel combustion
by-products. The composition pattern of the PAHs is most similar to samples taken in the
Auckland Region where coal tar was determined to be the major source of PAH contamination.
Indicator ratios present in the Lake Te Koo Utu samples where high (~1)
indenopyrene/benzo(ghi)perylene and benz(a)anthracene/chrysene ratios which would not be
consistent with petrogenic PAH sources such as tyres, diesel oil, or bitumen as displayed in
Figure 14 of (Depree & Ahrens, 2007). Indeno(1,2,3-c,d)pyrene / (Indeno(1,2,3-c,d)pyrene +
Benzo[g,h,i]perylene) (0.50-0.53) and Benzo[a]anthracene / (Benzo[a]anthracene + Chrysene)
((0.48 – 0.51) ratios align with that found in Motions Creek sediments in Auckland where it was
estimated there was a coal tar content of 0.1-1% contributing 85-95% of total PAH (Depree &
Ahrens, 2007).
Coal tar hasn’t been used since the 1980s but is still present in the older layers of many roads,
foot paths, and driveways and is a legacy contaminant which persists in sediments. Coal tar is
PAH rich when compared to the more modern bitumen now used and small amounts of it can
disproportionately elevate PAH levels compared to bitumen (Ahrens et al., 2007). Coal tar can
be released through road wear, road reconstruction, and the erosion of road side soils
contaminated by coal tar (Ahrens et al., 2007; Depree & Ahrens, 2007).
The full sediment quality data set is available in Appendix 3 and the certificate of analysis is
available in Appendix 4.
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Version: 3, Version Date: 17/04/20205.3 Botulism 3
There are frequent public reports of ducks displaying symptoms of botulism (reluctance to fly,
stumbling gait, and neck paralysis) and dead ducks are collected form the lake daily by the
Waipa District Council parks team.
Botulism is caused by waterfowl ingesting a neurotoxin produced by several strains of the
bacteria Clostridium botulinum (Thomas et al., 2008; Wobeser, 1997). The epizootiology
(factors which cause outbreaks of disease) of botulism is extremely complex with a large range
of contributing factors which can cause outbreaks and ultimately waterfowl deaths. Botulism can
occur in large, deep, and well oxygenated wetlands as well as river systems (i.e. waterbodies in
good health) and are relatively unpredictable based on comparison of environmental factors
between water bodies and even history of outbreaks (Thomas et al., 2008). Botulinum spores
are common in wetlands and lake sediments and are persistent in the environment.(Thomas et
al., 2008; Wobeser, 1997).
Optimal conditions for growth of C. botulinum bacteria depend on the strain, but in genera,l they
require high temperatures (optimal growth range of 25°C to 40°C), low oxygen levels, and high
protein substrates (Thomas et al., 2008). Early understanding of botulism suggested that the
bacteria grew and produced toxins in the “sludge bed” at the bottom of waterbodies caused by
accumulations of decaying organic material depleting oxygen. Research has since shown
rotting vegetation and stagnant water is a poor substrate for C. botulinum growth (Thomas et
al., 2008). Until recently the subsequent theory of “microenvironment concept” was used to
explain botulism outbreaks. This theory suggests that invertebrate carcasses or other decaying
matter provided a suitable substrate/microenvironment for C. botulinum growth and waterfowl
ate these toxin laden food particles. However, there is lack of conclusive evidence that shows
the role of invertebrate carcasses role in botulism outbreaks (Thomas et al., 2008).
Current research and understanding of botulism outbreaks focuses on the “carcass-maggot”
cycle of botulism where vertebrate carcasses (which contain C. botulinum spores from ingesting
them while feeding) provide an optimal high temperature, low oxygen, and high protein
substrate for bacteria growth and toxin production. Botulinum spores are widely found in
freshwater habitats and not an indicator of poor water quality. Spores are long lived and able to
withstand environmental extremes, however they are more common in areas where botulism
outbreaks have previously occurred. A large proportion of waterfowl, which feed in botulism
prone areas, may have botulinum spores in their liver or intensities (Thomas et al., 2008). Then
fly larvae (maggots), which appear to be unaffected by the toxin, feed on the decaying carcases
and concentrate the botulinum toxin. Waterfowl, which won’t typically feed on decomposing
carcasses, feed readily on these fly larvae, when they are exposed, which can contain
extremely lethal concentrations of botulinum toxin (one maggot can contain a lethal dose)
(Thomas et al., 2008; Wobeser, 1997). Instigating factors for outbreaks can therefore be
anything that kills waterfowl.
Control and management of botulism outbreaks therefore is best focused on the collection of
carcasses before they become maggot infested and therefore prevent the exposure of
waterfowl to botulinum toxin. In the case of Lake Te Koo Utu, this is already being done and as
the cause of mortality and ultimately the toxigenic carcasses may not be within the reserve’s
boundaries (and unlikely to be identified), then vigilance in preventing the spread within the
reserve’s boundary should remain the focus. It is important to note that there has never been a
case of botulism in humans associated with botulism in wild birds (Thomas et al., 2008).
3
This section provides an extremely brief summary of a very large and complex area of active, evolving research and
understanding and Thomas et al. (2008) should be referred to for more a detailed synopsis.
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Lake Te Koo Utu has elevated nutrient levels which, when coupled with high temperatures,
contributes to an excess of algal and plant growth within the water body of the lake. Excess
algal growth contributes to increased oxygen demand within the water body through overnight
respiration and through the build-up of dead organic matter in lake sediments being broken
down. The DO levels of the lake demonstrate this effect with low DO in deeper water during
warmer months. Depending on the species of algae that grows, elevated nutrient levels can
also contribute to toxic algal blooms such as cyanobacteria which are a threat to human and
animal health.
A “real-time” example of the impacts of elevated nutrient inputs can be observed within the
water quality data; chlorophyll a concentration of the lake indicates an algal bloom in September
which coincided with the depletion of oxygen from the bottom of the lake and the dissolved
oxygen stratification observed at the same time. This algal bloom and the following water quality
data also give support to Lake Te Koo Utu being nitrogen limited (Hobman, 2000). The water
quality data indicates that in the cooler months there is an oversupply/build-up of nitrogen. This
excess then appears to be used up during the warmer months with a trend towards nitrogen
limitation (nitrogen limitation is indicated by a total nitrogen : total phosphorus ratio of < 7:1
(Abell et al., 2010)). This indicates that, controlling the inputs and accumulation/build-up of
nitrogen is important to improve water quality and limit algal blooms during the warmer months
of the year.
Elevated suspended solids and the associated visibility (secchi depth) contributes to issues with
fish health such as blocking gills and disrupting food webs. However, the fish assemblage within
Lake Te Koo Utu is dominated by turbidity tolerant species and this is likely so for the food
species they rely on. High suspended solids can also inhibit plant and algal growth, however
there is no evidence for this in the data as there is no relationship between suspended solids
and Chlorophyll a concentration. It is unlikely that the turbidity of the water is inhibiting growth.
The most likely impact of elevated suspended solids in the lake is the transportation of
phosphorus and other contaminant laden sediment around the lake.
Elevated PAHs and metals found in the sediments close to the stormwater outlet for the western
residential catchments and the commercial catchment can have direct acute and potentially
long-term toxic effects on aquatic life within the lake.
The presence of elevated E. coli and Enterococci are indicators of faecal contamination and
indicate the elevated risk of contact with the water causing disease.
The elevated nutrients and contaminants in the lake are likely a result of run off from the urban
catchment. The sources of these are likely run off from residential houses, run off from roads,
and contamination of animal wastes. Elevated suspended sediment levels are likely a result
from a combination of urban stormwater runoff and the wind driven and mixing/resuspension of
lake bed sediments which can be a dominant driver sediment transport in shallow lakes (Bryers,
2000, p. 200). However, the accumulation of sediment does point to an ongoing discharge of
sediment via stormwater inputs.
The existing stormwater outlets at Lake Te Koo Utu are likely to have been contributing runoff to
the lake for the previous 50 years or more. Understanding how the catchment was developed
provides insights as to how long this build-up of contaminants has been occurring.
Prior to the roads being sealed, stormwater from residential lots (roof and driveway) would go to
ground soakage. The roads were gravel (pervious) with no runoff collected within a pipe and
therefore no outlet to the lake. The sealing of the road network necessitated a need for a kerb
and channel pipe network to manage the road runoff. Interestingly, there are still several minor
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grass berm with no kerb and channel. This is a good outcome in terms of stormwater treatment.
The issues began essentially with the sealing of the road network and parking for commercial
areas. The solutions therefore can also be found within these areas with a focus on the first
devices in areas of high use (traffic volume and intersections with vehicle braking) where most
contaminants are generated and where most treatment can be achieved.
6.0 Ecological restoration and stormwater
improvement approach
6.1 Key aspirations
The key aspirations for Lake Te Koo Utu for WDC, mana whenua and community are to
improve the water quality of the lake. The objective of the Lake Te Koo Utu concept plan, that
this report supports, is to improve water quality before it feeds into Te Awa O Waikato and to
enhance biodiversity within the boundaries of the reserve and Lake. The reason for this
constraint is that significant improvements require a catchment wide approach (integrated
stormwater management) beyond the boundaries of the Lake Te Koo Utu reserve which is
outside of the scope of the plan.
Through this process an aspiration is that there is an increase in the understanding of the
community of the ecological and cultural value and importance of Lake Te Koo Utu. This will
involve the community understanding the lakes water sources through education and what
impact they might have on it, educating community about significance of the lake to mana
whenua and understanding how the community can make changes to improve the water quality
outcomes for waterbodies they impact.
We urge caution in the expectation of water quality outcomes as the catchment use surrounding
the lake will not change there will be under similar water quality pressures in the future and
continue to receive untreated contaminants from the surrounding urban and commercial
catchment. Therefore, there needs to be an understanding that significant improvements to the
water quality of the lake will require stormwater management at source through improvements
of infrastructure to decrease contaminants entering the lake from the urban catchment.
We understand the WDC Waters Assets Team are proposing an LTP business case for a
pioneering study to look at catchment wide options to improve stormwater runoff to the lake. It is
likely these options will be like those presented in this report.
Without the significant improvement of stormwater treatment, the recommendations below, in
isolation, are unlikely to lead to measurable improvements in water quality.
With the implementation of stormwater treatment; the aspiration for the Lake’s water quality is
that it consistently maintains water quality that would put it above the Draft National Policy
Statement for Freshwater Management national bottom line for all attributes and consistently
meets the C band criteria or better.
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From the water and sediment quality data there are clear focal points for improving the water
quality of Lake Te Koo Utu. These are:
• Reduce or capture the first flush coarse sediment from the western residential
catchments and commercial catchment. Therefore, capturing phosphorus, metals, and
PAH contamination.
• Reduce nitrogen and phosphorus inputs. With emphasis on reducing the build-up and
input of nitrogen throughout the year to prevent, or at least reduce the magnitude of
algal blooms.
A comprehensive options report was provided to Waipa District Council in 2005 by Tonkin &
Taylor (Chizmar, 2005). GHD reviewed these options with updated costings in 2018 (Kirk,
2018). To avoid revisiting each option within this report we have provided comments on the
review of options provided by GHD in Appendix 5. What we consider the best options for
managing Lake Te Koo Utu water quality and the surrounding park are outlined in more detail
below. We urge caution in the expectation of water quality outcomes as the catchment use
surrounding the lake will not change it will be under similar water quality pressures in the future
and continue to receive contaminants from the surrounding catchment. Efforts should be in the
context of improving water quality with an understanding of the constraints of the potential water
quality outcome is not the same for an urban lake than a lake within a less intensive land use
catchment.
We have split our recommendations into actions that we consider should be undertaken in the
short, medium, and long-term and have focused on solutions that seek to address causes of the
degradation of water quality rather than the symptoms. These time frames are indicative with
many of the actions in the long-term section placed there as they are likely to occur over a long
time period but actions, such as installing stormwater treatment devices, should be addressed
as and when the opportunity is available within replacement and maintenance timeframes. All
options are addressed only as recommendations and subject to detailed design, optioneering,
and feasibility studies where required. It is important to note that restoration of water quality will
be a long-term proposition.
6.2.1 Short-term approach
Actions we recommend should be completed in the short term;
• Education of stakeholders in the catchment and highlight of issues within reserve
design – Educating residents that live in the catchments that feed into Lake Te Koo Utu
on the types of contaminants and their impacts on the health of the lake is important –
promoting a catchment wide approach to managing runoff from hard surfaces.
Education content should also provide effective ways on how to minimise the
environmental impact on the lake to reduce these contaminant loads. Signage and
landscape treatments within the reserve can also serve to contribute to this messaging.
Daylighting some of the ground water inputs along the southern side of the lake, with
interpretation panels, has the potential to provide information on different sources of
water that feed into the lake. This can be contrasted with highlighting stormwater inputs
and their effects. It is important to acknowledge and highlight these aspects rather than
hide them away to prevent the cognitive disconnect between actions in the catchment
and lake water quality outcomes. When carrying out the subsequent recommendations
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approaches will continue to be important.
• Integrate a planting shelf wetland into the lake outlet arm – we recommend that a
wetland (bands of shallow and deep marsh) designed to trap sediments and to provide
shading and nutrient attenuation in the lake outlet arm is installed. This wetland will
seek to minimise the impact of the stormwater inputs into the lake reaching further into
the wider environment and Karapiro stream. This wetland will also have the benefit of
moving ducks further into the wider, deeper, areas of the lake reducing potential faecal
contamination in the shallowest part of the waterbody.
• Adoption of business cases in the LTP which promote an integrated approach to
stormwater management within the Lake catchment.
6.2.2 Medium-term approach
Actions we recommend should be completed in the medium term;
• Naturalising the lake edge – The current lake edge consists of either vertical retaining
walls, bare soil banks, or, on the northern edge of the lake, large aggregate rock from
track construction. Remediation could be done by removing the retaining timber and re-
sloping the lake edge. Planting will assist in stabilising the slope, attenuating nutrients
and provide shading of the water around the edge. In the areas where there is a current
slope, but it is predominantly rock, coir-logs can be installed to enable planting and in
the medium term encourage the development of suitable growing conditions. This
naturalisation does not have be around the entire lake edge but should respond to the
needs and desires of the community for access and views of the lake.
6.2.3 Long-term approach
Actions we recommend should be completed in the long term;
• Options are limited at the stormwater outfall points into the lake due to restricted access
and narrow space around the lake perimeter. However, the space within the lake offers
an opportunity to implement ‘end of line’ water quality treatment measures which can be
closely aligned to landscape and amenity objectives for Lake Te Koo Utu. We
recommend formalising the naturally functioning wetland by providing a sediment
forebay in the western end of the lake to treat first flush (coarse sediment) runoff at the
pipe outlets from the western catchments. This wetland forebay would seek to contain
the current expansion of sediment deposition and vegetation in the western arm of the
lake. It should be designed to maximise nutrient attenuation and sediment trapping, and
habitat provided (however habitat should be secondary to other functions). The forebay,
would need to allow access for periodic maintenance and removal of sediment. This
option we consider requires a feasibility study from the point of view of constructability,
potential efficacy, consenting, and community feedback. During the wetland’s
construction options for removal of sediment which would reduce internal nutrient
loading should be consider; several options are provided and review in Appendix 5 that
should be considered.
• As stated earlier, the preferred option for stormwater management is to adopt an
integrated catchment approach with practical treatment options implemented when and
where possible in the catchment as part of either a new capital works programme or as
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improvement work as an ‘add on’ as part of a wider project – for example road upgrade
or pipe upgrade – is often a viable option which other local authorities are considering.
This provides for cost savings and efficiencies to achieve good stormwater treatment
outcomes while not being the core purpose of the project. It also allows WDC to
prioritise the best projects to deliver the best water quality outcomes such as focussing
on renewal programme for higher use roads with greatest contaminant loading or within
the commercial catchment. A full synopsis of stormwater treatment options is provided
in Appendix 5 and the approach taken at each individual upgrade will be dependent on
a multitude of factors and need to be identified within this context at the time of design.
Options are likely to include at source solutions such as catch pit inserts to collect
coarse sediments and litter and more comprehensive options such as ‘plug and play’
bio-retention devices such as pre-cast raingardens and tree pits and planted swales (as
part of road/pipe upgrades) all of which reduce the need for additional downstream
measures within the lake itself such as communal sediment forebays. Porous surfacing
for new commercial and higher density residential areas and use of inert roof materials
are also good outcomes for reducing contaminant runoff into the lake.
Bio-retention devices were not proposed in previous reports and we have provided a synopsis
of this option in Table 2.
Table 2: Bio-retention option synopsis
Control Improvement Description Indicative Boffa Miskell and Te
Area Option Costs Miro Water
Consultants
comments
Source Bio-retention Bio retention such as $10,000 fitted Bio-retention options
Control devices for raingardens can be for standard such as raingardens
road runoff sized to treat a HYNDS retrofitted during
discrete road sub 2mx3m precast road/sw reticulation
catchment leading to unit + growing upgrade. Focus on high
a single catchpit or – media and traffic areas and/or
depending on grade underdrain. commercial/industrial
– developed as One device is land use as part of new
larger communal required for consent or consent
units (generally < approximately variation.
100sqm) every 100m
length of road.
Overall a combined approach to managing stormwater runoff is recommended over the long
term. Measures can be implemented both within the upper catchment as well as at the point of
discharge to reduce the overall contaminant input into the lake. This approach is consistent with
the messages in previous reports.
6.3 Lake Te Koo Utu Reserve ecological enhancement
In addition to the recommendations directly related to improving water quality, our
recommendations to maintain and improve the ecological value of the reserve surrounding Lake
Te Koo Utu are:
• Develop a natural resource plan for the reserve; This plan should look at the different
availability of natural resources for native fauna over the course of the year (for example
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plantings should be selected, where possible, to fill in the gaps during the year for
specific resources. This will allow the reserve to provide a year-round resource to native
fauna and allow longer/permanent residence for species that currently are transient
visitors to the reserve. This resource plan can also look at habitat features that could be
created through varying the current maintenance practices such as; using natural timber
and tree trimmings to provide complex woody habitats for native skinks.
• Develop a vegetation management plan – with a focus on habitat enhancement and
succession planting; This plan should be developed to look at areas where
supplemental planting can increase the amount of habitat complexity in the reserve.
Approaches like underplanting and adding epiphytes like orchids or climbers should be
considered. Succession planning should look at the age and lifespan of all larger trees
and identify when and where trees will likely need to be removed and seek to transition
new planting and older trees to ensure a continuous rotation of habitats are available.
The environment within and surrounding the reserve is a complex of native and exotic
vegetation and while a “native plant first” policy could be considered for new plantings,
the removal of exotic plants, as policy, would do more harm than good. We consider
that managing and enhancing habitat values is a greater priority.
• Develop bat management protocols; the presence of large trees and confirmed bat
presence means there is a risk that, without appropriate management, tree
maintenance and felling can kill the “Threatened – Nationally Critical” long tailed bat.
Protocols should be developed for the planning of, and survey prior to works that impact
potential bat roosts. As a note; native bat species are ‘absolutely protected’ under the
Wildlife Act (1953, s63 (1) (c)) which is administered by the Department of Conservation
and disturbing, injuring, or killing is a breach of this act.
• Review of reserve management practices – current reserve practices should be
reviewed to determine if there are practices that could contribute to the degradation of
water quality or habitat in the reserve. Things to consider, for example, would be the
maintenance regime and methods for plantings, spraying of lake edges, and fertiliser
use.
7.0 Conclusion
The water quality of Lake Te Koo Utu has been degraded significantly by the inputs from the
catchments surrounding it. For example, sediment yields from road runoff are in the order of
100-200kg/year with lake inputs occurring over many decades since the roads were sealed and
pipe network installed.
We have provided recommendations to improve the water quality through a combination of
options to be implemented at the point of discharge in the lake reserve, within the wider lake
body, and at the likely source of contaminants to achieve overall improvements in lake water
quality.
In the long-term, a first flush forebay (with high flow bypass) combined with source controls in
the stormwater infrastructure to capture and treat first flush runoff volumes has high potential to
improve water quality of Lake Te Koo Utu. These options can integrate with the wider open
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enhancement project.
It is important to recognise that while we have provided many recommendations for works within
the reserve itself that the critical factor for improving the water quality of Lake Te Koo Utu is
treatment of the stormwater before it gets to the lake. Without a catchment wide approach to
improvement, stormwater treatment in lake and reserve options in isolation are unlikely to
achieve measurable improvements to water quality.
8.0 References
Abell, J. M., Özkundakci, D., & Hamilton, D. P. (2010). Nitrogen and phosphorus limitation of
phytoplankton growth in New Zealand lakes: implications for eutrophication control.
Ecosystems, 13(7), 966–977.
Ahrens, M., Depree, C., & Olsen, G. (2007). Environmental forensics: Cracking the case of the
contaminated streams. Water and Atmosphere, 15(1).
Auckland Regional Council. (2008). Urban sources of copper, lead and zinc (Auckland Regional
Council Technical Report TR 2008/023). Auckland Regional Council.
Australian and New Zealand Governments. (2018). Australian and New Zealand guidelines for
fresh and marine water quality. www.waterquality.gov.au/anz-guidelines
Bryers, G. (2000). Protocol for monitoring trophic levels of New Zealand lakes and reservoirs
(Issue 99/2). Lakes Consulting, Report.
Chizmar, J. (2005). Tonkin & Taylor letter to Waipa District Council; Options for the
management of Lake Te Ko Utu and surrounding park.
Deichmann, B., & Kessels, G. (2013). Significant Natural Areas of the Waipa district: Terrestrial
and wetland ecosystems (Waikato Regional Council Technical Report TR 2013/16).
Prepared by Kessels & Associates Ltd for Waikato Regional Council.
Depree, C., & Ahrens, M. (2007). Polycyclic aromatic hydrocarbons in Auckland’s aquatic
environment sources, concentrations and potential environmental risks (Auckland
Regional Council Technical Publication No. 378). Prepared by NIWA for Auckland
Regional Council.
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Version: 3, Version Date: 17/04/2020Gurau, A. L. (2014). The diet of the New Zealand long-tailed bat, Chalinolobus tuberculatus : a
thesis presented in partial fulfilment of the requirements for the degree of Masters in
Zoology at Massey University, Manawatu, New Zealand [Masters, Massey University].
http://hdl.handle.net/10179/5988
Hitchmough, R. A., Barr, B., Lettink, M., Monks, J., Reardon, J., Tocher, M., van Winkel, D., &
Rolfe, J. (2016). Conservation status of New Zealand reptiles, 2015 (New Zealand
Threat Classification Series No. 17). Department of Conservation.
Hobman, R. L. (2000). Lake Te Ko Utu, Cambridge: Studies on the Vegetation and Water
Quality, with Recommendations for Ecological Management. University of Waikato.
Kessels Ecology. (n.d.). Cambridge Lakefront Development ecological opportunities and
constraints report.
Kirk, A. (2018). GHD Letter to Waipa District Council; Lake Te Ko Utu - Options Review.
Leathwick, J. R., Clarkson, B. D., & Whaley, P. T. (1995). Vegetation of the Waikato Region:
current and historical perspectives. Manaaki Whenua Landcare Research.
O’Donnell, C. F. J., Borkin, K. M., Christie, J. E., Lloyd, B., Parsons, S., & Hitchmough, R. A.
(2018). The conservation status of New Zealand bats, 2017 (New Zealand Threat
Classification Series No. 21). Department of Conservation.
Robertson, H. A., Baird, K., Dowding, J. E., Elliott, G. P., Hitchmough, R. A., Miskelly, C. M.,
McArthur, N., O’Donnell, C. F. J., Sagar, P. M., Scofield, R. P., & Taylor, G. A. (2017).
Conservation status of New Zealand birds, 2016 (New Zealand Threat Classification
Series No. 19). Department of Conservation.
Rockell, G., Littlemore, J., & Scrimgeour, J. (2017). Habitat preferences of long-tailed bats
Chalinolobus tuberculatus along riparian corridors of the forested Pikiariki Ecological
Area, Pureora Forest Park (DOC Research & Development Series No. 349).
Department of Conservation.
Sedgeley, J. A. (2001). Quality of cavity microclimate as a factor influencing selection of
maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand.
Journal of Applied Ecology, 38, 424–438.
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