6.8.1 Overview
The list of effects from urban stormwater discharges and their associated impacts have been largely based on research undertaken by Auckland Council, which was completed in support of the Auckland Unitary
Plan’s Stormwater Management Provisions. These are summarised in: Auckland Unitary Plan stormwater management provisions: Technical basis of contaminant and volume management requirements, Prepared by Auckland Council. Auckland Council technical report, TR2013/035.
The above report from Auckland Council is a comprehensive source of information and is deemed appropriate for Tasman District Council because the effects from urban stormwater discharges that are present are not expected to be different from those experienced in other urban catchments around New Zealand. This has been demonstrated to be the case through the monitoring that has been completed in the Richmond catchment to date.
These sections primarily relate to the effects from operational stormwater discharges but does include a section 6.8.9 which discusses the effects of temporary discharges from maintenance and land disturbance activities which relate to the stormwater network.
6.8.2 Water Quality Effects
As outlined in section 2.4 of this application the stormwater quality is driven by the contaminants it contains or is likely to contain. The main stormwater contaminants present, and their effects, can be grouped and are outlined in the table below. These effects are then assessed in that order in the following sections.
Table 40: Water Quality Contaminant summary
Stormwater from an urban environment is inherently different to that from a natural catchment, or a more rural catchment. In a natural catchment the majority of rainfall either soaks into the soils, or is returned to the atmosphere through evapotranspiration. It is only during larger storm events that flood flows cause surface runoff into waterways. This run off is generally at a low velocity prior to entering waterways due to the natural roughness of the terrain. The contaminants derived from natural water courses tend to be limited to organic materials and natural soil derived sediments. The water courses then deliver these natural
contaminants to the freshwater and coastal receiving environments where they contribute to the natural systems of these receiving environments.
This consent considers the flows and contaminants from urban catchments. These are typified by significant increases in impervious surfaces such as roads, driveways, and roof tops. These surfaces, and the uses carried out within these areas, contribute to the wider variety of contaminants as broadly identified in the table above. There also tends to be an increase in velocity of discharges from these harder surfaces as
Contaminant Potential Effects on Environmental Values
Sediments Reduced water clarity and increased turbidity, potentially reducing primary production.
Reduced light levels, and smothering of the bed substrate.
Increased biochemical oxygen demand (BOD) from organic materials.
Can be harmful to fish directly via gill abrasion, reducing growth or resistance to disease, preventing successful egg and larval development, affecting natural migrations, or indirectly by reducing the abundance of their food.
Heavy Metals Toxic to plants and animals.
Nutrients Nuisance plant growth and increased BOD.
Some nutrients, e.g., ammonia, are toxic to aquatic life.
Hydrocarbons Increased chemical oxygen demand (COD) – oxygen depletion of waters.
Micro-organisms (pathogens)
Increased public health risk.
Temperature Elevated water temperatures impacting on health and survival of all aquatic organisms
Gross pollutants (human and naturally derived)
Water quality, and amenity effects.
surfaces more of the stormwater enters the stormwater system, and therefore the receiving environments, than is the case in less developed catchments. The effects discussed below are relevant to water quality effects of stormwater discharge from the UDA’s.
6.8.3 Sedimentation Effects
Sedimentation is often a feature of stormwater and is derived from a variety of sources within the urban environment. Typical sources are through the development of greenfield sites, or the redevelopment of brownfield sites. Also, hard surfaces such as roads can produce sedimentation through road wear, vehicles, and run off from dust and sediment that has been deposited on the road surface. Stream bank erosion is also a significant source of sediments. The effect of sediment varies depending on the sediment size and to an extent the composition of contaminants that attach to the sediments. The diagram below sets out the various categories of sediments. The courser sediments can be managed through settling or screening, while the finer sediments require filtration, adhesion or coagulation. The finer sediments also tend to carry a greater level of contaminants with them.
Figure 24: Particle size distribution ranges> Modified from solids size classification diagram (Roesner, Pruden & Kidner, 2007) source Auckland City Council Technical Report 2013/035.
The discussion below relates to the effect of sediment as a contaminant in its own right. As will be discussed further in this application there are additional effects of contaminants that adsorb onto sediment, including metals, nutrients, hydrocarbons, and other toxic chemicals.
To quote from the Auckland City Council Technical Report 2013/35:
Sediment loads in fresh and marine waters can cause adverse effects. In fresh water, the accumulation of fine sediment on stream beds may reduce the abundance and diversity of invertebrates. Many New Zealand fish species appear superficially unaffected by sub lethal turbidity levels, however evidence is increasing that some species show reduced feeding levels or avoidance behaviours at relatively low turbidity levels of >25 Nephelometric Turbidity Units (NTU) (Kelly, 2010). ….. Sediment that deposits in stream beds, fills interstitial spaces, which reduces the opportunity for fish and invertebrates to find refuge from high temperatures and predation during low flows. Recent NZ studies using radio tagged fish have demonstrated the importance of interstitial microhabitats to native New Zealand fish species (McEwan & Joy, 2011) (McEwan & Joy, 2013).
Sediment is a major marine contaminant that degrades coastal habitat and is toxic to many marine organisms. Thick, catastrophic deposits of sediment (>20mm) usually kill almost all bottom-dwelling organisms with a few days. Recovery is slow after catastrophic events, with the recovery of macrofauna lagging behind recovery of chemical and geotechnical properties (Kelly, 2010). Thin sediment deposits (1- 7mm) also reduce diversity and abundance of benthic organisms. The number of taxa and individuals has been observed to decline by around 50% within a number of days of sediment deposition (Kelly, 2010).
The effects experienced in the receiving environments to which stormwater flows are not solely attributed to discharges from the urban catchments that are subject to this application. However, these urban discharges undoubtable play a role in contributing sediments and the actions proposed through Tasman District
Council’s Stormwater Catchment Management Framework seek to reduce the level of sedimentation discharged. This reduction will occur through both the CMP’s and through many other actions of Council,
such as future reviews of the Tasman Resource Management Plan, the Land Development Manual and through increased education and enforcement.
This proposal, and other Council actions, result in appropriate management of sedimentation from stormwater and over time has the positive effect of reducing sedimentation transport into the receiving environment.
6.8.4 Effects of Metals
Metals are typically found in stormwater derived from urban catchments. While some metals are also found in runoff from natural catchments this is typically at lower levels than from urban sources. Metals in
stormwater tend to be in a soluble form and very often bound to sediments. Metals are sourced from a variety of land uses and activities within the urban catchments including building material such as zinc roofing, vehicle consumables such as brake pads and tyres and from natural sources. Typical metals found are zinc, copper and lead. Tasman District Council acknowledges that the composition of these
contaminants in urban stormwater runoff is very similar to that found in many other urban centres in New Zealand. For some of the small UDA’s, with a limited variety of land uses present the contaminants are likely to be of lesser concentration and variety than is the case in larger areas.
Assessed proportions of sources contributing zinc to stormwater (Kennedy & Sutherland, 2008)
Assessed proportions of sources contributing copper to stormwater (Kennedy & Sutherland, 2008)
The introduction of metals of a type or concentrations not found naturally in the local environment will have adverse effects on the freshwater or marine aquatic receiving environment. The actual level of these effects is difficult to establish. Due to the topography of the Tasman District Council the large natural backdrop behind most of the urban areas in the district results in most measurable contaminants being within acceptable levels although some have been shown to be increasing. This is due to ‘cleaner’ source of stormwater from the natural backdrops in the upper levels of most catchments providing dilution to the
stormwater derived from the urban portions of the catchment. However, this does not negate the goals of Council to reduce the levels of the various contaminants, including metals, in stormwater. This is with the intent of reducing the levels of adverse effects generated by stormwater discharges.
6.8.5 Effects of Nutrients
Nutrients commonly found in stormwater are nitrogen (N) and phosphorus (P) however urban nutrient loads are typically negligible compared to that contained in urban run-off. As stated in Auckland City Council Technical Report 2013/35:
Nutrients in the water column are delivered to freshwater bodies in short pulses, and have less effect due to the limited time they are present. However, in the urban environment, most of the nutrient load is associated with particulate material, and is only partly or slowly available for plant growth. This particulate material can be deposited in beds of streams, ponds and lakes, and the associated nutrients are then metabolised for plant growth (Griffiths & Timperley, 2005) (Williamson B., 1993)
The major sources of nutrients in the urban environment are fertiliser run-off, sewage overflows, and soil loss. Plant nutrients are present in moderate amounts in storm water. Concentrations are much higher in sewage, and therefore stormwater contaminated by wastewater overflow have considerable higher nutrient concentrations (Mills & Williamson, 2008).
The statement above is especially relevant in the Auckland situation where stormwater and sewer are contained within the same pipe. The TDC wastewater and stormwater networks are in separate pipes. Any over flows that do occur from the TDC wastewater network are generally from the lower parts of the network and in association with the pump stations. These flow to the Coastal Marine Area or streams and do not form a part of this application or impact on stormwater discharges directly. However, Motueka does receive some overflow from the wastewater network which may occur within areas of stormwater soakage to ground.
This interrelationship between wastewater overflows, and stormwater soakage has the potential to introduce nutrients to groundwater and forms part of the wider work program of Council around the three waters infrastructure in Motueka. Also see section 6.7 regarding the effects on groundwater quality and human health.
In more general terms and relating to surface water increased nutrient load can lead to increased plant growth, including unwanted organisms, algae blooms, eutrophication and in extreme situations, anoxia. This can be particularly pronounced in lakes and wetlands with a local example being Lake Killarney in Takaka.
This freshwater lake is located within the urban area but also receives rural runoff inputs from nearby farm land. Since 2014 algae blooms have been observed in the lake for the first time in living memory. Trevor James, Tasman District Council’s Resource Scientist, has reported to Council that “Over successive summers, these blooms appear to cover more of the lake, are more dense, and occur for longer periods’.
Discharges to the lake have been observed from the Council system, including from a sump on neighbouring farmland. Samples have been tested which have showed very high levels of nitrogen and phosphorus are present and have built up in the sediments that have been discharges to the lake. Mr James notes that these are more than enough to cause these algae blooms. Council is taking action to address this situation by removing the nutrient input to the lake by diverting the flows through grassy swales which can remove some nutrients and are looking at options to address the nutrients already in the pond. These nutrients are contained within the 700mm of sludge that has built up in the bottom of the lake.
This local example shows the effect of excess nutrients (and sediments) in an enclosed water body. In this case the effects are significant. While the source of the contaminant is from a rural activity within the UDA, it is Council’s stormwater system that has delivered it to the lake. Despite the fact Council did not cause the contamination it is taking steps to avoid this adverse effect (by diverting the stormwater run-off from land used for rural purposes for treatment in a swale) and to remedy the situation by managing or removing the sediments and contaminants already in the lake.
This is also an example of the type of issues that would be identified through the Catchment Management Plan process. The actions taken to resolve or lessen the adverse effects of the issue would then be identified and set out within the CMP.
6.8.6 Effects of Hydrocarbons
Hydrocarbons come in a variety of forms but of predominant interest in this case are those derived from vehicle use. This is mostly from vehicle exhaust emissions and oil leaks. Other sources relate to point source discharges or spills entering the stormwater system. This could be from a vehicle accident or other
difficult to identify specific adverse effects from hydrocarbons in stormwater however they are known to be toxic in fresh water and marine water environments. The reduction of these inputs into these receiving environments is therefore a desirable outcome to be achieved through Council’s Catchment Management Planning framework.
6.8.7 Effect of Micro-organisms (pathogens)
Micro-organisms are ubiquitous in the environment and in most cases are an essential part of the wider ecological system. It is the subset of micro-organisms that are pathogens which are of interest when considering the effects of stormwater. Pathogens are micro-organisms which are capable of producing infection or disease. In stormwater pathogens are most commonly transmitted through faeces which are introduced to the stormwater system through bird, rodent, cat and dog faeces being washed into the stormwater system. Overflows, or leakages from wastewater (sewerage) systems are also a contributor of pathogens to surface water and in the case of Motueka potentially the stormwater system. In general concentrations of pathogens in stormwater is low when compared to wastewater. Also, the pathways for infection are limited due to the general lack of contact with stormwater flows. The main risk of infections is from food gathering and contact from swimming at beaches soon after rainfall events where there have been discharges and elevated micro-organism levels occurring. This is primarily a human health effect and one which can be addressed through ongoing reduction of cross contamination from the sewerage network and through behavioural changes in the community such as picking up after dogs. This is also discussed in the recreational effects section 6.11.2.
6.8.8 Effect of Temperature
The following sections are from the Auckland City Council Technical Report 2013/35 and accurately sets out the reasons for, and the effect of temperature changes in stormwater. This is applicable to the urban catchments of Tasman District.
In a natural catchment, the heating effect of solar radiation on the surface is significantly mitigated by the shading and evapotranspiration provided by vegetation. Shading intercepts radiant energy, whilst
evapotranspiration removes heat due to the energy required to change the phase of water from a liquid to a gas. As a consequence of these effects, the surfaces of natural catchments are relatively cool. When rain falls, most infiltrates into the soil, where it quickly equilibrates with the groundwater temperatures. The small amount of surface runoff that occurs does not have a significantly elevated temperature. Streams in these catchments have low temperatures because they have high baseflow from low temperature groundwater stores. Typical stream temperatures are in the range of 15 to 20 °C, depending upon degree of shading (Neale, 2012).
In an impervious catchment, the lack of shading and evapotranspiration causes the surface to become significantly hotter. Impervious surfaces, such as asphalt or concrete, may reach temperatures in excess of 45 °C. When rain falls, there is no infiltration and most gives rise to runoff. The temperature of this runoff is elevated due to energy transfer from the hot surface, and the result is a pulse of high temperature
stormwater entering streams. During summer the temperature of stormwater from impervious catchments is typically in the range of 20 to 25 °C, even in temperate climates. Where these catchments have stormwater ponds and wetlands for water quality and flow control purposes, stormwater temperatures may become even further elevated. Water temperatures in the range of 25 to 35 °C are routinely recorded downstream of stormwater ponds.
Water temperature is an important factor influencing the health and survival of all aquatic organisms, including native fish, amphibians, and invertebrates, and influences the physicochemistry of streams and other water bodies. Most fish, insects, zooplankton, phytoplankton, and other aquatic organisms that operate in only narrow ranges of temperature can be killed by sudden temperature changes. High water
temperatures lower the dissolved oxygen content, and this is stressor to most aquatic organisms.
Consistently higher stream temperatures can adversely affect growth, reproduction, species competition, and disease progression within aquatic communities. High temperature water may also act as a fish barrier.
Reducing stream temperature is a key goal of Council from the stream health point of view. This is part of a wider work programme than just that directly as a result of this stormwater project. Recognising the impact of stormwater flows on stream temperature will assist with determining methods of reducing the adverse effects of stormwater discharges.
6.8.9 Effect of Gross pollutants
Gross pollutants delivered through the stormwater system to the receiving environments include larger solids which have a working definition of being over 5mm in size. These include coarse sediments such as sand, gravels and road aggregate. There are also organic particles such as leaves, twigs, grass clippings and similar. Human activities in the urban environment also introduce a variety of rubbish such as paper, plastics, metals, glass, cloth, cigarette butts, packaging materials and many others. The majority of these are washed off roads, or blown from rubbish bins and private properties and end up in the receiving environments.
The effects of the organic materials are those that represent a biological oxygen demand (BOD) through reducing dissolved oxygen as part of the natural decomposition process. An example of this occurring is if large quantities of grass clippings ended up in a low flow stream system.
Human derived rubbish has significant visual and recreational amenity effects as well as adverse
environmental effects through decomposition in the environment. Visually rubbish is unattractive in water ways and can detract from people’s enjoyment of these areas. It can also prevent, or limit, the ability to use areas recreationally. Environmentally rubbish has adverse effects on aquatic life through becoming
entangled in these materials, and through ingestion of either larger items, or smaller has the item decomposes. Plastics in particular can have significant adverse effects due to their persistence in the environment. This is a well-recognised, and worldwide, environmental problem which the Tasman area is not immune from. While stormwater is not the direct cause of this issue, it’s network is often the delivery mechanism by which these pollutants reach the receiving environments.
6.8.10 Effects on Surface Water Quality of Maintenance and associated Land Disturbance Discharges
This effects assessment also includes the effects from maintenance works and associated land disturbance.
The general works that this involves has been set out in Section 2.5. As noted in that section Council already holds a global river works consent which allows for works which involve physical disturbance of the rivers themselves. The effects of works under that consent have therefore already been considered. This section considers the effects of maintenance or land disturbance that discharges to rivers from the
stormwater network.
One of the key limitations of this application is that it only includes small to moderate scale maintenance activities. Larger scale works may require an individual application, so those larger projects will have their effects specifically considered.
Erosion and sediment control is an important requirement to reduce effects. To ensure that this is designed to appropriately manage each individual project a condition of consent is volunteered to prepare a dust, erosion and sediment control plan for any maintenance works that have the potential to introduce sediment to rivers. This is an effective control that allows the best solution to be designed for each situation. In addition, a condition is volunteered for water removed from de-watering to pass through a sediment control system prior to discharge. This ensures sediment laden water is not discharged back to rivers.
Spill management of fuels or other hazardous substances during maintenance works is an expected part of project management for any work on the stormwater system. These materials can be kept in a secure area and any refuelling or machinery maintenance activities undertaken in an area where any potential spill cannot reach the system and therefore the water ways at the end of the pipe. As the works in the scope of this application are small to moderate in scale the extent of machinery and the time on site is reduced which inherently lowers the potential for spillage to occur.
The effects on surface water quality from maintenance and land disturbance is considered to be of low probability with the measures set out above in place. Enabling these works to be undertaken also provides positive benefits in maintaining the stormwater network in good condition without the need for larger scale works if smaller matters are dealt with promptly.
6.8.11 Effects on Aquatic Habitat and Stream Health
Urban development as a whole typically degrades aquatic ecosystem health in both the fresh water and coastal environments. This is due to short term changes through land disturbances during the construction