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How Hydrology Helps - Sam Trowsdale

(Sam Trowsdale is an Auckland based hydrologist working on solutions to contamination of waterways and harbours. The following is an extract from a recent study he has been working on and demonstrates the scientific proof of the conclusions the environmentalists have long been aware of:  Too many heavy metals and other pollutants are entering our waterways and harbours. Sam’s full report is available on request.)

Auckland is New Zealand’s largest conurbation. The regions 1.4 million inhabitants comprise 32 per cent of the nations population (source: statistics NZ estimated resident population as of 30^th June 2004). The size and population density of the city raise the issue of environmental protection, which needs to be addressed urgently. Auckland increases in size by 49 people and 21 houses per day escalating the rate of environmental deterioration.

Urbanisation significantly alters the hydrology and hydrochemistry of natural watersheds. Changes to the hydrology are largely attributed to an increase of impervious surface and hydraulically efficient piping, which generates increased runoff volumes. Increased runoff volumes lead to flashy stream flows that degrade stream habitat by causing bank erosion and flushing life from streams. Contaminants associated with urban runoff often comprise suspended sediments, heavy metals, hydrocarbons, nutrients, and pathogens causing problems such as silting of streams and receiving environments, nutrient contamination and biological kills. Elevated levels of contaminants in urban runoff have been recorded in New Zealand and have been shown to be accumulating in receiving environments such as waterways, estuaries and harbours.

Initiatives to protect the aquatic environment in New Zealand’s urban areas have been the focus of recent national, regional and local government action. Contaminated urban runoff is clearly identified as a major issue to tackle. The required expenditure for Aucklands stormwater systems to meet recreational, human health, ecological and economic goals has been estimated at US$2000 - 5000 million over the next 20 years.

As a step towards more confident assessment of the impacts of urban stormwater on the aquatic environment of New Zealand, we present initial data from a six-year ‘low impact urban design and development’ (LIUDD) programme aiming to promote the provision and uptake of more sustainable and cost-effective urban water strategies. To evaluate the potential impact to streams that receive runoff discharges from different urban landuse we compare the quality and quantity of runoff from three catchments in Auckland each with different degrees of urbanisation.

The study focused on three geographically similar catchments, each with different land cover. The land cover ranges from: the regenerating bush and pastoral Cantwell catchment; the peri-urban Waikumete catchment, characterised by low density housing in its lower reaches and significant areas of regenerating indigenous vegetation and vegetated riparian margin in its headwaters; and the urbanized Tangutu catchment, comprising low to medium density residential housing and more than 50 per cent of natural perennial streams piped or contained within lined channels. The Cantwell, Waikumete and Tangutu catchments are located in the Glen Eden/ Titirangi area of Waitakere City, Auckland, New Zealand. Each catchment contains a principle stream, after which it is named after, and a number of tributaries depending on topography. As sub catchments of the Oratia Stream they drain the lower eastern slopes of the Waitakere Ranges into the Waitemata Harbour through Henderson Creek. The boundaries of the study catchments were defined using digital elevation models. Digital elevation models were built using a 2 metre contour resolution for the Waikumete and Tangutu catchments and 20 metre resolution for the Cantwell catchment.

Impervious surfaces were mapped in the three catchments using Ikonos multispectral satellite imagery with a pixel size of 4 x 4 metres. This spatial resolution was sufficient to represent features such as roads, which are typically 8 – 12 metres in width, and house roofs, which are seldom less than 10 metres across. Soil types, Topography, Rainfall and Stormwater infrastructure were similarly taken into account.

Summary of water quality data. N = number of samples

	Cantwell				Waikumete				Tangutu				Comparison
Parameter	n	Median		Range	n	Median		Range	n	Median		Range	Kruskal-W allis	Probability
TSS mgL-1	53	207	1 – 1365		101	184.5	1 – 2146		171	153	9 – 1194		1.0257	0.5
EC msm-1	53	127	68 – 203		101	127	52 – 283		171	85	44 – 252		39.7991	<0.001
TN mgL-1	53	1.58	0.5 – 3.6		101	1.26	0.37 – 4.2		171	1.39	0.6 – 3.7		5.5972	0.05
TP mgL-1	53	0.23	0.04 – 0.72		101	0.17	0 – 0.71		171	0.23	0.03 – 0.99		8.3371	0.01
Cu µgL-1	53	43.8	3 – 479		101	41.0	2 – 158		171	57	5 – 247		22.6145	<0.001
Pb µgL-1	51	28.1	4 – 106		93	31.4	2 – 73		155	35.3	7 – 390		6.2159	0.02
Zn µgL-1	53	26.6	2 – 119		101	81.2	17 – 504		171	154	47 – 783		140.0329	<0.001

Conclusions:

Data presented within clearly illustrates elevated peak flows and less sustained base flows with increasing urbanisation. Hydrographs confirm the hydraulic efficiency of urban development and the flashy nature typical of urban streams.

Generally, the peak stormflows in the more impervious Tangutu and Waikumete catchments are substantially greater than those discharged from the pasture and regenerating bush-covered Cantwell. The reduced rates of base flow discharged from the Tangutu and Waikumete catchments reflect the reduced infiltration of rain water into soil and groundwater storages due to their impervious surface cover. Sustained flows from these storages are essential for maintaining stream flows and aquatic habitats during periods of little rainfall.

The study highlights the capacity urban runoff to transport contaminants. The exceptional geographic location of Auckland means contaminants move directly into a much-valued Harbour. Significant impact on usability, biodiversity and desirability. Alarming when we consider the total area of these catchments is X per cent of total city.

Quality and quantity are clearly related to landuse. However, quality and quantity not easily related. - ANZECC guidelines are exceeded for all landuse. - Pb has reduced significantly over past decades.

There is a large source of zinc in the urban catchment. Zinc concentrations exceed national guidelines.  If we are to continue benefiting from our clean, green image we must question the appropriateness of the current urban design in New Zealand.

Cu, Pb and Zn usually exceed both the USEPA (1986) criteria for long and short-term exposure and the ANZECC (2000) guideline for 80 per cent species protection. It is unacceptable to discharge water from a point source with a similar chemical signature into natural waters (RMA). Urban runoff is clearly an undesirable input into our waters. With this realised we should force rethought of our current urban paradigm.

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