Delineating land for the beneficial reuse of treated wastewater in New Zealand : a GIS approach.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Environmental Sciences
Degree name
Master of Science
Publisher
Journal Title
Journal ISSN
Volume Title
Language
English
Date
2022
Authors
McIntosh, Sam
Abstract

The irrigation of treated wastewater onto land may be preferable to disposal into water bodies, where the plant nutrients in treated wastewater can exacerbate eutrophication, cause algal blooms, and degrade water quality, among other problems. Land application can provide economic benefits because treated wastewater can offset freshwater requirements for irrigation, and the high concentrations of plant nutrients, particularly nitrogen and phosphorus, can reduce fertiliser needs. The development of a land application system requires the consideration of multiple environmental, technical, and social factors. Consequently, it has remained a limited practice in New Zealand. One of the challenges of developing a land application system is site selection, specifically the delineation of candidate land areas that could be potentially suitable to be irrigated with treated wastewater.

This study aimed to use GIS analysis to delineate candidate land that may be suitable for treated wastewater irrigation. It further sought to qualitatively rank the potential of the candidate land to receive treated wastewater. Twelve GIS datasets were used in the analysis. Potentially suitable sites were inferred from land used for primary industry activities, specifically agriculture, pine forestry, and horticulture, that had a topographic slope of ≤15°. Non-applicable land types were removed from the Candidate Land layer, which included urban areas, water bodies, protected conservation land, the New Zealand road network, and culturally significant sites including Maraes and Pā sites. A weighted multi-criteria analysis was developed to qualitatively rank the potential suitability of the identified candidate land. The factors considered in the multi-criteria analysis included groundwater properties, soil permeability, flood risk, buffer zones around protected areas, and annual water deficit.

Analysis results were reported for each local Council and showed that 42.23% of the total land area in New Zealand was potentially suitable to be irrigated with treated wastewater. It was calculated that just 1.20% of the identified candidate land (133,000 ha) would be required to reuse 100% of the treated wastewater produced in New Zealand at a conservative application rate of 1 mm/day. Sixty-three of sixty-six councils in New Zealand had sufficient candidate land that 100% reuse of wastewater onto land may be possible, with respect to the delineated land area. There were just three councils (Wellington City, Hamilton City, and Lower Hutt City Councils) that would require land outside of their Council territories, or alternatively a higher rate of application to achieve 100% wastewater reuse. This study demonstrated how geospatial investigations can be used as a cost-effective analysis tool for the delineation of suitable land sites using free-source, national-scale GIS datasets. Future research on this topic could involve local-scale analyses including the incorporation of legal property boundaries and land values for specific wastewater treatment plants investigating the conversion to land application.

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