Assessment of Spatial Variation of Groundwater Quality and Its Relationship With Land Use in Perth Metropolitan

To determine the effects of land use on groundwater quality in Western Australia, a quantitative analysis is carried out using groundwater quality data supplied by the Department of Water from over 500 groundwater wells across the Perth metropolitan area. We analyzed four main groundwater quality indicators; nutrients, physical parameters, inorganic non metals and trace metals. We found that groundwater beneath agricultural land was found to be particularly susceptible to nutrient loading due to the application of fertilizers. Nutrient levels were found to be rising over time due to increasing agriculture and urban development. Industrial areas were also found to have numerous contamination plumes that continue to migrate with the groundwater flow. According to Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines and the Australian Drinking Water Guidelines (ADWG), several areas including rural areas like Carabooda lake, Gnangara and Jandakot Mounds, Cockburn Sound, Forrestdale, Joondalup, and Ellenbrook and high density urban areas like Balcatta and Neerabup, industrial areas like North Fremantle, Welshpool and Kwinana are indentified as the vulnerable areas for groundwater quality.

• Groundwater
• Quality
• Land use
• Perth

1. Introduction

Fresh water is fast becoming a scarce resource in Australia. The largest available source of freshwater is currently groundwater. As groundwater has a huge potential to ensure future demand for water, it is important that human activities on the surface do not affect negatively on this precious resource. It is important to Western Australia as it supplies approximately 70% of Perth’s fresh water for industrial, agricultural and municipal use [1]. Reduced rainfall in recent years and diminishing inflow has led to depleted dam levels in Perth [2]. Coupled with Western Australia’s population growth rate of 2.3% and concerns about climate change, Perth is facing significant water shortages and demand for groundwater continues to rise [3]. It is becoming increasingly important that the quality of Perth’s groundwater system be monitored and sustained for years to come.

Pollution of groundwater occurs when waste products or any foreign substance alters the biological or chemical characteristic of water and degrades the quality so that animals, plants or humans are affected [4]. Numerous studies have examined this correlation between land use and contamination of groundwater [5-8]. There are widely using tools such as Multivariate statistics analysis to analyze the variations in water quality and sources of contamination [9-11]. Although the progression of underground contaminants depends on numerous microbiological, physical and chemical processes; the most significant factor controlling contamination of groundwater is the source of contamination on the surface including its type, strength and location relative to the water source [5]. By studying the relationship between groundwater contamination and land use, issues of sustainability can be addressed and integrated with better land use practices and water protection strategies.

Perth groundwater is particularly susceptible to groundwater pollution as Perth has predominantly sandy soils that are generally correlated with low adsorption potentials; allowing for easy leaching of foreign chemicals through the soil profile [12]. Pesticides are one of the significant problems in Perth groundwater [13]. Another concern in regards to Perth land use is the historical use of septic tanks. There have been many studies that have correlated polluted groundwater with septic tanks [14].

This study mainly concerns the spatial distribution of groundwater quality in Perth Metropolitan basin and investigates the relationship between historical land developments and temporal variation of groundwater quality. Also the study addresses the critical levels of contaminants in Perth groundwater highlighting Australian water quality standards.

2. Methodology

2.1. Study Area: Perth Basin

The Perth Basin is selected as the study area. The geologic formations of the Perth Basin have been grouped into six distinct aquifers: The Superficial, Rockingham, Kings Park, Mirrabooka, Leederville and Yarragadee aquifers. These aquifers are locally, hydraulically connected or separated by confining beds or geologic formations [12]. Drainage patterns and hydraulic characteristics are categorised into distinct groundwater flow systems known as groundwater mounds. The Gnangara Mound is Perth’s most important water source supplying 380GL of fresh water per year. Recent studies have found that extractions from the mound are close to its sustainable limits [15].

In the past, Perth’s groundwater has been described as generally good in quality and predominantly pollutant free. However in recent years Perth investigations have shown increased levels of dispersed contaminants as well as a significant number of localised contamination plumes [16,17]. Land use has developed considerably since the development of the Swan River Colony in 1829. As Perth continues to grow and expand, there are increasing agricultural (primarily horticulture) and aquaculture developments over the Perth Basin. Furthermore, urban developments are intruding on the boundaries of the Gnangara and Jandakot Reserves [18]. It is becoming increasingly more important to have more stringent protection and management of Perth’s groundwater sources to limit the effects of urbanization on groundwater quality.

2.2. Data and Materials

The Department of Water (DoW) supplied qualitative data on 500 groundwater wells within the Perth metropolitan area. Available data is based on samples tested from 1984 to 2008. Based on available data, water quality data were grouped into historic (1984 - 1994), recent (1995 - 1999) and current (2005 - 2009) periods. For each period, data were analysed across the Perth metropolitan area to determine if there are any correlations between groundwater quality parameters and location. Data was assessed for spatial and temporal trends. A geo-statistical method of analysis was utilised that uses mathematical models to determine time and spatial trends in data. The SURFER Mapping System by Golden Software [19] was used to find trends in concentration distributions of groundwater quality parameters. The following groundwater quality indicators were analysed:

Nutrients: Total Nitrogen (TN), Nitrate (NO₃), Ammonia, Total Phosphorus (TP)

Physical: Dissolved Oxygen (DO), Acidity (pH), Alkalinity Inorganic non metals: Arsenic (As), Chloride (Cl), Boron (Bo)

Trace metals: Zinc (Zn), Lead (Pb), Nickel (Ni)

Spatial distributions were examined and compared with assessment levels for groundwater quality adopted from the Department of Environment and Conservation (DEC). The DEC evaluates contaminated groundwater sites according to the Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines [20] and the Australian Drinking Water Guidelines (ADWG) [21]. Results were discussed comparing to land use for the study area. A map of land use for the Perth Metropolitan Region Scheme (MRS) was supplied by the Department of Planning and Infrastructure through their Mapping and Geospatial Data [22]. The maps specify land use across the Perth metropolitan area including urban, industrial, state forests, parks and private recreational areas and rural and agricultural lands.

3. Results & Discussions

3.1. Comparison of Spatial Distribution of Groundwater Quality and Land Use Pattern

Several water quality parameters were analysed for historic, recent and current periods and their spatial distribution are mapped using the SURFER software. Figure 1 shows the spatial distribution of Total Nitrogen (TN), Alkality, Arsenic (As) and Zinc (Zn) to represent the distribution of nutrients, physical parameters, and inorganic parameters and trace metals in current period (2005 - 2009). Results for Total Nitrogen showed that levels in Perth are significantly high. Historic samples had an average total nitrogen content of 3.2 mg/l. This amount exceeds the ANZECC trigger limit for freshwater of 700 µg/L. Furthermore, the Total Nitrogen average increased to 3.6 mg/L for the 2005 - 2009 period. The spatial distribution indicates high levels of Nitrogen in northern rural areas around Carabooda Lake. This result is a concern as this area lies over the Gnangara Mound which is Perth’s major groundwater source.

Figure 1 further shows that low alkalinity is evident in northern suburbs of Joondalup and Ellenbrook. These

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