The pollution risk of the Hutt aquifer system

Abstract

The Lower Hutt aquifer system provides 25% of the potable water supply for the Wellington Metropolitan area. Land use overlying the aquifer is predominately urban with associated contaminant hazards from industry, commerce, and waste disposal. The aquifer is therefore vulnerable to contamination. The aim of this study was to quantify the groundwater pollution risk. The results of the risk assessment are used to develop implications for the sustainable management of the groundwater resource. This study examines the hydrogeological characteristics that are of importance for the pollution risk assessment. The hydraulic characteristics of the unconfined aquifer were investigated by conducting three salt water tracer tests. Results from two tests were reliable with hydraulic conductivities of 4.1 x 103m·day-1 - 1.9 x 103m·day-1 and 8 x 103m·day-1 - 2 x 104m·day-1. The third test produced only indicative results with a maximum hydraulic conductivity of 25m·day-1 - 350m·day-1. Existing water quality data for the aquifer was reviewed and additional analyses conducted on three samples from bores in the unconfined aquifer. The baseline water quality of the aquifer was high. No trends were observed to indicate that the water quality is being affected by contamination. The hazard of three potential contaminant sources, Underground Storage Tanks, Above-Ground Bulk Storage Tanks and sewage have been assessed. An Underground Storage Tanks hazard map based on the relationship between increasing age and leakage from corrosion was compiled. A large number of sites did not have information on their age and to take a precautionary approach it was consider that they present a high hazard. A trend towards increased hazard in the Seaview area is also observed. A map showing the hazard posed by breakage of the sewage network from liquefaction, shows the most hazardous areas are located on the western and southern branches and a small section on the eastern branch in the Naenae area. The hazard of the four Above-Ground Bulk Storage Tank premises were assessed using the Dow hazard index adapted to groundwater hazard. The two most hazardous sites border onto the Petone Foreshore. The potential size of an earthquake induced contaminant event at any of the premises was estimated as the contents of a single tank (1-3.6 x106 litres). The groundwater contamination vulnerability of the unconfined and Waiwhetu confined aquifer were assessed using the DRASTIC method. A DRASTIC map of the unconfined aquifer shows areas of high vulnerability surrounding the Hutt River and the Petone Foreshore. A DRASTIC map for the Waiwhetu confined aquifer and the section of the unconfined aquifer (north of the Kennedy Good Bridge) that is in hydraulic contact with Waiwhetu confined aquifer was also compiled. The unconfined section is clearly the most vulnerable area with a zone surrounding the Hutt River being critically sensitive. These maps are compatible with the DRASTIC map of the Wellington Region. A contaminant risk map of the sewage network shows the area at greatest risk to be in the Naenae area. Modelling of possible contaminant events in this area show there is no risk of polluting the downstream bores. An underground storage tank contaminant risk map shows several sites to be in high risk locations. Modelling of possible leakages indicate there is a potential pollution risk from underground storage tanks. Modelling of a contaminant spill from Above-ground Bulk Storage Tanks, indicate a pollution risk to the Petone Foreshore. The maps, models and hazard information in this assessment can be used as management tools to reduce the pollution risk of the Hutt Aquifer system. An important step in risk reduction is the process of communicating risk assessment information to interested parties and the general public. Developing an informed perception of risk enables individuals and organisations to manage them more effectively.