Developing Source Water Risk Management Plans: National Case Studies

H. Ross, Tonkin & Taylor Ltd

Source water risk management plans (SWRMPs) are comprehensive plans designed to help manage and protect the quality and quantity of drinking water at its source. The Water Services Act 2021 mandates the development and implementation of SWRMPs for all registered water suppliers in New Zealand. 

While limited guidance exists for SWRMP development, Tonkin & Taylor Ltd has worked with clients to provide a robust and scalable methodology across varied water supplies and multiple regions, including Auckland, Canterbury, Wellington, and the Hawkes Bay. This paper outlines our process and methodology, which includes identification of potential contaminant sources (hazards), catchment risk assessments (CRA) and the development of source-specific SWRMPs. An overview of several case studies is also presented to illustrate the process and outcomes.

Before identification of potential hazards can be undertaken, we develop a source-specific conceptual hydro(geo)logical model to identify pathways for potential contaminants to enter the water source; either into an aquifer or directly into surface water. Using this understanding of the hydro(geo)logy of the source water, we delineate catchment areas for the water source, known as Source Water Risk Management Areas (SWRMA). These areas are defined using various analytical or numerical tools depending on data/model availability and water source characteristics.

Hazard data within the SWRMAs are assimilated into CRAs, which form an integral part of understanding the risks to a water source. These assessments comprise desk-based components and physical catchment surveys. CRAs rely on a detailed understanding of the historic, current and future activities in the source water catchments which could pose a risk to the source water quality from potentially contaminating activities. In addition, they incorporate publicly available data from climate change models, such as sea level rise and flooding events. 

Once hazard data and hydro(geo)logical pathways have been assessed, each potential contaminant source is identified and rated based on a qualitative risk rating. Hazards are assigned a risk rating, based on a likelihood and consequence risk matrix, resulting in the development of a source-specific CRA for each water supply. These CRAs are then used in the creation of SWRMPs for each water supply, by reviewing the identified risks and developing management solutions for those risks in partnership with our clients and their stakeholders. 

SWRMPs support the long-term, multi-barrier approach to understanding the health and well-being of the source water, acknowledging Te Mana o Te Wai. The SWRMPs are dynamic documents that include proactive management strategies which can trigger specific actions once new risks are detected in the catchments, as evidenced by the devastation of the January 2023 Auckland floods and Cyclone Gabrielle. Furthermore, they play a crucial role in supporting Water Safety Plans.