The Motueka-Riuwaka Plains are situated in the Tasman District, north-west of Nelson. The groundwater system has been formed from gravels out-washed into Tasman Bay from the Motueka and Riuwaka rivers. We have worked with Tasman District Council (TDC) over many years to develop and refine an integrated surface water-groundwater model of the system and have applied this model to evaluate various water management strategies.

Key water management objectives include:

• Protecting against saltwater intrusion around the coastal margins;

• Maintaining flows in coastal springs;

• Ensuring adequate flow regimes for the Motueka and Riuwaka rivers; and

• Providing a reliable water supply for existing and future groundwater users.
  
  

The model has been developed over multiple stages since 1999. As the work has progressed, TDC has installed new monitoring bores and collected data, which we have incorporated into subsequent stages of model development.

The model is an integrated MODFLOW surface water-groundwater model that simulates both flow and contaminant transport (specifically nitrate-nitrogen) over the period 1990-2012. We calibrated the model against groundwater levels measured in 18 dedicated monitoring bores, flows in three lowland drains and nitrate-nitrogen concentrations in multiple bores.

TDC’s principal concern for managing the Motueka-Riuwaka groundwater system relates to saltwater intrusion, and as part of this study various criteria were developed (collaboratively with TDC) which provided limits to protect against saltwater intrusion occurring. These criteria include groundwater levels in a coastal monitoring bore, which is designed to ensure a minimum subsurface flows across the coastline. With these limits in place, various allocation regimes were trialled (including restrictions and locations of takes) under historic and projected future climates to arrive at a sustainable allocation volume, which have been incorporated into TDC’s Tasman Resource Management Plan.

The Motueka Riuwaka Plains Groundwater Model report is available on the Tasman District Council’s website.