The Integrated Catchment Study of Auckland City (New Zealand): Long Term Groundwater Behaviour and Assessment

Long term groundwater behaviour under existing and future scenarios in a fractured basalt aquifer system is presented for Auckland City (New Zealand). The Auckland Isthmus is comprised of unconfined basalt aquifers and semi-confined Waitemata and Tauranga Groups (sandstone and mudstone) aquifers. The basalt aquifers are used to dispose stormwater via soakholes, provide groundwater supply in Onehunga and other industrial and commercial sites, constitute baseflow in Motions and Meola Creeks and feed important springs in Western Springs and Onehunga. Regional groundwater models were developed in Visual MODFLOW and MIKE-SHE, covering approximately 45 km² with a north-south grid alignment and 100 x 100 m grid cells. Vertically the models are divided into a single basalt aquifer hydrogeological unit. The models were calibrated to five years of historical records. Two long term scenarios and were simulated: (1) existing conditions (20-year simulation with current climate and existing development land use) and (2) future conditions (15-year most probable 2050 climate and maximum probable development 2050 land use). Spare capacity exists in the aquifers to accommodate additional stormwater disposal. Additional groundwater breakout (surface flooding) is predicted in expected areas during high rainfall recharge years. Stream and spring flows, and groundwater supply are not anticipated to be adversely affected by groundwater management recharge practices. A major unresolved issue is to find practical ways to capture and inject large volumes of stormwater generated over short, high intensity storms.