HYDROGEOLOGICAL CONTROLS ON METEORIC DERIVED WATER IN AN ERODED VOLCANIC TERRAIN: BANKS PENINSULA, NEW ZEALAND

This study updates hydrogeological models for the loess (surface cover) and volcanic (bedrock) springs on Banks Peninsula. Spring types in order of abundance on Banks Peninsula are as follows: mixed colluvium, volcanic colluvium, loess, volcanic bedrock (jointed lavas and breccias), and alluvium.

In volcanic springs, unweathered to slightly weathered brecciated layers of lava flows are the principal areas of lateral groundwater flow. Maximum flow occurs where a permeable layer is underlain by an impermeable layer, the most common example being a brecciated layer underlain by ash or tuff. Typically flow within volcanic units follows the dip of the lava flows, commonly dipping away from the now heavily eroded volcanic center. Cooling joints within lava flows control the vertical flow of water while sub-horizontal joints and sub-horizontal flow exists mainly in brecciated layers. Intrusive bodies (dikes and domes) can block or divert groundwater movement and result in localized springs.

In loess springs fluid flow and spring abundance is relative to the permeability of the surficial unit. Colluvium with high proportions of volcanic content tend to host more springs, exist at a higher altitude, and be more permeable, acting as aquifer layers. Colluvium with greater proportions of loess content tends to have less springs, be at a lower altitude, and be less permeable. Channelized tunnels control the way that springs flow and form in aquifer layers closely underlain by less permeable layers. Loess derived springs appear to reflect pre-existing eroded lava drainages that were draped by periods of loess deposition. Intervening and post deposition of loess drainages persisted reworking loess and depositing volcanic-rich colluvium units which now act as aquifer horizons.

Inherited volcanic structures (i.e. lava dips, ash beds, and intrusives) and primary cooling joints control fluid flow of meteoric water within volcanic units. Within surficial loess derived deposits, primary depositional features and reworking of loess through slopewash and drainage development control fluid flow and location of springs. Characterization, mapping, and modeling of the springs on Banks Peninsula has identified sources and mechanism of fluid flow, an important initial component in the allocation and management of water resources.