How does it behave?: a conceptual model of the Omaka Aquifer

Abstract

The Omaka Aquifer is likely to come under stress from irrigation pumping in the short to medium term if the current rate of horticultural development continues. A conceptual understanding of key aquifer processes was a prerequisite for refining estimates of the sustainable aquifer yield and simulating aquifer behaviour. A conceptual model of the Omaka Aquifer was postulated following a review of existing information. The information review showed gaps in the knowledge base with in an inadequate understanding of in particular the deeper aquifer levels. Future work should focus on: aquifer recharge, regional groundwater flow patterns, flow boundaries and hydraulic properties. Essential features of the aquifer were its low permeability resulting in sluggish flow at depth combined with significant surface spring losses. At least 4 aquifer layers consisting of thin, discontinuous lenses with varying degrees of interconnection were tentatively identified. Regional interaction with neighbouring aquifers and the effective rate of recharge remain uncertain. Aspects of the conceptual understanding were shown to be workable using a simple numerical flow model. While further work is required to reduce the uncertainty over key aquifer processes, the conceptual model currently represents the best understanding of the Omaka Aquifer and forms a good basis for further research such as refining the figure of aquifer safe yield.