A conceptual model of flow to the Waikoropupu Springs, NW Nelson, New Zealand, based on hydrometric and tracer (sup18/supO, Cl,sup3/supH and CFC) evidence

摘要

The Waikoropupu Springs, a large karst resurgence 4 km from the coast, aresupplied by the Arthur Marble Aquifer (AMA) underlying the Takaka Valley,South Island, New Zealand. New evidence on the recharge sources in thecatchment, combined with previous results, is used to establish a newrecharge model for the AMA. Combined with the oxygen-18 mass balance, thisyields a quantitative description of the inputs and outputs to the aquifer.It shows that the Main Spring is sourced mainly from the karst uplands(74%), with smaller contributions from the Upper Takaka River (18%)and valley rainfall (8%), while Fish Creek Spring contains mostly UpperTakaka River water (50%). In addition, much of the Upper Takaka Rivercontribution to the aquifer (58%) bypasses the springs and is dischargedoffshore via submarine springs. The chemical concentrations of the MainSpring show input of 0.5% of sea water on average, which varies with flowand derives from the deep aquifer. Tritium measurements spanning 40 yr,and CFC-11 measurements, give a mean residence time of 8 yr for the MainSpring water using the preferred two-component model. Our conceptual flowmodel, based on the flow, chloride, oxygen-18 and age measurements, invokestwo different flow systems with different recharge sources to explain theflow within the AMA. One system contains deeply penetrating old water withmean age 10.2 yr and water volume 3 km3, recharged from the karstuplands. The other, at shallow levels below the valley floor, has muchyounger water with mean age 1.2 yr and water volume 0.4 km3,recharged by Upper Takaka River and valley rainfall. The flow systemscontribute in different proportions to the Main Spring, Fish Creek Springsand offshore springs. Their very different behaviours, despite being in thesame aquifer, are attributed to the presence of a diorite intrusion belowthe surface of the lower valley, which diverts the deep flow towards theWaikoropupu Springs and allows much of the shallow flow to pass over theintrusive and escape via submarine springs.