Complementary use of tracer and pumping tests to characterize a heterogeneous channelized aquifer system in New Zealand

Abstract

The combined use of pumping and tracer test data enabled the derivation of equivalent average hydraulic conductivities (Kavg) for each test in a heterogeneous channelized alluvial aquifer, whereas K values of the preferential flow paths were two orders of magnitude higher. Greater and earlier drawdown was generally observed along preferential flow lines in a pumping test, within an array of 21 wells. The study aim was to characterize hydraulic properties of a channelized aquifer system in New Zealand by combining tracer and pumping test data. Estimates were able to be made of the percentage of highly permeable channels within the profile (∼1.2%), effective porosity that reflected the maximum fraction of highly permeable channels within the aquifer (ϕeff–pc ∼0.0038), and flows through highly permeable channels (∼98%) and the sandy gravel matrix material (∼2%). Using ϕeff–pc, a tracer test Kavg value (∼93 m/day) was estimated that was equivalent to pumping test values (∼100 m/day), but two orders of magnitude smaller than K calculated solely from transport through permeable channels (Kpc ∼8,400 m/day). Derived K values of permeable and matrix material were similar to values derived from grain size distribution using the Kozeny-Carman equation.