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 (K avg) 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.