Single-well injection-withdrawal tests as a contaminant transport characterisation tool for landfilled waste

A new single well injection withdrawal (SWIW) test was trialled at four landfills using the tracers lithium and deuterium, and by injecting clean water and measuring electrical conductivity. The aim of the research was to develop a practical test for measuring lateral contaminant transport to aid in the design of landfill flushing. Borehole dilution tests using dyes were undertaken prior to each SWIW test to determine background flow velocities. SWIW tests were performed at different scales by varying the volume of tracer injected (1 to 5,800 m(3) ) and the test duration (2 to 266 days). Tracers were used individually, simultaneously or sequentially to examine repeatability and scaling. Mobile porosities, estimated from first arrival times in observation wells and from model fitting ranged from 0.02 to 0.14. The low mobile porosities measured rule out a purely advective-dispersive system and support a conceptual model of a highly preferential dual-porosity flow system with localised heterogeneity. A dual-porosity model was used to interpret the results. The model gave a good fit to the test data in 7 out of 11 tests (where R-2 >= 0.98), and the parameters derived are compatible with previous experiments in MSW. Block diffusion times were estimated to range from 12 to 6,630 h, with a scaling relationship apparent between the size of the test (volume of tracer used and/or the duration) and the observed block diffusion time. This scaling relationship means affordable small-scale tests can inform larger-scale flushing operations. (C) 2021 The Authors. Published by Elsevier Ltd.

Automated summary

The new SWIW test using tracers like lithium and deuterium aimed to measure lateral contaminant transport for landfill flushing design. Tests were conducted at different scales, with mobile porosities ranging from 0.02 to 0.14 indicating a dual-porosity flow system in landfills. The dual-porosity model provided a good fit to the data in most tests, with block diffusion times ranging from 12 to 6,630 hours, showing a scaling relationship with test size.