Doublet tracer tests to determine the contaminant flushing properties of a municipal solid waste landfill

Abstract

This paper describes a programme of research investigating horizontal fluid flow and solute transport through saturated municipal solid waste (MSW) landfill. The purpose is to inform engineering strategies for future contaminant flushing. Solute transport between injection/abstraction well pairs (doublets) is investigated using three tracers over five separate tests at well separations between 5 m and 20 m. Two inorganic tracers (lithium and bromide) were used, plus the fluorescent dye tracer, rhodamine-WT. There was no evidence for persistent preferential horizons or pathways at the inter-well scale. The time for tracer movement to the abstraction wells varied with well spacing as predicted for a homogeneous isotropic continuum. The time for tracer movement to remote observation wells was also as expected. Mobile porosity was estimated as ~ 0.02 (~ 4% of total porosity). Good fits to the tracer breakthrough data were achieved using a dual-porosity model, with immobile regions characterised by block diffusion timescales in the range of about one to ten years. This implies that diffusional exchanges are likely to be very significant for engineering of whole-site contaminant flushing and possibly rate-limiting.

Abbreviations
b, Thickness of saturated zone [mg/L]; bb, Half-width of an immobile block [m]; B, Block Geometry Function (Barker, 1985) [−]; cX, Background-corrected concentration at location X (e.g. X = M for monitoring point) [mg/L]; CA, Concentration in abstraction well [mg/L]; Cb, Background concentration [mg/L]; CI, Concentration in injection well [mg/L]; CM, Concentration at the monitoring point [mg/L]; CP, Concentration at any point within the waste (e.g. at an observation well) [mg/L]; CR, Concentration returned to the injection well [mg/L]; CT, Tracer input concentration [mg/L]; D, Spacing between injection and pumping well [m]; Da, Apparent diffusion coefficient [m2/d]; M, Transfer function for transport through return pipework to monitoring point [−]; P, Point in the waste (defined by horizontal coordinates x, y); q, Darcy velocity [m/d]; Q, Pumping (and injection) flow rate [m3/d]; rw, Well radius [mm]; R(s), Transfer function for transport through return pipework to injection well [−]; s, Laplace variable [d− 1]; sd, Slope of ln(concentration) against time in a dilution test [log(mg/L)/d]; t, Time [d]; ta(ψ), Advection time for a streamtube [d]; tA, Time constant of abstraction well [d]; tb, Time for fastest advection of tracer from injection to abstraction well [d]; tcb, Characteristic diffusion time to/from immobile zone [d]; tcf, Characteristic diffusion time to/from mobile zone [d]; tfd, Time of first detection of tracer [d]; tI, Time constant of injection well [d]; tM, Advection time from abstraction well to monitoring point [d]; tP, Advection time from injection well to point P in waste [d]; tR, Return time from abstraction well to injection well [d]; tT, Duration of tracer input for a top-hat input [d]; T(s), Transfer function for transport from tracer injection point to injection well [−]; W(s), Transfer function for transport through waste; z, Distance along a streamtube [m]; α, Dispersivity [m]; αL, Dispersivity per unit distance of travel, α /z [−]; γ, Specific weight [N/m3]; θ, Total volumetric water content (porosity) [−]; θim, Immobile volumetric water content (porosity) [−]; θm, Mobile volumetric water content (porosity) [−]; ψ, Angle from line joining doublet wells to streamline entering abstraction well [radians]

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Identifiers

ORCID for Nicholas Woodman: orcid.org/0000-0002-5571-0451

ORCID for Tristan Rees-White: orcid.org/0000-0001-9009-8432

ORCID for Richard Beaven: orcid.org/0000-0002-1387-8299

ORCID for Anne Stringfellow: orcid.org/0000-0002-8873-0010

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