Contaminant Transport in Groundwater

Contaminant transport describes how dissolved and immiscible pollutants move and spread in groundwater through advection, dispersion, and chemical and biological processes.

Definition

Contaminant transport in groundwater is the movement and fate of dissolved or immiscible pollutants in the subsurface, governed by advection with the flowing water, hydrodynamic dispersion, and chemical and biological reactions that retard or transform them.

Scope

This topic covers the physical transport of solutes by advection and dispersion, the retardation and degradation of contaminants by sorption and reaction, the behavior of non-aqueous phase liquids, and the basis for predicting plumes and remediation. It applies groundwater flow to the movement of dissolved and immiscible contaminants.

Core questions

• How do advection and dispersion control the movement of a contaminant plume?
• How do sorption and degradation retard and attenuate contaminants?
• How do non-aqueous phase liquids behave differently from dissolved solutes?
• How are plumes predicted and aquifers remediated?

Key concepts

• Advection and seepage velocity
• Hydrodynamic dispersion and diffusion
• Advection-dispersion equation
• Sorption and retardation factor
• Biodegradation and natural attenuation
• Non-aqueous phase liquids (NAPLs)

Key theories

Advection-dispersion equation

Solute transport is modeled by combining advection at the mean groundwater velocity with hydrodynamic dispersion and diffusion, yielding the advection-dispersion equation that predicts how plumes spread over time.

Retardation and attenuation

Sorption to aquifer solids retards reactive contaminants relative to the water, while biodegradation and other reactions attenuate them, processes central to predicting plume length and to natural and engineered remediation.

Mechanisms

A dissolved contaminant is carried along by the average groundwater flow (advection) while spreading by mechanical dispersion, as flow paths vary through the pore network, and molecular diffusion. Reactive contaminants partition onto aquifer solids by sorption, which slows them relative to the water (retardation), and may be transformed by chemical or microbial reactions; immiscible liquids form separate phases that act as long-term sources.

Clinical relevance

Understanding contaminant transport is essential for assessing risks to drinking-water supplies, delineating and predicting pollution plumes from spills, landfills, and agriculture, designing monitoring networks and remediation systems, and evaluating natural attenuation as a cleanup strategy.

History

Contaminant transport theory drew on the advection-dispersion framework formalized by Bear and others in the 1970s; rising concern over groundwater pollution from the 1970s onward made contaminant hydrogeology a major field, extending to sorption, biodegradation, and the difficult behavior of dense and light non-aqueous phase liquids.

Debates

Scale dependence of dispersivity

Field-measured dispersivity tends to increase with the scale of observation because of aquifer heterogeneity, raising debate over whether the classical advection-dispersion equation with constant dispersivity adequately represents transport at field scale.

Key figures

• Jacob Bear
• John A. Cherry
• C. W. Fetter

Seminal works

• freeze1979
• bear1979
• fetter1999

Frequently asked questions

Why does a contaminant plume spread out as it moves?

Beyond being carried by the mean flow, the contaminant spreads because water follows many tortuous paths at different speeds through the pores (mechanical dispersion) and because of molecular diffusion, so the plume grows in extent and its concentrations dilute downgradient.

What is a NAPL?

A non-aqueous phase liquid is a contaminant such as oil or a chlorinated solvent that does not readily mix with water; NAPLs move as a separate phase and can persist for decades, slowly dissolving and acting as long-term sources of groundwater contamination.