An illustrated handbook of DNAPL transport and fate in the subsurface

Executive summary Dense non-aqueous phase liquids (DNAPLs) such as creosote, coal tar, chlorinated solvents and polychlorinated biphenyl oils represent a particular class of soil and groundwater contaminant that exist as a separate liquid phase in the presence of water. DNAPLs come to rest in the subsurface as disconnected blobs and ganglia of liquid referred to as residual DNAPL, and in potentially mobile distributions referred to as pools. The region of the subsurface containing residual and pooled DNAPL is referred to as the source zone. Groundwater flowing through the source zone slowly dissolves the DNAPL, giving rise to aqueous phase plumes of contamination hydraulically down-gradient of the source zone. Some DNAPL compounds are resistant to biodegradation and sorb little; they can therefore give rise to substantial aqueous phase plumes. Other DNAPL compounds are relatively immobile in groundwater and, therefore, are highly retarded relative to the rate of groundwater flow. In unsaturated media, volatile DNAPLs give rise to vapour phase contamination. Because DNAPLs are only slightly soluble in water, DNAPL source zones can persist for many decades and, in some cases, even hundreds of years. Some DNAPLs are highly toxic and even very low concentrations in groundwater or the atmosphere can pose an unacceptable risk to human health or the environment. The fact that DNAPLs are denser than water allows them to migrate to substantial depths below the water table in both unconsolidated deposits and fractured bedrock. Delineating the spatial extent of the DNAPL source zone at a site can be a substantial undertaking, requiring at times several years of investigation and significant financial resources. Remediation strategies are site-specific, with separate approaches often warranted for the DNAPL source zone and its associated aqueous phase plume. There has been limited success in removing all DNAPL from below the water table at sites, particularly in a fractured rock environment. Remediation strategies are therefore often directed towards source zone containment or stabilisation, partial mass removal, plume management or plume interception, within the framework of appropriate risk-management objectives. The purpose of this handbook is to provide a user-friendly overview of the nature of DNAPL contamination in a UK context. It is intended to assist site investigators, site owners and regulators in conducting site investigations, conducting risk assessments and selecting remediation approaches. While this handbook reflects the state-of-the-art at the time of publication, it should be noted that the discipline of groundwater and soil contamination by hazardous organic liquids is evolving continuously and is relatively ‘young’ compared with many other areas of science and engineering. Readers are therefore advised to keep abreast of the new advances in understanding and approaches expected in the foreseeable future.