Manual for Site Assessment at DNAPL Contaminated Sites in South Africa
Jan 2008
INTRODUCTION 1501/2-5/08

The potential for serious long-term contamination of groundwater by some DNAPL chemicals at many sites is largely due to their toxicity, limited solubility (but much higher than drinking water limits) and significant migration potential in soil gas, groundwater, and/or as a separate phase.  DNAPL chemicals, especially chlorinated solvents, are among the most prevalent groundwater contaminants identified in groundwater supplies and at waste disposal sites (Cohen and Mercer, 1993).

The South African Department of Water Affairs and Forestry (DWAF) have adopted a risk-based approach to managing contaminated sites.  In order to determine the risk that contamination may pose at a site and to manage that risk, it is necessary to have a good understanding of the nature and distribution of contaminants in the subsurface, and to understand how the contaminant will behave within the soil and/or groundwater system.

Collecting environmental data is expensive. Owners of contaminated sites must balance costs against the need for adequate information about the subsurface conditions and contaminant distribution to be able to make informed decisions about whether the contamination represents a threat to human health or the environment.  Contamination of fractured rock aquifers by non-aqueous phase liquids (NAPLs), is a worldwide problem.  The resulting groundwater contamination is difficult to manage, due in part to the problems associated with collecting the necessary data to understand contaminant behaviour within the system, properly.

The subsurface movement of DNAPL is controlled substantially by the nature of the release, the DNAPL density, interfacial tension and viscosity, porous and/or fractured media capillary properties and usually, to a lesser extent, by hydraulic forces.  Below the water table, non-wetting DNAPL migrates preferentially through permeable pathways such as soil and rock fractures, root holes and sand layers that provide relatively little capillary resistance to flow.  Visual detection of DNAPL in soil and groundwater samples may be difficult where the DNAPL is transparent, present in low saturation or distributed heterogeneously.  These factors confound characterisation of the movement and distribution of DNAPL, even at sites with relatively homogeneous soil and a known, uniform DNAPL source.  The difficulty of site characterisation is further compounded by fractured bedrock, heterogeneous strata, multiple DNAPL mixtures and releases, etc.

Obtaining a detailed delineation of subsurface DNAPL, therefore, can be very costly and may be impractical when using conventional site investigation techniques. Although DNAPL may greatly complicate site characterisation, failure to define adequately its presence, fate and transport may result in misguided investigation and remedial efforts.  Large savings and environmental benefits can be realised by conducting studies and implementing remedies in a cost-effective manner.  Cost-effective DNAPL site management requires an understanding of DNAPL properties and migration processes and of the methods available to investigate and interpret the transport and fate of DNAPL in the subsurface.

This report’s purpose is therefore to document the applicable assessment procedures for a DNAPL contaminated site.  Consideration was given to the following aspects: