Field and Laboratory
Investigations to Study the Fate and Transport of Dense Non-Aqueous
Phase Liquids (DNAPLs) in Groundwater
1501/5/08
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:
- DNAPL contaminant properties
- South African aquifer characteristics
- Available technology in South Africa
- Innovative and cost-effective assessment methodologies.