Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 333

relatively few inputs can be applied at early stages of modeling contaminant transport
to and from groundwater, to and from soil or from soil and groundwater to ambient
and indoor air (BIOSCREEN, 1997; VLEACH, 2007; MOFAT, 1991; BIOCHLOR,
2002; NAPL Simulator, 2010; and other Domenico-based models). In these models,
the mathematical function provides information about the system's behavior without
the need for measured data and values. Numerical models can account for a more
complex nature of the site such as heterogeneity, time-dependent transport, complex
transformation reactions (e.g. CHEMFLUX, 2013; MODFLOW, 2005; MODPATH,
2008; MT3DMS, 2010; SWMS 3D, 1995; PHREEQC, 1999). There are several soft-
ware packages on the market such as the currently popular Hydrus 3D for Windows,
to simulate water, heat, and solute movement in two- and three-dimensional variably
saturated media (Hydrus, 2013).
The theoretical basis of environmental transport and fate modeling has been
laid down in the 1980-1990s by US EPA together with ASTM standardized trans-
port and fate modeling, its documentation and integration into the contaminated site
characterization procedure (ASTM, 1999).
In Europe, associated with the development of the generic risk assessment method-
ology of chemical substances for legislative purposes, multimedia fate and transport
models have been recommended and applied such as the multimedia fate conceptual
model (Mackay et al., 1996) and the fugacity approach (Mackay, 2001).
The fugacity models were developed from the simplified Level I models, which
assume equilibrium partitioning of a non-reacting chemical in an equilibrium and
steady-state closed system, followed by Level II, III and IV models, which are more and
more dynamic, assuming degradation and advective loss in an open system (Level II),
no equilibrium compartments, intermedia transport (Level III) or unsteady state (Level
IV). Fugacity is not a new phenomenon, but its environmental application in these
transport and fate models is new, and is used in the sense of the escaping tendency of a
chemical substance from environmental compartments. The fugacity belonging to the
equilibrium is the half-partition value (Mackay, 1991).
In the EUSES system (European Union System for the Evaluation of Substances)
created for the evaluation of chemical substances in the environment, a local model is
nested in a regional model that, in turn, is nested in a continental model (EUSES, 1996).
The latest version of EUSES is recommended as a decision-support instrument which
enables government authorities, research institutes and chemical companies to carry
out rapid and efficient assessments of the general risks posed by substances to man and
the environment (EUSES 2.1.2, 2012). It is mainly intended for initial and refined risk
assessments rather than comprehensive assessments. The Joint Research Centre of the
European Commission (EU JRC, 2013) has published useful support material for the
fate and transport modeling of chemicals in the environment using GIS tools (Fate JRC,
2013). The availability of pan-continental datasets allows the development of spatially
explicit GIS-based models. Spatial models can predict chemical concentration at a given
location when the emissions to different media at continental scale are known. The
model is able to tell where the pollutants go. The inverse use of the same model shows
where the pollutants originate in cases when measured concentrations are the input
information.
To justify the use of a numerical model, the results from the model have to be
verified through a calibration and validation procedure (Troldborg, 2010). Model