Environmental Engineering Reference
In-Depth Information
Prediction and modeling for biotransformation and biodegradation and their effects on
fate and transport require a different approach. Yong and Mulligan (2004) have provided an
accounting of some of the more popular analytical-computer models used in the application
of natural attenuation schemes. These in effect are fate and transport models since biotrans-
formation and biodegradation are the primary attenuation processes—a principal feature
in fate and transport of organic chemicals. For example, the analytical model BIOSCREEN
(Newell et al., 1996) developed for the Air Forces Center for Environmental Excellence by
Groundwater Services (Houston, Texas) assumes a declining source concentration with
transport and biodegradation processes for the soluble hydrocarbons that include advec-
tion, dispersion, adsorption, aerobic, and anaerobic degradation. Most of the available ana-
lytical-computer models developed to handle biotransformation and biodegradation in fate
and transport modeling have the essential items contained in BIOSCREEN. The principal
distinguishing factors between the available computational packages such as BIOPLUME
III (Rifai et al., 1997), MODFLOW and RT3D (Sun et al., 1996), BIOREDOX (Carey et al., 1998),
and BIOCHLOR (Aziz et al., 2000), include (a) structure of the outputs, (b) manner in which
degradation is handled, such as order of degradation and degradation rates, (c) types of
organic chemicals, (d) inclusion of heavy metals and some other inorganics, (e) availability
and types of electron acceptors, and (f) adsorption-desorption. There are various versions
of the groundwater low model MODFLOW including MODFLOW-96, MODFLOW-2000,
and MODFLOW-2005. The various versions of the solute transport model are MT3DMS
(v5.3), MT3D, RT3D (v2.5), and MT3D99 (available from S.S. Papadopulos & Associates).
MT3D (Scientiic Software Group, 1998) is a groundwater solute transport model for com-
plex transient and steady-state lows, anisotropic dispersion, irst-order decay, and produc-
tion reactions to include biodegradation and sorption (linear and nonlinear). The newer
version, MT3D99, adds the capability for simulating multispecies reactions and simulate or
assess natural attenuation within a contaminant plume. MT3D99 works with MODFLOW.
Other models have also been developed according to the Center for Subsurface Modeling
Support (USEPA, 2013). Footprint is based on the Domenico model (1987), which was
released in 2008 by the EPA (Martin-Hayden and Robbins, 1997). It can be used to estimate
the extent of a plume using either a zero- or irst-order decay rate or can simulate the bio-
degradation of BTEX and/or ethanol. The Remediation Evaluation Model for Chlorinated
Solvents (REMChlor) released in 2007 is used to simulate the irst-order sequential decay
and production of several species of chlorinated solvents. It is applicable for simulating
contaminants in groundwater source and remediation whereas REMFuel, released in 2012,
is applicable for fuel hydrocarbons.
Solution of the transport relationships shown as Equations 9.7 and 9.8 and other similar
relationships can be achieved using analytical or numerical techniques. For well-deined
geometries, initial and boundary conditions, and processes, analytical techniques provide
exact solutions that can further one's insight into the processes involved in the problem
under consideration. Numerical techniques such as inite difference, inite element, and
boundary element are useful and are perhaps the techniques favored by many because of
their capability to handle more complex geometries and variations in material properties
and boundary conditions.
9.7.3 Geochemical Speciation and Transport Predictions
Abiotic reactions and transformations, together with the biotic counterparts, form the
suite of processes that are involved in the transport and fate of contaminants in the
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