80
Pesticides, Organic Contaminants, and Pathogens in Air
environment of the model has been modifed to better describe the behavior
of highly water-soluble chemicals (Cahill et al., 2003). An additional feature
is the ability to simulate cycling and branching in the degradation pathways.
This article used chlorpyrifos, pentachlorophenol, and perfuorooctane sulfonate to demonstrate the capabilities of this new model (Cahill et al., 2003).
Khan and Ongerth made a theoretical model using the fugacity concept
that provides predictions of pharmaceutical concentrations, behaviors, and
fates in raw, primary, and secondary sewage (Khan and Ongerth, 2004). The
model incorporates two main features: (1) the prediction of pharmaceutical
concentrations at the sewage treatment plant inlet, and (2) the prediction of
pharmaceutical concentrations, behavior and fate during primary and secondary sewage treatment. Although the model currently has a relatively
“high degree of uncertainty,” it does provide (1) a basis for estimating the
relationship between the quantity of pharmaceuticals used and the observed
concentrations of the measured compounds; (2) an estimate of concentration
of compounds that have not been measured; and (3) an estimation of future
concentrations of new drugs.
5.4.2 Soil/Root Zone Models
PRZM (pesticide root zone model) is a one-dimensional, fnite-difference
model that accounts for pesticide and nitrogen fate in the crop root zone
(Table 5.4). PRZM3 includes modeling capabilities for such phenomena as
soil temperature simulation, volatilization, and vapor-phase transport in
soils, irrigation simulation, microbial transformation, and a method of characteristics algorithm to eliminate numerical dispersion. PRZM is capable of
simulating transport and transformation of the parent compound and as
many as two daughter species. VADOFT is a one-dimensional, fnite-element
code that solves the Richard’s equation for fow in the unsaturated zone. The
user may make use of constitutive relationships between pressure, water content, and hydraulic conductivity to solve the fow equations. VADOFT may
also simulate the fate of two parent and two daughter products. The PRZM
and VADOFT codes are linked together with the aid of a fexible execution
supervisor that allows the user to build loading models that are tailored to
site-specifc situations. In order to perform probability-based exposure assessments, the code is also equipped with a Monte Carlo pre- and post-processor
(www.epa.gov/exposure-assessment-models/przm-version-index).
Root Zone Water Quality Model 2 (RZWQM2) simulates major physical,
chemical, and biological processes in an agricultural crop production system (Table 5.4). RZWQM2 is a one-dimensional (vertical in the soil profle)
process-based model that simulates the growth of the plant and the movement of water, nutrients, and pesticides over, within, and below the crop root
zone of a unit area. It has a quasi-two-dimensional macropore/lateral fow. It
responds to agricultural management practices including planting and harvest practices, tillage, pesticide, manure and chemical nutrient applications,
Pesticides, Organic Contaminants, and Pathogens in Air
environment of the model has been modifed to better describe the behavior
of highly water-soluble chemicals (Cahill et al., 2003). An additional feature
is the ability to simulate cycling and branching in the degradation pathways.
This article used chlorpyrifos, pentachlorophenol, and perfuorooctane sulfonate to demonstrate the capabilities of this new model (Cahill et al., 2003).
Khan and Ongerth made a theoretical model using the fugacity concept
that provides predictions of pharmaceutical concentrations, behaviors, and
fates in raw, primary, and secondary sewage (Khan and Ongerth, 2004). The
model incorporates two main features: (1) the prediction of pharmaceutical
concentrations at the sewage treatment plant inlet, and (2) the prediction of
pharmaceutical concentrations, behavior and fate during primary and secondary sewage treatment. Although the model currently has a relatively
“high degree of uncertainty,” it does provide (1) a basis for estimating the
relationship between the quantity of pharmaceuticals used and the observed
concentrations of the measured compounds; (2) an estimate of concentration
of compounds that have not been measured; and (3) an estimation of future
concentrations of new drugs.
5.4.2 Soil/Root Zone Models
PRZM (pesticide root zone model) is a one-dimensional, fnite-difference
model that accounts for pesticide and nitrogen fate in the crop root zone
(Table 5.4). PRZM3 includes modeling capabilities for such phenomena as
soil temperature simulation, volatilization, and vapor-phase transport in
soils, irrigation simulation, microbial transformation, and a method of characteristics algorithm to eliminate numerical dispersion. PRZM is capable of
simulating transport and transformation of the parent compound and as
many as two daughter species. VADOFT is a one-dimensional, fnite-element
code that solves the Richard’s equation for fow in the unsaturated zone. The
user may make use of constitutive relationships between pressure, water content, and hydraulic conductivity to solve the fow equations. VADOFT may
also simulate the fate of two parent and two daughter products. The PRZM
and VADOFT codes are linked together with the aid of a fexible execution
supervisor that allows the user to build loading models that are tailored to
site-specifc situations. In order to perform probability-based exposure assessments, the code is also equipped with a Monte Carlo pre- and post-processor
(www.epa.gov/exposure-assessment-models/przm-version-index).
Root Zone Water Quality Model 2 (RZWQM2) simulates major physical,
chemical, and biological processes in an agricultural crop production system (Table 5.4). RZWQM2 is a one-dimensional (vertical in the soil profle)
process-based model that simulates the growth of the plant and the movement of water, nutrients, and pesticides over, within, and below the crop root
zone of a unit area. It has a quasi-two-dimensional macropore/lateral fow. It
responds to agricultural management practices including planting and harvest practices, tillage, pesticide, manure and chemical nutrient applications,