A Modeling-Based Evaluation of the
Effect of Wastewater Application Practices on
Groundwater Quality
CHARLES M. REYNOLDS AND ISKANDAR K. ISKANDAR
INTRODUCTION
processes. Moreover, each process directly in-
volved in altering a particular compound is, in
Wastewater treatment in soil by land applica-
turn, influenced by a multitude of other variables.
tion is a proven and effective technology that ad-
Furthermore, the effects of these variables on dif-
vantageously uses the natural ability of soil to de-
ferent chemicals are not necessarily in the same
grade organic molecules on a long-term, regenera-
magnitude or direction.
tive basis. Because there are numerous interacting
Many of the processes that occur are controlled
processes involved, mechanistic dynamic simula-
by microbial, and thereby enzymatic, activity. The
tion models can provide insight into a system's
importance of microbial and enzymatic activities
response to a number of conditions. The model
in transforming or degrading compounds contain-
WASTEN was developed by Selim and Iskandar
ing energy or nutrient sources, such as nitrogen,
(1981) to predict nitrogen transport and transfor-
carbon, phosphorus and sulfur, is reasonably in-
mations in soils receiving wastewater.
tuitive. Nevertheless, physio-chemical processes,
CRREL has assisted the Philadelphia District
such as sorptiondesorption and diffusion, are also
of the Corps of Engineers by providing technical
important in determining the fate of compounds.
consultation and WASTEN simulations for condi-
Microbial or enzymatic processes can also influ-
tions appropriate for a proposed treatment facil-
ence compounds, such as metals, that are not used
ity at Fort Dix, New Jersey. We compared several
as nutrient or energy sources. In many instances,
nitrogen input scenarios and predicted the levels
microbial-induced changes in microsite environ-
of nitrate in the groundwater for the proposed site.
ments can induce fixation, immobilization or sol-
vation of metals.
Overview
In recent years the combined impacts of inten-
Bioremediation
tional and accidental releases of chemicals and
The soil-mediated processes involved in the
waste solutions in and on soils, and our aware-
land treatment of wastewater are not new or un-
ness of the vulnerability of groundwater to pollu-
natural. Microbes are known to adapt over time to
tion from surface sources, have received increased
transform or degrade compounds in their environ-
attention. The rate of movement or transport of
ment, and their enzymes can readily transform or
chemicals in soil and subsoils can vary tremen-
completely degrade many non-anthropogenic or-
dously, and consequently the time required for a
ganic compounds. Microbial enzyme systems may
solute to reach a given depth varies. Transport de-
not be as efficient in transforming and breaking
termines the residence time of a compound in each
down more complex, anthropogenic molecules.
particular soil zone. During a chemical's residence
However, there are enzymes that catalyze many
in a soil zone, the chemical is subject to a host of
of the reactions needed to degrade more recalci-
biological, chemical and physical processes that
trant, complex molecules. Often, a series of reac-
combine to ultimately determine its fate. The gov-
tions must occur in a specific sequence to yield a
erning processes are intimately linked to one an-
desirable product. The conditions that favor any
other in myriad, complex chains of reactions and
one of these processes may inhibit another pro-
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