Numerical modeling of arsenic mobility - In-Situ Remediation of Arsenic-Contaminated Sites - page 34

CHAPTER 2

Numerical modeling of arsenic mobility

Ilka Wallis, Henning Prommer & Dimitri Vlassopoulos

2.1 INTRODUCTION

Assessment and successful remediation of arsenic contaminated sites requires a rigorous under-
standing of the factors influencing arsenic fate and transport as well as the ability to predict the
behavior of arsenic in soil and aquifer systems under future conditions. The fate of arsenic in both
naturally and anthropogenically impacted aquifer systems, however, is determined by interactions
between physical, chemical, and biological processes. In many cases it can be difficult to discern
or distinguish between specific geochemical or biogeochemical processes controlling the fate of
arsenic, or whether its fate in fact might be predominantly controlled by physical transport. The
underlying causes of elevated arsenic concentrations will generally vary from case to case and
can be affected by several site-specific factors including the nature and magnitude of histori-
cal or ongoing releases, the chemical and mineralogical composition of the aquifer matrix, the
composition of the ambient groundwater and hydrogeological conditions.
Due to the complexity of the interactions between hydrologic, chemical, and biogeochemical
processes which are generally nonlinear, arsenic fate can often be non-intuitive and it becomes
obvious that analyses and predictions based on single-solute transport models are overly simplis-
tic and of limited value. Therefore numerical models that integrate these processes, specifically
geochemical and reactive transport models, provide an important pathway to asses and quan-
tify biogeochemical processes. Over the last two decades continuous advances in computational
performance have facilitated an increasing use of numerical simulators that fully couple both
geochemical and hydrological processes. Based on knowledge derived from either theoretical
work, laboratory investigations and/or site-specific field work, these numerical models provide a
quantitative framework to evaluate arsenic fate in complex subsurface systems, thereby allowing
simulation of time- and space-varying flow, transport and biogeochemical reactions.
In this chapter we provide a brief overview of the approaches and mathematical descriptions
that are used to simulate and quantify the processes affecting As behavior in aquifers.

2.2 MODELLING APPROACHES, TYPES OF MODELS AND

COMMON MODELLING TOOLS

Analysis and prediction of arsenic behavior in groundwater may be carried out over different time
and spatial scales at widely differing levels of geochemical complexity. Therefore, the appropri-
ate choice of modeling tools and approaches can vary considerably, depending on the nature and
objectives of the study as well as the types and amount of data that are available to constrain the
models. Prior to the use of numerical models (i) the clear definition of the modeling objectives
and (ii) the development of preliminary conceptual models of the chemistry and chemical changes
within the groundwater systems are important prerequisite that cannot be overemphasized. In the
early phases of model development, geochemical indicators provide information on groundwater
provenance, flow directions and rates, and age which form the basis of conceptual hydrogeolog-
ical models. Vice versa , knowledge of aquifer hydrostratigraphy, predominant groundwater flow
patterns and similar types of information help to develop conceptual models of hydrogeochemical
processes.

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