Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 280

contaminant mass does not exceed the holding capacity of the soil. Soils or sedi-
ments composed of sand and gravel generally have lower organic carbon content
compared to soils composed of clay. An example of high relative organic carbon
content is shown in Figure 2.36 where a sand deposit is interbedded with layers of
ash primarily composed of organic carbon.
• Soil chemistry. The pH, redox potential, and other soil chemistry factors influence
the contaminant migration of many different types of compounds. Many metals,
for instance, are particularly sensitive to pH differences in soil, and these differ-
ences—along with the characteristics of each metal—influence their migration
patterns in the environment.
• Stratigraphy. This is where heterogeneity and the anisotropic nature of the geo-
logic sediments play a significant role at both a micro- and macroscale. As shown
in Chapter 2, the geology beneath the surface can and does change dramatically
in just a few meters in any direction. The result is differing sediment types and
chemical composition, including pH and redox potential, of subsurface layers act-
ing to impede or enhance contaminant migration.
• Unconformities. From Chapter 3, the presence of unconformities influences
the migration of groundwater as well as the migration of contaminants.
Hydrogeologically, the presence of an unconformity indicates there is a surface or
plane in the subsurface geologic environment. This space often produces a signifi-
cant difference in the hydraulic conductivities within the soils or sediments above
and below the unconformity, especially if these units are fine-grained sediments
such as silts or clays. As a result, contaminants released in this type of location use
the unconformity as a sink and migrate much further than expected.
Specific climatological factors affecting migration of contaminants in soil include
(USGS 2006a)
• Freeze-thaw cycles. Freeze-thaw cycles (Chapter 3) can lead to the development of
vertical fractures in the soil to depths approaching 10 m. These vertical fractures
are a type of secondary porosity and, if present, can greatly increase the migra-
tion potential of contaminants vertically through the soil column. In addition, the
freezing of near surface soils may trap contaminants at the surface and lead to
increased contaminant loading during warmer periods when the ice melts.
• Rainfall. Because water is the universal solvent, geographic locations receiving
abundant rainfall play a significant role in enhancing the migration of contami-
nants, especially if they are soluble. Rainfall also enhances the migration of con-
tamination through the physical transport of particles with sorbed contamination
on their surfaces.
• Snowfall. Airborne deposition of contaminants may become temporarily trapped
in seasonal snowpack. Increased contaminant loading to the environment may
occur during warmer periods when the snowpack melts (Wania et al. 1998).
• Wind. Many locations within the United States contain significant amounts of
wind-blown deposits, especially in the southwest (Chapter 2). Contaminants with
a high sorption potential may become attached to fine wind-blown sediment
grains and transported over long distances (Section 7.8.12). In addition, volatile
contaminants released as a gas are routinely transported by wind.