Geoscience Reference
In-Depth Information
As they persist or age, organic contaminants in the subsurface become pro-
gressively less available. In a review on contaminant bioavailability during aging,
Alexander (
2000
) shows evidence that organic compounds aged in the field are
less bioavailable than the same compounds added freshly to samples of the same
soil. Table
8.12
compiles the time required for a number of contaminants to
become less available for microbial degradation as result of aging. Bioavailability
to microorganisms decreases with time, until reaching a value below which further
decline is not detectable. The time required to reach this value varies among soils,
compounds, and environmental conditions.
Sequestration is considered as the decrease in availability of a compound, for
uptake by a living organism and for nonvigorous extraction by an organic solvent.
Nam et al. (
1998
) studied phenanthrene sequestration by fresh and aged soil OM,
where the contaminant presence was determined by both bioavailability (e.g.,
mineralization by an added bacterium) and mild solvent extraction (1-butanol)
procedures. Sequestration of phenanthrene as measured by bacteria-induced
mineralization was appreciable in samples with [2 % organic C and was not
evident in samples with \2 % organic C. Phenanthrene aged 200 days was more
slowly degraded than the freshly added compound in soils with [2 % organic C.
However, only a small effect was evident when the soil organic C was\2 %. Mild
extraction with 1-butanol showed that the quantity removed by the solvent
diminished as the compound persisted and the rate of decline in extractability
generally diminished with decreasing organic C. Subsequent extraction by Soxhlet
led to complete recovery of the compound, showing that the decline in availability
comes from sequestration and does not reflect a loss of phenanthrene during aging.
It is questionable whether or not the value obtained by bioassays or the
sequestration value can be used to define contaminant-bound residues. Aging-
sequestration relationships in the subsurface, as determined through bioavailabil-
ity, may provide an answer for short-term assessment of the contamination status
at a specific site. To predict long-term hazards caused to the subsurface by organic
contaminants of anthropogenic origin, where potential changes in environmental
parameters may occur, the use of a reliable solvent extraction of bound residues is
required
8.7 Contaminants in Urban Geosystems
Land surface deposits in the urban environment comprise a mixture of natural
earth materials and disposed residues during many years of human activity. The
term ''urban geochemistry'' was initially used by Thornton in the 1990s and
reactivated recently by Jartun and Ottensen (
2011
) and Lyons and Harmon (
2012
).
Urban geochemistry is characterized by intense toxic chemical disposal on a soil-
subsurface landscape already disturbed by previous anthropogenic manipulation.
The redistribution of contaminants in an urban environment differs from that
occurring in a natural geosystem, due to differential impact of microrelief, water,
