Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 305

and are considered soluble in water and mobile—especially in the case of nitrates and
phosphorus. Once in surface water, nitrate and phosphorus can promote excessive algal
growth. Significant algal growth can deplete the dissolved oxygen in surface water and
cause suffocation and death to aquatic organisms. The solubility of some fertilizers, com-
bined with the hydraulic linkage between surface water and groundwater, may lead to
groundwater contamination.
The natural process of enrichment of surface waters with plant nutrients is termed
eutrophication . When anthropogenic activities such as fertilization or sewage discharges
accelerate this natural process, cultural eutrophication occurs (McGucken 2000).
8.4.6.4 Cyanide
In the atmosphere, cyanide is most often present as hydrogen cyanide. When present in
surface water, cyanide compounds will form hydrogen cyanide and then enter the atmo-
sphere through evaporation. When released to soil, cyanide compounds are considered
fairly mobile when the retention capacity of the soil is exceeded and may migrate and
contaminate groundwater. Cyanide compounds are degraded by microorganisms when
present at low concentrations. When concentrations of cyanide compounds are elevated,
they tend to be toxic to microorganisms and resist degradation (ATSDR 2006b).
The half-life of cyanide in the atmosphere ranges between 1 and 3 years (ATSDR 2006b).
In soil and water, the half-life of cyanide compounds is much more difficult to estimate
because the concentration, distribution, and presence of microorganisms available to
degrade the cyanide compounds vary.
8.4.6.5 Asbestos
Asbestos fibers do not degrade, evaporate, or dissolve in water and remain virtually
unchanged in the environment (ATSDR 2001c). Asbestos originates from naturally occur-
ring minerals and is therefore present in the environment. Average background concen-
trations of asbestos in air range from 0.00001 to 0.0001 fibers per milliliter of air and are
highest in urban areas (ATSDR 2001c).
Small diameter asbestos fibers can remain suspended in the atmosphere for a long
period of time compared to larger fibers (those larger than 10 microns; ATSDR 2001c).
Since asbestos was widely used in building materials, it is most common in urban areas
and where natural deposits are present. Asbestos can become airborne through the dis-
turbance of asbestos-containing materials during demolition or remodeling activities. The
asbestos threat is tied to its mineralogy (Chapter 7).
8.4.6.6 Acids and Bases
When released into soil, acids and bases neutralize rapidly. They are diluted when they
come into contact with water if a difference in pH levels exists. Therefore, if environmental
impairment occurs, it must be realized rapidly before the acid or base becomes neutral-
ized. This impairment occurs with the majority of sudden and accidental releases but does
not hold true for acid rain, which generates effects with slower onsets.
Acids and bases may migrate a significant distance—sometimes more than 1.6 km
(1 mi)—when released in the atmosphere and may cause significant impairment to living
organisms exposed to their vapors (ATSDR 2004b,c). The characteristics of acid rain are
covered during the discussion of sulfur dioxide later in this chapter.