fatally), by methylated mercury in fish. The methyl mercury, a waste product of
the plastics industry, had been discharged into coastal waters near the village.
The bioavailability of mercury is complex, controlled by fundamental biogeo-
chemical parameters such as type of microbial communities and Eh/pH (as dis-
cussed above), but also by its partitioning between solid and liquid phases and its
complexation particularly with dissolved organic ligands (see Box 6.4). Labora-
tory studies show that humic acids acting as ligands (see Box 6.4) drastically
increase the solubility of otherwise rather insoluble Hg^0 at near-neutral pH.
Moreover, the soluble complex formed is much less reactive than Hg^0 to mineral
surfaces in river sediments. This finding is highly significant for tropical rivers
like the Amazon that contain high levels of dissolved organic matter and humic
acids. It implies that mercury will remain mobile rather than fixed to mineral sur-
faces in river sediments, and may be transported to sites where oxidation and
methylation is occurring, increasing the risk of toxicity. Once in local soils and
water, mercury is also difficult to remove. In the Sierra Nevada region of the
western USA, millions of kilograms of mercury were lost to the environment
during the late 19th century gold mining. Much of this legacy remains today,
slowly releasing mercury into surface and groundwaters.5.7 Contamination of groundwater
Although aspects of groundwater chemistry have been discussed elsewhere in this
chapter, this section highlights issues relating to the contamination of ground-
water. Groundwater is critically important to humans since it is a major source
of drinking water. For example, in the USA over 50% of the population rely on
groundwater as a source of drinking water. Groundwater quality is therefore very
important and, in most developed countries, water must conform to certain stan-
dards for human consumption. Groundwater may fail to meet water quality stan-
dards because it contains dissolved constituents arising from either natural or
anthropogenic sources. Typical anthropogenic mechanisms of groundwater con-
tamination are shown in Fig. 5.16. In the USA, major threats to groundwater
include spillage from underground storage tanks, effluent from septic tanks and
leachate from agricultural activities, municipal landfills and abandoned hazardous
waste sites. The most frequently reported contaminants from these sources
include nitrates, pesticides, volatile organic compounds, petroleum products,
metals and synthetic organic chemicals.
The chemistry of contaminated groundwater is little different from that of
surface waters, except that most groundwaters are anaerobic. This has posed a
major problem for remediation of benzene (C 6 H 6 ), one of the most prevalent
organic contaminants in groundwater and of concern because of its toxicity.
Although much is known regarding aerobic degradation of benzene (see Section
4.10), until recently no pure culture of an organism capable of anaerobic degra-
dation of benzene existed, which made remediation almost impossible. It is now
known, however, that the bacterium Dechloromonas aromaticastrain RCB oxidizes
benzene anaerobically, using nitrate as the electron acceptor.174 Chapter Five