eqn. 5.25In addition to benzene this bacterium also oxidizes toluene, ethylbenzene
and xylene and thus offers great potential for the treatment of petroleum-
contaminated aquifers (Section 5.7.1).
Natural degradation processes that occur in days or weeks in surface waters
may take decades in groundwater, where flow rates are slow and microbiological
activity is low. This limits the potential for natural purification through flushing
or biological consumption. Once contaminated, groundwater is difficult and
expensive—in many cases impossible—to rehabilitate.
The location of older sites of contamination may be imprecisely known, or
even unknown, and hydrogeological conditions may dictate that contaminated
groundwater discharges at natural springs into rivers or lakes, spreading conta-
mination to surface waters.
The following sections highlight different styles of groundwater contamina-
tion where chemical considerations have proved important. Nutrient element
contamination of groundwater was discussed in Section 5.5.1.5.7.1 Anthropogenic contamination of groundwater
Landfill leakage—Babylon, Long Island, New York, USA
At Babylon landfill site, New York, shallow groundwater contamination of a
surface sand aquifer has resulted from leakage of leachate rich in Cl-, nitrogen
compounds, trace metals and a complex mixture of organic compounds. Land-
filling began in the 1940s with urban and industrial refuse and cesspool waste.
The refuse layer is now about 20 m thick, some of it lying below the water table.
Chloride behaves conservatively (see Section 6.2.2) and is thus an excellent tracer
of the contaminant plume, which is now about 3 km long (Fig. 5.17).
Close to the landfill, most nitrogen species are present as NH 4 +, indicating
reducing conditions resulting from microbial decomposition of organic wastes.
With increasing distance from the landfill, NO 3 - becomes quantitatively impor-
tant due to the oxidation of NH 4 +(Fig. 5.12), brought about by mixing of the
leachate plume with oxygenated groundwater. This demonstrates how nitrogen
speciation can be used to assess redox conditions in a contaminant plume.
Reducing conditions within the leachate plume also cause metal mobility,
particularly of manganese and iron. The plume near the landfill has a pH of
6.0–6.5 and is reducing (-50 mV), making Fe^2 +stable (Box 5.4). The transition
to oxidizing conditions down gradient in the aquifer allows solid iron oxides
(e.g. FeOOH) to precipitate, dramatically, reducing the mobility of metals which
co-precipitate with iron.
This relatively inoffensive example illustrates the importance of redox condi-
tions in contaminated groundwater. Worse scenarios are known where toxic
chlorophenolic compounds in very alkaline groundwaters (pH 10) ionize to neg-C H NO H CO H O
benzene66 ()aq 66 6 3 ()aq aq 2 aq^3 N 2 g^6 2l- ()
+++ ()+ +()
()
()176 Chapter Five