Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 754

world. Sewage lagoons, landfills, leaking sewage systems (Lerner 2002 ), along with
waste disposal facilities, for example, can negatively affect groundwater quality.
Individual sewage treatment systems (septic systems) for homes in rural areas and
buildings not connected to the city sewer system - although in a quantitative sense
generally of limited importance - are another factor to consider when evaluating
risks to groundwater resources. An example of the large scale anthropogenic impact
on groundwater is described in Santos et al. ( 2002 ), who measured zinc, cadmium,
lead and copper in Andalusian groundwater, in the south of Spain resulting from the
Aznalcóllar accident. In April 1998, the failure of the tailings dam of the lead zinc
mine at Aznalcóllar near Seville, Spain, released nearly five million cubic meters of
toxic tailing slurries into the valley of the Guadiamar River. As a result, the alluvial
aquifer of Rio Agrio became contaminated.
Moreover, organic contaminants are frequently found in groundwater in all
developed countries worldwide. In fact, in all situations where chemicals are han-
dled, the groundwater beneath the location will be contaminated to a greater or
a lesser extent (Lerner and Tellam 1992 ). The most frequently found contami-
nants in the groundwater are mobile contaminants that are not readily attenuated
in the groundwater. Amongst these are the groups of chlorinated hydrocarbons
and petroleum hydrocarbons, denser (Dense Non-aqueous Phase Liquids; DNAPL)
and less dense (Light Non-aqueous Phase Liquids; LNAPL) than water, respec-
tively. Moran et al. ( 2007 ), for example, registered at least one VOC in 12% of
the samples from 1,926 rural private wells in the USA. The seven most frequently
detected VOCs were: trichloromethane, methyl tert -butyl ether, tetrachloroethene,
dichlorodifluoromethane, methylbenzene, 1,1,1-trichloroethane, and 1,2-dibromo-
3-chloropropane. Solvents and trihalomethanes were the most frequently detected
VOC groups in private wells. The concentrations of the most detected VOCs were
relatively small and only 1.4 percent of the samples had one or more VOC con-
centrations that exceeded a federally established drinking water standard or health
criterion. Kelsh et al. ( 2003 ), in yet another example, registered perchlorate in
Californian groundwater. During the last few years, MTBE (methyl tert -butyl ether)
emerged as a frequently found contaminant in groundwater in the USA (e.g., Deeb
et al. 2003 ) and in Europe. Kolb and Püttmann ( 2006 ), for example, showed detec-
tion frequencies of MTBE in groundwater in Germany that were 58% at 29 known
fuel-contaminated sites, 63% at 67 urban sites and even for non-urban sites they
were 24%.
With regard to immobile contaminants in the upper soil layers, leaching to
groundwater may continue for many decades. Mandocdoc and Primo David ( 2008 ),
for example, detected dieldrin in concentrations that exceeded the drinking water
standards in several wells at the former United States military facility Clark Air
Base in the Philippines, 16 years after dieldrin was last used.
In addition, diffuse sources contribute to contaminated groundwater. The use of
fertilisers in agriculture, for example, is the main cause of high nitrate concentra-
tions in groundwater (e.g., Tomer and Burkart ( 2003 ), who demonstrated the impact
of different agricultural management procedures on nitrate leaching to groundwater
in Iowa, USA).