Geoscience Reference
In-Depth Information
Table 17.1
Mean
concentrations
of
dissolved
heavy
metals
in
shallow
groundwater
in
a
Michigan
urban
watershed
(modified
after
Murray
2006
).
Modified
from
Murray
(
2006
).
Copyright (2006) with permission
Ba
Cd
Cr
Cu
Pb
Hg
Ni
Se
Ag
Zn
Residential
0.25
0.02
0.37
0.03
0.042
0.0004
NA
0.005
0.009
0.10
Commercial
0.61
0.005
0.02
0.05
0.045
0.015
0.12
0.078
0.008
0.31
Industrial
1.21
0.076
4.87
0.04
0.053
0.10
0.27
0.015
0.02
0.34
MCL (mg/L)
2.0
0.005
1.3
0.015
0.015
0.002
NA
0.05
0.1
5.0
Diffuse contamination: Heavy metals reach groundwater via diffuse sources,
when they are distributed over a broad urban area, and via point sources, when
leaching from waste disposal sites and landfills. A few examples of irreversible,
diffuse and point source groundwater contamination by heavy metals are discussed
below.
Murray (
2006
) reported the anthropogenically induced presence of dissolved
heavy metals in shallow groundwater in a Michigan urban watershed. A total of 1,140
samples were collected from the watershed of an urban area divide in residential,
commercial, and industrial areas. The samples were analyzed for barium, cadmium,
chromium, copper, lead, mercury, nickel, selenium, silver, and zinc, and compared to
their corresponding mean concentrations at the near surface (Table
17.1
). The
comparison demonstrated that the groundwater is significantly polluted with heavy
metals; in some locations, the mean concentration of Cr, Cd, and Pb exceeded the
allowable mean contamination level (MCL) by 20, 12, and 10 times, respectively.
The solubility and mobility of Cr species are specific for each oxidation state. Thus, a
high concentration is associated with the presence of toxic, anthropogenic Cr
6+
which has a high solubility and mobility. The high Cr
6+
concentration can be
attributed to significant releases from industrial sites. Because natural attenuation of
Cr, Cd, and Pb contaminants in groundwater is unlikely to occur, such anthropo-
genically induced contamination can be considered as irreversible.
Diffuse anthropogenic contamination of groundwater by heavy metals in Erasi-
nos Basin (E. Attica, Greece), due to intense urbanization and intense agriculture,
was reported by Champidi et al. (
2011
). The study area occupies 212 km
2
and was
sampled at 3 spring outlets, 40 wells, and 12 deep boreholes. Heavy metals (Sr, Fe,
Mn, Cu, Cr, Ni, Pb, Cd, and Zn) were detected in the sampled groundwater. In
general, the trace element concentrations exceeded the maximum acceptable level
according to European Union standard, although significant variation was found
among the collected samples: Cr 0.000-0702 ppm; Pb 0.000-0.434 ppm; Fe
0.000-0.271 ppm; Cu 0.003-0.157 ppm; Mn 0.000-0.392 ppm; Zn
0.010-2.343 ppm; Ni 0.000-0.297 ppm; and Sr 0.270-7.240 ppm. By application
of R-mode factor analysis, used to characterize relationships among different data
sets and variables, it was found that 30 % of the variance in the data matrix may be
considered as a ''heavy metal factor,'' and the spatial distribution is depicted in
Fig.
17.14
. The high heavy metal concentration observed in the western part of the
basin is associated with an industrial facility and intense agriculture.
