Environmental Engineering Reference
In-Depth Information
TABLE 9.8
Metal Mobility and Contamination Ranking (n = 59)
Ratio
(gw-Surface)
Geologic
Unit
Measured Conc./
MCL (Rank) (mg/L)
Metal
Hg
0.126
Clay
17.0 (3)
Se
0.083
Outwash
(36)
Se
0.076
Silty clay
(18)
Cd
0.067
Sand
(5)
a
Cr
0.071
Sand
(2)
Cr
0.059
Clay
(1)
As
0.054
Clay
(9)
Hg
0.053
Silty clay
(12)
Cd
0.039
Moraine
(14)
As
0.029
Silty clay
4.6 (11)
a
Missing top 10 places: 4th = Pb in sand @ 14.0; 6th = Cr in
silty clay @ 8.9; 7th = Pb in silty clay @ 8.0; 8th = Cd in clay
@ 7.6; 10th = Pb in clay @ 5.3.
moraine geologic units. There is generally lower mobility observed for Ag, Ba, Cu, Ni, Pb,
and Zn.
Referring again to Table 9.8, groundwater contamination is represented by the ratio
between the mean concentration of the metal and its MCL in column 4. The rank of each
metal appears in parentheses after the ratio. The two highest contamination ratios and
ranks belong to Cr in clay and sand, respectively. Other significant rankings exist for Cd
and Pb in the sand unit, which were ranked fourth and fifth, respectively. Thus, three
metals had measured mean concentrations which were over 10 times the MCL within a
geologic unit with high hydraulic conductivity.
These results demonstrate that the groundwater resource within the Rouge River water-
shed is significantly polluted with heavy metals. Most significant are the occurrences of
elevated concentrations of heavy metals within the sand unit, with the mean concentra-
tions of Cr, Cd, and Pb exceeding the MCL by 22, 12, and 14 times, respectively. The main
commercial uses of Cr include chrome alloy in chromium metal products and in chrome
plating, and to a lesser extent as compounds in paints, dyes, explosives, ceramics, glass,
photography, and paper. The principal use of Cd is as an alloy in electroplating, in Ni-Cd
batteries, solders, electronic equipment, photographic supplies, glass, ceramics, and plas-
tics (Jaagumagi 1993). The high concentration of Cr and Cd in groundwater confirms the
principle of increased metal mobility in acidic, sandy soil.
The presence of elevated levels of Pb in groundwater may be due to the industrial mix
within the watershed. Metal industries use Pb in smelting and electroplating, but the
single largest use of Pb is in the production of lead acid batteries. Other uses include
paints, glassware, electronic equipment, plastics, solder, and construction materials.
Notwithstanding, this result is somewhat surprising, as Pb has a soil retention and
adsorption capacity higher than any other metal and is generally considered immobile
in subsurface soil (McLean and Bledsoe 1992). However, the mobility of Pb within south-
east Michigan soils was suggested by Murray et al. (1997) in their study of Pb at an out-
door shooting range. Because of its low MCL, even small amounts of Pb which manage
to migrate to groundwater can generate a high-measured mean concentration to MCL
ratio.
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