Metal Contamination in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 333

3.5

3

Arsenic
Barium
Cadmium
Chromium
Copper
Lead

2.5

2

1.5

1

0.5

0

(a)

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

Mercury

Nickel

Selenium

Silver
Zinc

(b)

FIGURE 9.4
West-to-east distribution of heavy metals in groundwater for (a) arsenic, barium, chromium, copper, and
lead; (b) mercury, nickel, selenium, silver, and zinc (west is to the left on each graph). (From Murray, K.
et al.: Dissolved heavy metals in shallow ground water in a southeastern Michigan urban watershed. J. Am.
Water Resour. Assoc ., 2006. 42. 777-792. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with
permission.)

Metal mobility is assessed by computing the ratio between the mean concentrations of
metals in groundwater and at the soil surface. Columns 1-3 of Table 9.8 show the metals
with the 10 highest ratios and the geologic unit where this ratio occurs; column 4 will be
referenced during the discussion on contamination.
High ratios are observed for several metals within the clay and silty clay units (Hg, Se,
Cr, and As), indicating some vertical mobility does occur for these metals within clay-
rich soils and subsoils. Two of these same metals (Se and Cr) are also mobile within units
consisting of larger-grained soil particles, which have higher hydraulic conductivities
(sand, moraine, and outwash). Cadmium also exhibits high ratios within the sand and