Metal Contamination in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 329

To evaluate the mobility of heavy metals, the ratio of the mean concentrations of each
metal in groundwater to surface soil was computed. The metal concentrations within
surface soil were chosen since this is where the highest concentrations occur, and this
location also accounts for the longest vertical distance metals could migrate before
reaching groundwater. Although the measurement units of the mean concentrations
within soil (mg/kg) and groundwater (mg/L) are different, the ratio of mean ground-
water concentration to mean surface soil concentration provides a consistent and unbi-
ased way to compare the amounts of different metals present along this vertical profile.
These ratios are then ranked to identify the most mobile metals and their associated
geologic units.
To demonstrate the magnitude of contamination, another ratio is calculated between the
mean concentration of each metal and the metal's maximum contaminant level (MCL).
Because numerous drinking water supply wells exist within the watershed, the presence
of a specific metal within groundwater is considered significant if its mean concentration
within a geologic unit or land use category exceeds the current drinking water standard—
the MCL in mg L −1 set by the USEPA (1992, 2003).
9.4.2 Results
Table 9.7 contains the mean concentrations of heavy metals within the vertical soil/
groundwater column for each of the six geologic units of the watershed. The mean
concentrations of the sites within the three categories of land use are also presented.
Significant concentrations of metals in groundwater exceeding the MCL are denoted by
an asterisk.
As shown in Table 9.7, there are 66 total possible combinations of metals and geologic
units (11 metals * 6 geologic units) within the study area. Subtracting the seven instances
of missing data leaves 59 combinations. Significant metal concentrations occur in 19 of
these 59 cases (32%), which suggest groundwater contamination by heavy metals is wide-
spread throughout the watershed. The metals showing significant concentrations within
the most geologic units are Pb (5), Cd (4), and Cr (4). Mean concentrations for each of these
metals in the sand exceeds the MCL, with Cr exhibiting the highest mean concentration.
Copper, Ba, and Zn do not exceed their MCLs within groundwater across any of the geo-
logic units; Ni has no established standard—the 0.1 mg/L standard was remanded in 1995
(USEPA 2003).
All of the metals tested—with the exception of Se—exhibited the highest mean concen-
trations at industrial sites. In the Murray et al. (2004) soil study, the mean concentration
of metals generally increased across the watershed in a west-to-east direction. This same
pattern of increase is also seen with metals dissolved in groundwater (Figure 9.4a and b),
and with the vertical scale of contamination in Figure 9.4b indicating these metals occur at
generally higher mean concentrations than those metals graphed in Figure 9.4a. The MCLs
were exceeded by five metals (As, Cd, Cr, Pb, Hg) at industrial properties by As Cd, Pb,
and Hg at commercial sites and Cd and Pb at residential locations. Thus, the MCLs were
exceeded by cadmium and lead within all land use categories.
All of the metals exceeding the MCL within the industrial land use category (As, Cd,
Cr, Pb, Hg) also exceeded their MCLs within the clay unit. This characteristic illustrates
the historical urban development pattern of the region, which saw the heaviest con-
centrations of industry located within the older, more urbanized eastern portion of the
watershed.