• Hazard rating
Using the equation
P ¼ c 1 Â Z1 þ c 2 Â Z2 þ c 3 Â Z3 þ c 4 Â Z4 þ c 5 Â Z5 þ c 6 Â Z6 þ c 7 Â Z7;
Then W ¼ 0:22 Â 10 þ 0:17 Â 1 þ 0:16 Â 8 þ 0:1 Â 8 þ 0:09 Â 10 þ 0:12 Â
5 þ 0:14 Â 10 ¼ 7:27.
The hazard index of the site is calculated to be 7.27, rated Grade III.
(b) Vadose zone mitigation rating
• Indicators and scores for pollution control
According to the hydrogeological conditions of the site, the groundwater depth to
water table is 1.70–2.00 m. The mixed soil on the top is 1.30 m thick on average
and 1.3–1.5 m deep relative to water table. As to silty clay, the average thickness
and depth to water table are 1.22 m and 1.00–4.80 m respectively. It is apparent the
vadose zone of the site is comprised of miscellaneous fill and silty clay.
Miscellaneous fill refers to waste earth left or piled in people’s lives and production
activities. It is hardly described by a unified permeability index due to differences in
the types and duration of existence of such earth. In light of this, the mitigation
performance considers the silty clay only, instead of the vadose zone of miscellaneous fill. According to the results of field survey, the permeability coefficient of
silty clay reaches 0.25 m/d. As shown in Table 3.10, the single-layer thickness of
the rock (earth) layer Mb ! 1.0 m; the permeability coefficient K ! 10
−4 cm/s.
Hence, the mitigation performance of the vadose zone is rated “weak” with a score
of 0.
• Indicators and scores for vulnerability
Considering the weak performance in pollution control, the comprehensive evaluation of vadose zone is not conducted, so is the vulnerability assessment.
(c) Rating and evaluation of groundwater pollution source intensity
Based on the above results, the hazard of pollution sources is rated Grade II and the
vadose zone is rated weak in pollution control. Without consideration to the
comprehensive evaluation of vadose zone, the intensity of groundwater pollution
sources for the site is rated Grade II.
(4) Health risk assessment of groundwater environment
As indicated in Table 5.52, the situation is grim that pollutants far exceed the limits
at the three sampling sites. The overall groundwater quality at the sampling sites is
classified into Grade V, wherein at No. 1 sampling site, ammonia nitrogen and
permanganate index reach Grade V standards and arsenic Grade IV standard; at the
other two sampling sites, permanganate index reaches Grade V standard and arsenic
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193
Using the equation
P ¼ c 1 Â Z1 þ c 2 Â Z2 þ c 3 Â Z3 þ c 4 Â Z4 þ c 5 Â Z5 þ c 6 Â Z6 þ c 7 Â Z7;
Then W ¼ 0:22 Â 10 þ 0:17 Â 1 þ 0:16 Â 8 þ 0:1 Â 8 þ 0:09 Â 10 þ 0:12 Â
5 þ 0:14 Â 10 ¼ 7:27.
The hazard index of the site is calculated to be 7.27, rated Grade III.
(b) Vadose zone mitigation rating
• Indicators and scores for pollution control
According to the hydrogeological conditions of the site, the groundwater depth to
water table is 1.70–2.00 m. The mixed soil on the top is 1.30 m thick on average
and 1.3–1.5 m deep relative to water table. As to silty clay, the average thickness
and depth to water table are 1.22 m and 1.00–4.80 m respectively. It is apparent the
vadose zone of the site is comprised of miscellaneous fill and silty clay.
Miscellaneous fill refers to waste earth left or piled in people’s lives and production
activities. It is hardly described by a unified permeability index due to differences in
the types and duration of existence of such earth. In light of this, the mitigation
performance considers the silty clay only, instead of the vadose zone of miscellaneous fill. According to the results of field survey, the permeability coefficient of
silty clay reaches 0.25 m/d. As shown in Table 3.10, the single-layer thickness of
the rock (earth) layer Mb ! 1.0 m; the permeability coefficient K ! 10
−4 cm/s.
Hence, the mitigation performance of the vadose zone is rated “weak” with a score
of 0.
• Indicators and scores for vulnerability
Considering the weak performance in pollution control, the comprehensive evaluation of vadose zone is not conducted, so is the vulnerability assessment.
(c) Rating and evaluation of groundwater pollution source intensity
Based on the above results, the hazard of pollution sources is rated Grade II and the
vadose zone is rated weak in pollution control. Without consideration to the
comprehensive evaluation of vadose zone, the intensity of groundwater pollution
sources for the site is rated Grade II.
(4) Health risk assessment of groundwater environment
As indicated in Table 5.52, the situation is grim that pollutants far exceed the limits
at the three sampling sites. The overall groundwater quality at the sampling sites is
classified into Grade V, wherein at No. 1 sampling site, ammonia nitrogen and
permanganate index reach Grade V standards and arsenic Grade IV standard; at the
other two sampling sites, permanganate index reaches Grade V standard and arsenic
5.2 Northwest China
193