3.1.4 Characteristic Factors for Mitigation Performance
of Vadose Zone in Contaminated Sites
Ground and shallow surface pollutants must first pass through the vadose zone to
reach the groundwater. The vadose zone is an environmentally sensitive zone with
complex lithology and structure, rich material composition, and three-phase (gas,
liquid, solid) coexistence. It is closely related to the atmosphere, biosphere lithosphere and human sphere, and serves as an important natural shield against
groundwater contamination through pollutant convection, dispersion, adsorption
and degradation. Therefore, the mitigation performance of vadose zone is of high
significance for the prevention and control of groundwater contamination (Xi
2016).
3.1.4.1 Screening Main Factors Based on Inherent Vulnerability
The DRASTIC method offers a classic model for evaluating groundwater vulnerability and has been widely used in groundwater vulnerability assessments around
the world. Seven parameters are used in the DRASTIC model, including depth to
water table (D), net recharge (R), aquifer media (A), soil media (S), topography (T),
impact of vadose zone media (I), and hydraulic conductivity (C). They have different effects on the mitigation performance of vadose zone. Net recharge represents
the total quantity of water that infiltrates and returns from irrigation into the aquifer,
which is difficult to accurately quantify in practice. It may bring more pollutants
into the ground, and meanwhile dilutes and reduces pollutant concentrations to a
certain extent by increasing the amount of groundwater. The dual effects make the
final R value controversial. Aquifer media refers to the media in which groundwater
is stored in the aquifer, and can be ignored when screening the mitigation factor of
vadose zone. Topography affects the possibility of pollutants entering the
groundwater by affecting the infiltration of land surface precipitation, and it is
negligible for site-scale pollution sources. Soil media denotes the uppermost portion
of the vadose zone characterized by significant biological activity, and vadose zone
denotes the zone from the water table to the bottom of soil biological activity layer.
They are collectively referred to as impact of vadose zone media when the
DRASTIC model is generalized. Hydraulic conductivity affects the velocity of
pollutant migration after entering the aquifer, rather than the process of pollutant
entry into the groundwater from land surface. Therefore, its influence can also be
embodied in the impact of vadose zone media. In short, only D and I in the
DRASTIC model are considered as the main factors of mitigation performance of
vadose zone.
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3 Rating and Evaluation of Pollution Source Intensity …
of Vadose Zone in Contaminated Sites
Ground and shallow surface pollutants must first pass through the vadose zone to
reach the groundwater. The vadose zone is an environmentally sensitive zone with
complex lithology and structure, rich material composition, and three-phase (gas,
liquid, solid) coexistence. It is closely related to the atmosphere, biosphere lithosphere and human sphere, and serves as an important natural shield against
groundwater contamination through pollutant convection, dispersion, adsorption
and degradation. Therefore, the mitigation performance of vadose zone is of high
significance for the prevention and control of groundwater contamination (Xi
2016).
3.1.4.1 Screening Main Factors Based on Inherent Vulnerability
The DRASTIC method offers a classic model for evaluating groundwater vulnerability and has been widely used in groundwater vulnerability assessments around
the world. Seven parameters are used in the DRASTIC model, including depth to
water table (D), net recharge (R), aquifer media (A), soil media (S), topography (T),
impact of vadose zone media (I), and hydraulic conductivity (C). They have different effects on the mitigation performance of vadose zone. Net recharge represents
the total quantity of water that infiltrates and returns from irrigation into the aquifer,
which is difficult to accurately quantify in practice. It may bring more pollutants
into the ground, and meanwhile dilutes and reduces pollutant concentrations to a
certain extent by increasing the amount of groundwater. The dual effects make the
final R value controversial. Aquifer media refers to the media in which groundwater
is stored in the aquifer, and can be ignored when screening the mitigation factor of
vadose zone. Topography affects the possibility of pollutants entering the
groundwater by affecting the infiltration of land surface precipitation, and it is
negligible for site-scale pollution sources. Soil media denotes the uppermost portion
of the vadose zone characterized by significant biological activity, and vadose zone
denotes the zone from the water table to the bottom of soil biological activity layer.
They are collectively referred to as impact of vadose zone media when the
DRASTIC model is generalized. Hydraulic conductivity affects the velocity of
pollutant migration after entering the aquifer, rather than the process of pollutant
entry into the groundwater from land surface. Therefore, its influence can also be
embodied in the impact of vadose zone media. In short, only D and I in the
DRASTIC model are considered as the main factors of mitigation performance of
vadose zone.
38
3 Rating and Evaluation of Pollution Source Intensity …