(d) OCS
• Pore water
Where the direction of groundwater flow is known, a site background monitoring
well is laid 30–50 m away in the upstream; and pollution diffusion monitoring well
(s) are installed 5–30 m away from the buried oil tank in the downstream;
Where the direction of groundwater flow is not unknown, a site background
monitoring wells is laid 30–50 m away in the upstream; at least two pollution
diffusion monitoring wells are installed 5–30 m away from the buried oil tank in the
downstream. The three monitoring wells present triangular distribution with the
largest possible spacing.
• Karst water
In principle, at least two monitoring wells are installed on the main channel in the
karst water investigation area. According to the distribution and flow direction of
the underground river, two monitoring wells are laid on the upper and lower
reaches, which are used for monitoring site background and pollution respectively.
Where there is well-developed karst and complex underground rivers, one or two
monitoring wells can be added, depending on the site circumstances. Two are added
if the primary tributary is more than 2 km long, and one added if the primary
tributary is less than 2 km long. The deployment method for fissure water investigation can be referred to.
• Fissure water
Two monitoring wells are placed in the fissure water background area, and three to
four monitoring wells in the pollution diffusion area.
(3) Quality control of monitoring well construction
(a) Depth of monitoring wells
Where municipal solid waste (MSW) and hazardous waste are landfilled above the
water level of the shallow aquifer, monitoring wells should be installed in the
shallow aquifer. Where municipal solid waste and hazardous waste are landfilled
below water level of the shallow aquifer, the bottom of monitoring wells should be
3–5 m lower than the waste.
As the characteristic pollutants in a gas station site are largely light non-aqueous
phase liquids (LNAPLs), groundwater samples should be taken as close as possible
to the upper aquifer, and preferably within 1 m from groundwater table. The depth
of monitoring wells should be determined according to monitoring purpose and
aquifer type, depth and thickness. Preferably, monitoring wells are located 2 m
below the known maximum groundwater depth.
(b) Waterstop selection
Waterstops are especially important to the construction of monitoring wells, so that
monitoring wells can take water from the target aquifer section while avoiding
2.2 Content of Investigation
25
• Pore water
Where the direction of groundwater flow is known, a site background monitoring
well is laid 30–50 m away in the upstream; and pollution diffusion monitoring well
(s) are installed 5–30 m away from the buried oil tank in the downstream;
Where the direction of groundwater flow is not unknown, a site background
monitoring wells is laid 30–50 m away in the upstream; at least two pollution
diffusion monitoring wells are installed 5–30 m away from the buried oil tank in the
downstream. The three monitoring wells present triangular distribution with the
largest possible spacing.
• Karst water
In principle, at least two monitoring wells are installed on the main channel in the
karst water investigation area. According to the distribution and flow direction of
the underground river, two monitoring wells are laid on the upper and lower
reaches, which are used for monitoring site background and pollution respectively.
Where there is well-developed karst and complex underground rivers, one or two
monitoring wells can be added, depending on the site circumstances. Two are added
if the primary tributary is more than 2 km long, and one added if the primary
tributary is less than 2 km long. The deployment method for fissure water investigation can be referred to.
• Fissure water
Two monitoring wells are placed in the fissure water background area, and three to
four monitoring wells in the pollution diffusion area.
(3) Quality control of monitoring well construction
(a) Depth of monitoring wells
Where municipal solid waste (MSW) and hazardous waste are landfilled above the
water level of the shallow aquifer, monitoring wells should be installed in the
shallow aquifer. Where municipal solid waste and hazardous waste are landfilled
below water level of the shallow aquifer, the bottom of monitoring wells should be
3–5 m lower than the waste.
As the characteristic pollutants in a gas station site are largely light non-aqueous
phase liquids (LNAPLs), groundwater samples should be taken as close as possible
to the upper aquifer, and preferably within 1 m from groundwater table. The depth
of monitoring wells should be determined according to monitoring purpose and
aquifer type, depth and thickness. Preferably, monitoring wells are located 2 m
below the known maximum groundwater depth.
(b) Waterstop selection
Waterstops are especially important to the construction of monitoring wells, so that
monitoring wells can take water from the target aquifer section while avoiding
2.2 Content of Investigation
25