Environmental Engineering Reference
In-Depth Information
• Bulk density
• Cation exchange capacity
• Organic carbon content
• pH
• Specific gravity
• Mineral content
4.3.2 Groundwater Sampling Methods
If groundwater is encountered during the drilling of soil borings or excavating test pits,
then in most cases, the environmental investigation must also evaluate the potential for
groundwater contamination. Sampling groundwater usually requires the installation of a
monitoring well, but this is not always the case. Occasionally, a water sample is collected
directly from the borehole or excavation pit, or a temporary monitoring well is installed
and removed after a groundwater sample is collected.
A groundwater sample collected directly from an open borehole is called a grab sample.
Groundwater samples collected from a temporary monitoring well may also be called grab
samples, depending on the construction of the temporary well, how long the well had
been in place before sampling, and how the well was developed. In general, groundwater
samples collected from open boreholes or temporary monitoring wells installed without
great care are used as a rough screening of groundwater to evaluate the need for a more
permanent monitoring well. Collecting and analyzing groundwater samples from an open
borehole or temporary monitoring well is not always recommended because the analytical
results may not be representative of the actual groundwater in the aquifer.
Figure 4.21a through e show the process of collecting a groundwater grab sample
from a borehole advanced into the saturated zone using a Geoprobe. Figure 4.21a shows
a stainless steel well screen which is then placed inside the lead drill rod. Once the
desired depth is reached, the outer rod is pulled up to expose the screen. A polyvinyl
chloride (PVC) flexible hose is inserted into the inside of the drill rods as shown in
Figure 4.21b, and groundwater is pumped from the subsurface using a pump (Figure
4.21c). Groundwater is then pumped into a drum as depicted in Figure 4.21d. After the
pumping continues for several minutes, a groundwater sample is collected in an appro-
priate container (Figure 4.21e).
4.3.2.1 Installing a Monitoring Well
Collecting a representative groundwater sample requires the installation of a monitoring
well. The objectives of installing a monitoring well include
• Collecting a representative sample of groundwater for analytical testing to evalu-
ate groundwater quality
• Evaluating the direction of groundwater flow
• Estimating the hydraulic conductivity
• Evaluating specific yield
• Monitoring seasonal fluctuations in flow direction
• Future monitoring of groundwater quality
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