Environmental Sampling Techniques - Fundamentals of Environmental Sampling and Analysis - page 87

Step 3 (B E) E)

Groundwater
flow direction
(E

East well

Step 1 (A Step 1 (A B) B)

West well

(E B) B)

20.4 m 20.4 m 20.0 m

A A D D

300 m 300 m

B B

E E

Step 2 (C Step 2 (C D) D) 90 o 90 o

200 m

α α

Equipotential Equipotential line

South well

C C

20.2 m 20.2 m

DB = AB/2 = 150 m
Tan α = DB/BC = 150/200 = 0.75, α = 37 o
EB = BC

DB = AB/2 = 150 m
Tan α = DB/BC = 150/200 = 0.75, α = 37 o
EB = BC × × sin sin α α = 200 = 200 × × sin 37 o = 120 m sin 37 o = 120 m

Figure 4.10 Determine the groundwater flow direction and hydraulic gradient

from the higher to the lower hydraulic head along line EB, meaning that the
water flows from E to B rather than from B to E.
4. Determine the hydraulic gradient:

Head of well C --Head of well B
Distance from B to the equipotential line (CD)

Hydraulic gradient ¼

20 : 220 0

120 ¼ 0:00167 ðDimensionlessÞ

:

¼

Well Purging

Purging is used to remove stagnant water in the well borehole and adjacent sandpack
so that the water to be sampled will be stabilized prior to sampling and representative
groundwater can be obtained. Various methods for determining the necessary extent
of well purging have been recommended.
The USGS recommends pumping the well until water quality parameters are
stabilized. A ''stabilized'' condition can be determined by in situ measurement of
the following parameters: DO: 0.3 mg/L; turbidity: 10% (for samples greater
than 10 NTUs); specific conductivity: 3%; oxidation-reduction potential (ORP):
10 mV; pH: 0.1 unit; temperature: 0.1 C. This approach is also specified in the
U.S. EPA's Groundwater Guidelines for Superfund and RCRA Project Managers
(EPA, 2002). The most commonly used parameters are pH, temperature and
conductivity (Popek, 2003). The U.S. EPA also recommends the removal of three
well-casing volumes prior to sampling. A handy formula in English measurement
unit system is as follows (Bodger, 2003).

48 pr 2 h V ¼ 7 : ð4 : 2Þ