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Groundwater investigation techniques 143

Fig. 5.2 General designs of (a) well, (b) observation borehole and (c) piezometer for the measurement of groundwater level. After
Brassington (1998).

σ + P = σ + P eq. 5.1 P + dP + γψ′= P + dP eq. 5.4
T A e w A A w w
where P is atmospheric pressure, σ is the total stress which, on substitution of equation 5.2 in equation
A T
created by the weight of the overlying aquitard, σ is 5.4, gives:
e
the effective stress acting on the aquifer material and
P is the fluid pressure in the aquifer. The fluid pres- dP − dP = γ(ψ − ψ′) eq. 5.5
w A w
sure creates a pressure head, ψ, that is measured in
the well penetrating the aquifer. At position Y in the Since dP − dP is greater than zero, then ψ − ψ′ is
A w
well, the balance of pressures is: also greater than zero, proving that an increase in
atmospheric pressure leads to a decrease in water
P + γψ = P eq. 5.2 level (Fig. 5.5). In a horizontal, confined aquifer the
A w
change in pressure head, dψ = ψ − ψ′ in equation 5.5,
where γ is the specific weight of water. If, as shown in is equivalent to the change in hydraulic head, dh, and
Fig. 5.4b, the atmospheric pressure is increased by an so provides a definition of barometric efficiency, B,
amount dP , the change in the stress field at position expressed as:
A
X is given by:
=
B γd h eq. 5.6
dP = dσ + dP eq. 5.3
A e w P d
A
Now, it can be seen that the change in dP is greater The barometric efficiency of confined aquifers is usu-
A
than the change in dP such that at position Y in the ally in the range 0.20–0.75 (Todd 1980). Jacob (1940)
w
well, the new balance of pressures is: further developed expressions relating barometric