Page 189 - Hydrogeology Principles and Practice

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172 Chapter Five

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δ O of pre-event streamwater =−7‰ For point piezometers that are open for a short
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δ O of event water =−9‰ interval at their base (Fig. 5.30a), the interpretation of
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δ O of total discharge =−8‰ the water level versus time data commonly employs
the Hvorslev (1951) method. Hvorslev (1951) found
Substituting in equation 5.24 gives: that the return of the water level to the original, static
level occurs at an exponential rate, with the time
8
Q −− (−9 ) 1 taken dependent on the hydraulic conductivity of the
P = =
eq. 5.25
Q −− (−9 ) 2 porous material. Also, the recovery rate depends on
7
T
the piezometer design; piezometers with a large area
and it becomes apparent that Q = Q /2 is equal available for water to enter the response zone recover
P T
3 −1
to 0.3 m s and Q = (Q − Q ) is also equal to more rapidly than wells with a small open area. Now,
E T P
3 −1
0.3 m s . In this example, the baseflow component if the height to which the water level rises above the
calculated by hydrochemical means was a higher per- static water level immediately at the start of a slug
centage of the total discharge than was interpreted by test is h and the height of the water level above the
o
the graphical hydrograph separation method. static water level is h after time, t, then a semilog-
The hydrochemical separation technique was read- arithmic plot of the ratio h/h versus time should
o
ily applied in the above example in that the pre-event yield a straight line (Fig. 5.30b). In effect, using the
and event stable isotope compositions were easily ratio h/h normalizes the recovery between zero and
o
distinguishable as a result of evaporative enrichment one. If the length of the piezometer, L, is more than
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of the heavier isotope ( O) in groundwater stored in eight times the radius of the well screen, R, then the
the peaty catchment soils. In general, the technique hydraulic conductivity, K, can be found from:
is best applied to small catchments of the order of
2
2
10 km . In larger catchments, variation in catchment r log ( L R)
/
=
e
K eq. 5.26
geology may obscure the chemical signatures of indi- 2 LT
o
vidual components of baseflow and storm runoff.
where r is the radius of the well casing and T is the
o
time lag or time taken for the water level to rise or fall
5.8 Field estimation of aquifer properties to 37% of the initial change (Fig. 5.30b).
The Hvorslev method as presented here assumes
5.8.1 Piezometer tests a homogeneous, isotropic and infinite material and
can be applied to unconfined conditions for most
Piezometer tests are small in scale and relatively piezometer designs where the length is typically greater
cheap and easy to execute and provide useful site than the radius of the well screen. Hvorslev (1951)
information, but are limited to providing values of also presented formulae for anisotropic material and
hydraulic conductivity representative of only a small for a wide variety of piezometer geometries and
volume of ground in the immediate vicinity of the aquifer conditions. For slug tests performed in fully
piezometer. or partially penetrating open boreholes or screened
It is possible to determine the hydraulic conductiv- wells, the reader is referred to the method of Bouwer
ity of an aquifer by tests carried out in a single and Rice (1976) for unconfined aquifers and Bouwer
piezometer. Tests are carried out by causing a sudden (1989) for confined aquifers. The approach is similar
change in the water level in a piezometer through the to the Hvorslev method but involves using a set of
rapid introduction (slug test) or removal (bail test) curves to determine the radius of influence of the test.
of a known volume of water or, to create the same
effect, by the sudden introduction or removal of a
solid cylinder of known volume. Either way, the 5.8.2 Pumping tests
recovery of the water level with time subsequent to
the sudden disturbance is monitored and the results Pumping tests are generally of larger scale and
interpreted. duration compared with piezometer tests and are