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Groundwater investigation techniques 163

BO X
Continued
5.1

porewater or shallow groundwater and so measures the effect of where i is the recharge or infiltration rate and q is the moisture con-
evapotranspirative loss of precipitation. For example, using repres- tent. For the recharge rate calculated above (eq. 2), and for a mois-
entative data for Senegal, where P = 443.2 mm, C = 1.1 mg L −1 ture content of 0.02 measured in 1988, a calculated downward
p
−1
−1
and C = 11.2 mg L , then: water movement is 0.045/0.02 = 2.25 m a . Thus, for this year,
s
the soil water is estimated to infiltrate a distance of 2.25 m. By
. 114 inspecting the chloride value in each sampling interval, it is then
R = 443 .2 = mm a −1 eq. 2
45
d
11 .23 possible to calculate, from dividing the sampling interval by the rate
of downward water movement, the residence time of the water in
2
Seven unsaturated zone profiles from a small, 1 km area to the the interval. Hence, it is possible to build up a recharge chronology
west of Louga in Senegal gave a mean C value of 82 mg L −1 cor- for the unsaturated zone profile.
s
−1
responding to a long-term recharge rate of 13 mm a . When The oscillations in chloride concentration shown in Fig. 1 indicate
considered on a regional scale, this long-term recharge represents that the recharge rate has not been constant during the con-
a sufficient volume to supply the present domestic water needs of structed, 60-year recharge chronology for Senegal. The period of
the traditional villages, even during a drought when the recharge Sahel Drought (1968–1986) is clearly visible as a zone of higher
rate may be halved (Edmunds 1991). chloride concentrations. The drought is also emphasized by the
The recharge history can also be calculated if the moisture con- deuterium stable isotope data (d H in ‰) which show that water
2
tent profile of the unsaturated zone is known. The rate of downward from this period is enriched in the heavier isotope as a result of
water movement, 2, can be calculated from: greater evaporation. The water quality data for nitrate show values
often in excess of 10 mg L −1 as N, although these high values are
=
2 i eq. 3 unrelated to surface pollution and instead arise from natural
q fixation of nitrogen by plants and micro-organisms (see Fig. 6.25)
with subsequent concentration by evaporation.
enough upstream to attain ambient velocity before Table 5.6 Coefficients by which the maximum surface velocity
entering the reach. By measuring the distance of the of a river should be multiplied to give the mean velocity in the
reach and the time taken for the float to travel the measuring reach.
length of the reach, the water velocity can be calcu-
Average depth in reach (m) Coefficient
lated by dividing the length by the time. The proce-
dure is repeated a number of times to obtain the 0.3 0.66
average maximum surface velocity, converted to mean 0.6 0.68
velocity using coefficients (Table 5.6). By measuring 0.9 0.70
1.2 0.72
the flow area upstream and downstream of the reach
1.5 0.74
and taking the average value, the mean flow area for 1.8 0.76
the reach is obtained. The river discharge is then found 2.7 0.77
by multiplying the mean velocity by the mean flow area. 3.7 0.78
4.6 0.79
≥6.1 0.80
Current metering
Current metering of stream discharge is another calculated given a calibration equation linking velo-
velocity–area method and commonly employs one city and count rate. Another type of measuring device
of two types of current meter (cup type or propeller is the electromagnetic current meter which measures
type) to obtain point measurements of velocity the voltage resulting from the motion of a conductor
(Fig. 5.19). At each measurement point the meter is (water flow velocity) through a magnetic field. The
allowed to run for about 60 seconds and the number magnetic field is produced by a coil in the sensor and
of revolutions made by the cup or propeller is obtained the voltage is detected by electrodes on the surface of
from a counter. The velocities can subsequently be the sensor. The sensor has no moving parts and the