Page 91 - Water Engineering Hydraulics, Distribution and Treatment

Drawdown at the well,
a = 304.8m
r = 610 mm∕2 = 305 mm = 0.305 m
w
s = 3.5 log(2 × 304.8∕0.305) = 11.55 m.
w
Drawdown at 152.4 m (or 500 ft) from the stream:
r = 152.4m
r = a + r = 304.8m + 152.4m = 457.2m
i
s = 3.5 log(457.2∕152.4) = 1.67 m.
Drawdown at 457.2 m (or 1,500 ft) from the stream: 3.13 Aquifer Boundaries 69
r = 457.2m
r = a + r = 304.8m + 457.2m = 762 m
i
s = 3.5 log(762∕457.2) = 0.78 m.
The results are shown in Fig. 3.10.
For a well located near a stream, the proportion of waste disposal (or artificial recharge) and for water supply in
the discharge of the well diverted directly from the source the same area. The recirculation can be minimized by locating
of recharge depends on the distance of the well from the the recharge well directly downstream from the discharge
recharge boundary, the aquifer characteristics, and the dura- well. The critical value of discharge and optimum spacing
tion of pumping. Figure 3.11 shows an inflatable rubber dam for no recirculation can be evaluated.
that is inflated seasonally (late spring to late autumn) to raise The permissible distance, r , between production and
c
the river stage by 3 m. The higher stage allows greater pump- disposal wells in an isotropic, extensive aquifer to prevent
ing rates to be maintained in the collector wells that supply recirculation is given by
water to Sonoma County residents in Maui, Hawaii. The
contribution from a line source of recharge and distribution r = 2Q∕( ∕TI) (3.38)
c
of drawdown in such a system can be evaluated and are
extremely useful in determining the optimal location of well
where r is in ft or m, Q is the equal pumping and disposal rate
c
fields. 3 3
in gpd or m /d, T is the transmissivity in gpd/ft (m /d/m), and
The problem of recirculation between a recharge well
I is the hydraulic gradient of the water table or piezometric
and a discharge well pair is of great practical importance
surface.
because of the use of wells (or other devices) for underground
3.13.2 Location of Aquifer Boundaries
In many instances, the location and nature of hydraulic
boundaries of an aquifer can be inferred from the analysis of
aquifer test data. The effect of a boundary when it reaches
an observation well causes the drawdowns to diverge from
the curve or the straight-line methods. The nature of the
boundary, recharge, or barrier is given by the direction of
departures. An observation well closer to the boundary shows
evidence of boundary effect earlier than does an observation
well at a greater distance. The theory of images can be used
to estimate the distance to the boundary. The analysis can be
extended to locate multiple boundaries.
For the estimation of the formation constants, only those
observations should be used that do not reflect boundary
effects, that is, the earlier part of the time–drawdown curve.
For the prediction of future drawdowns, the latter part of the
Figure 3.11 Inflatable rubber dam (Courtesy USGS). curve incorporating the boundary effects is pertinent.