GROUNDWATER - Water Quality Engineering in Natural Systems - page 145

leaves sufficient water for leaching and maintaining salt
and chemical balances in the root zone. Irrigation effi-
ciencies of 100% theoretically would only be sustain-
able without rainfall if distilled water were used for
irrigation.

and the leaching ratio is given by Equation (5.4) as

Q
Q

1 1

1 0 20

i

= = = 1 25 .

1 − LR − .

e

This result indicates that the minimum irrigation
plus effective rainfall required to keep the root zone
TDS concentration less than 300 mg/L is 1.25( Q e )
= 1.25(30 cm) = 37.5 cm. In this case, the irrigation
plus effective rainfall is 40 cm (≥37.5 mg/L) and
yields an adequate root-zone TDS concentration.

EXAMPLE 5.1

An avocado crop can tolerate water in the root zone
with a TDS concentration of up to 300 mg/L, and avo-
cados require 10 cm of water to support growth during
the spring planting season. Available irrigation water
and effective rainfall combined has a TDS content of
60 mg/L, soil evaporation during the spring planting
season is 30 cm, and the effective rainfall is 25 cm.
(a) Estimate the amount of irrigation water required
and the expected TDS concentration in the root zone.
(b) Determine the leaching ratio and the maximum
requirement for irrigation plus rainfall to avoid exces-
sive TDS in the root zone.

5.2.5 Solid Waste Disposal Sites

Solid waste disposal sites are commonly called landills .
modern landfills are constructed with leachate collec-
tion and treatment systems, but most older landfills
are simply large holes in the ground filled with waste
and covered with dirt. Leaking liquids and leachate
from older landfills can be a significant source of ground-
water contamination. Typical landfills are shown in
Figure 5.5. modern landfills are sophisticated engineer-
ing operations employing resource recovery (collection
of methane and subsequent conversion to energy),
leachate collection and subsequent treatment, and daily
covering of wastes with soil. After ceasing operation, a
landfill site can be reclaimed for other uses.
Although solid waste disposal sites are considered
point sources of pollution, leachate from unsanitary
landfills and dumps can pollute large portions of ground-
water and appear as contaminated baseflow in rivers
and streams. Dangerous toxic compounds are com-
monly part of the overall composition of landfill leach-
ate, especially when the landfill is used for the disposal
of toxic chemicals. Table 5.1 shows the ranges in concen-
tration for various chemical constituents of typical
leachate from municipal solid waste disposal sites.
There are several methods for managing leachate:
natural attenuation by soils, prevention of leachate
formation, collection and treatment, pretreatment to
reduce volume and solubility, and detoxification of
hazardous wastes prior to landfilling. Leachate under-
goes natural attenuation by various chemical, physical,
and biological processes as it migrates through soil.
Whether natural attenuation will be adequate to prevent
groundwater contamination should be evaluated for
each site.

Solution

(a) The irrigation requirement is determined on a volu-
metric basis according to the relation

irrigation requirement
crop requirement evaporation rainfa = + − ll

= 10 cm + 30 cm − 25 cm

= 15 cm

From the data given, Q i = rainfall + irrigation =
25 cm + 15 cm = 40 cm, c i = 60 mg/L, and Q e = 30 cm.
Equation (5.1) gives the resulting TDS in the root
zone, c aq , as

c Q
Q Q

( 60 40

40 30

)( )

i i

c = = = 240 mg/L

aq

− −

i e

Therefore, the TDS in the root zone is expected
to be 240 mg/L. This is less than the allowable
maximum for avocados of 300 mg/L. In the event
that the root-zone TDS concentration turned
out to be greater than 300 mg/L, the irrigation
requirement would need to be increased beyond
the volumetric requirement of 15 cm, such that
c aq ≤ 300 mg/L.
(b) The LR is given by Equation (5.3) as

5.2.6 Waste Disposal Injection Wells

Waste disposal injection wells are used to inject con-
taminated water, surface runoff, and hazardous wastes
deep into the ground and away from drinking-water
sources, but poor well design, faulty construction,

c
c

60
300

i

LR = = = 0 20 .

aq