905R80128
tor Identifica tion
aluation of thf
Extent oj- PoJLlutjLon from Sal.t Water Intrusion
CONTENTS
Introduction. , 1
Causes of salt water Intrusion ,. 2
Sea Water Intrusion in Coastal Aquifers 2
Upstream Encroachment of Sea Water. 3
Intrusion in Inland Aquifers , U
Extent of Pollution from Salt Water Intrusion S
Identification of Pollution
from salt Water Intrusion 6
Evaluation of the Effects of Pollution
from salt Water Intrusion. lu
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flu;*, da no P. rcr Tdont j *"< cation and F valuation
and Fxtpnt c Po-11»t^on fro re Salt Vat or Intrusion
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accordance with the previsions of Section 208 of the Federal
Water Pollution Control Act as Amended.
of Salt Water Intruson
Salt water intrusion whether into surface cr ground
water is a complex situation controlled by the geoloqic and
hydroloqic characteristics of the area. Natural water
systems are dynamic. They respond in quality and quantity
to natural phenomena and to man's activities such as chanqes
in land use, stream channel linings, and consumptive
withdrawal. Identification and evaluation of the nature and
extent of salt water intrusion heqins with an understanding
of the qeneral mechanisms by which intrusion occurs.
Sea_Water_Intrusign_in_Coastal__Aauif ers
Under natural conditions fresh ground water in coastal
aquifers is discharqed into the ocean at or seaward of the
coastline. Where coastal aquifers are overpuinped, lowered
by natural drainage, or natural recharge is impeded by
construction or other activities, the ground water 1^-vel,
whether water table in unconfined aquifers or niezometric
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surface in confined aquifers, is lowered thereby reducing
the fresh water flow to the ocfan. The interface between
the fresh and saline water has a parabolic form with the
saline water tending to underride the less dense fresh
litter. The reversal or reduction of fresh water flow allows
' ^e heavier saline water to move into areas where only fresh
water previously existed. Thus, even with a seaward
pressure aradient, sea water can advance inland. Because of
the hiqh salt content of sea water, as little as two n^rcent
of it in fresh ground wat<=r will make the water unusable in
relation to U.S. Public Health Service drinking water
standard for total dissolved solids. Only a small amount of
intrusion can have serious implications regarding the future
use of an aquifer as a water sunply source.
The interaction of river flow and tidal currents
results in a net upstream movement of sea water along the
bottom with fresh water overriding this wedge in a seaward
direction. The position cf the interface between the fresh
water and the sea water is dependent on channel geometry,
riv^r discharge, and high tide height. A change in any of
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these oarameters will cause the salt water/fresh water
interface to migrate. The most common causes o* upstream
encroachment of sea water are deepening of navigation
channels, construction of sea level canals, and reduction of
stream flow. Reduction of stream flow or deepening of
channels results in landward migration of the sea water
wedge while increased stream flow results in a seaward
migration. Sea water encroachment can contaminate both
surface and subsurface water supplies, render fish and
wildlife habitats unsuitable for native populations, and
through increased corrosion shorten the life expectancy of
engineering structures.
Intrusign in Inland Aguifers
Large quantities of saline water exist under diverse
geoloaic and hydrologic environments in the United States.
Most of the Nation's largest sources of fresh ground water
are in close proximity to natural bodies of saline around
water. Interaquifer transfer of saline waters results from
two basic mechanisms. One involves the upward migration of
saline waters into fresh water aquifers as a result of man-
induced changes in the hydroloaic pressure regime. The
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other involves the direct transfer of saline waters
vertically through wells or other penetrations. Because of
the relatively slow movement of ground water, any saline
water intrusion may produce detrimental effects on ground
>ater quality that could persist for months or years after
: be intrusion has ceased.
of Pol lution from Salt Water Intrusion
Salt water intrusion problems are ubiquitous in coastal
areas and surprisingly widespread in inland areas. On the
highly peculated Atlantic Coast, between Massachusetts and
Florida, each of the States has reported problems with sea
water intrusion. The seriousness of the problem is usually
dependent on th
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solids, and the problem of salt water intrusion in inland
aquifers can be the samp as in coastal areas. Only eiqht of
the fifty states do not report significant salt water
intrusion problems.
ion 2l H2iilition £-°.!B Salt Water Intrusion
Most intrusion of salt water into fresh wa^er can be
ascribed to one of three primary mechanisms: the reversal or
reduction of fresh water discharge which allows the heavier
saline water to move into an area where only fresh water
previously existed; the accidental or inadvertant
destruction of natural barriers that formerly separated
bodies of fresh and saline waters; or the accidental or
inadvertant results of the disposal of waste saline water.
Maior elements in an assessment of the occurrence and
extent of salt water intrusion should include:
1. spatial delineation of primary aquifers and
streams,
2. analysis of historical water quality (salinity)
data for suspect areas,
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3. establishment of a salinity monitoring network for
surface and around water,
H. monitoring of the fresh water/salt water
interface,
5. basin wide hydrogeologic investigations where
saline intrusion occurs,
6, identification of causal factors.
Prime areas for consideration should include rapidly
developing coastal areas where demands for fresh water
result in a reduction or reversal of flow gradient; and
areas of coastal waterway or embayment construction, or
deepening of navigation channels where natural barriers to
salt water flow may be breached. Another prime example of
breachinq of confining strata is encountered in drilling
operations, especially in oil producing areas where salt
water may move great distances along broken or corroded well
casings or improperly abandonded wells. Not to be
overlooked as a source of pollution is any operation that
disposes of waste saline waters, whether disposal is
directly to surface streams or to the ground water through
evaporation pits or other methods.
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In other than oil prcducinq areas salt water intrusion
is seldom the direct result of waste disposal. More often
it is the natural adjustment of the hydroloaic system to the
many stresses placed upon it. Fundamental to an evaluation
of the extent of salt water intrusion is the need for
comprehensive hydroqeoloqica1 investigations of the surface
and subsurface water systems. Identification and evaluation
of the extent of salt water intrusion should b^ an inteqral
part of each State's water quality monitorinq proaram
required under section 106 (e) (1) of the Act, with salinity
one of the parameters routinely monitored throuqhout the
water quality network.
As an initial step in the evaluation of the nature and
extent of salt water intrusion principal aquifers must be
spatially defined, and historical water quality records for
both surface and qround waters should be collected and
contour maps of salt concentration compiled. In this way,
natural or base line conditions can be established and the
location of the salt water/fresh water interface can be
displayed in relation to the water requirements of the
hydroloqic basin. Updatinq of such maps from current
monitorinq data provides a rapid indication of the advance
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or retreat of the salt water wedge. Under normal conditions
monitoring points should te measured for salinity (or total
dissolved solids) or checked for electrical conductivity at
one to two month intervals. More frequent measurements may
Hf» warranted if °ncroachment is in the proximity of major
water supply sources.
Most salt water intrusion problems will he encountered
in heavily populated coastal areas. In many cases extensive
water quality monitoring programs will have been in effect
and will provide most or all of the water quality data
required for determining the present extent of salt water
intrusion in that area. Salinity measurements cf both
surface and ground waters should be an integral part of the
State's wa-*-er quality monitoring program and forms the basic
data input for continuous evaluation of the extent of salt
water intrusion.
An inventory of existing monitoring points for both
surface and ground waters which may be used in determining
the salinity of streams and principal aquifers should be
undertaken by each State, and alditional monitoring stations
installed as part of the State's water quality monitoring
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10
net.work where necessary for adequate spatial coverage. In
situ measurement of electrical conductivity can provide an
indication of salt content in surface and qround waters
without collecting water samples for laboratory analysis.
Sampling information for each surface or subsurface
monitoring station should include:
1. location by latitude, longitude and elevation,
2. stream or aguifer identification and date,
3. depth or depths of samples,
4. stream velocity,
5, temperature,
6. electrical conductivity, TDS, or chloride concentration.
Where a rise in electrical conductivity is noted,
samples should be analyzed for increased salinity.
Automatic recordina devices can be installed for continuous
electrical conductivity mcnitoring, and should be
incorporated in the State's water quality monitoring
network. Any water samples that are taken for laboratory
analysis should be secured and preserved according to
standard methods as described in Methods for Examination of
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11
er and Hastes, (U.S. Environmental Protection Agency,
1971).
Where salt water intrusion in either surface or around
water is suspected or know to exist, a comprehensive
hydrogeological investigation should be designed to provide
requisite information for planning and control programs.
The type of information that may be required could include:
1. the geoloqic structure of the surface and ground
water basins and their boundaries;
2. the nature and hydraulic characteristics of the
subsurface formations including:
a. rock type
* b. degree and type of porosity
c. permeability
d. reservoir pressure
e. deqree of hydraulic continuity with surface
waters.
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12
3. surface water and ground water levels, and
directions and rates of movement and seasonal
fluctuations;
1. surface water and qround water quality,
particularly natural chlorides content;
5. sources, locations, amounts, and quality of
natural recharge;
6. locations, amounts, and quality of artificial
recharge;
i
7, locations and amounts of extractions.
Historical information of this type is generally
available, to some degree, in published form from Federal,
State, and local agencies that are concerned with water
resources. Additional information of this type can be
derived from a variety of investigative techniques including
but not limited to:
1. . geoloqic reconnaissance,
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13
2. geophysical surveys,
3. examination of well logs,
U. test holes,
5. well pumping tests,
6. measurement of surface and ground
water levels,
7. chemical analysis of samples of surface
and ground waters,
8. analysis of precipitation and runoff records.
Techniques for predicting the location and extent of
salt water intrusion mainly rely on mathematical analysis of
aquifer and stream parameters, and tidal characteristics,
The level of sophistication and predictive ability of
analytical techniques varies from simple extrapolation of
the time of arrival of the salt water/fresh water interface
at successive observation wells to highly complex numerical
models of the entire hydrologic system. Discussion of the
application of these techniques is beyond the scope of this
report but selected references to detailed explanations are
included at the end of the document.
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The areal extent and depth of detail of the
investigations will vary v,ith the extent of the water basin
or aquifer that has betn or may be affected, and the present
and prospective uses of the water resources. The
investigations should be designed to define the water budget
of the basin or aquifer in sufficient detail to allow
prediction of the volumes and rates of surface and ground
water flow necessary to arrest and reverse the salt water
advance. Such information will be an integral part of the
data base used in basin wide water use planning, management,
and pollution control programs.
Evaluation 2f the Mf§2£§ 2f Salt Water Intrusion
Surface and ground waters are integral parts of the
same hydrologic whole, changes in the salinity concentration
of one will most likely affect the salinity concentration of
the other. Ideally the objective of any salt water
intrusion control program should be to maintain zero
increase in the salinity cf fresh water resources. This
objective is seldom attainable, however, especially in areas
of high water use. Nor is it possible to define a single
optimal or tolerable salinity concentration for "fresh
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waters". These concentrations are dependent on the use that
is to be made of the water. Water devoid of dissolved
materials is intolerable in nature because pure water will
not support lif°. Natural waters contain endless varieties
of dissolved materials in concentrations that differ widely
from one locality to another as well as from time to tir.ie.
The chlorides, carbonates, and silicates of sodium,
potassium, calcium, and magnesium are generally the most
common salts present. Different organisms vary in their
optimum salinity requirements as well as in their ability to
live and thrive under variations from the optimum.
Any evaluation of the potential effects of salt water-
intrusion must be performed in the context of its effect on
the total dissolved solids of the receiving water and the
water use requirements.
Optimal and tolerable salinity concentrations will be
different for such uses as: public water supplies, fish and
wildlife production, and agricultural uses. Waters with
less than about 500 ma/1 total dissolved solids are
aenerally considered suitable for domestic purposes, while
waters with greater than about 5,000 mg/1 TDS generally are
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unsuitable for irrigation purposes. Maximum salinity
concentrations for livestock consumption vary from less than
3,000 mq/1 TDS for poultry to as much as 12,000 mg/1 TDS for
sheep. A more detailed analysis of salinity requirements
for various water uses is contained in Water Quality
ia, (U.S. Environmental Protection Agency, 1972) .
Evaluation of the nature, extent and effects of salt
water intrusion may vary from simple plots of water quality
that indicate the position of the salt water/fresh water
interface to sophisticated mathematical models of the entire
surface and ground water basin. Such models can be used to
predict the response of the salinity concentration to
various types of stresses at any point in the system and
allow for long-range basin planning and comprehensive
intrusion control programs. The degree of sophistication of
analysis required will vary in proportion to the complexity
of the hydrologic system and the water demands for the area.
Regardless of the level of analysis involved the objective
of the water quality monitoring and hydrogeologic
investigations should always be to relate salt water
intrusion to its causal factors. Only in this way can water
use planning be accomplished in a manner that will maintain
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Chicago, Illinois 60606
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the hydrologic balances necessary to control salt water
intrusion.
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References
1. Fayers, F.J., and Sheldon, J.W., 1962, The Use of a
High Speed Digital Computer in the Study of the
Hydrodynamics of Geologic Basins: Jour. Geophys, Res., *
V. 67, no. 6, p. 2421-2431. J
2. Freeze, R.A., 1971, Three-dimensional, Transent, i
Saturated-Unsaturated Flow in a Ground Water Basin:
Water Resour. Res., v. 7, no. 2, p 347-366.
3. Freeze, R.A., and Witherspoon, P.A., 1966-fl,
Theoretical Analysis of Regional Ground Water Flow:
Part 1, Water Resour. Res. v. 2, no. 4, p. 641-656,
1966 Part 2, Water Resour. Res. v. 3, no. 2, p. 623-
634, 1967 Part 3, Water Resour. Res. v. 4, no. 3, p.
581-590, 1968
H. Ippen, Arthur T., "Salt-Water Fresh-Water Relationships
in Tidal Channels", Proceedings of the Second Annual
American Water Resources Conference, 1966,
5. Kashef, Abdel-Azis, F., "Model Studies of Salt Water
Intrusion", Water Resources Bulletin, Vol. 6, No. 6,
P944-967, 1970.
6. Pinder, George F., "A Numerical Technique for
Calculating the Transient Position of the Salt Water
Front", water Resources Research, Vol. 6, No. 3, P 875-
882, 1970.
7. Pinder, G.F., and Frind, E.O., Application of
Galerkin's Procedure to Aquifer Analysis: Water Resour.
Res., V.8, no. 1, p. 108-120. 1972.
8. Witherspoon, P.A., Javandel, I., and Neuman, S.P.,
1968, Use of the Finite Element Method in solving
Transient Flow Problems in Aquifer Systems: p. 687-698 .
in The use of Analog and Digital computers in Hydrology •
(vol. 2): Internat. Assoc. Sci. Hydrol. Publ. No. 81.
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