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Chapter XV. Groundwater and Its Outflows into Local Ecosystems
T H E E N V I R O N M E N T S OF G R O U N D W A T E R
ground structure so isolates them that separate water budgets must be
cast for each one. In California alone, 500 groundwater basins are
objects of investigation and usually management (Peters, 1972).
Stallman (1964) points out that ”the distribution of water might be
viewed as dependent on either supply and/or environmental factors.”
For example, surface runoff is a direct function of supply, that is, the
input of rainfall, mediated through relatively small storages in the
interception zone and the surface-detention zone of an ecosystem.
Groundwater, on the other hand, ”is so greatly dependent on envircnment that groundwater investigations are almost completely devoted
to an interpretation and description of environmental factors alone,”
i.e., the geologic structure of the rock below the ecosystems at the
earth’s surface. The spatial differentiation in groundwater environments does not necessarily display the same pattern as do the
ecosystems at the surface, except where seepage and recharge areas in
shallow groundwater are prominent or where such intrusive systems
as landfills have been established. With increasing depth these spatial
differences tend to diminish and more uniform aquifer conditions
underlie the mosaic of surface ecosystems.
Two writers on groundwater outline the opportunities-indeed,
urgencies-for research on the ”kaleidoscopic variety of environments
in which ground water occurs, and of modifications made by man”
(Thomas and Leopold, 1964). Emphasis on the eiivironment indicates
that often it is the capacity, not solely the water as such, that may be
most important. In this respect the reservoir serves much as do the
underground spaces in the Middle West in which natural gas is stored
during the summer for use the following winter. Essential qualities are
porosity and means of rapid input and output. ”Underground space”
is now recognized as a resource having value in itself, which is
enhanced where it provides a secure, clean environment.
Some underground reservoirs are managed like surface reservoirs,
water being withdrawn in dry seasons or dry years and replaced in
seasons of abundance. Both input and output would be artificially
accelerated under integrated administrative control. often by the
agency that also manages the surface reservoirs. For example, an
irrigation district that coordinates well pumping also provides for
artificial recharge, as described earlier. In such joint use the underground reservoir is likely to be utilized for long-term storage, i.e.,
from year to year, while the surface reservoir, more rapidly filled and
emptied, serves shorter-term balancing purposes.
This management technique, however, becomes unworkable if
drawdown of fresh water allows the inflow of contaminants from
G R O U N D W A T E R A N D ITS OUTFLOWS
adjacent rock layers. Saline water in deeper layers or from the ocean,
or contaminants from industry or cities, can render the space useless
for further freshwater storage. Dozens of coastal aquifers exist in
environments that are threatened by incursion of salt water if they
should be drawn down too far (American Society of Civil Engineers,
Percolation downward from the soil almost always can be looked at
from the converse point of view, by which it becomes a replenishment
to the groundwater body, or recharge. The exception occurs when the
ground water is being destroyed and its water table is receding into
the depths faster than the percolation stream can follow it, as is the
case near Phoenix.
Groundwater storage is the balance between incoming water, which
results from percolation that is either widespread or localized, and
various kinds of outflow. We have discussed the intermittent character
of percolation, and now should note how the size of storage affects the
response of the groundwater body to this episodic inflow. This
relation can be expressed as the ratio of stored water to the amount of
water annually flowing into storage. Such a ratio, with the dimension
of time (mm divided by mm yip'), is small where inflow is large
relative to the size of the body.
In small, rather tight basins it may be of the order of 0.01-0.05 yr. In
the drainage basin of the Goulburn River in New South Wales, the
storage-to-flow ratio, 1.05, represents a combination of one year's
equivalent of flow stored in the consolidated rocks of the whole basin,
plus 0.05 year's equivalent flow stored in river alluvium, which
probably is the principal area of recharge (Chapman, 1964). In the
adjacent Hunter River basin, an area with extensive areas of alluvium
and pervious upland rock formations, the ratio is about 5 yr (McMahon, 1964).
Ratios smaller than about 50 yr indicate that groundwater storage
is not great enough to absorb large fluctuations in the input when
rainfall and percolation change, as a group of wet years gives way to a
group of drier ones. Such bodies are vulnerable to fluctuations in
percolation. Larger ratios suggest "inherited" groundwater rather than
"cyclic" (Nace, 1969), an example being the water in the Ogalalla
formation in west Texas, which has been 13,000 yr in transit. Management of inherited water is difficult because demands can easily come
THE V O L U M E OF S T O R E D U N D E R G R O U N D WATER
to exceed inputs. When these deposits have mined out the overlying
ecosystems (largely cotton fields in this case) that have been irrigated
from them will be confronted with change.
A problem with large amounts of water in storage is that changes
are difficult to detect and calculation of inputs and yield are subject to
large error. Even in the small basin of Aquitaine, in southern France,
2.530 millennia are required for water to pass through, and Schoeller
(1959) points out that during these millennia great changes might have
taken place in the pattern of recharge to the basin by percolation.
THE VOLUME OF STORED UNDERGROUND WATER
The volume of water in groundwater storage is determined by the
specific yield fraction and the depth of the saturated layer. The lower
edge of this layer usually is indefinite, as the small fissures or fractures
pinch out with depth.
Another lower ievel is analogous to the economic depth of mining
used in estimating coal reserves, and is set by the energy and dollar
costs of raising water to the ground surface. Water at greater depths
may be regarded under present conditions as unrecoverable. This
depth often 1s assumed as 60 m, but obviously varies with the local
energy costs of pumping (e.g., extension of natural gas pipelines into
eastern Colorado has led to a large increase in pumping for irrigation),
as well as the local value of pumped water. In many parts of the
southwestern United States, pumping lifts are as great a s 200 m. One
wonders how much this definition of lower depth will change as
energy becomes more costly in the United States.
Volume of groundwater can be compared with the vapor mass in the
atmosphere, or with the liquid water in lakes and streams at the
earth’s surface. In the United States about 100 times a s much water
(8000 kg m P ) is estimated to exist below the ground as in rivers and
lakes on its surface (75 kg mp2),although much of the groundwater is
below the economically accessible limit mentioned above. *
Vapor in the atmosphere averages about 20 kg m-2 over the United
States, 11400 of the volume that exists, nearly immobile, beneath the
ground. Vapor in the local air, the zone overlying surface ecosystems,
is 5-5-10 kg mp2. While this mass is important in its mobility and its
exchanges with ecosystem water (Chapter XIII) it is small in comparison with the tons per square meter below surface ecosystems.
* Volume of groundwater in Australia is estimated a t 1500 kg m - 2 , river and lake
water at 5 kg rn? (Chapman, 1964).
GROUNDWATER A N D ITS OUTFLOWS
The Water Table
The upper boundary of the groundwater body is more definite than
the lower. It is the familiar water table. Except in geologic environments of very high permeability this surface is not leveI but follows
the ups and downs of the land surface above it, from which it receives
percolation. In one small area of Sweden, for example, the level of the
water table was 3 4 m below the ground surface ”irrespective of the
height of the terrain” (Gustafsson, 1968), which varied from an
elevation of 4 m above sea level to 38 m. Contour maps of the water
table are a recommended means of inventorying aquifers and are
useful for validating models of groundwater budgets (Peters, 1972).
The water table can be seen at the bottom of wells. Large galleries
just above it, excavated for skimming wells taking water from the
basal aquifer of Oahu, present a rather impressive sight to tourists. It
also is visible in landscapes pocked with lake basins, as it forms the
actual surface of some of the lakes. Such ”spring ponds” have
particular value as fish habitats because of their coldness.
Annual variation in the water table, i.e., in groundwater storage
volume, indicates the season when most percolation occurs. The
specific yield fraction determines the change in elevation. For instance, if the input from percolation during the winter of 1939-1940
from the chaparral site at North Fork, California, was 590 mm,
movement into a groundwater environment with 0.07 specific yield
would raise the water table by 59010.07 = 8430 mm = 8.4 m.
In some terrains the highest level of the water table comes above
parts of the land surface, forming temporary ponds or marshes.
Recession of the water table subsequently allows these water bodies to
If groundwater is confined by a n upper impermeable layer, there is
no free water table, and for purposes of observation its place is taken
by the equivalent pressure (piezometric) surface. Short-term and
seasonal changes in pressure are not usually as marked as in a free
water surface. This is. partly because a confined water body does not
receive brief pulses of percolating water, but rather a slow steady
inflow, perhaps by leakage from other aquifers.
Changes in the volume of water may be buffered by changes in the
aquifer volume itself. It will recover spatiai volume when the water
volume increases if its behavior under heavy loading from above is
elastic. If, on the other hand, its behavior is plastic, the structure
collapses when dewatered and permanent loss of storage capacity
THE V O L U M E O F S T O R E D U N D E R G R O U N D WATER
Long-Term Changes in Volume
Slower changes also take place in the volume of groundwater
bodies. For example, the amount of percolation from the overlying
layers may decrease with the paving of the surface or the replacement
of cesspools or septic tanks by sewers. Mining of groundwater
deposits inherited from the past inevitably reduces their volume.
These slow changes are evident in several ways, depending on the
environment of the groundwater body.
(1) If it is confined under pressure, there is a loss in head; if early
wells were artesian, they cease to flow.*
(2) If the groundwater body is confined laterally in its own basin,
any decrease in its volume is evident as a drop in the water table. In a
basin near Phoenix, Arizona, for instance, the water table has been
receding for several decades at a rate of one to several meters per year.
(3) If the groundwater body is in a basin that it shares with ocean
or other mineralized water, the decrease in volume of fresh water is
evident as a shift in the position of the interface between the two
waters of differing density. Sea water encroaches into the aquifer
space vacated by the shrinking freshwater volume.
Sfability in Oahu Long-term balance in groundwater volume is
exemplified in an aquifer in the island of Oahu. Long ago the nature
of the aquifers that exist on this volcanic island was established,
although neither the high-level bodies of water held by vertical dikes
nor the basal lens of fresh water floating on salt water are common
elsewhere in the world (Fig. XV-1) (U.S. Water Resources Council,
Concern has long been felt for the state of health of the basal lens in
particular, which is under the ever-present threat of salt-water encroachment at its lower interface. Much attention has been given to
how fast it responds to changes in inflow or outflow (Wentworth,
Invisible though it is, it is generally recognized as the principal
resource in the water economy of the island (U.S. Water Resources
Council, 1968, pp. 1-28), and is treated with the consideration this
warrants. "'The probability of overdraft and resultant salt water intrusion is directly related to increased land development for residential,
industrial, and resort use" (U.S. Water Resources Council, 1968, p. 6* Though they frequently appear in textbook illustrations, flowing artesian wells are
not common any more, the exploitation of groundwater being so widespread.
G R O U N D W A T E R A N D ITS OUTFLOWS
Fig. XV-1. Cross section of the geology and groundwater of Oahu (U.S. Water
Resources Council, 1968).
19-5). As a result, the outlook is for “permanent operation of monitoring wells; continued studies to maximize safe production” (American
Society of Civil Engineers, 1969).
In addition to the threat of salt-water encroachment, the basal
aquifer has acquired other mass budgets. Nitrates in the percolate
from fertilized cane fields increased nitrate concentration in the upper
laver of the lens from 1 to 8 ppm- The concentration of silica has
doubled, due to heavy percolation of irrigation water from the cane
fields because the percolate speeds up the weathering of the rock
mantle (Visher and Mink, 1964).
W a s t e in the Salt River Valley A long-term out-of-balance situatiop.
that contrasts with the well-managed basal freshwater lens under
Oahu is illustrated by the groundwater body enciosed in the alluvia2
basin of the Salt River Valley in Arizona.
High groundwater was a problem that early irrigators in this valley,
the Hohokam, had to contend with to about tne fourteenth century. It
was probably the reason their occupation of the region ended. When
irrigation from the river resumed in the late nineteenth century, and
was accelerated in the first decade of the twentieth century by the
federally built Roosevelt Dam (Skibitzke ef al., 19611, swamping
promptly showed up again. This led to local pumping to lower the
water table, a practice that rapidly spread.
This body of water, like others in the Southwest, is now in decline
and has been since at least 1923. During the forty years from 1923 to
1964, its shrinkage in volume is indicated by a drop in the water table
that totals, on the average over the whole alluvial plain, 45 m
The situation is well known to both the irrigators and other groups
in the state; the management of water is ”a matter of primary interest
to the general public and the various interest groups” (Mann, 1963, p.
THE VOLUME O F S T O R E D U N D E R G R O U N D WATER
67). This knowledge, however, has had little apparent effect, for the
shrinkage continues unabated. Recent values of the average drop in
the water table are 3 m yr-’ (U.S. Water Resources Council, 1968, p. 614-4).
It is interesting to look for possible reasons for the way people
perceive this situation. One reason is its remoteness: far out of sight.
Moreover, “recent economic growth, in Arizona at least, has not been
stimulated by irrigation and has not been impeded by water scarcity”
(National Research Council, Committee on Water, 1968, p. 15), so the
problem is also remote economically. A more detailed economic
analysis found that much of the pumped water makes little contribution to either the agricultural or the overall economy of the region
(Kelso ef al., 1973, p. 27). ”The problem is a ’man-problem’ rather than
The legacy that hampers clear sight of the problem is the view of
groundwater in past times (although Arizona is one of our newest
states), when court cases and territorial legislation were setting the
framework for the present day. Groundwater was looked on as having
potential value chiefly for domestic wells and for watering livestock on
the range-the windmill and tank combination so vital in settling the
West. ”There appeared to be little necessity of vesting it with a public
character similar to surface water” (Mann, 1963, p. 43). Institutions
created at that time hardly fit the current situation in which ground
water outshadows surface water.
Another problem lies in a myth about great underground rivers,
which influenced what courts wrote into their judgments. This myth
is expressed in most nineteenth century textbooks in physical geography, with vivid descriptions of caves and streams in limestone karst
terrain. Widely believed in many places, this myth has nothing to do
with groundwater in the Salt River Valley. This ignorance of the
environment of groundwater carries over into a present reluctance to
support research on the geological environment of the basins in which
water is found (Mann, 1963, p. 237).
A prediction of continued decline in the volume of this body of
groundwater has been made from an analog modelling study. Over
the period 19641974, the model indicates an average decline of 16 m
in the water table. Over the period 1964-1984, it predicts a 32-m
average drop, with a shift in areal distribution (Anderson, 1968). Such
a decline is, again, part of the local perception of the situation. While
some of the recent developers of groundwater ”were interested in a
permanent livelihood on the desert, many were of the ’suitcase’
variety, willing to make an investment for short-term profits with full
G R O U N D W A T E R . A N D I T S OUTFLOWS
knowledge that the resource eventually would play out” (Mann, 1963,
MASS BUDGETS ASSOCIATED WITH GROUNDWATER
Water in its underground environment is affected by the temperature and chemical composition of the rocks. The progressive mineralization of meltwater with an initially diverse but small content of
dissolved matter was studied in the Sierra Nevada by Feth ef a l .
(1964), using measurements of chemical composition of water that had
spent varying lengths of time in the soil and rock of this granite
batholith (Table I). Their data illustrate how water charged with the
bicarbonate ion from carbon dioxide in the soil air vigorously dissolves minerals from the underlying rocks.
Mineral content of the water increases seven times (to 36 ppm) as i t
passes through the soil and shallow layers of the ground water en
route to ephemeral springs or seeps. It doubies again in water that
passes through deeper rock layers before coming out in perennia:
springs (75 ppm).
Water in thermal springs, which here mostly originated as snow or
rain, shows further mineralization, to 410 ppm. It acquires both heat
and salts in its long journey from the soil through the rocks and
probably into deep fault zones. Chlorides, sulfates, and nitrates,
which display only an initial or terminal increase, do not seem to be
derived from the rocks but from the atmosphere or deep fault-zone
water. Decaying litter is important for nitrates.
Water long buried underground often becomes highly mineralized.
That in the Great Artesian Basin of Queensland, for example, is too
salty for irrigating crops-fortunately for the conservative management it requires.
Movement of nitrate in percolate water into bodies of groundwater
was noted in the preceding chapter, and many other examples of manmade contamination of ground water by rural and urban systems
could be cited. Removal of pollutants is often slow and sometimes
impossible, and this useful space remains contaminated. A mismanaged ecosystem at the overlying surface has left a permanent blot
below ground. Walker (1974) notes that some techniques for reducing
air or water pollution produce residues that managers want to dump
somewhere, and that ”the ’somewhere’ undoubtedly will be a landfill,
seepage lagoon, spray irrigation plot or some other ground-water
recharge-disposal system,” and is concerned over the time-bomb
MASS BUDGETS ASSOCIATED WITH GROUNDWATER
Dissolved Solids in Meltwater after Varying Periods of Contact with Granite Rocks of the
Sierra Nevada, California"
No. of samples
' Units: ppm, Source: Feth et a / . (1964).
aspects of buried toxic chemicals. Groundwater IS in motion, and
leachate plumes from landfills on Long Island have been measured as
long as 3 km (Kimmel and Braids, 1974).
To avoid such irreversibIe changes in a valuable resource, the
budget of mass inflows, storages, and outflows is commonly modeled.
These budgets, if used properly within their limits (Evenson et a l . ,
1974), can aid the understanding of the mass budget of water and help
the land planners and managers. Even if special bodies of groundwater are dedicated to the function of servicing urban waste water and
the solid-waste jandfills, these bodies must be monitored. Qut-ofsight, out-of-mind cannot be the guide! Rather, it will be imperative
to watch the volumes of these special water bodies, their spread or
migration, and the concentrations both of the substances carried into
them by landfill leachate and wastewater applications and of new
G R O U N D W A T E R A N D I T S OUTFLOWS
chemical compounds resulting from the mixture of a great many
reactive substances in aqueous solution. Mass budgets of each ion as
well as of the water itself need to be cast for each of these groundwater
LOCAL OUTFLOWS OF WATER FROM UNDERGROUND STORAGE
Groundwater bodies generally extend over greater areas than typical
ecosystems do, and tend, like the local air discussed in a preceding
chapter, to be a "common" element shared by many ecosystems. As
water moves within a groundwater body it traverses distances greater
than the diameter of the ecosystem from which it initially percolated;
the characteristics of regimen or dissolved salts impressed upon it by
the originating ecosystem are carried beneath other ecosystems and
eventually mixed into a composite typifying the whole groundwater
This is, of course, one of the problems with leachates from solidwaste disposal sites or pathogens percolating from suburban wastes.
Outflows from groundwater carry a mix of influences from the
numerous ecosystems from which the aquifer was recharged. They
begin to represent a movement of water away from the generating
ecosystem, an off-site flow similar to those to be discussed in
Outflows should be seen within the contexts of the hydrogeologic
environment and the groundwater regime. "The hydrogeologic environment is defined by characteristics of the topography, geology, and
climate. The groundwater regime is described by parameters such as
the amount of water, the pattern of groundwater flow, the rate of
discharge, the chemical composition of groundwater, the temperature
of groundwater, and the variations of these properties" (Tbth, 1971).
Outflows therefore appear as "springs, seepages, quicksand, soap
holes, geysers, frost mounds, pingos, groundwater lakes and marshes,
and certain near-surface and surface accumulations of salts, landslides,
slumps, soil creep and gullying. . ." (Tbth, 1971), all of which shape
the ecosystems in their vicinities.
Outflows may be restricted locally, and forced to occur at a distance
from the site where percolation fed the groundwater body, a topic we
defer to Chapter XVII.
Outflows that occur locally from groundwater can be classified in
terms of the environment into which the water is discharged. (a) At
linear depressions, the emerging groundwater is concentrated in
L O C A L O U T F L O W S O F W A T E R FROM U N D E R G R O U N D S T O R A G E
volume sufficient for its potential energy to power its flow into a river
system and away from the local area altogether. This route taken by
the water-yield from the locality is discussed in a later chapter. (b)
Outflow in small trickles into high-energy environments allows the
absorption of radiant energy by wet soil or wet-soil plants to power
the evaporation of most or all of the water. These sites provide
conditions favoring water flux from the groundwater body into the
local air, quite a different departure route than that taken by ground
water that feeds into rivers.
Outflows into Vaporizing Environments
Several outflows from groundwater bodies exist in nature, and
others have been added by man. The natural ones often bear an
element of mystery, since their water comes from an invisible source.
They represent a local cycling of water from the local rock formations
to the local landscape and its ecosystems, occurring on a small spatial
Small Seeps and Springs Springs have long been regarded as magical, if not actually sacred, each with its own goddess, and there is a
friendly air about even a seep of water that supports a little patch of
green in a dry landscape or an outflow that keeps only a few meters of
The seeps of water at the foot of a forested slope represent a delayed
near-surface flow following a storm. If they are sustained after a storm
or snowmelting period ends, they represent outflow from a shallow
groundwater body. Wet-weather seeps are outflows from groundwater
bodies that have a longer life during the season when heavy percolation keeps the water table high, and persist for a month or more after
such percolation ceases. In the geological view, where less permeable
rocks force water to the ground surface, the area (and its ecosystems)
becomes a seepage area (as in Fig. XV-2). In a storm, such an area is
quick to generate off-site flow (Chapter XVI).
These small outflows from different groundwater bodies are common in the basin of the Central Sierra Snow Laboratory, both from
glacial deposits and from the contact zone where permeable volcanic
layers overlie the basal granite. “The atmospheric waters easily penetrate the porous volcanic rocks and, collecting on the bed-rock surface
below them, reappear at points along the contact” (Lindgren, 1897). In
late summer, long after the store of soil moisture has been exhausted,
these seeps continue to trickle water out to small meadows, strips of
willows and shrubs down the dry hill slopes, and along the upper