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Chapter XV. Groundwater and Its Outflows into Local Ecosystems

Chapter XV. Groundwater and Its Outflows into Local Ecosystems

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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



393



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,

the

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water being withdrawn in dry seasons or dry years and replaced in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



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XV.



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,

1969).



GROUNDWATER RECHARGE



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



395



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).



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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

dry UP.

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

ensues.



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



397



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,

1968).

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,

1947).

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.



398



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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

(Anderson, 1968).

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



399



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

a ’nature-problem’.’’

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



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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,

p. 51).



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



401



MASS BUDGETS ASSOCIATED WITH GROUNDWATER



TABLE I



Dissolved Solids in Meltwater after Varying Periods of Contact with Granite Rocks of the

Sierra Nevada, California"



Snow



No. of samples



77



Snowmelt

runoff in

streams



Base

flow in

streams



Ephemeral

springs



Perennial

springs



34



34



15



56



70

21



20



6.8

57



12



16



25



pH

HCO,



-



2.9



7.0

14



SiO,



0 16



8 1



Cations (Alkali)

Na

K



0.46

0.32



1.5

0.7



2.7

1.0



3.0

1.1



6.0

1.6



Divalen t

Ca

Mg



0.40

0.17



2.4

0.6



4.4

0.9



3.:

0.7



10

1.7



0.95

0.50

0.07



1.3

0.4

0.1



2.0

1.3

0.2



1.0

0.5

0.02



2.4

1.1

0.3



62



Thermal

perennial

springs

8



8.6

180



51

122

2.6

13



1.0



Anions



so4

Cl

NO,

Total dissolved

solids



4.7



22



35



36



75



59

59

0.2

410



' 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



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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

entities.



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

body.

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

following chapters.

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



403



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

scale.



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

channel moist.

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



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