EVALUATION OF GROUNDWATERRESOURCES STATUS

Q.4 - How do I assess the extent of urban groundwateruse and the status of aquifer development?

Written specifically for this Web-site by Stephen Foster with contributions by Brian Morris.

Estimated average annualgroundwater rechargePIEZOMETRIC

SURFACEABSTRACTION

RATE

150

100

50

0

1850

1800

1750

1700Ave

rage

aqu

ifer p

iezo

met

ric le

vel (

m A

SL)

Tota

l gro

undw

ater

abs

trac

tion

(Mm

3 /a)

2000s1990s1980s1970s1960s

Actual

Predicted followingimplementation ofmanagement measures

EVALUATION OF GROUNDWATER RESOURCES STATUS

Susceptibility to Exploitation Side-Effects

Over Exploitation: The Problem of Definition

• All groundwater exploitation through pumping wells and boreholes results in a decline in aquifer piezometric level over a certain area. In fact in some cases a reduction may be desirable to aid land drainage and to maximise potential aquifer recharge. The fall in piezometric levels, however, can have adverse consequences that become more pronounced with increases in the number of wells and the volume of groundwater pumped.

• Over-exploitation is often used (in the narrowest sense) to describe the state of the

groundwater balance where the average rate of groundwater abstraction exceeds the average rate of aquifer recharge, leading to a near-continuous reduction in the volume of groundwater in permanent storage. Such conditions are sometimes known as groundwater mining or overdraft (Custodio, 1992), because there is effectively a consumption of reserves.

• While the term over-exploitation can be useful in general resource administration terms, it

is in reality an emotive expression without precise scientific definition (Foster, 1992). This is because use of the term immediately prompts certain questions:

? For what area should the groundwater balance be evaluated, especially in situations where pumping is very unevenly distributed? ? Over what period should this balance be established, especially in more arid climates where major recharge episodes occur once a decade or even less frequently?

• The response to these questions in practice varies considerably with the exploitable storage

volume of the aquifer system and the periodicity of the recharge episodes involved. All other things being equal, localised aquifers with low storage coefficient and small sustainable yield will be the more susceptible to side-effects. While almost all groundwater development will have some negative side-effects, the principal concern is with those which have significant adverse consequences, and the term over-exploitation is better used in those cases (Custodio, 1992).

Exploitation Side Effects: Reversible or Irreversible?

• Excessive groundwater abstraction can have a number of side effects, which are detailed in

Table 1. It is important to distinguish between reversible side-effects, such as water-table or piezometric surface decline from interference between neighbouring wells, boreholes and springs, from those which are quasi-irreversible, such as land subsidence due to the compaction of aquitards interbedded within aquifer systems. This distinction is fundamental for sustainability and critical in a full cost-benefit analysis of groundwater abstraction.

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Table 1 Summary of main side-effects of excessive groundwater abstraction REVERSIBILITY SIDE-EFFECT OF

EXCESSIVE ABSTRACTION

UNDERLYING CAUSE FREQUENCY OF

OCCURRENCE REVERSIBLE Increases in well pumping

costs

Reductions in well yield

Reductions in spring flows

Falling groundwater table/piezometric surface

Widespread

?

?

Impact on stream and/or wetland ecosystems

Aquifer saline intrusion

As above with reduction in natural aquifer discharge

Reversal of horizontal hydraulic gradients in aquifer

Widespread

Quite Common

IRREVERSIBLE Induced recharge of polluted water

Induced changes in natural hydrogeochemistry/quality of groundwater

Land subsidence

Aquifer transmissivity reduction

Reversal of vertical hydraulic gradients in aquifer

Drawing of oxygen into naturally anaerobic aquifers and transforming/mobilising some minerals (As, S, Fe, Mn).

Reduction in porewater pressure and compaction of interbedded aquitards

Compaction of aquifer matrix

Quite Common

Rare

Localised

Rare

• In uncontrolled situations, the exploiter of groundwater resources in effect receives all of the benefits of water-supply but pays only part of the costs, namely the capital cost of well construction plus the recurrent cost of pumping. The cost of the side-effects, whether reversible or irreversible, is not taken into account. Unrestricted groundwater development resulting in significant side-effects can lead to serious problems of social inequity (Shah 1993), which could frustrate public policy goals. Such a situation often arises for example where many domestic and small-scale periurban agricultural users only have ready access to the shallowest parts of an aquifer system as a result of limitations in the operating head of traditional water-lifting techniques.

• It is also important to note that not all aquifer over-exploitation is negative. If planned with

specific aims, and as long as the consequences have been technically evaluated and are economically acceptable, it may represent logical resource exploitation policy. This would be the case where groundwater mining enables a cycle of economic development to occur, which would more than compensate for the substitution of more expensive water at a later date, or lead to new technology improving the efficiency of water use or to effective demand management.

• The most common quality impact of inadequately controlled aquifer exploitation,

particularly in coastal areas, is the intrusion of saline water. As groundwater levels fall, reversal of flow direction can occur causing the freshwater-saline water interface in the aquifer to advance landward. For some aquifers this takes the classical wedge-shaped form,

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but in thicker, multi-aquifer sequences (characteristic of most major alluvial and some other formations) salinity inversions often occur with the intrusion of modern seawater or retention of palaeosaline water in near-surface horizons and fresh groundwater in deeper formations. This can greatly complicate both the diagnosis of the condition and management responses, as in-depth hydrogeological studies are required to unravel the distribution of freshwater. The effect of saline intrusion in most aquifer types is quasi-irreversible. Once salinity diffuses into the pore water of the fine-grained aquifer matrix, its elution will take decades or centuries, even after a coastward flow of fresh groundwater is re-established.

• A frequent consequence of inadequately controlled exploitation is contamination of deeper

(semi-confined) aquifers that underlie shallow, phreatic aquifers, by poor quality water resulting from anthropogenic pollution and/or saline intrusion. This induced pollution has two origins. Firstly inadequate well construction can cause direct flow down wells accidentally linking aquifer horizons, the borehole acting as a vertical conduit. Secondly, pumping-induced vertical leakage can occur, caused by head differences as the piezometric surface of the lower aquifer declines below the water-table of the phreatic aquifer.

• Such conditions cause the penetration of the more mobile persistent contaminants. Polluted

recharge to aquifers is also often induced where there is major abstraction of groundwater from riverside aquifers, especially downstream of towns and cities where surface watercourses are commonly polluted, at least for some kilometres.

Assessing Probability of Adverse Abstraction Side-Effects

• The probability of serious adverse side-effects of intense or excessive groundwater

abstraction varies quite widely with hydrogeological environment (Table 2). The potential severity of such side-effects can be estimated in a general way if some quantitative information is available on the hydrogeology of the aquifer system concerned (Table 3). These tables only provide a preliminary estimate of susceptibility, and detailed hydrogeological investigations are required to make a full diagnosis of the situation and to model probable future scenarios under various management options.

• Serious saline intrusion is confined to relatively few hydrogeological settings and major

land subsidence is largely restricted to those coastal alluvial and intermontane valley-fill formations which contain significant thicknesses of interbedded unconsolidated clays/silts of lagoonal/lacustrine origin. However, a much greater variety of aquifers are susceptible to the induction of polluted recharge if they experience excessive abstraction in urban areas.

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Table 2 Susceptibility of hydrogeological environments to adverse side-effects during excessive abstraction

TYPE OF SIDE-EFFECT HYDROGEOLOGICAL ENVIRONMENT SALINE

INTRUSION OR UPCONING

LAND SUBSIDENCE

INDUCED POLLUTION

Major Alluvial Sediments Coastal ✔✔ ✔✔ ✔✔ Inland ✔ ✔ ✔✔ Intermontane Colluvium and Volcanics: with lacustrine deposits ✔✔ ✔✔ ✔ without lacustrine deposits ✔ ✔ ✔✔ with permeable lavas/breccias ✔✔ - ✔✔ without permeable lavas/breccias ✔ - ✔ Consolidated Sedimentary Aquifers ✔✔ - ✔ Recent Coastal Limestones ✔✔ - ✔ Glacial Deposits ✔ ✔ ✔ Weathered Basement Complex - - ✔ Loessic Plateau Deposits - ✔ - ✔✔ major effects ✔ occurrences known - not applicable or rare

Table 3 Factors affecting the susceptibility of aquifers to adverse side-effects from

excessive abstraction

SUSCEPTIBILITY TO ADVERSE SIDE-EFFECTS FACTOR SYMBOL UNITS

High Moderate Low

aquifer response characteristic T/S m2 /day 100,000 1,000 100 10

aquifer storage characteristic S/R - .1 .01 .001 .0001

available drawdown to productive aquifer horizon

s m 10 20 50 100

depth to groundwater table h m 2 10 50 200

proximity of saline-water interface to abstraction zone

L km 0.1 1 10 100

vertical compressibility of associated aquitards Ø m2 /N 10-6 10-7 10-8 10-9

T = transmissivity (m2 /day) S = storativity (dimensionless) R = average annual recharge rate (mm/year)

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• The degree to which well yields decrease due to local or regional over-abstraction of groundwater resources depends on certain detailed hydrogeological features, such as the available drawdown above major groundwater flow horizons within the aquifer which are associated with high-permeability geological features (exceptionally coarse or highly fractured beds). Where these occur at a shallow depth spectacular yield reductions and unexpected well failures can sometimes occur. In other situations there will be a gradual reduction over periods of decades as the saturated thickness and length of producing screen decline, and/or individual producing horizons are progressively dewatered .

Sociopolitical & Institutional Issues Particular To Developing Nations • While there are wide variations between developing nations, there are a number of conditions

in common which the emerging economies share when faced with controlling groundwater exploitation and correcting aquifer overexploitation. The following need to be borne in mind when discussing groundwater resources management issues in a developing nation context.

• First, the pressing need to expand food production and to provide for rapid urban growth

continues to place groundwater resources under strong development pressure. This is especially the case in the more arid regions where water rather than land is the limiting constraint to development. This situation can be complicated where there is reluctance by government to allow a realistic price to be levied for urban water-supply or there exists a policy decision to subsidise beyond realistic limits agricultural irrigation from groundwater. An accompanying lack of political will to enforce controls upon economically-powerful lobbies is also not uncommon, and while this of course is a global human characteristic, it can be particularly pernicious where income differences are extreme.

• Second, the traditional patterns of groundwater exploitation in some developing nations have

led to very large numbers of small abstractors many of whom are highly vulnerable to interference effects (Shah, 1993). Such patterns tend to favour the initial exploitation of shallower groundwater, such as that found in major aquifer discharge areas or in perched aquifers. This has been due to technological limitations on well construction and pumping plant.

• Third, the implementation of controls on groundwater resource exploitation, make significant

organisational and technical demands on central or local government. This is especially the case if these controls are intended to be comprehensive in their application. The institutional capacity and budgets of regulatory departments or agencies are commonly insufficient to cope with such demands and the legal provision by which they are backed is often insufficient to control established abstractors.

• Fourth, control of abstraction to reduce risk of aquifer overexploitation presupposes that

groundwater resources have been reliably evaluated. In many developing nations there is simply inadequate information to make such an evaluation and there are major uncertainties remain over the size of the groundwater resource, both in terms of replenishment and storage and about the potential side effects of overexploitation. The inadequacy of information results from the generally scanty nature of many fundamental hydrogeological databases, the fact that very limited hydrogeological process research has been undertaken and the lack of historic operational data on aquifer abstraction rates and water-levels (Foster, 1992). It may for instance be difficult even to diagnose incipient signs of overexploitation because of the lack of time-series piezometric data from monitoring boreholes.

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The following examples, drawn from Foster (1992) encapsulate some of the challenges for resource management in fast-developing economies.

Example 1 Lima-Peru

The alluvial fan aquifer of Lima and its port Callao was exploited in the late 1980s by more than 320 municipal production boreholes, which provided a supply of up to 650 Ml/d. Many other private industrial abstractors also tap the same resource. This is an extremely arid area and diffuse recharge from excess rainfall is virtually negligible. Total abstraction since the mid-1970s has considerably exceeded the other forms of aquifer recharge. Over a substantial area the water-table was falling by rates of more than 2 m/a, and in extreme cases by more than 5 m/a. The consequences were to reduce dramatically the yield of production boreholes, especially in areas where the most permeable horizons of the alluvial aquifer occur relatively close to the original groundwater table (Figure 1).

. static water-level falling continuously during 1964-84

A : 10 - 1964

A : 7 - 1978

A : 11 - 1979

A : 4 - 1981A : 8 - 1982

A: 10

- 1984

production boreholewell screen/slotted lining tubes

more permeable horizon inalluvial sand-gravel aquifer

40

0

dept

h (m

BG

L)

20

60

80

0 20 40 60

yield (l/s)

Figure1 Decline in operational perfromance of a production boreholke in a heavily over-

exploited alluvial aquifer, Lima Perú In other areas borehole yields have been less affected. The overall effect on the municipal water-supply situation is that the number of operational boreholes has had to be steadily augmented since 1980 to maintain groundwater abstraction at the same volume and the unit energy costs of water

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production have increased by 25% during 1975-85 (Table I). The average depth and capital cost of new production boreholes has also increased very significantly Table 1. Operational history of municipal groundwater abstraction from the overexploited Lima alluvial-fan aquifer YEAR 1975 1980 1985

production boreholes (number in operation) 151 219 264

total groundwater abstraction (Mm3/a) 155 199 208

average borehole yield (1/s) 32 29 25

unit energy consumption (kWh/m3 )

0.70 0.82 0.88

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Example 2 Hermosillo-Mexico Another example of gross aquifer overexploitation and unsustainable groundwater development is found on the Hermosillo coast of northern Mexico (Figure 2), where highly productive irrigation of wheat and maize from both surface water and groundwater has been practised since the 1960s. This is also an arid region in which groundwater recharge is largely derived from excess irrigation and (very infrequent episodes of) excess rainfall associated with summer storms. Groundwater abstraction by some 500 production wells with an average capacity of 80 1/s has led to depletion of storage reserves in the 60-200 m thick alluvial aquifer, at rates of 400-750 Mm

3/a (Tinajero, 1989). Invasion

of saline water for distances of up to 25 km from the coast has resulted, despite the fact that finer-grained sediments close to the coastline in places form a partial hydraulic barrier. This saline intrusion has led to the abandonment of many production wells and of much agricultural land.

GU

LF

OF

CA

LI F

OR

NI A

Hermosillo

Boundary of irrigation district

-60

-40

-40

-20

-20

0

+20

Position of generalised frontof saline water intrusion intoalluvial aquifer in 1985(due to pumping 750-1100 Mm3/aduring 1965-85)

Borehole monitoring network

II

I

II

IIII

III

II

II

II

II

I

II

I

II

I

IIIIII

II

I

❘❘

❘❘❘❘

❘❘❘❘❘❘❘❘❘❘❘❘❘❘

❘❘❘❘

❘❘

❘❘❘❘

❘❘❘❘

❘❘❘❘

❘❘❘❘

❘❘❘❘

❘❘❘❘

❘❘❘❘❘❘❘❘❘❘

❘❘❘❘

❘❘❘❘❘❘❘❘

❘❘❘❘

❘❘

IIII

N

PREDICTION FOR YEAR 2000(if pumping continues at 850 Mm3/a)

(a) water-table contours (msl)

(b) generalised front of saline water

-20

❘❘❘❘❘❘❘❘

0 20km

Figure 2 Current and probable future intrusion of saline water front into the

overexploited alluvial aquifer of the Hermosillo coast, Mexico (afeter Tinajero, 1989)

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Example 3 Chihuahua-Mexico

The larger towns of the interior of northern and central Mexico are also highly dependent upon groundwater both for municipal water-supply and for agricultural irrigation. In many cases this has led to continuously falling groundwater levels at rates similar to those observed in Lima, with fierce competition between municipal, industrial and agricultural water-supply interests (Figure 3). In some cases this overexploitation is inducing infiltration and deep penetration of polluted water. In the alluvial aquifers of the Chihuahua area, for example, deterioration in groundwater quality has resulted from infiltration of unsewered sanitation effluent in parts of the city itself and of urban sewage effluent downstream, either directly through the riverbed or following over-irrigation of agricultural land.

IMPERMEABLEBEDROCK

SACRAMENTO VALLEY CHUVISCAR-TABALAOPA VALLEY

Formerly zone of groundwaterdischarge upstream of impermeablebarrier; condition reversed due toaquifer overexploitation withinfiltration of urban effluents

shallowgroundwater

quality

Cl

NO3

5 - 15

<25

30 - 80

50 - 100

Riverflow (mainlyurban wastewater)infiltrating andcreating perchedwater-table

Localised aquiferoverexploitation with fallinggroundwater table

Groundwaterdischarge to riverand via vegetationupstream ofbarrier

40 - 100

30 - 150

5 - 15

<25

>50

<25

(mg/l)

(mg/l)

1000

1200

1400

1600

(mA

SL)

Chihuahua

1000

1200

1400

Direct evapotranspirationfrom shallow water table

0 10kilometres

vertical exaggeration x20

Groundwater flow directions

Water table of alluvial aquifer

Alluvial aquifer dewatered

Equipotentials of alluvial aquifer

Figure 3 Overexploitation of intermontane alluvial aquifers in Chihuahua, Mexico

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Example 4 Cochabamba-Bolivia

Examples of physically-sustainable but irrational exploitation of groundwater resources are found very widely. Common occurrences include the competitive drilling of increasingly deep production boreholes, which obtain almost all of their yield from induced vertical leakage and simply dewater shallower aquifer levels, which were fully exploited by earlier production boreholes. There are also frequent instances of borehole drilling to capture groundwater that was largely being discharged immediately downstream in (geological contact) springflow already captured for public water-supply. The reverse type of situation also occurs quite widely as is the case in the Cochabamba valley of Bolivia (Figure 4). Here natural groundwater discharge has been very inefficiently developed by a large number of individually small-scale abstractors, dependent upon artesian pressures for shallow well or spring discharge. In such situations major loss of water occurs due to under-utilisation, evaporative loss and other wastage. Such installations also have no capacity to exploit groundwater storage in drought years. Community pressure exerted by their users, however, can be a formidable obstacle to subsequent more efficient exploitation of groundwater storage system by municipal wellfields up- hydraulic gradient in the same aquifer .

UNCONFINED

Major recharge area(excess rainfall/influent streams)

2400

2600

2800

3000

(mA

SL)

2200

2400

2600

SEMI-CONFINED CONFINED

Transition zoneArtesian

discharge area

MODERATE TMODERATE THIGH T

Decreasing south with reduction of average aquifer grain-size

0 10kilometres

vertical exaggeration x20

Aquifer water-table/piezometric surface

Generalised groundwater flow lines

Yield of individual boreholes and wells

Municipal water-supply wellfields

(25-35 l/s) (10-20 l/s)

(<0·5 l/s)

More than 400 shallowoverflowing wells and springs

Rio Rocha

(10-20 l/s)

Sierra de Tunari

N S

Figure 4 Section of Rocha Valley downstream of Cochabamba Bolivia illustrating irrational

pattern of groundwater exploitation

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REFERENCES/FURTHER READING Custodio E 1992. Hydrogeological and Hydrochemical Aspects of Aquifer Overexploitation.

IAH Hydrogeology Selected Papers Vol 3, pp 3-28. VH Heise, Hannover Foster SSD 1992. Unsustainable Development and Irrational Rxploitation of Groundwater

Resources in Developing Nations-An Overview. IAH Hydrogeology Selected Papers Vol 3 pp321-336. VH Heise, Hannover

Shah T 1993. Groundwater Markets and Irrigation Development: Political Economy and Practical

Policy. Oxford University Press, Oxford . Tinajero J A 1989. Economic Assessment of the Consequences of Groundwater Use.

Developments in Water Science, 39 pp 133-152