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2 7Hydrological Sciences - Journal - des Sciences Hvdrologiques, 32, 2, 6/1987

O 8 7! .IBRARY

Groundwater in Colombia INTERNATIONAL REFERENCE CENTKEFOR COMMUNITY VVATRR SUPPLY AMD

AU3ERTO LOBC-GUERRERO U,Consulting Geologist, Apartado Aereo 100777,Bogota D.E., ColombiaYMKOV GILBOAConsulting Geologist, 34 Lochamei Hageto St.,Petah Tiqwa 49408, Israel

ABSTRACT In tropical Colombia, where 75% of groundwaterexploitation is concentrated in two limited areas,successful agricultural ventures proved its economicvalue. It is therefore imperative to assess the develop-ment possibilities of this almost untouched resource tocomplement surface water in irrigation and water supplyprojects, and in some cases to replace contaminatedsources. The present paper introduces the major aquiferunits of the country and discusses the state of exploita-tion and approximate recharge values .

Les eaux souterraines en ColombieRESUME Dans la Colombie tropicale ou 75% de 1' exploita-tion des eaux souterraines est concentree dans deux zoneslimitees , le succes des entreprises agricoles a prouveleur valeur economique. Aussi , 1' evaluation des possibi-lites de developper 1 'exploitation de ce genre deressources en eau a peine utilise est indispensable pourcompleter ou remplacer les eaux de surface polluees, dansles projets d'irrigation et d'alimentation en eau. Getarticle presente les aquiferes principaux du pays etdiscute du stade de leur exploitation et des valeurs dutaux de recharge approximatif .

INTRODUCTION

Colombia, the fourth largest country in South America, covers1 138 914 km2. Due to its position within the "tropics and boundedby two oceans, Colombia enjoys precipitation rates above 2000 mmyear"1 (Fig.l). Rainfall less than 1000 mm year"1 is limited to theextreme northern lowlands, to the semiarid Guajira Peninsula, and tosome highlands on the Andean ridges.

The abundant rainfall, resulting in severe floods on the lowerparts of the large unregulated catchment areas (Magdalena, Cauca andmany others) , combined with the low water demand are the mainreasons why groundwater is barely exploited in Colombia. Largecentres of water consumption such as big cities are supplied bysurface water installations. The only two important centres ofgroundwater exploitation are encountered in the Cauca Valley and inthe Sabana of Bogota.

Open for discussion until 1 December 1987.

161

162 Alberto Lobo-Guerrero U. & Yaakov Gilboa

CARIBBEAN SEA

PANAMA

6'

,>>0 - ISOHrETl CURVE .'m/r,<

• - SDUHOAPr Of MORPHOLOGICAL UNIT

0 SC <00 700 »m

.1

Fig. 1 Isohyetal map of Colombia.

Topographic and economic constraints are reducing the feasibilityof further surface water development through expensive regulationprojects and the widely occurring groundwater should be used to meetmany new water requirements. Leading and flourishing agriculturalenterprises, supplied partially by groundwater, like the sugar canecrops in the Cauca Valley, the banana plantations in Uraba and inthe delta of the Magdalena River, the flowers in the Sabana ofBogota and the cotton in the Cesar Valley, prove the importance of

Groundwater in Colombia 163

this resource.Even though most of the existing water supply systems are based

on surface water intakes, a re-evaluation of their soundness iscalled for. Recent natural calamities like the eruption of theNevado Ruiz Volcano, which buried the town of Armero, and smallerdisasters like the floods in Villavicencio and Cucuta, demonstratethe vulnerability of these systems. Increasing pollution hazardsand elevated costs of treatment also make surface water lessattractive. Therefore, thousands of villages and small towns areto be considered for installation of an appropriate groundwatersupply system.

Most of the numerous existing reports on groundwater in Colombiadeal with local aspects of groundwater supply. Only a few regionsare covered by extensive studies (Cauca Valley, Sabana of Bogota,La Guajira, parts of the northern Atlantic Coast and the CesarValley, Cucuta). Recently, a first attempt was made to elaborate,on a large scale and as part of the National Water Study, theinformation on groundwater, and the country was divided into 28hydrogeological zones (Mejia et al., 1983; Lobo Guerrero, 1984).This attention is a logical response to the proven economicadvantage of groundwater use in irrigation schemes. The presentpaper provides a preliminary evaluation of groundwater in Colombia.

GENERAL DESCRIPTION

Colombia is divided into two major and different regions:(a) The low and flat eastern region, 100 to 500 m a.m.s.l.,

covers 670 000 km2 and is crossed by large rivers that drain eastand south into the bounding Orinoco and Amazonas Rivers. This vastregion is divided into the northern "Llanos Orientales" (plains)and the southern, jungle covered, Amazonas basin. In this easternregion, relatively thin Palaeozoic-Cenozoic sediments lie on top ofPrecambrian metamorphic and igneous rocks of the Guayana Shield.

(b) The remaining 469 000 km cover the central and westernregions. They consist of three, NNE-SSW trending, mountain ranges(the Andean Cordilleras; Oriental, Central and Occidental) and thehumid coastal plains. The Cordilleras, with elevations up to 5000 ma.m.s.l., are separated by low, 100 to 500 m intermontane valleys(Magdalena and Cauca). Along the northern Pacific Coast and thePanama border, remnants of a low coastal range separate the plainsfrom the ocean. The highest peak of Colombia, 5800 m a.m.s.l.(Sierra Nevada of Santa Marta), is located in an isolated range onthe Atlantic coast (Fig.l).

About 20% of Colombia is formed by igneous and metamorphic rocks,concentrated in the three Cordilleras, the isolated Sierra Nevada ofSanta Marta and in the Guayana Shield (INGEOMINAS, 1976). The restof the Andean mountain ranges are composed of Palaeozoic and thickMesozoic and Cenozoic sedimentary rocks. The low plains anddepressions are filled with thick Tertiary and Quaternary sediments.

The climate is tropical, with an average precipitation above2000 mm year"1 and average temperature of 28°C at sea level (Fig.l).Temperatures decrease as altitude increases. The pleasant tempera-tures attracted the major part of the population to the highlands.


For instance, the capital, Bogota, with its 5 million inhabitants,is located in a flat valley as high as 2650 m a.m.s.l., enjoying anaverage temperature of 14 C.

Except for streams in the Guajira Peninsula and in a few smallinland areas like Northern Huila (Neiva), all the rivers areperennial. They are grouped in 12 major hydrographical basins, ofwhich the most important are: the Amazonas, Orinoco, Magdalena andCauca (Fig.2).

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THE AQUIFERS

Excluding 237 000 km formed by igneous and metamorphic rocks and502 000 km by impervious shales, claystones and turbidites, theremaining 400 000 km contain aquifers of some importance (INGEO-MINAS, 1976, 1977; Rubio et al., 1978). They are, from east towest (Figs 3 and 4):

the Quaternary aquifers of the Llanos Orientales, Cucuta andthe Amazonas basin;

the Quaternary fill and the Cretaceous-Tertiary sediments inthe synclinal troughs of the Cordillera Oriental;- alluvial fill and Tertiary beds of the Upper and MiddleMagdalena Valley;

alluvial fill and Tertiary beds of the Cauca Valley;- alluvium and Quaternary deposits of the Pacific coastal area;

Quaternary beds and Tertiary sediments of the Lower MagdalenaValley, Cesar Valley and the Atlantic coast; and

Quaternary-Tertiary aquifers of La Guajira Peninsula.Details of the different aquifers are summarized in Table 1 andFig.5.

The volcano-sedimentary beds in the south, between Popayan andPasto, have unknown potential. The small Caribbean Island of SanAndres, of Tertiary and Pleistocene karstic limestones, suffersfrom serious problems of sea water encroachment. Several of theTertiary and Quaternary aquifers of the Pacific coast, LowerMagdalena, Atlantic coast and La Guajira are brackish.

While in the igneous-metamorphic domain, water circulation islimited mainly to fault zones, the occurrence of detritic andcalcareous horizons and lenses within the otherwise imperviousCretaceous-Tertiary shales and clays enables local groundwaterextraction.

The Quaternary aquifers of the Llanos Orientales, Cucuta and theAmazonas basin

The recent unconsolidated alluvium that covers the flat drainageareas of the rivers that end in the Orinoco and the underlyingQuaternary beds form the large aquifers of the Llanos Orientales.Their granulometry changes from very coarse and poorly sorted inthe piedemont alluvial cones in the west to fine grained loose sandintercalated with clay in fluvial braided deposits eastwards. Nearthe Andean Cordillera, boulders and coarse gravels present drillingdifficulties as do artesian pressures in low-lying zones.

This vast humid region, covered by grass and by marshes formedby frequent floods and with a sparse population, has a very smallwater demand. The actual groundwater consumption, mainly fordomestic use, is less than 15 million cubic metres (MCM) per year,extracted by a few boreholes, with depths around 100 m. Withmodification of the water supply schemes in the largest and rapidlygrowing towns in the region (such as Villavicencio, Yopal andArauca), and plans for secondary recovery by water injection wellsin the recently discovered oil fields near Arauca and Villavicencio,future consumption will increase and wells are expected to be deeperand of larger capacities.

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CORDILLERA CORDILLERA

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Fig. 4 The aquifers of Colombia.

The vast region to the south, covered by dense tropical forests,belongs to the Amazonas basin. The igneous-metaraorphic basement,exposed in a few marginal areas, is covered by Tertiary, slightlycompacted, clays and silts. The alluvial fill of recent river

Table 1 The aquifers of Colombia

Totalrecharge Number Depth

LocationArea

Aquifer (km2)

LLANOS ORIENTALES Alluvium 107000

Amazonas basin

CORDILLERAORIENTALSabana ofBogota

Other valleys:Ubate-ChiquinquiraTunja. Paipa-Duitama-Sogamoso,Bucaramanga,Cucuta, etc.

UPPER ANDMIDDLEMAGDALEN A

Alluvium 50000

Sabana Fm 1 250

Tilata Fm 1 500

Guadalupe Fm 4000

Alluvium and 2000Tilata FmGuadalupe Fm 34000

FracturedTertiary-CretaceousFormationsAlluvium 15000

Gigante and 21 000Mesa Fms,Honda GroupPyroclastic 20 000sediments

ThicknessLithology (m)

Boulders, 100gravels, sand.clayGravels, sand, 30clay

Clay, gravel, 300sandGravel, sand, 200claySandstone, 700chert, shalesSand, clays, 400gravelSst., chert. 250-600shaleSst.,conglomerate.limestones

Sand, silt, clay, 80gravel, bouldersSst., claystones 4000conglomerates

Tuff, 25-350aglomerate.claystone,boulders

(MCM of of wellsSource of recharge year" 1 ) wells (m)

Rainfall and 2500-7500 50 50-150infiltration fromrivers

Yieldof wells(m5 h"1 )

115-300

Quality of water

Good

Infiltration from 850-4300 10?rivers

Rainfall 35

Rainfall, infiltration 30from rivers

-60 50-300

-60 150° 50-500

Rainfall 80-120 50 100-700

Rainfall, infiltration 60-100 200 10-300from riversRainfall

300Rainfall 100

10 100-400i-0 20 100-375

Rainfall, infiltration 600-1200 100 50-80from riversRainfall, infiltration 300-600 100 50-200from rivers

Infiltration from 200-800 50 25-140rivers

3-50

3-125

3-350

3-35

3-70

2--40

5-65

5-170

3-30

Good, slightlyferruginousGood

Good, slightlyferruginousGood, slightferruginousGood

Good to poor

Good

Good to calcareous

Hard, slightlyferruginous.manganese

CO

?ocrofj)co>

Estimated 3production ™(MCM °year1 ) F

C?15 •<o>

Ql

O

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20

20

5

<5

20

<20

5

CAUCA VALLEY Valle Fm 3160 Sand,gravels,clay, boulders

150-450 Rainfall, infiltrationfrom rivers

400-600 1500 100-200 250-400 Good, in northern 800part regular

"ainfnll 9f 1 1 00 •70? Brackish, some <5

PACIFIC COAST Alluvium 22000 Sand, gravel, Rainfall 900-1800clay

20? Brackish, some <5good

ATLANTIC COASTCesar Alluvium

Lower Magdalena Alluvium

Bolivar

Atlantico

Sucre

Uraba

Alluvium

Tertiary Fms

Brackishwater aquifers

Alluvium

LAGUAJIRA Alluvium

Mongui Fm

12000 Sand, gravel, 25-150 Rainfall 400-700clay

4000 Sand, gravel, 50-150 Rainfall, infiltration 140-240clay from rivers

23000 Sand, silt, 50 Rainfall 1400-2300gravel, clay

15000 Sandstones, 1000 Rainfall 150-600conglomerates.claystones

38000 Sst.. conglom- 75-750erates lime-stones, clay-stones

8000 Gravel, sand, 200 Rainfall, infiltration 300-900clay from rivers

9000 Sand, silt 50-100 Rainfall, infiltration 30-120from rivers

10000 Calcareous sst., 200 Inflow through major 10-30limestones, fault zoneconglomerates

?50 50-100 10-300 Good 30

?50 50-150 10-100 Good, locally to 20the north —brackish

7100 15-120 5-50 Brackish; some 20good

7100 12-260 5-150 Hard 20

— Brackish to saline —connate water

60 100-200 150-250 Good, some sites 30brackish

50-100 3-10 Brackish 15

500 180-300 20-130 Good to brackish <5

OTHER AQUIFERSPopayan-Pasto PyroclasticsCordillera Central Pyroclastics-

sedimentaryrocks

25000 400-1200 750 <5

Cesar-Guajira Fractured

TOTAL

ConsolidatedPalaeozoic-Mesozoic Fms

400000 4970 1150 9o————————————————————————————————————————————————————— C3Q_

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No of wrlltDepth O Aquitrr

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A — ALLUVIUMT- TERTIARYC - C R E T A C E O U S

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Fig. 5 Water wells distribution in Colombia.

courses and outcrops of fractured Tertiary consolidated sedimentsform separate aquifers, bounded by impervious layers. In thislarge jungle area, with abundant rainfall, very little groundwateris pumped.


The Quaternary fill and the Cretaceous-Tertiary sediments in theCordillera Oriental

Aquifers are encountered within the sedimentary strata of theCordillera Oriental in the Cretaceous to Quaternary formations thatcompose the sub-surface of the elevated plateaus and depressions.Examples are the 1250 km2 Sabana of Bogota, the Valleys of Ubate-Chiquinquira, Tunja, Paipa-Duitama-Sogamoso, the low plateau ofBucaramanga and the Cucuta Valley (Figs 3 and 4). Due to the highpopulation density and the intensive economical activities of thisregion, these superimposed aquifers play an important role in watersupply schemes and in industrial and agricultural ventures.

The aquifers vary from highly permeable unconsolidated recentalluvial deposits through Cretaceous-Tertiary consolidated rockswith primary permeability (Cacho Sandstone) or with secondarypermeability due to fracturing (like the Guadalupe Sandstones) orsolution (limestones), to the semiconsolidated beds with highpermeability such as the Tilata formation or with moderate per-meability like the fluvio-lacustrine sediments of the Sabanaformation, the Chicamocha formation in Paipa-Duitama-Sogamoso andthe Tilata formation of Boyaca.

The most important centre of groundwater extraction in the regionis the Sabana of Bogota, where the aquifer overlies the imperviousshaley Villeta Group. Due to faulting and erosion, the strati-graphic sequence above the Villeta Group is not complete. Along thecentre of the Cordillera the lowermost aquifers are composed of the700 m thick fractured sandstones of the Guadalupe formation. Wherenot exposed, the Guadalupe Formation is covered by the imperviousGuaduas formation and younger sediments, and is considered to be aconfined aquifer. It yields important amounts of somewhat ferrugi-nous groundwater. Where the Cacho sandstones are conserved abovethe Guaduas formation, boreholes achieve moderate yields. Thedepression of the Sabana of Bogota is filled with the fluvio-lacustrine semiconsolidated sandy, argillaceous and locally gravellySabana and Tilata Formations. More than 1500 shallow boreholes,less than 100 m deep, and a few deep wells extract water from thegravel and sand beds encountered within these formations.

All the aquifers in the Sabana of Bogota are recharged directlyby precipitation, some also by infiltration from river courses. Asshown by the schematic cross section, some of the aquifers may beinterconnected (Fig.6). The high rates of exploitation from theSabana and Tilata Formations are reaching the estimated safe yieldvalues and question future large-scale development of this resource.Even though extraction from the Guadalupe aquifer is relativelysmall, concentrated pumping seriously affects certain zones of theSabana of Bogota. In spite of recent efforts by the responsiblelocal authority to control drilling activities, no accurate informa-tion regarding the actual state of exploitation is available.

Alluvial fill and Tertiary beds of the Upper and Middle MagdalenaValley

The Upper and Middle narrow reaches of the Magdalena Valley separatethe Cordillera Oriental from the Cordillera Central. Within the

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topographic low valley, Quaternary alluvial deposits near rivers andstreams form highly permeable aquifers. The Quaternary-TertiaryGigante, Mesa and Honda Formations, that consist of poorly-cementedsands and gravels, sandstones and conglomerates with some claystones,form phreatic and confined aquifers. Pyroclastic beds and mudflowsthat form large fans, mesetas and high terraces, like the IbagueFan, present possibilities as aquifers, but are exploited by only afew wells, due to difficult drilling conditions. The aquifers arerecharged by precipitation, water courses and from bounding Cor-dillera formations. Salinization from deeper horizons is a per-manent danger for certain aquifers.

Alluvial fill and Tertiary beds of the Cauca Valley

The Cauca Valley, drained by the river with the same name, separatesthe Cordillera Central from the Cordillera Occidental. In thissection, the 3160 km2 of the low lying valley consist mainly ofRecent-Quaternary coarse alluvial fill, 50 m to a maximum of 450 mthick. These beds contain the most exploited, and also best studiedand controlled, aquifers in Colombia. About 1500 wells, 100-200 mdeep, with yields of 250-400 m3h-1 and total installed pumpingcapacity of 400 000 m3h~1 are extracting 800-1000 MCM year"1 fromthese phreatic and semiconfined aquifers.

Locally, beneath the Valle formation and in the foothills of theCordilleras, are other aquifers of Tertiary age: the ZarzalFormation (Pliocene) with semiconsolidated and semiconfined aquifers;La Paila Formation (Miocene), with consolidated, primary permea-bility and good confined-to-semiconfined aquifers; and the Caucagroup (Palaeocene-Miocene) with consolidated sandstones and conglo-merates, with secondary permeability.

The groundwater extracted is used mainly for the irrigation oflarge sugar cane plantations.

The aquifers are recharged by precipitation, rivers and inflowfrom the bounding Cordilleras.

Alluvium and Quaternary deposits of the Pacific coastal area

Located in the wettest region of Colombia, along the Pacific coast,the aquifers are covered by dense tropical forests or inundated bymarshes. The scarce population, concentrated mainly in a few smalltowns along the coast, is supplied by surface water installations.Sporadic shallow wells represent the minimal groundwater exploita-tion. In spite of their potential, it seems that the aquifers inthis region will remain practically untapped.

Quaternary beds and Tertiary sediments of the Lower Magdalena Valley,Cesar Valley and the Atlantic coastThe Recent-Quaternary alluvium and coarse fluvio-lacustrine Holocenedeposits are by far the most important aquifers in this region viz.Cesar, Delta of the Magdalena River, Mompos and Uraba. Reaching athickness of 150 m in certain areas, these aquifers give rise toborehole yields above 250 m3h~:.

The Tertiary sequence, thousands of metres thick, contains


moderate to poor aquifers. These are composed mainly of detriticformations, of continental or marine origin, with primary orsecondary permeability.

Over large areas between the Sierra Nevada of Santa Marta and theGulf of Uraba, these aquifers contain brackish to saline water andare not considered as exploitable aquifers. Where they containfresh water, the aquifers supply, with moderate-to-high boreholeyields, various small towns in the northern Departments of Atlantico,Bolivar and Sucre.

Other aquifers, like the Popa coralline limestone along the coast(Bolivar-Atlantico) and the fractured consolidated clastic orcalcareous formations that bound or are confined below the Quater-nary fill of the Cesar Valley (La Luna Formation), complement theaquifer potential of this region.

£>uaternary-Tertiary aquifers of La Guajira Peninsula

The northernmost peninsula of La Guajira, with a rainfall of 300-1000 mm year"1, is the most arid region of Colombia (Hoffman, 1975).The search for water there started several decades ago, and a fewpoor to moderate aquifers that yield fresh to highly brackish waterwere discovered.

The alluvial fill and semiconsolidated Quaternary terraces in themiddle and lower zones of the peninsula have watertable aquifersthat yield small amounts of water to hundreds of boreholes and dugwells, with the quality deteriorating from south to north. Under-lying the recent fill of the plains are fractured limestones in theUpper Guajira, the semiconsolidated Mongui Formation in the MiddleGuajira and other rocks in the Lower Guajira. These aquifers arealso saline toward the coast. A centre of fresh groundwaterpumping from the Mongui Formation is located near the capital of theDepartment, Rioacha, on the Atlantic Coast.

Recharge is attributed to rainfall and to inflow through majorfault lines from the Sierra Nevada of Santa Marta.

REPLENISHMENT POSSIBILITIES

Direct rainfall is the main source of groundwater replenishment, andinfiltration rates depend on the amount and frequency of precipita-tion, the evapotranspiration, the type of soil, the vegetation andthe depth to the water table. Many untapped aquifers in Colombiaare saturated up to ground level and thus infiltration is rejected.

Absence of pertinent data impedes the establishment of therelationship between the aquifers and the baseflow of rivers.However, it can be observed that the outflow from the shallowaquifers forms the baseflow of streams. Aquifers with deep ordepleted water tables are recharged from the river courses. It isassumed that a high percentage of the total runoff serves as tran-sitory recharge, appearing later as baseflow in the rivers.

Rapid evaluations, based on baseflow separation of the river flowdata, were undertaken recently in one or two small areas, in orderto obtain some idea of the order of magnitude of groundwaterrecharge. The computational method introduced in Sao Paulo, Brasil,


was modified for Colombian conditions (Gilboa et al., 1976). Deepinfiltration rates, varying between 0.1 and 5% of total precipita-tion, were used in accordance with the topographic, morphological,lithostratigraphic and aquifer saturation conditions.

The total runoff of Colombia is estimated to be about 1500 x 109m3year~1, part of it as torrential surface runoff and most of it astransitory flow. Transitory recharge taking place in the crystal-line rocks through faults and fractures feeds the baseflow of therivers or recharges connected aquifers.

Deep recharge, attributed only to direct precipitation, isestimated to be 9-25 x 109m3year~1, and is less than 2% of totalrunoff (Table 1). In the Magdalena basin, deep groundwater rechargeis about 3-7% of total runoff. The recharge estimates do notinclude additional amounts infiltrating from river courses andcanals, return flow from irrigation and inflow from adjacentaquifers, since there is no way yet to evaluate these. Rechargevalues are considered conservative, as demonstrated by the followingexample.

The Cauca Valley is an elongated and narrow valley, extendingnear Cali over 3160 km2, and with limited outlets between twobounding Cordilleras, the Central and Occidental. Consequently, inits lower reaches, the main aquifer drains into the river course.By using high rainfall infiltration rates, the recharge of theaquifer is estimated to be 400-600 MCM year"1. These values arebelow the discharge rates through pumping from wells (800 MCMyear" ), but except for small areas where pumping is heavilyconcentrated and leading to local water table depletion, no seriousoverpumping has been detected. The lack of negative effects isattributed to entrances from other sources: infiltration fromstreamflow, inflow through Cordillera faults, and return flow fromirrigation of the sugar cane plantations. This supplementaryrecharge may even be enhanced in the vicinity of pumping centres.In the areas where the water table has been depleted, the rate ofrecharge enhancement is probably too small to balance the amounts ofgroundwater extracted. In spite of the fact that only 6% of totalrunoff recharges the aquifer, a flow of 13 x 109m3year~ is gaugedat the lower end of the Cauca Valley (HIMAT, 1983). The aquifer isthus the largest regulating reservoir in the whole Cauca basin.

In the Sabana of Bogota, the sand lenses of the Sabana and TilataFormations are recharged by rainfall and infiltration from stream-flow. Intensive pumping led to water table depletion and someisolated lenses even dried up. However, to compensate for theamounts extracted, additional sources such as inflow from theGuadalupe Formation are assumed to recharge both aquifers (Fig.6).The Guadalupe Formation itself has limited natural drainage outlets.Overlying the impervious Villeta Shales, the Guadalupe aquiferremains saturated even at low rates of recharge, as is demonstratedby a series of springs. By increasing the extraction of ground-water, this spring discharge may be reduced and result in anincrease of replenishment rates. Nevertheless, intensive faultingdissects the aquifer into deep buried and sometimes isolated blocks,and calls for special attention. Wherever pumping is concentratedin such a block (e.g. Madrid-Bojaca) considerable water depletionoccurs. The increasing demand for groundwater has led in recent


years to drilling deeper than the Guadalupe Formation and it hasbeen found that the Villeta Shales are also fractured and yieldsome water from jointed zones.

Large areas in the lower reaches of the Magdalena basin, as wellas in the Orinoco-Amazonas and the Pacific basins, are frequentlyflooded and remain under water. Due to high rates of evaporation,inundated aquifers are salinized and have to be discarded asexploitable aquifers.

Deterioration of water quality was observed also in the semiaridLa Guajira Peninsula, where salinization is related to mineralizedaquifer beds. Sea water encroachment is observed, following hightides, near the Pacific and Atlantic coasts and on San AndresIsland, where pumping has depleted the karstic limestone aquifer todangerous levels.

DISCUSSION

About 5000 wells in Colombia produce annually 1100-1400 MCM ofgroundwater, 95% of this amount coming from the Magdalena-Caucabasin. The wells in the Cauca Valley alone extract 70% of the totalamount pumped in Colombia.

The total extracted amount of groundwater is only a smallproportion (6-15%) of conservative recharge estimates, thus leavingroom for more development. Drilling costs, including casing,screens, gravel pack and well testing, vary from US$ 100/m for smalldiameter and less than 100 m deep boreholes, to US$ 200-250/m forlarge diameter deeper boreholes. The actual drilling costs combinedwith the low costs of energy make the exploitation of groundwater inColombia economically attractive.

Even within the Magdalena-Cauca basin where groundwater pumpingreaches 10-25% of the estimated recharge and where the majoreconomic activity of the country takes place, there are manypossibilities for additional quantities of groundwater to supplycentres of demand. Only in two areas should measures be introducedto prevent irrevocable damage to these important resources: theCauca Valley where exploitation is approaching the estimated safeyield, and in the Sabana of Bogota where production reaches 40-60%of total recharge.

In the Cauca Valley, reducing development in the upper aquiferand enhancing exploitation, already initiated, from the deeperaquifer is the correct step to attenuate or reverse negativeeffects. Another measure, still to be studied, is the augmentationof infiltration rates in the depleted areas by artificial rechargethrough the construction of spreading basins. The possibility ofrecharge through existing wells that remain idle more than 6000 h peryear should not be discarded, in spite of the problematic clogginghazard.

To contain the alarming depletion in the aquifers of the Sabanaof Bogota, controlled exploitation is imperative. This calls forthe implementation of a tight programme with stress on effectivelicensing. A thorough groundwater investigation of the whole basinis needed to identify clearly the problem areas. Recharge of theaquifers could be augmented by spreading basins along various stream


courses like the Lower Tunjuelo, the Subachoque and possibly the RioFrio.

The regulation capacity of the large aquifers is an asset tosurface runoff. The huge aquifers in the Orinoco-Amazonas and inthe Pacific basins, located in high rainfall regions with insig-nificant water demands, will remain practically untapped in thelong term future. Therefore, groundwater integration in Colombiawill be concentrated in the Magdalena-Cauca basin with the twolargest exploitation centres, the Cauca Valley and the Sabana ofBogota, nearing the limits of their capacity.

Moderate demands in smaller or marginal aquifers call forinnovative approaches and appropriate solutions, such as:

(a) Dispersal of pumping and sealing of certain sections infresh water aquifers to reduce the danger of deterioration fromnearby saline formations (Uraba, northern coastal area, middlereaches of the Magdalena Valley).

(b) Artificial recharge combined with sewage reclamation to haltsea water encroachment in San Andreas Island.

(c) Increasing the well production capacity in La GuajiraPeninsula and development of agricultural ventures based on small-scale brackish water projects.

(d) Conveyance of fresh groundwater to towns located on salineaquifers, alleviating the serious water supply deficiencies. Thefeasibility of this solution was recently demonstrated by two wells,drilled near the Ariguani River (tributary of the Lower MagdalenaRiver), that with yields of 250 m3h~J supply the town of El Dificil,located some 20 km to the west.

The actual drilling capacity for water in Colombia is about 200boreholes per year. The 50-600 m deep wells are drilled mainly bythe direct rotary method and, on a smaller scale, by reversecirculation and percussion methods. There is a need to at leastmaintain and probably increase this capacity.

REFERENCES

Gilboa, Y., Mero, F. & Mariano, I.E. (1976) The Botucatu Aquifer ofSouth America, model of an untapped continental aquifer. J.Hydrol. 29(1-2).

HIMAT (1983) Anuario Hidrologico 1980. HIMAT, Bogota.Hoffman, J.A.Y. (1975) Atlas Climatico de America del Sur. WMO-

UNESCO.INGEOMINAS (1976) Mapa Geologico de Colombia, Scale 1:1 500 000.

INGEOMINAS, Bogota.INGEOMINAS (1977) Mapa Hidrogeologico de Colombia, Scale 1:3 000 000.

INGEOMINAS, Bogota.Lobo Guerrero, A. (1984) Zonas hidrogeologicas de Colombia. Bol.

Geologia, Universidad Industrial de Santander 30, 21-36, Bucara-manga, Colombia.

Mehia, Millan & Perry (1983) Estudio Nacional de Aguas-Fase Ianexo B; Las aguas subterraneas en Colombia (por Lobo Guerreroy Gomez). Inedito.

178 Alberto Lobo-Guerrero U. &Yaakov Gilboa

Rubio, C. et al. (1978) Recursos Hidricos de Colombia. INGEOMINAS,Recursos minerales de Colombia, Bogota.

deceived 23 June 1986; accepted 19 November 1986.

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