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Revista Geolgica de Amrica Central, 46: 161-178, 2012ISSN: 0256-7024

ENGINEERING GEOLOGY MAPPING IN THE SOUTHERNPART OF THE METROPOLITAN AREA OF SAN SALVADOR

MAPEO DE INGENIERA GEOLGICA EN PARTE SUR DEL REA

METROPOLITANA DE SAN SALVADOR

Jos A. Chvez1,2*, Jan Valenta2, Jan Schrfel2,

Walter Hernandez3 & Ji ebesta4

1Oficina de Planificacin del rea Metropolitana de San Salvador (OPAMSS),San Salvador, El Salvador

2Czech Technical University in Prague, Faculty of Civil Engineering,Department of Geotechnics, Czech Republic

3Servicio Nacional de Estudios Territoriales, San Salvador, El Salvador4Czech Geological Survey, Prague, Czech Republic

*Autor para contacto: [email protected]

(Recibido: 17/02/212; aceptado: 29/08/2012)

Abstract: The use of classic geologic maps, where geological layers are grouped according to their age or origin,makes difcult the interpretation and use for civil engineer design or urban planning to people without deep knowledgein geology. Due to this reason engineering geological mapping has been carried out in the southern part of the Metro-politan Area of San Salvador using the stripe method. The objective of the methodology is that geological information,

geological hazards and geotechnical recommendations as well, can be represented and grouped depending on theintrinsic characteristics of each zone. This information can be easily interpreted for urban planners, private buildersand government agencies. The weakness in the compilation and research of geological and geotechnical information inEl Salvador, are some of the reasons for the current problems that experiment the region, indicating the importance ofimproving risk management, as well as soil and rocks mechanics.Key words: Engineering geology, stripe method, geology, San Salvador Formation, Balsamo Formation, Tierra Blanca.

Resumen: El uso de mapas geolgicos clsicos que agrupan los estratos por edad u origen, diculta la interpretacin yuso para diseos de ingeniera civil o planicacin urbana, para las personas sin conocimientos profundos en geologa.Debido a esto se ha llevado a cabo mapeo de ingeniera geolgica en sector sur del rea Metropolitana de San Salvador,haciendo uso de la metodologa de bandas. El objetivo de la metodologa es que la informacin geolgica, peligrosidad

CHVEZ, J.A., VALENTA, J., SCHRFEL, J., HERNNDEZ, W. & EBESTA, J., 2012: Engineering geology mapping in thesouthern part of th metropolitan area of San Salvador.- Rev. Geol. Amr. Central, 46: 161-178.

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162 REVISTA GEOLGICA DE AMRICA CENTRAL

Revista Geolgica de Amrica Central, 46: 161-178, 2012 / ISSN: 0256-7024

INTRODUCTION

This project is the beginning of missing sys-

tematic engineering geology mapping for civilengineering purposes in El Salvador. One of thegoals is to introduce a systematic method to rep-resent the geological materials and potential geo-hazards; this will aid in urban planning and earlystages of civil engineer design. The study area waschosen in the southern part of the MetropolitanArea of San Salvador (MASS) where pressure forurban growth was expected at the time.

Central America is located between thePacific and Atlantic Oceans; tectonic and vol-canism of the area is related to the Ring of Fire.This region (Gonzalez et al., 2004) is exposedto volcanic eruptions, earthquakes, floods, massmovements, drought and heavy rains that causehuman victims, damages in the infrastructure andeconomical losses that prevent sustainable devel-opment of the countries.

These past years the problematic is more fre-quent in the countries of the area; for example inEl Salvador heavy rains connected with tropicaldepressions or hurricanes like Mitch (1998), Stan(2005), Ida (2009), Alex (2010), Agatha (2010)and 12-E (2011), as well as earthquakes in 1986and 2001 forced the local authorities to adjust thenational and municipal budgets and ask for inter-national loans to rebuild the infrastructure and as-sist the affected people. (MARN, 2009 and 2010;La Prensa Grafica, 2011; El Diario de Hoy, 2011)

According to the government the damagecosts of the rains in October of 2011 (12-E) were4 % of the Gross Domestic Budget (El Diario de

Hoy, 2011) and for the 2001 earthquakes the eco-nomic losses were around 12% the GDB of 2002(Gonzalez et al., 2004).

The lack of proper knowledge prior to buildmost of the housing projects and infrastructure inthe MASS has been one of the reasons of the el-evated vulnerability to geological hazards. Othersreasons are rapid urbanization, connected withan elevated population density of 2 569 hab/km2(Ministerio de Economa. 2008). Also the migra-tion to the city and persons of scarce economicresources that live in risky areas (Gonzalez et al.,2004) like steep slopes, next to the scarps or in-vading the floodplains and stream fans, raise thevulnerability to geological hazards.

For land-use planners, information like geol-ogy, geomorphology, geotechnical data, seismici-ty, hydrogeology among others; can give informa-tion like the necessary type of foundation, masswasting processes, water resources and floodhazard. Recognition of these factors (Dearman,1987) will improve planning and development ofthe city allowing rational decisions to be taken inthe territory.

One of the tasks of the project of the CzechRepublic Foreign Development Programme underguarantee of the Czech Ministry of EnvironmentRP/6/2007 was improving land use decisions

by proposing a methodology of engineering

geolgica y recomendaciones geotcnicas puedan representarse y agruparse dependiendo de las caractersticas intrn-secas de cada zona. Esta informacin puede ser fcilmente interpretada por los planicadores urbanos, constructoresprivados y agencias gubernamentales. La debilidad en la recopilacin e investigacin de informacin geolgica y geo-tcnica en El Salvador, son unas de las razones de la problemtica que experimenta la regin, indicando la importanciade mejorar el manejo del riesgo, as como la mecnica de suelos y de rocas.Palabras clave: Ingeniera geolgica, mtodo de bandas, geologa, Formacin San Salvador, Formacin Blsamo,Tierra Blanca.

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163CHVEZ et al.: Engineering geology mapping in the southern part...


geology mapping. The results of the field workand mapping completed during the year 2008 are

presented next.

AREA OF STUDY

The Metropolitan Area of San Salvador(MASS) has an area of 609,9 km2 (Fig. 1) consist-ing of 14 municipalities and is located between theSan Salvador Volcano and the Ilopango Caldera(two active volcanoes) and the major part is insideof the Central Graben, a tectonic depression (Lexa

et al., 2001), which is connected to the subductionprocess in the Ocean. Inside the MASS most ofthe governmental agencies, industries, economicand financial institutions are concentrated.

The study area at the present is part of the nat-ural protection zone proposed by the Ordinanceof Protection and Conservation of the NaturalResources (Diario Oficial, 1998). Some of thereasons for been chosen for mapping, was the re-cent improvement of the roads in the sector thatcould increase the pressure for urbanization in thearea; also the area is almost in its natural state andthis made the mapping job easier.

Fig. 1: Location of the study area in the Metropolitan Area of San Salvador, black lines indicate the approximate area of theCentral Graben

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The zone is in the upper part of the Bocanade Toluca basin. The actual land-use of the slopesin the region is coffee plantations, woods and ru-ral environmental with crops in some slopes; the

more problematic areas (mass wasting) are most-ly located in the artificial road cuts(Fig. 2).

Some landfills and dumps were placed toincrease the area of construction of some of the

properties. During the field work it was observedthat most of them werent compacted properlyand as a result cracking, mass movements andcollapse was observed (Fig. 3).

Geology and GeomorphologyThe basic geological map of the whole El

Salvador, which is currently in use, was madeby the German Geological Survey (Bosse et al.,1978) in a 1:100 000 scale; after that, the geo-logical mapping was only limited to some areas,for example the 1:15 000 scale map of a sec-tor of San Salvador (Centro de InvestigacionesGeotcnicas, 1987) and the geological map 1:50000 made for a seismic zoning after the 1986earthquake (Consorcio, 1988). Lexa et al. (2011)made a 1:50000 geological map of the southern

part of the MASS where the study area is located.Geomorphological assessments of the MASS was

performed by ebesta (2006, 2007a, 2007b) andebesta & Chavez (2010, 2011)

According to the geological map of Lexaet al. (2011), the study area has presence of theLate MiocenePliocene Blsamo Formation andthe Late PleistoceneHolocene San Salvador

Formation (Fig. 4)The Blsamo Formation (Lexa et al., 2011)

is conformed by andesite lavas, tuffs and epiclas-tic volcanic breccias/conglomerates representingremnants of andesite stratovolcanoes. The SanSalvador Formation includes products of basalt-andesite stratovolcanoes associated with the evo-lution of the Central Graben as well as interstrati-fied silicic tephra/ignimbrites of the Coatepequeand Ilopango calderas.

According to ebesta (2006) and Lexa et al.(2011) some faults connected with the CentralGraben cross the area and for this reason part ofthe study area (Fig. 5) is composed by tectonicscarps and descending diastrophic blocks thatare distributed very chaotically. Relative tectonic

Fig. 2: Small mass movements in the artificial road cuts ofthe area.

Fig. 3: Landfills and dumps in the area where collapse and cracks were observed.

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uplift and depression is inferred by the juxtaposi-tion of recent and older deposits. The region has

been impacted historically by earthquakes (Larde,2000 and ebesta, 2007a).

Mass wasting processes (ebesta, 2006 andLexa et al., 2011) are connected with slopes cov-ered by tuffs of the San Salvador Formation,

but also with other regions where rocks of theBalsamo Formation are exposed and there is pres-ence of laterite or weathered tuffs. This situationhappens principally in the steep slopes of the ero-sion hillsides in the upper part of the basin.

On the southern part of the Central Graben,remnants of old stratovolcanoes are present,

where the thickness of the laterite is important. InFinca la Labranza there is an area of polygeneticfill within a tectonic depression (Fig. 5) which iscomposed by accumulated material of volcanic

eruptions and eroded material.

ENGINEERING GEOLOGICAL MAPPING

Part of the project, was to observe how thegeotechnical work and risk management is made inEl Salvador including the usual lab and field tests.

Currently in El Salvador the practice of soiland rock mechanics is weak, and thats one of the

Fig. 4: Geological map of the area, scale 1:50 000. SS- San Salvador, PL- Plan de la Laguna, TB- Tierra Blanca, LL- Loma Larga

(Lexa et al., 2011).

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reasons why the civil infrastructure like housingprojects, roads, slopes and bridges are easily dam-aged after a major event (rain or earthquake), in-creasing the costs of rebuilding

When a project is prepared, the geologicalrisk is not fully taken into consideration, and oc-casionally the project will originate, increase or

be affected by a natural hazard. Investors, tryingto save money, build in conditions, which some-times are inadequate for construction; in additionthe concentration of buildings in the cities is un-reasonably high.

According to Anon (1972) one of the short-comings of conventional geological maps, fromthe point of view of civil engineer, is that rockswith different engineering properties are charac-

terized as a single unit because they are of thesame age or origin.

The aim of the engineering geological map-ping (Commission on engineering geologicalmaps, 1976) is the selection of the geologicaland engineering characteristics of rocks and soilswhich are more related, and group the layers con-sequently. The degree of simplification dependson the purpose and scale of the map, the accuracyof the information and the techniques of represen-tation.

The most important tool during mappingwas the correlation of soil and rocks in different

places. The stratigraphy according to Hernandez(2008) was very helpful through the field work.Other aspects used were the interpretation

Fig. 5: Geomorphological map scale 1:25 000 of the area (ebesta & Chavez, 2011).

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of existing geological maps, field work, andevaluation of probable rock behavior from

previous knowledge.The engineering geology mapping was

done without any drilling boreholes or geo-physical measurements and it was based on theexistent topographical cartography 1:25 000(CNR, 1980, 1981).

The stripe method was chosen to symbol-ize the engineering geological units that exist inthe area. Dearman (1987) explains that the stripemethod was originated in Czechoslovakia in the60s and is analogous to trenching down throughthe surface layer of soil to the next layer below(rock base). The stripe method is the representa-tion of the character of the existing layers and

Fig. 6: Engineering geology map of the area.

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the thickness of the top layer. The basementor intermediate layers can be shown by stripes(simulating a window) underneath the surfacelayer.

In the stripe method a single color representsa single unit and the map unit patterns describethe lithological character (Dearman, 1987); forexample lava, pyroclastic flows, epiclastic brec-cia, weathered lava and ashes.

An engineering geological map illustratingthe aptitude for urbanization is presented in fig-ure 7. To build this map information like masswasting processes, slope inclination, human dis-turbance, flooding, lithology, weathering and

permeability were identified in the field. Thenthe areas with the same level of aptitude for ur-

banization were grouped according to the infor-mation compiled above, and general recommen-dations were proposed for the more problematic.Some hazards like the seismicity, extreme rainsor volcanism were not evaluated though, be-cause of the lack of complete information at thetime of mapping.

The map legend represent: dark gray color forinadequate areas because of hazards; light graycolor for suitable areas if some conditions are ful-fill (more detailed research); light gray color withhatch lines for suitable areas if some conditions

Fig. 7: Engineering geological map illustrating aptitude for urbanization.

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are fulfill and where complementary geotechnicalmeasures are necessary (walls, gabions etc.) butthe solution has to be based on a detailed research;and gray color for areas where is possible to build

from the geotechnical and engineering geologi-cal point of view, but always based on a properresearch.

The engineering geological map (Fig. 6)evaluates geological aspects of the area (mostlyquaternary geology), hardness, properties of soilsand rocks, exogenous processes (weathering),morphology and hydrogeology using the stripemethod.

The map is divided between the cover lay-ers and the rock basement. Since most of thesurface is covered by Tierra Blanca Joven (TBJ),the stripe method is able to present what is un-derneath it. The outcrops visited during the fieldwork helped to define the areas of stripes in themap, representing simulated windows of layerswith different level below TBJ. The surface lay-ers of Plan de la Laguna and San Salvador scoriaidentified in the field were grouped together. Alsothe combined set of tuffs between TBJ and therock basement (Arce and Congo, TB4, G1, TB3,TB2, G2, IB and PL) were grouped into one unitand represented with stripes in the map. Romannumerals in the map indicate the thickness of theengineering geological units.

The ISRM (1981) rock classification wasused for the different rock basement units and isrepresented as text in the map above the corre-sponding unit (Figure 6 and table 1). The descrip-tion of the units is presented next.

Description of rocks and soilsAccording to Hernandez (2008) the volca-

nic tuffs predominates in the surface, belongingmostly to the San Salvador volcano and to the

Ilopango Caldera, decreasing the thickness andchanging the grain particle size as they moveaway from their source center. The location of thetuffs was controlled depending on the direction of

winds, erosion processes and explosive force dur-ing the eruption; for these reasons in the outcropstheres no presence of all the layers.

Engineering geological units presented in themap are (Fig. 6):

The bedrock (Balsamo Formation) is com-prised mainly of solid lava rocks with presenceof boulders and blocks on the slopes, as well asepiclastic rocks product of debris flows and mudflows of the ancient volcanoes (Fig. 8). Lava anddebris flows are located in different places andthe depth decrease and disappear indicating howthe original morphology depressions was used fortheir transport. This rock base is usually massiveand its affected by selective weathering (Figure9). The result is that slightly weathered blocks oflava flow are surrounded by completely weath-ered lava flow (almost clayey material). Theweathered material is exposed in slopes and thereis a hazard of mass movements in many cases.According to Lexa et al. (2011) the weatheredrocks (laterites) have presence of illite, Kaolinite/halloysite, smectite and goethite. The montmo-rillonite (smectite) presence is important sinceswelling can occur.

The surface layers described by Hernandez(2004, 2008) and Lexa et al. (2011) are thegroup of volcanic tuffs (Tierra Blanca 4 (TB4),G1, Tierra Blanca 3 (TB3), Tierra Blanca(TB2), G2, IB, Plan de la Laguna (PL), TierraBlanca Joven (TBJ) including Arce and Congo)which are placed above the bed rock (Figure 10

and 11). This group is characterized by someweathering on the top (paleosoils) which are in-terbedded in the volcanic materials, being veryimportant for engineering geology purposes.According to Hernandez (2008) the deposits

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from the Ilopango Caldera that have paleosoilsare TB4, TB3, TB2; and G1, G2 from the SanSalvador Volcano. Congo and Arce are the de-

posits originated from Coatepeque Caldera andare weathered as well. The third unit is a sur-face of Tierra Blanca Joven (TBJ) cover withunknown thickness (Fig. 12), and is located inareas with no information due to poor acces-sibility or lack of outcrops; for this reason, ex-trapolation and interpolation was necessary.

Landfills were identified also, in order toavoid future problems if urban growth or con-structions are established on them (Figs 3 and 6).

Geotechnical knowledge

In El Salvador the moisture content, particlesize analysis, Atterberg limits, cohesion, angleof internal friction, compressibility, permeabil-ity and compressive strength are the parameters

Class Approx. Range of

Strenght c (MPa)

Strenght Field Denition

R0 < 1 Extremely weak rock Crumbles in hand

R1 1 to 5 Very weak rock Thin slabs break easily under hand pressure

R2 5 to 25 Weak rock Thin slabs break easily under heavy hand pressure

R3 25 to 50 Medium strong rock Lumps on core broken by light hammer blows

R4 50 to 100 Strong rock Lumps on core broken by heavy hammer blows

R5 100 to 250 Very strong rock Lumps only chip with heavy hammer blows. Dull ringing sound

R6 > 250 Extremely strong rock Rocks ring on hammer blows. Sparks y

Table 1

Simple eld identication compressive strength of rock (From ISRM, 1981)

Fig. 8: Epiclastic breccias and lavas of Balsamo formation, A are volcanic epiclastic breccias of the intermediate and distal zone,photo B represents jointed lava flow of the proximal zone (photos Lexa, J,, 2010.).

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usually obtained. Standard penetration tests(SPT) and Modified Proctor are the tests thatnormally are done. The triaxial and shear boxtests, for saturated soils, using the ASTM normsare done also. For the rock mechanics drill-hole

bores, geophysics and compressibility strengthare done sometimes, depending on the impor-tance of the project.

Tierra Blanca Joven (TBJ) is the most impor-tant volcanic tuff and is present in important sec-

tors of the MASS (as is shown in Figure 6). TBJis quite sensitive to erosion, water content change,natural or artificial vibrations and compressibil-ity (Rolo et al. 2004; Hernandez, 2004; ebesta,2007b; ebesta & Chavez, 2010). This materialis also quite sensitive for slope stability problems(Figure 2 and 13).

Fig. 9: Weathered lavas of Balsamo formation (photo Lexa,J,, 2010).

Guzmn & Melara (1996); Amaya &Hayem (2000); Rolo et al. (2004); Hernndez(2004); Molina et al. (2009) and Avalos &Castro (2010) studied the geotechnical and lith-

ological aspects of the younger Ilopango tuffsTierra Blanca Joven (TBJ). All of the authorscharacterized TBJ as silty sands and sandy silts.

At the present, the soil mechanics and the-ory for saturated soils is used in El Salvador.There are two distinct seasons throughout theyear: summer and rainy season, and usually thegroundwater level is deep, (35 m in urban areasaccording to Rolo et al., 2004), remaining al-most the same all the year; this means that mostsoils in the country are unsaturated and thereare capillary forces that act on the soil structuremaking that an apparent cohesion (suction)improve the strength of the soil; this situationmakes that the slopes are almost vertical andtemporally stable, but will collapse when wet-ted or dried.

For the present publication, the values ofcohesion and friction angle of different au-thors were compiled and compared, (figure 14)showing scatter in the data. The authors thatmade the tests concluded that the strength ofundisturbed or disturbed samples with naturalmoisture decreases, if theyre saturated. In spitethat most of the tests were made with the shear

box, the results show disparity; since for un-saturated soils the combination of total stress ,

pore water pressure uw, pore air pressure ua anddegree of saturation are needed. For unsaturat-ed soils characterizing (Fredlund & Rahardjo,1993), the use of two independent stress vari-ables: net stress -u

aand suction u

a-u

ware

needed. Murray & Sivakumar (2010) say thatthe most recent stage in the understanding ofunsaturated soils is the analysis of the behav-ior in terms of constitutive relations linkingthe volume change, shear stress and defor-mation due to shear stress in elasto-plasticmodels. Nowadays considerable progress has

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been achieved through this approach, becauseits possible to explain some of the characteris-tics of soil behavior.

This brings out the necessity of research-ing about the behavior and constitutive model-ing using the critical state and unsaturated soilmechanics, to avoid the dispersity of the results

and have consistency into a more coherentframework. Rolo et al (2004, table 3) comparedsuction values from TBJ samples and concluded

that the negative pore pressure, weak interpar-ticle bonding or cementation have a contribu-tion to the shear strength, creating an apparentcohesion that is lost when is saturated or duringseismic events. Guzman & Melara (1996) andHernandez (2004) conclude the same.

There is almost no geotechnical informa-

tion available for the other geological layersbecause its not usual to identify the name ofthe layer that is tested during a project.

Fig. 10. Almost complete stratigraphy of the San Salvador formation; photo for reference was taken about 3 kms north of thestudied area during the beginning of engineering geology mapping.

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Fig. 11: San Salvador formation layers in the area of study.

Fig. 12: Sector of Tierra Blanca Joven (TBJ) with unknownthickness.

CONCLUSIONS

There is a need to simplify the geologicalmaps in order to help the interpretation for the de-velopers and urban-planners. The experience hasshown the necessity to introduce in each project a

proper geological risk assessment to avoid futureproblems; taking into account the mass move-

ments, seismic effects, floods, groundwater situ-ation, geological material properties and erosion

principally.The engineering geological mapping can serve

as first or final steps in planning an infrastructureand can have an emphasis according to the con-tent or interests to evaluate. In the rocks and soils(Anon, 1972) the description of color, grain size,texture, structure, fracture state, weathered state,strength properties and permeability can indicatethe properties and characteristics of them.

The presence of layers of laterites andpaleosoils are very problematic in the sector

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Fig. 13: Tierra Blanca Joven (TBJ) problems, on the left erosion problems; and in the right liquefaction of TBJ after vibrations inthe back of pick up vehicle.

Fig. 14: Cohesion (kPa) and friction angle (0) obtained using shear box and triaxial test (UU) of Tierra Blanca Joven surface layersby different authors and compiled for this publication.

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but is important to note that in areas where thereis significant presence and thickness of TierraBlanca Joven (TBJ) is necessary to be carefulwith the loss of apparent cohesion, experienced

when the suction or cementing decrease or whenan earthquake affects the area (Rolo et al, 2004).Also the mass wasting processes acts intensivelyin the surface and above the ground of TBJ,the anthropic disturbance (for example broken

pipes and urban growth) increase the problems.The volume changes (collapse or swelling)and the dynamic properties in the geologicallayers can cause damage to the structures; thissituation makes imperative the introduction ofthe unsaturated soils mechanics in the design ofthe projects that will reduce the problems that theMASS is experimenting at the present.

It is recommended to continue systematicengineering geological mapping and data col-lection. The scientific research, probing, fieldand lab testing will improve the knowledgeof the risk and properties of all the geologicallayers. The built of a public archive like theGeofond (http://www.geofond.cz/en/home-

page) in the Czech Republic; with geotechnicaland geological hazard data; which will be opento all (designers, engineering geologists, geo-technicians, state and town authorities, agricul-tural engineers, environmentalist etc.) wouldhelp improving the knowledge applied duringthe conception of the projects.

ACKNOWLEDGEMENTS

The work has been carried out in the

framework of cooperation between theCzech Geological Survey and the Oficina dePlanificacin del rea Metropolitana de SanSalvador (OPAMSS) that was a part of the Projectof the Czech Republic Foreign DevelopmentProgramme under auspices of the Czech Ministryof Environment RP/6/2007. Authors acknowledgesupport of the Czech Geological Survey, thePlanning Office of the Metropolitan Area of SanSalvador de San Salvador (OPAMSS) and theGeological Survey of the Environmental and

Natural Resources Ministry of El Salvador. We

are grateful to the reviewers and editors whoseremarks improved the quality of the paper.

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