U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006-5207

Evaluation of Baseline Ground-Water Conditions in

the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins,

Republic of Cape Verde, West Africa, 2005-06

Evaluation of Baseline Ground-Water Conditions in

the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins,

Republic of Cape Verde, West Africa, 2005-06

Prepared in cooperation with the

MILLENNIUM CHALLENGE CORPORATION, MILLENNIUM CHALLENGE ACCOUNT, AND 

INSTITUTO NACIONAL DE GESTÃO DOS RECURSOS HÍDRICOS
Cover photos: 

Background—Rainwater catchment structure in the Ribeira Fajã Basin, São Nicolau.

Foreground (left)—Gathering water from a well in the Mosteiros Basin, Fogo.

Foreground (right)—USGS and INGRH technicians making a discharge measurement at a 

spring in Ribeira Paul Basin, Santo Antão. 

(Photographs by Victor Heilweil.)
Evaluation of Baseline Ground-Water 

Conditions in the Mosteiros, Ribeira Paul, 

and Ribeira Fajã Basins, Republic of Cape 

Verde, West Africa, 2005-06

By Victor M. Heilweil, John D. Earle, Jay R. Cederberg, Mickey M. Messer, 

Brent E. Jorgensen, and Ingrid M. Verstraeten, U.S. Geological Survey

Miguel A. Moura, Arrigo Querido, Fernanda Spencer, and Tatiana Osório, 

Instituto Nacional de Gestão dos Recursos Hídricos

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the 

MILLENNIUM CHALLENGE CORPORATION, MILLENNIUM CHALLENGE ACCOUNT, 

AND INSTITUTO NACIONAL DE GESTÃO DOS RECURSOS HÍDRICOS

 

Scientific Investigations Report 2006-5207
U.S. Department of the Interior

Dirk Kempthorne, Secretary 

U.S. Geological Survey

P. Patrick Leahy, Acting Director

Reston, Virginia: 2006

For additional information write to:

U.S. Geological Survey

Director, USGS International Water Resources Branch

MS 420, National Center

Reston, Virginia 20192

http://international.usgs.gov/disciplines/water.htm

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

 Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Scientific Investigations Report 2006-5207
iii

Contents

Abstract............................................................................................................................................................ 1

Introduction...................................................................................................................................................... 2

Purpose and Scope............................................................................................................................. 2

Acknowledgments .............................................................................................................................. 2

General Conceptual Model of Volcanic Island Hydrogeology ................................................................ 2

Methods............................................................................................................................................................ 4

Ground-Water Level Data.................................................................................................................. 4

Electric-Tape Measurements ..................................................................................................... 5

Pressure-Transducer Measurements ....................................................................................... 5

Water-Quality Data ............................................................................................................................. 5

Field Parameters........................................................................................................................... 5

Environmental Tracer Analyses.................................................................................................. 5

Spring and Stream Discharge Data ................................................................................................. 9

Hydrogeologic Setting ................................................................................................................................... 9

Mosteiros Basin, Fogo........................................................................................................................ 9

Ribeira Paul Basin, Santo Antão..................................................................................................... 12

Ribeira Fajã Basin, São Nicolau ..................................................................................................... 15

Baseline Ground-Water Conditions........................................................................................................... 17

Mosteiros............................................................................................................................................ 17

Ground-Water Budget................................................................................................................ 17

Ground-Water Levels ................................................................................................................. 21

Water Quality and Environmental Tracers.............................................................................. 22

Ribeira Paul ........................................................................................................................................ 25

Ground-Water Discharge .......................................................................................................... 25

Ground-Water Levels ................................................................................................................. 31

Water Quality and Environmental Tracers.............................................................................. 32

Ribeira Fajã......................................................................................................................................... 33

Ground-Water Discharge .......................................................................................................... 33

Ground-Water Levels ................................................................................................................. 34

Water Quality and Environmental Tracers.............................................................................. 35

Measured Ground-Water Discharge as a Percentage of Precipitation.............................................. 37

Challenges in Evaluating Ground-Water Conditions in the Three Basins........................................... 38

Summary and Conclusions.......................................................................................................................... 38

References Cited........................................................................................................................................... 40
iv

Figures

Figure 1. Location of the study basins within the Republic of Cape Verde, West Africa ...................... 3

Figure 2. Generalized conceptual model of volcanic island hydrogeology ............................................. 4

Figure 3. Relation between land-surface altitude and water temperature for springs in 

Cape Verde ......................................................................................................................................... 7

Figure 4. Estimated initial and decay-corrected tritium concentration of precipitation in Cape 

Verde, 1950-2005................................................................................................................................ 9

Figure 5. Location of wells, springs, and precipitation stations in the Mosteiros Basin, Fogo, 

Cape Verde ....................................................................................................................................... 10

Figure 6. Geology of the Mosteiros Basin, Fogo, Cape Verde.................................................................. 11

Figure 7. Location of wells, springs, and precipitation stations in the Ribeira Paul Basin, 

Santo Antão, Cape Verde............................................................................................................... 13

Figure 8. Geology of the Ribeira Paul Basin, Santo Antão, Cape Verde................................................. 14

Figure 9. Location of wells, springs, and precipitation stations in the Ribeira Fajã Basin, São 

Nicolau, Cape Verde ....................................................................................................................... 15

Figure 10. Geology of the Ribeira Fajã Basin, São Nicolau, Cape Verde.................................................. 16

Figure 11. Historical precipitation at Posto Cachaço, Ribeira Fajã Basin, 1961-2005. ........................... 17

Figure 12. Total annual well withdrawal and the portion of Monte Vermelho spring discharge 

used for public supply in Mosteiros Basin, 1998-2005 .............................................................. 17

Figure 13. Water level in well FF07, Mosteiros Basin, December 2005 through March 2006 ............... 21

Figure 14. Depth profile of specific-conductance values at well FF07, Mosteiros Basin, 

December 2005 ................................................................................................................................ 22

Figure 15. Relation between δ18O and δ2H values in precipitation and ground water from the 

Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde............................................... 23

Figure 16. Temperature lapse band and the range of possible dissolved-gas ground-water 

recharge temperatures for sites in Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, 

Cape Verde ....................................................................................................................................... 24

Figure 17. 3Helium:4Helium ratios relative to the helium isotopic ratio of air and total 4Helium 

concentration in ground water from the Mosteiros, Ribeira Paul, and Ribeira Fajã 

Basins, Cape Verde......................................................................................................................... 25

Figure 18. Relation between the 3Helium:4Helium ratio and fraction of 4Helium derived from 

atmospheric gases in ground water from the Mosteiros, Ribeira Paul, and Ribeira 

Fajã Basins, Cape Verde ................................................................................................................ 30

Figure 19. Water level in well FA93, Ribeira Paul Basin, August 2005 through June 2006. ................... 31

Figure 20. Galleria Fajã discharge and annual precipitation at nearby Posto Cachaço, Ribeira 

Fajã Basin, 1984-2005...................................................................................................................... 34

Figure 21. Water level in well FN41, Ribeira Fajã Basin, 1980-86............................................................... 35

Figure 22. Water level in well FN21, Ribeira Fajã Basin, 1984-87............................................................... 36

Figure 23. Water level in well FN39, Ribeira Fajã Basin, December 2005 through March 2006. .......... 36
Tables

Table 1. Area, slope, and average and maximum altitude of the Mosteiros, Ribeira Paul, and 

Ribeira Fajã Basins, Cape Verde................................................................................................... 10

Table 2. Annual precipitation at selected stations in the Mosteiros, Ribeira Paul, and Ribeira 

Fajã Basins, Cape Verde .............................................................................................................. 12

Table 3. Records of selected springs and streams in the Mosteiros, Ribeira Paul, and Ribeira 

Fajã Basins, Cape Verde ................................................................................................................ 18

Table 4. Records of selected wells in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, 

Cape Verde ....................................................................................................................................... 20

Table 5. Selected field measurements, stable isotopes, dissolved-gas concentrations, and 

estimated recharge temperatures of samples collected from the Mosteiros, Ribeira 

Paul, and Ribeira Fajã Basins, Cape Verde................................................................................. 26

Table 6. Tritium and helium concentration and estimated age of samples collected from the 

Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde............................................... 29
vi

Conversion Factors, Datums, and Abbreviated 

Water-Quality Units

Multiply By To obtain

Length

millimeter (mm) 0.0394 inch (in.)

meter (m) 3.2808 foot (ft)

kilometer (km) 0.6215 mile (mi)

Area

square kilometer (km2) 247.0966 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

milliliter (ml) 0.0338 ounce, fluid (fl. oz)

liter (L) 2.1133 pint (pt)

liter (L) 1.0566 quart (qt)

liter (L) 0.2642 gallon (gal)

cubic meter (m3) 264.201 gallon (gal)

cubic centimeter (cm3) 0.0610 cubic inch (in3)

cubic meter (m3) 35.3107 cubic foot (ft3)

cubic meter (m3) 1.3079 cubic yard (yd3)

Flow rate

cubic meter per day (m3/d) 35.3107 cubic foot per day (ft3/d)

liter per minute (L/m) 0.2642 gallon per minute (gal/min)

Density

cubic centimeter per gram (cm3/g) 0.0160 cubic foot per pound (ft3/lb)

Hydraulic conductivity

meter per day (m/d) 3.2808 foot per day (ft/d)

Hydraulic gradient

meter per kilometer (m/km) 5.2798 foot per mile (ft/mi)

Transmissivity1

meter squared per day (m2/d) 10.7643 foot squared per day (ft2/d)

1

Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.
vii

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32) / 1.8.

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Vertical 

coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Altitude, as 

used in this report, refers to distance above the vertical datum. 

Specific conductance is reported in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C). 

Concentration of chemical constituents in water is reported in milligrams per liter (mg/L). Concentration of 

dissolved gases is reported in cubic centimeters at standard temperature and pressure per gram (cm3 STP/g). 

Tritium concentration is reported in tritium units (TU).
viii
Abstract

This report documents current (2005-06) baseline 

ground-water conditions in three basins within the West 

African Republic of Cape Verde (Mosteiros on Fogo, Ribeira 

Paul on Santo Antão, and Ribeira Fajã on São Nicolau) based 

on existing data and additional data collected during this study. 

Ground-water conditions (indicators) include ground-water 

levels, ground-water recharge altitude, ground-water discharge 

amounts, ground-water age (residence time), and ground-water 

quality. These indicators are needed to evaluate (1) long-term 

changes in ground-water resources or water quality caused by 

planned ground-water development associated with agricultural projects in these basins, and (2) the feasibility of artificial recharge as a mitigation strategy to offset the potentially 

declining water levels associated with increased ground-water 

development.

Ground-water levels in all three basins vary from less 

than a few meters to more than 170 meters (m) below land 

surface. Continuous recorder and electric tape measurements 

at three monitoring wells (one per basin) showed variations 

between August 2005 and June 2006 of as much as 1.8 m. Few 

historical water-level data were available for the Mosteiros or 

Ribeira Paul Basins. Historical records from Ribeira Fajã indicate very large ground-water declines during the 1980s and 

early 1990s, associated with dewatering of the Galleria Fajã 

tunnel. More-recent data indicate that ground-water levels in 

Ribeira Fajã have reached a new equilibrium, remaining fairly 

constant since the late 1990s.

Because of the scarcity of observation wells within each 

basin, water-level data were combined with other techniques 

to evaluate ground-water conditions. These techniques include 

the quantification of ground-water discharge (well withdrawals, spring discharge, seepage to springs, and gallery 

drainage), field water-quality measurements, and the use of 

environmental tracers to evaluate sources of aquifer recharge, 

flow paths, and ground-water residence times. 

In the Mosteiros Basin, measured well and spring discharge is about 220,000 cubic meters per year (m3

/yr). For the 

Ribeira Paul Basin, measured well discharge, spring discharge, 

and ground-water seepage to springs is about 1,600,000 m3/yr. 

Ribeira Fajã Basin is the driest of the three basins with a precipitation rate of about half that of the other two basins. The 

only measurable ground-water discharge from this basin is 

from Galleria Fajã, estimated to be about 150,000 m3/yr. Measured discharge for all three basins does not include submarine 

outflow or agricultural/phreatophyte consumptive use (Paul 

Basin, only) and is assumed to be less than total ground-water 

discharge. 

Ground-water ages indicate that recharge to wells and 

springs occurred from more than 50 years ago at some locations to within the past decade at other sites. Ground water 

in Paul is younger than that in the other two basins, indicating that recharge generally occurred within the past 50 years. 

Ground water at all the dateable sites using tritium/helium in 

both the Mosteiros and Ribeira Fajã Basins show that recharge 

occurred more than 50 years before the sampling dates. 

Ground-water tritium/helium age dating was not possible at 

some sites in Mosteiros and Ribeira Fajã Basins because of the 

presence of helium in the aquifer derived from the mantle or 

aquifer matrix. However, this helium was useful for accurate 

age dating of the unaffected ground-water sites. 

Dissolved gases indicate that most ground-water recharge 

occurs at mid and high altitudes within all three basins; calculated recharge altitudes ranged from 700 to more than 2,000 

m. In the Mosteiros and Ribeira Fajã Basins, recharge altitudes 

are much higher than the wells and springs. This suggests that 

it may take many years for artificial recharge to result in a 

beneficial impact on the aquifer in areas where the agricultural 

projects are implemented. Recharge altitudes in Paul Basin 

also were generally higher than their respective ground-water 

discharge sampling sites. Generally younger ground-water 

ages in Paul, however, indicate the existence of some short 

flow paths where artificial recharge may possibly enhance 

available water resources within a few years.

The salinity of wells and springs is generally low in the 

Ribeira Paul and Ribeira Fajã Basins, but somewhat higher 

in Mosteiros Basin. Specific-conductance measurements of 

wells and springs in Ribeira Paul and Ribeira Fajã ranged from 

about 200 to 700 microsiemens per centimeter at 25 degrees 

Celsius (μS/cm). Although the Monte Vermelho spring in 

Evaluation of Baseline Ground-Water Conditions in the 

Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Republic 

of Cape Verde, West Africa, 2005-06

By Victor M. Heilweil, John D. Earle, Jay R. Cederberg, Mickey M. Messer, Brent E. Jorgensen, and Ingrid M. 

Verstraeten, U.S. Geological Survey; Miguel A. Moura, Arrigo Querido, Fernanda Spencer, and Tatiana Osório, 

Instituto Nacional de Gestão dos Recursos Hídricos
   Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Mosteiros Basin also has very low salinity (200 μS/cm), 

water from the wells along the coastal plain has specific-conductance measurements of as much as 16,000 μS/cm. These 

higher values indicate some brackish water intrusion. Additional ground-water development of the Mosteiros coastal 

plain may exacerbate this situation.

Introduction

The Republic of Cape Verde is an island archipelago 

located about 750 km off the west coast of Africa (fig. 1). Its 

400,000 inhabitants live on 9 separate small islands with a 

combined area of 4,000 km2, resulting in one of the highest 

population densities among the West African nations (Langworthy and Finan, 1997). The majority of the population 

resides in rural areas and derives its livelihood from rain-fed 

agriculture. Farming is challenging in the Sahelian climate, 

dominated by the cool, dry northeast trade winds. Total annual 

precipitation ranges from 0 to 50 mm along the populated 

coastal areas to 400-800 mm in the highlands and is extremely 

variable from year to year (Haagsma, 1995). About 80 percent 

of this precipitation is associated with the equatorial monsoon 

(August-October; Vailleux and Bourguet, 1974) and comes 

during only a few days each year. The majority of the precipitation is lost to evapotranspiration and runoff to the sea. The 

evaporation and runoff losses are caused by a combination 

of warm climate, thin soil cover, steep topographic gradients, 

and short distances to the sea. Surface-water resources on the 

islands are virtually non-existent; there are no lakes and only 

a few small perennial streams. Nearly all of the water for consumption comes either from rainfall catchment structures or 

ground water. Because of the lack of rainfall and sparse water 

resources, Cape Verde currently imports about 70 percent of 

its food (Instituto Nacional de Gestão dos Recursos Hídricos, 

written commun., 2005). 

To decrease this dependence on imported food, agricultural development projects funded by the U.S. Millennium 

Challenge Corporation (MCC) are currently underway in three 

basins on three islands: Mosteiros on Fogo, Ribeira Paul on 

Santo Antão, and Ribeira Fajã on São Nicolau. These projects 

are dependent on enhanced availability of water resources 

through additional rainfall catchment infrastructure and/or 

ground-water development. Artificial recharge of ground 

water is being planned as a mitigation strategy to offset the 

potential of declining aquifer water levels and/or deteriorating 

water quality associated with increased use. An important metric for the success of the agricultural development projects is 

the identification of current ground-water baseline conditions 

in order to evaluate any long-term changes in ground-water 

resources or water quality. 

The U.S. Geological Survey (USGS) received support 

from the MCC to provide training in ground-water data collection and to assess the current (2005-06) baseline ground-water 

conditions in the three project basins. Ground-water conditions 

include ground-water levels, ground-water recharge altitudes 

and sources, ground-water discharge quantities, ground-water 

age (residence time), and ground-water quality. These tasks 

were accomplished, in coordination with the Instituto Nacional 

de Gestão dos Recursos Hídricos (INGRH), during and after 

two technical missions to Cape Verde.

Purpose and Scope

The purpose of this report is to summarize and interpret 

the ground-water data collected during the project. In addition 

to this report, ground-water monitoring equipment, training 

classes and manuals, and field demonstrations were provided 

in all three study basins. Because of the scarcity of observation wells within each basin and the inability to drill additional 

wells within the scope of this project, sparse water-level data 

were combined with other techniques to evaluate ground-water 

conditions. These other methods include (1) the use of pressure transducers and historical ground-water levels to evaluate 

temporal changes in potentiometric surfaces in each basin, (2) 

the measurement of well withdrawals, spring discharge, and 

gallery drainage in order to estimate ground-water discharge 

budgets for each basin, and (3) the use of environmental tracers to evaluate sources of aquifer recharge, flow paths, and 

ground-water residence times.

Acknowledgments

The authors thank Sra. Elsa Samoes, Millennium Challenge Account Cape Verde, for providing historical precipitation data and artificial recharge development plans; Sra. Maria 

Modesto, Ministereo de Infrastructuras, Transportes, e Mar, 

Cape Verde, for providing GIS topographic coverages; Tony 

Mulhern and Kayo Shiraishi, U.S. Peace Corps volunteers in 

Santo Antão, for collecting precipitation samples and providing translating assistance during the USGS missions; D. Kip 

Solomon and Alan Rigby, University of Utah Dissolved-Gas 

Service Center, for their laboratory analysis and interpretive 

assistance with dissolved-gas samples; Keith Halford and 

Edwin Weeks, USGS, for their assistance with interpreting the 

Galleria Fajã aquifer drawdown data; Patrick Tucci, Richard 

Healy, and Steven Gingerich, USGS, for technical support; 

and Lourdes Lima, Instituto Nacional de Gestão dos Recursos 

Hídricos, Cape Verde, for overseeing in-country logistical support and technical assistance

General Conceptual Model of Volcanic 

Island Hydrogeology

Cape Verde consists of a chain of volcanic islands 

believed to have been formed over an oceanic hot spot (Lodge 

and Helffrich, 2006). The islands are predominantly basaltic 

layered lava flows interspersed with pyroclastics and breccias 
Figure 1. Location of the study basins within the Republic of Cape Verde, West Africa.

San Tiago

Fogo

Boa Vista

 Sal

Santo Antão

Maio

São Nicolau

São Vicente

Brava

Santa Luzia

Raso

Branco

 Ilheus Seco

Mosteiros

Basin

Ribeira Fajã

Basin

Ribeira Paul

Basin

Mosteiros

Basin

Ribeira Fajã

Basin

Ribeira Paul

Basin

Praia

EXPLANATION

15°

16°

17°

25° 24° 23°

Study basin

0 40 80 KILOMETERS

0 20 40 MILES

Base from Environmental Systems Research Institute, Inc. 

Digital Chart of the World, 1993, 1:1,000,000

Cabo Verde National Grid Projection

Coastal

West Africa

Mali

Mauritania

Guinea

Senegal

Liberia

Sierra Leone

Guinea-Bissau

The Gambia

Cape Verde

Islands

Dakar

Bissau

Conakry

Monrovia

Freetown

Nouakchott

Saint Louis

Coastal

West Africa

Mali

Mauritania

Guinea

Senegal

Liberia

Sierra Leone

Guinea-Bissau

The Gambia

Cape Verde

Islands

Dakar

Bissau

Conakry

Monrovia

Freetown

Nouakchott

Saint Louis

Banjul

General Conceptual Model of Volcanic Island Hydrogeology 
 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

and intruded by near-vertical mafic dikes (Barmen and others, 

1990). Volcanic islands, especially those recently formed like 

Cape Verde, typically have high topographic relief, shallow 

soils, and rapid runoff. Rainfall distribution and infiltration is 

dependent upon basin altitude and morphology, orientation 

with respect to the tropical trade winds, strength of monsoonal 

patterns, and the thickness and type of soil cover. As is common on most islands with much topographic relief, the islands 

of Cape Verde typically have wet and dry sides that result from 

wind and precipitation patterns. In general, however, most 

infiltration occurs in the higher-altitude parts of the islands, 

where precipitation rates are highest. Part of this moisture is 

subsequently lost as either evapotranspiration or lateral flow 

to perched springs. The component of infiltration that continues downward to recharge the aquifer is herein referred to as 

net infiltration. Water-table altitude and slope of this aquifer 

would generally be dependent on both aquifer permeability 

and net infiltration (recharge) rate. Hydraulic conductivity of 

basalt is reported to range from about 0.1 to 1,000 m/d (Freeze 

and Cherry, 1979). Studies from Hawaii indicate that highly 

transmissive volcanic island basalts generally have hydraulic-conductivity values of about 100 to 1,000 m/d (Gingerich 

and Voss, 2002), whereas low-transmissivity basalts generally 

have hydraulic-conductivities of about 0.1 to 10 m/d (Izuka 

and Gingerich, 2003). Watersheds with high recharge rates 

and/or low permeability basalt aquifers would have steeper 

water-table slopes and higher-altitude water tables; watersheds with low net-infiltration rates and/or high permeability 

basalt aquifers would have lesser slopes, with water tables 

only a few meters above sea level. Relatively high horizontal 

hydraulic-conductivity values (hundreds of meters per day) are 

often reported for basaltic volcanic islands (Prada and others, 

2005) and result in a regional (basal) water table within tens 

of meters of sea level. Low-permeability intrusive mafic dikes, 

where present, are thought to act as horizontal flow barriers. 

Similarly, low-permeability breccias, tuffs, and pyroclastics 

may act as either horizontal or vertical flow barriers. Horizontal and vertical flow barriers may cause perched ground water, 

high-altitude springs, and/or ground-water compartmentalization resulting in a stepped (rather than smooth) overall water 

table (fig. 2)

Measurable ground-water discharge from volcanic island 

watersheds typically includes discharge to springs, well withdrawals, seepage to streams, and evapotranspiration in areas 

of shallow water table. Also, in Cape Verde and some other 

volcanic islands, horizontal tunnels have often been constructed to drain ground water as an additional water-supply 

source. Ground-water discharge, however, is often dominated 

by submarine outflow, which is difficult to quantify. Submarine discharge has been documented in other volcanic island 

sites (Izuka and Gingerich, 2003; Prada and others, 2005) and 

quantified in other settings (Bokuniewicz, 1980; Cable, 1997; 

Cambareri and Eichner, 1998; Michael and others, 2003). 

Small islands typically have a freshwater lens of ground 

water (lower density) floating on top of higher-density brackish and salt water. The fresh water/brackish water boundary 

is not fixed or sharp; rather it is controlled by pressure and 

density and dependent upon the balance between ground-water 

recharge and ground-water discharge. Aquifer drawdown 

associated with excessive pumping, therefore, may lower 

water levels in the aquifer and move the fresh/brackish water 

boundary inland, causing salt-water intrusion.

Methods

Ground-Water Level Data

In order to evaluate ground-water conditions and provide 

a baseline for the future assessment of changes in groundwater storage in the three study basins, ground-water levels 

were measured periodically with electric tapes. In addition, pressure transducers and data loggers were installed in 

selected observation wells to provide a continuous record of 

water-level changes. 

Figure 2. Generalized conceptual model of volcanic island hydrogeology. EXPLANATION

Fractured basalt

Unconsolidated materials

Direction of freshwater movement

Brackish waterBrackish water

Submarine

outflow

Galleria Tunnel

Well

Brackish water

Atlantic

Ocean

Atlantic

Ocean

Freshwater

Saltwater

Saltwater

NOT TO SCALE Modified from Macdonald and others (1983)

Water table

Perched

water body

Dike-compartment

water table
Electric-Tape Measurements

Manual water-level measurements in production and 

observation wells were made by the INGRH and the USGS 

with calibrated electric tapes in all three project basins. The 

accuracy of this method is about 0.01 m, which is of sufficient 

precision to provide a baseline for monitoring both seasonal 

and longer-term water-level changes. Three lengths of electric 

water-level tapes (60-, 100- and 250-m) were used, depending 

on the depth to water at each well. Water-level measurements 

were corrected by subtracting the distance from the measuring point above land surface. Land-surface altitudes were 

estimated with hand-held GPS, topographic maps, or from 

drillers’ logs. Although the accuracy of these altitudes is only 

about plus or minus 10 m, this study was concerned with relative depth below land surface and temporal changes in groundwater levels, rather than absolute altitude.

Pressure-Transducer Measurements

Two types of pressure transducers, vented and nonvented, and data loggers were installed in observation wells 

to continuously monitor water levels. A vented transducer 

was installed in observation well FA93 in the Ribeira Paul 

Basin. These transducers are vented to the atmosphere and 

automatically compensate for changes in atmospheric pressure. They do not require barometric pressure corrections, but 

have a fixed-length cable that cannot be modified. Non-vented 

transducers were installed in observation wells FF07 in the 

Mosteiros Basin and FN39 in the Ribeira Fajã Basin. The nonvented transducers require independently measured barometric 

pressure data in order to correct for changes in atmospheric 

pressure; thus loggers also were installed in FF07 and FN39 to 

record barometric pressure. The non-vented transducers hang 

on a small-diameter steel cable rather than requiring a fixedlength cable and, therefore, are better suited for wells with 

large depths to water and/or where sharp objects in the well 

may damage the more-delicate vented cable. All the transducers were programmed to record water-pressure and barometric 

pressure or water-level data every 6 hours, which was later 

corrected for instrument drift based on independent electrictape water-level measurements (Freeman and others, 2004).

Because of both ocean and earth tides and the relatively 

small daily absolute water-level change, tidal effects were 

corrected in two of the four transducer wells. The tidal corrections were done with a digital Butterworth filter program 

(Umari, M.J., and Earle, J.D., U.S. Geological Survey, written 

commun., 2002) using a low pass cutoff frequency of 0.8 

cycles per day. This is based on frequencies of tidal harmonic 

components ranging from 0.85 to 2 cycles per day (Merritt, 

2004).

Water-Quality Data

Field parameters were measured and laboratory analysis 

of environmental tracers was conducted on water samples 

collected from springs, wells, streams, and tunnels in the three 

study basins. This information is used for evaluating sources 

of recharge, ground-water residence times, ground-water flow 

paths, and salt-water intrusion. It will also provide a baseline 

geochemical data set for potential evaluation of future changes 

to the ground-water systems.

Field Parameters

Field water-quality parameters were measured at groundwater and surface-water sites according to USGS procedures 

(Wilde and Radtke, 1998). Parameters include specific conductance, pH, temperature, dissolved oxygen, barometric and 

total dissolved-gas pressure. These parameters were measured 

with a multimeter probe, which was calibrated daily to a representative set of standards.

Specific conductance was used as a proxy for dissolvedsolids concentration because geochemical laboratory analysis 

of dissolved solids was not within the scope of this study. 

Dissolved-solids concentration is generally about 55 to 75 

percent of the specific-conductance value, depending on the 

chemical makeup of the water (Hem, 1992). The pH of a water 

sample is often indicative of the movement of ground water 

through differing geologic layers, such as volcanic or alluvial deposits. Low pH values may indicate geothermal water 

sources. Water temperature also is useful for evaluating the 

influence of upwelling geothermal waters and is needed for 

the dissolved-gas solubility calculations used in determining 

recharge temperatures and for tritium/helium age dating. Dissolved oxygen is an indicator of oxidation conditions within 

an aquifer, including both anaerobic and aerobic bacterial 

activity associated with the decomposition of organic matter. 

Barometric and total dissolved-gas pressures are used to guide 

appropriate dissolved-gas sampling methods and are needed 

for the determination of absolute dissolved-gas concentrations 

from in-situ diffusion samplers. Total dissolved gas pressures 

can also be used to evaluate hydrologic processes such as 

denitrification, anaerobic decay and gas production (CH4, H2, 

CO2), degassing of deep geothermal fluids (CO2 production), 

and air entrapment in the recharge zone.

Environmental Tracer Analyses

Ground-water samples were analyzed for the stable 

(deuterium and oxygen-18) isotopes of the water molecule. 

Samples for deuterium (2H) and oxygen-18 (18O) were collected from open boreholes by using polyvinyl chloride 1,000-

ml sample bailers. Water samples from production wells were 

collected by using existing submersible pumps. Water samples 

from springs and tunnels were collected by gravity flow. 

Stable-isotope samples were collected according to procedures 

Methods   
described by Coplen and others (1996), stored in clean polyethylene bottles, and analyzed by the USGS Stable Isotope 

Laboratory in Reston, Virginia. The stable isotopes (2H, 18O) 

are used to evaluate the relative altitude of source precipitation 

that recharges a ground-water system. These stable isotopes 

are analyzed by measuring the ratio of the heavier, less-abundant isotope to the lighter, more-abundant isotope and are 

reported as differences to a known standard. The isotope ratios 

are reported as delta (δ) values expressed as parts per thousand 

(permil). The δ value for an isotope ratio, R, is determined by:

δR = (Rsample/ Rstandard –1)x1000	 (1)

where:

δR is the δ value for a specific isotope in the sample 

(

2

H or 18O), 

	 Rsample is the ratio of the rare isotope to the common isotope for a specific element in the sample, and

	Rstandard is the ratio of the rare isotope to the common isotope for the same element in the standard reference 

material. 

A positive δR value indicates that the sample is enriched in 

the heavier isotope with respect to the sample. A negative 

δR value indicates that the sample is depleted in the heavier 

isotope with respect to the sample. In general, precipitation 

falling at a higher altitude is more isotopically depleted in 

2

H and 18O than precipitation at lower altitudes. Therefore, 

these stable isotopes are generally helpful for evaluating the 

precipitation source areas of recharge to an aquifer (Mazor, 

1991). The relation between δ2H and δ18O values generally 

plots along a trend line known as the global meteoric water 

line. This trend is described by the equation:

δ2H = 8 (δ18O)+ d (2)

where:

	 d is defined as the 2H excess (Dansgaard, 1964). The 

mean global value for d in freshwater is 10 (Craig, 

1961).

Dissolved-gas samples were collected to evaluate 

ground-water recharge temperature (temperature of water 

entering the water table). In contrast to stable isotopes, these 

tracers are useful for evaluating the altitude where recharge 

actually enters the aquifer rather than the altitude where the 

precipitation occurs. A large fraction of gases dissolved in 

ground water originate in the atmosphere and their solubility in water is temperature-dependent (Stute and Schlosser, 

2001). As recharging water enters an aquifer, these dissolvedgas concentrations are “locked in” based on the temperature, 

barometric pressure, amount of excess air, and salinity at the 

water table (Aeschbach-Hertig and others, 1999; Ballentine 

and Hall, 1999; Stute, 2001). Because the gases are relatively inert, their concentrations generally remain unchanged 

along ground-water flow paths to points of discharge (wells 

and springs). Dissolved-gas concentrations in discharging 

ground water, therefore, can be used to estimate ground-water 

recharge temperatures. For this study, dissolved concentrations 

of N2, 40Ar, 84Kr, 20Ne, and 129Xe were used in the closed system equilibration model (Aeschbach-Hertig and others, 2000; 

Kipfer and others, 2002) to calculate estimated ground-water 

recharge temperature, pressure, excess air, and a fractionation 

factor (related to the partial dissolution of trapped air bubbles). 

Because there are five known parameters (the individual 

dissolved-gas concentrations) and four unknowns, this is an 

over-determined problem that can be solved (optimized) with 

a system of linear equations. 

Dissolved-gas samples were collected as water samples 

in copper tubes and as gas samples with in-situ diffusion samplers. The copper-tube collection method for dissolved gases 

is described by Stute and Schlosser (2001). The diffusion 

sampler method is described by Sheldon (2002). The diffusion sampler consists of a small copper chamber (about 0.82 

cm3) with gas-permeable membranes. Samplers were installed 

in wells and springs and allowed to equilibrate with the water 

for 24 hours. The samples were quickly removed and coldwelded by using a pinch-off tool. Dissolved-gas concentrations were analyzed by the University of Utah’s Dissolved Gas 

Service Center using both quadrapole and sector-field mass 

spectrometers. The mass-spectrometer analysis provides the 

relative mole fractions of dissolved gases. The dissolved-gas 

concentrations of the water sample are then calculated based 

on Henry’s Law relations by using field measurements of total 

dissolved-gas pressure and water temperature. The University 

of Utah’s analytical precision generally results in a ± 0.5°C 

uncertainty in dissolved-gas recharge temperatures for samples 

with small amounts of excess air and known recharge altitude 

(K. Solomon, University of Utah, oral commun., 2006).

Recharge altitude is an unknown parameter, a typical 

situation in locations with high topographic gradient, such 

as the three study basins in Cape Verde. Because recharge 

temperature and recharge altitude are correlated, it is common to generate a range of estimated dissolved-gas recharge 

temperatures for each ground-water sample using (1) the landsurface altitude (the minimum altitude where recharge could 

occur) at each sample site to calculate a maximum dissolvedgas recharge temperature (Trmax), and (2) the highest possible 

water-table altitude (the maximum altitude where recharge 

could occur) within the watershed to calculate a minimum 

dissolved-gas recharge temperature (Trmin). This possible range 

of dissolved-gas recharge temperatures for each sample is then 

compared to the atmospheric temperature lapse curve to determine the “most-probable” dissolved-gas recharge temperature 

and recharge altitude.

Temperature lapse rates vary at different locations. 

Factors affecting temperature lapse rates include strength of 

solar radiation (season, slope, aspect), proximity to oceans, 

topography (cold air drainage), and relative humidity. Air 

temperature lapse rates are generally about -6°C per 1,000 m 

(Rolland, 2003), but can vary from -10°C per 1,000 m (dry air 

adiabatic lapse rate) to -4°C per 1,000 m (saturated adiabatic 

lapse rate). Water-table temperature lapse rates are assumed 

   Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06
to be similar to air temperature lapse rates in places having 

shallow depth to water and can be generated from measurements of spring altitude and water temperature (Manning and 

Solomon, 2003). However, if there is a thick vadose zone and 

large depth to water, the apparent temperature lapse rate may 

be lower because of the geothermal gradient. Therefore, apparent recharge altitudes derived from dissolved-gas recharge 

temperatures in this study are likely a minimum and may 

underestimate actual recharge altitudes towards the interior of 

the islands where there is the possibility of a thick vadose zone 

and deep water table.

A local temperature lapse rate for Cape Verde was constructed based on the relation between altitude and measured 

water temperature for seven springs within the three study 

basins, as well as other published data. These published data 

include 10 springs in Fogo but outside the Mosteiros Basin 

study area (Kallrén and Schreiber, 1988; Barmen and others, 

1990) and 34 springs on the island of Santiago (Barmen and 

others, 1984). In order to minimize the influence of geothermally affected waters, only springs with water temperatures of 

< 26°C were used when constructing the lapse curve. Spring 

altitude and water temperature are closely correlated (r2 = 

0.75; fig. 3). The resulting temperature lapse rate is about 

-5°C per 1,000 m. This lapse rate is within the reported range 

in lapse rates (Rolland, 2003). The following relation can be 

used for estimating water-table temperature lapse rate (fig. 3):

A = (24.3 – T) / 0.005	 (3)

where:

A is altitude, in m, and 

T is temperature, in °C. 

Figure 3. Relation between land-surface altitude and water temperature for springs in Cape Verde.

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

10 12 14 16 18 20 22 24 26 28 30

WATER TEMPERATURE, IN DEGREES CELSIUS

LAND-SURFACE ALTITUDE, IN METERS 

Maximum

temperature

Minimum

temperature

Best fit

Methods 
Because of uncertainty in the measured temperature lapse 

curve, it is often presented as a band of altitude-dependent 

temperatures. Residuals calculated by subtracting the measured temperature from the theoretical temperature for each 

spring by using equation 3 result in a standard deviation of 

1.6°C. A 2-sigma temperature band of 3.2°C for each altitude 

was used, therefore, in order to encompass 95 percent of the 

samples. 

The “most probable” range of recharge temperature/altitude combinations is then determined by the intersection of 

the maximum range of recharge temperatures (Trmin/Trmax) and 

the temperature lapse band. Estimated recharge altitudes have 

an uncertainty that is difficult to quantify, caused primarily by 

uncertainty in the temperature lapse rate.

Ground-water samples also were collected and analyzed 

for the tritium (3H) isotope in the water molecule. As with the 

stable isotopes, 3H samples were collected from open boreholes by using polyvinyl chloride 1,000-ml sample bailers. 

Water samples from production wells were collected by using 

existing submersible pumps. Water samples from springs 

and tunnels were collected by gravity flow. All samples were 

stored in clean polyethylene bottles. Tritium samples were 

analyzed with the tritium in-growth method (Clarke and others, 1976) at the University of Utah’s Dissolved Gas Service 

Center.

Tritium is a radioactive isotope of hydrogen and decays 

to tritiogenic helium (3Hetrit) with a half life of 12.3 years and, 

therefore, can be used for evaluating the relative age of ground 

water. As with the stable isotopes, tritium is analyzed by 

measuring the ratio of the heavier and less-abundant isotope to 

the lighter and more-abundant isotope. Tritium concentrations 

are reported in tritium units, where one tritium unit represents 

one molecule of 3H1HO in 1018 molecules of 1H2O (Solomon 

and Cook, 2000). Tritium is naturally produced in earth’s 

upper atmosphere through cosmic-ray neutron bombardment 

of nitrogen. Much larger amounts were introduced into the 

atmosphere during thermonuclear testing in the 1950s and 

1960s. On the basis of International Atomic Energy Agency 

records from network stations at similar latitudes (Barbados, 

Ascension Island, St. Helena), Barmen and others (1990) 

estimated that the natural background concentration of 3H in 

precipitation in Cape Verde was about 4 TU prior to 1953 and 

after 1980. This conclusion is consistent with five precipitation samples collected on Santiago Island during 1984 with 3

H 

concentrations ranging from 2.3 ± 0.1 to 4.4 ± 0.2 TU (Akiti, 

1985) and one sample collected on Fogo Island in 1987 with 

a 3H concentration of 3.0 ± 1 TU (Barmen and others, 1990). 

The samples collected during the 1980s, however, may still 

have contained some anthropogenic 3H. Sampling during 

2005, as part of this study, indicates a current atmospheric 

concentration of 1 to 3 TU, which may be a more accurate 

natural background concentration for Cape Verde. Relatively 

large amounts of anthropogenic 3H were introduced into the 

atmosphere by thermonuclear testing in the 1950s and 1960s, 

with estimated peak precipitation 3H concentrations in Cape 

Verde reaching about 175 TU in 1964 (Kallrén and Schreiber, 

1988). 

Based on its 12.3-year half life, decay-corrected 3H concentrations can be calculated with:

C = Coe-tλ (4)

where: 

C is the measured 3H concentration in water, 

Co is the initial 3H concentration, 

t is the number of years since the precipitation 

occurred, and 

	 λ is the decay constant (0.0565/yr for 3H). 

The estimated initial and decay-corrected 3H concentration of precipitation in Cape Verde from 1950 through 2005, 

modified from Barmen and others (1990), is shown in figure 

4. Assuming that precipitation recharging aquifers in Cape 

Verde prior to the early 1950s had an initial 3H concentration of 3.0 TU, these waters in 2005 would have had a 3

H 

concentration of ≤0.3 TU. Recharge that occurred during 

above-ground nuclear testing in the late 1950s and 1960s 

(assuming initial 3H concentrations in precipitation of 10 to 

175 TU) would have current 3H concentrations of about 1 to 

17 TU. Water recharging the aquifer during the 1970s (assuming initial 3

H concentrations in precipitation of 4.0 to 14 TU) 

would have current 3H concentrations between 0.6 and 2.0 TU. 

Water recharging the aquifer during the 1980s (assuming an 

average 3H concentration in precipitation of 3.5 TU) would 

have current 3H concentrations between 0.9 to 1.4 TU. Water 

recharging the aquifer during the 1990s to present (assuming 

an average 3H concentration in precipitation of about 2.0 to 

3.0 TU) would have current 3H concentrations between 1.4 

and 3.0 TU. Ground water discharging from springs and wells 

often contains a mixture of flow paths having water of various 

ages. Ground water having a 3H concentration less than 4 TU 

but greater than 0.3 TU may represent a mixture of both young 

and old waters rather than representing a discrete flow path 

and recharge year. Except for ground-water recharge occurring 

during the 1963 3H peak, therefore, it is generally not possible 

to uniquely determine modern (post-1950s) ground-water ages 

from tritium alone. 

Tritium/helium (3H/ 3Hetrit) age dating is a technique, 

however, that can be used for dating relatively young ground 

water recharged within the past 50 years. 3H in the water 

molecule decays to 3Hetrit, a gas which is readily lost in the 

atmosphere but is geochemically conservative (inert) when 

dissolved in ground water. The age-dating clock begins, therefore, when recharge crosses the water table rather than when 

it infiltrates the subsurface. The 3H/ 3Hetrit ratio can be used 

to determine the number of years since the water entered the 

aquifer as recharge with the following equation:

t = λ-1 ln(3Hetrit / 3H +1) (5)

 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06
Figure 4. Estimated initial and decay-corrected tritium concentration of precipitation in Cape 

Verde, 1950-2005.

0

20

40

60

80

100

120

140

160

180

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

TRITIUM CONCENTRATION, IN TRITIUM UNITS

Decay-corrected tritium

concentration in precipitation

Initial tritium concentration 

in precipitation

Modified from Barmen and others (1990)

where:

t is the apparent age in years, and 

	 λ is the 3H decay constant of 0.0563/yr (Solomon and 

Cook, 2000). 

In areas with thick vadose zones and low recharge rates, 

this apparent recharge age may underestimate the actual time 

since the infiltration of precipitation occurred. Also, the apparent age of ground water at sites with convergent flow, such 

as at springs or wells with large perforated intervals, will be 

a weighted average age of water traveling along various flow 

paths. The 3H/ 3Hetrit ratio only yields the age of the modern 

component of ground water. The ratio of the reconstructed 

initial 3H concentration in the ground-water sample (sum of 3H 

plus 3Hetrit) divided by the expected initial 3H concentration in 

precipitation (fig. 4) yields the modern (post-1950s) fraction 

of water.

Spring and Stream Discharge Data

Two methods were used to quantify ground-water 

discharge to springs and streams. A bucket of known volume 

(generally 10 L) and stopwatch was used for smaller discharge 

measurements where the flow could be diverted into a freefalling column. A current (pygmy) meter to measure velocity 

(Buchanon and Somers, 1969) was used for higher discharge 

rates (greater than 300 L/min) and where the flow could not be 

diverted to a free-falling column.

Hydrogeologic Setting

Mosteiros Basin, Fogo

Mosteiros is the southernmost of the three study basins 

and is located on the island of Fogo (fig. 1), which has the 

only active volcano in the archipelago. The island is a relatively new addition to the archipelago, having been formed 

within the last 500,000 years. Its most recent eruption 

occurred in 1995. The island is a spherical cone with a peak at 

2,900 m, the second tallest point in the Atlantic Ocean. 

Because of the island’s conical shape, the Mosteiros 

Basin (fig. 5) is uniquely composed of eight smaller watersheds on the flank of the volcano that each drain directly 

to the ocean. In contrast, the other two basins (Ribeira Paul 

and Ribeira Fajã) only consist of one watershed each draining to the ocean. The Mosteiros Basin is also larger than the 

other two basins, with a total area of 42 km2 (table 1). It has a 

maximum altitude of 2,400 m, about 1,000 m higher than the 

other basins. The average topographic slope of the basin is 27 

degrees, steeper than the Ribeira Fajã Basin (23 degrees) but 

not as steep as the Ribeira Paul Basin (38 degrees).

Hydrogeologic Setting 
Figure 5. Location of wells, springs, and precipitation stations in the Mosteiros Basin, Fogo, Cape Verde.

FF29

FF07

FF06

FF21

FF22

Monte Vermelho

Mosteiros

Monte Velha Pãu Cortado

Monte Barro Fonsaco

Atalaia

Fajazinha

Mosteiros

Pai Antonio

Ribeira Ilheu

Fajazinha Airport

Cha das

Caldeiras

Mosteiros Basin

Fogo

FF29

FF07

FF06

FF21

FF22

Monte Vermelho

Mosteiros

Monte Velha Pãu Cortado

Monte Barro Fonsaco

Atalaia

Fajazinha

Mosteiros

Pai Antonio

Ribeira Ilheu

Fajazinha Airport 200

400

600

800

1000

1200

1400

1600

1800

2000

2200

Cha das

Caldeiras

Mosteiros Basin

Fogo

24°24' 24°22' 24°20'

15°0'

15°2'

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Cabo Verde National Grid Projection Spring

Precipitation station

Town

Altitude, in meters

2,400

0

Basin boundary

200-meter topographic contour

100-meter topographic contour

Road

Stream

Well

EXPLANATION

FF06

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

Table 1. Area, slope, and average and maximum altitude of the Mosteiros, 

Ribeira Paul, and Ribeira Fajã Basins, Cape Verde

Basin

Total area 

(square kilometers)

Average 

slope 

(degrees)

Average 

altitude 

(meters)

Maximum 

altitude 

(meters)

Mosteiros 42 27 820 2,400

Ribeira Paul 15 38 620 1,500

Ribeira Fajã 16 23 480 1,300

10 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06
The geology of the Mosteiros Basin (fig. 6) is dominated 

by basaltic lava flows with intercalated pyroclastic deposits 

(Barmen and others, 1990). The basin is also dotted with 

numerous cinder cones made of coarse volcanic fragments 

(lapilli). The basement rocks within the basin consist of an 

alkaline-carbonatite basaltic complex. Lava flows associated with the basement rocks are highly weathered to clays, 

presumably acting as a low-permeability barrier. The basement 

rocks are also cut by a dense network of igneous dikes and 

contain granular xenoliths, further reducing their permeability.

Basaltic lava flows of the Principal Eruptive Complex 

overlie these basement rocks. These basalts have prominent 

columnar jointing (Barmen and others, 1990), as observed 

along the sea cliffs in the northernmost part of the Mosteiros 

Basin. Pillow lavas also are associated with these basalts. The 

basalts are intercalated with both coarse-grained pyroclastic tuffs and thin layers of clays. The lava flows form fairly 

continuous steeply dipping layers (20 to more than 30 degrees) 

from the island’s central caldera complex (Cha das Caldeiras) 

to the sea that likely are important conduits of ground-water 

flow. In addition to columnar jointing, widespread fracturing 

Figure 6. Geology of the Mosteiros Basin, Fogo, Cape Verde.

Cha das

Caldeiras

Monte Vermelho

Fajazinha Airport

Fonsaco

Atalaia

Mosteiros

Pai Antonio

Ribeira Ilheu

FF21

FF22

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

Mosteiros Basin

Fogo

24°24' 24°22' 24°20'

15°0'

15°2'

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Geologic data from Barmen and others (1990)

Cabo Verde National Grid Projection

FF07

FF06

FF29

Fajazinha

Fajazinha Airport

FF29

Recent basaltic lava flow

Basalts with intercalated pyroclastic 

 deposits of the Principal Eruptive Complex

Pre-Caldera cinder cones

Pyroclastic deposits intercalated with 

 basalts of the Upper Principal Eruptive 

 Complex

Recent cinder cones

Caldera wall

EXPLANATION

Town

Basin boundary

Road

Stream

FF06 Well

Approximate location

of contact

Hydrogeologic Setting 11
12 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

also increases the permeability of these basaltic lava flows. In 

contrast to the basement rocks, the noticeable absence of dikes 

cutting across the basalts of the Principal Eruptive Complex 

reduces the likelihood of flow barriers, thus further enhancing 

the probability of ground-water flow in this layer. 

The southern (highest-altitude) part of the Mosteiros 

Basin is composed of the Upper Principal Eruptive Complex

associated with Fogo’s upper volcanic cone. In contrast to the 

prominent lava flows of the Principal Eruptive Complex, this 

layer is dominated by pyroclastic tuffs, lapilli, and agglomerates and is cut by a dense network of basaltic sills and dikes, 

which are less transmissive for ground-water flow.

The morphology of the Mosteiros Basin is dominated 

by the steep slope of the main volcanic cone and numerous 

recent lava flows and cinder cones (fig. 6). Superimposed on 

this slope are narrow incised gullies, indicative of the highenergy runoff from sporadic but intense precipitation events. 

Because of the recent creation of the island and the intensity of 

runoff, soil layers are generally very thin or non-existent. The 

exception to this is along the northeastern coast, where volcanic cliffs form a very steep escarpment juxtaposed against 

a nearly flat coastal plain with rich volcanically derived soils. 

The southern border of the Mosteiros Basin is defined by the 

1,000-m near-vertical wall of the island’s 8-km-wide caldera. 

This feature acts as a prominent surface-water divide. Water 

falling on the north-facing slope of the caldera wall runs off 

toward the coastal areas of the Mosteiros Basin; water falling 

on the south-facing slope drains into the closed basin of the 

caldera. Because the caldera bottom is covered by highly 

permeable volcanic deposits (lapilli, scoria, and basalt flows) 

and receives relatively large amounts of precipitation, this area 

likely provides a substantial amount of recharge to the island’s 

ground-water system.

Seven-year (1996-2002) average annual precipitation was 

207 mm at the Mosteiros station (75-m altitude), 346 mm at 

the Monte Barro station (348-m altitude), and 379 mm at the 

Pau Cortado station (460-m altitude) (fig. 5, table 2; Instituto 

Nacional de Meteorologia e Geofisica, written commun., 

2005). These data follow the expected trend of higher precipitation rates at higher altitudes. Although precipitation data 

for the same 7-year period are not available in the highland 

areas of the basin, historical data from the Monte Velha station 

(850-m altitude) and the Cha das Caldeiras station (1,650-m 

altitude) indicate that annual rainfall exceeds 1,000 mm in the 

upper reaches of the basin (Barmen and others, 1990). This 

precipitation supplies sporadic runoff and flow to gullies, but 

there are no perennial streams in the basin. The range from 72 

to 453 mm/yr at Mosteiros (table 2) shows the annual variability in precipitation within the basin.

Ribeira Paul Basin, Santo Antão

Santo Antão is the northwesternmost island of Cape 

Verde and the Ribeira Paul Basin is located on its northeastern coast (fig. 1). The basin area, not including the adjacent 

La Cova closed basin, is 15 km2 (table 1). The basin’s vertical relief of 1,500 m (fig. 7) is much less than that of the 

Mosteiros Basin; however, it is a more deeply incised basin, 

as indicated by its higher average topographic slope of about 

38 degrees, compared to 23 and 27 degrees for the other two 

study basins. 

As with the Mosteiros Basin, the geology of the Ribeira 

Paul Basin is dominated by volcanic rocks. In contrast to 

Fogo, the island of Santo Antão is geologically much older, 

Table 2. Annual precipitation at selected stations in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde 

[Precipitation reported in millimeters]

Station name Mosteiros Monte 

Barro

Pau 

Cortado

Figeuiral 

Paul Passagem Covão Posto Canto Fajã Cachaço

Altitude 

(meters) 75 348 460 200 300 810 410 575 715

Basin Mosteiros Mosteiros Mosteiros Paul Paul Paul Fajã Fajã Fajã

1996 72 435 207 599 1054 773 187 251 319

1997 149 213 272 246 525 509 237 371 378

1998 158 410 366 276 384 409 230 213 296

1999 453 504 620 1,044 1,391 958 430 386 541

2000 76 117 196 695 1,172 940 366 187 344

2001 279 403 731 263 314 438 189 207 352

2002 264 339 259 443 658 636 411 501 677

7-year average 207 346 379 509 785 666 293 302 415

12 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa
thus there has been more time for erosion to create a welldeveloped canyon system. The predominant exposed rocks 

within the basin include basaltic lava flows and pyroclastic deposits (fig. 8). Many near-vertical igneous dikes in 

the basalts are exposed along the upper canyon headwalls. 

Beneath these rocky headwalls, most of the sloping terrain 

within the basin has only a veneer of soils. Alluvial deposits 

thicker than a few meters are only present along the wash at 

the lower end of Ribeira Paul.

Annual precipitation in the Ribeira Paul Basin is much 

higher than the other two study basins, even though the average basin altitude (620 m) is less than the Mosteiros Basin 

(820 m; table 1). This difference is caused by the orientation 

and steep topographic relief of the basin and its ability to trap 

the moisture of ascending air masses associated with both the 

northeastern trade winds and the equatorial monsoon winds 

(Haagsma, 1995). Seven-year (1996-2002) average annual 

precipitation was 509 mm at the Figueiral Paul station (200-m 

altitude), 785 mm at the Passagem station (300-m altitude), 

and 666 mm at the Covão station (810-m altitude; fig. 7, table 

2; Instituto Nacional de Meteorologia e Geofisica, written 

commun., 2005). The larger amount of precipitation at Passagem, at an altitude about 500 m lower than that of Covão, 

illustrates the spatially erratic nature of precipitation in the 

Figure 7. Location of wells, springs, and precipitation stations in the Ribeira Paul Basin, Santo Antão, Cape Verde.

1000

800

600

200

400

600

400

600

800

200

400

1400

800

1200

1000

1200

400

200

R. Joao Deaninha R. Cringa - Cha de Padre Cima

Covão

Ladeira

Passagem

Chazinha

Paul (Vila das Pombas)

FA93

FA57

R. Cringa

R. Cabouco

R. Cabelao

R. das Morcas

R. Oio de Agua

R. Calistrinho diversion

R. Zeagrios #5

R. Zeagrios #4

R. Zeagrios #3

R. Calistrinho

R. Cha Joao Vaz

R. Covada Clara

R. Fidelo Diversion

R. Cabelao Diversion

Covão

Passagem

Figueiral Paul

NA21

Tabulera

Seladinha

Ribeirinha

Dragueiro #1

Dragueiro #2

R. Joao Deaninha R. Cringa - Cha de Padre Cima

Covão

Ladeira

Passagem

Chazinha

Paul (Vila das Pombas)

FA93

FA57

R. Cringa

R. Cabouco

R. Cabelao

R. das Morcas

R. Oio de Agua

R. Calistrinho diversion

R. Zeagrios #5

R. Zeagrios #4

R. Zeagrios #3

R. Calistrinho

R. Cha Joao Vaz

R. Covada Clara

R. Fidelo Diversion

R. Cabelao Diversion

Covão

Passagem

Figueiral Paul

NA21

Tabulera

Seladinha

Ribeirinha

Dragueiro #1

Dragueiro #2

Ribeira Paul Basin

Santo Antão

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Cabo Verde National Grid Projection

25°4' 25°2'

17°6'

17°8'

La Cova

Altitude, in meters

1,500

0

Stream site

Spring

Precipitation station

Town

Basin boundary

200-meter topographic contour

100-meter topographic contour

Road

Crater wall

Stream

FF06 Well

EXPLANATION

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

Hydrogeologic Setting 13
14  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Figure 8. Geology of the Ribeira Paul Basin, Santo Antão, Cape Verde.

basin. These varying microclimates are likely caused by sharp 

vertical temperature gradients, cloud cover, wind velocity, and 

topography (Haagsma, 1995). Five-year average precipitation 

(1945-46, 1948-50) at Vila das Pombas (altitude 20 m) was 

320 mm/yr (Vailleux and Bourguet, 1974).

Ribeira Paul is unique among the three study basins in 

its abundance of springs and spring-fed streams (about 100). 

The basins on the northeastern side of Santo Antão, including 

Ribeira Paul, are the only ones in all of Cape Verde containing 

perennial streams. Although Ribeira Paul is perennial (except 

for its lower reaches near well FA93; fig. 7), there are no 

historical streamflow records. In addition to the larger amount 

of annual precipitation, there are other possible contributing 

factors for the abundant ground-water discharge. Its moreincised morphology, compared to that of the other two basins, 

likely causes the locally perched or regional water table to be 

intersected by land surface; many of the larger springs and 

streams discharge from altitudes between 600 to 900 m at the 

base of the near-vertical canyon walls. Abundant low-permeability mafic dikes (exposed in outcropping rock throughout 

the basin), tuffs, and breccias may impede lateral flow and 

also cause ground-water discharge. Also, subsurface inflow 

from the adjacent La Cova volcanic caldera likely provides 

additional recharge to the ground-water system of the Ribeira 

Paul Basin (fig. 7). La Cova receives substantial precipitation 

and is a closed basin. None of its rainfall leaves the basin as 

surface water. Because of its small size and permeable soils, 

however, there is no reported streamflow within the basin, 

even after large precipitation events. Rainfall in La Cova is, 

therefore, either lost to evapotranspiration or becomes subsurRibeira Paul Basin

Santo Antão

La Cova Covão

Ladeira

Passagem

Chazinha

Paul (Vila das Pombas)

FA93

FA57

Ribeira Paul Basin

Santo Antão

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Geologic data from Burgeap (1974)

Cabo Verde National Grid Projection

25°4' 25°2'

17°6'

17°8'

La Cova Covão

Ladeira

Chazinha

Paul (Vila das Pombas)

FA93

FA57

"

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

Ribeira Paul Basin

Santo Antão

La Cova Covão

Ladeira

Passagem

Chazinha

Paul (Vila das Pombas)

FA93

FA57

EXPLANATION

Basalts with intercalated 

 pyroclastic deposits

Marine tuffs and breccias

Older tuffs and breccias

Pyroclastic deposits

Town

Basin boundary

Road

Stream

FA57 Well

14  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa
face recharge, similar to that in the Cha das Caldeiras caldera 

located adjacent to the Mosteiros Basin. 

Ribeira Fajã Basin, São Nicolau

São Nicolau is located near the center of the Cape Verde 

island archipelago. The Ribeira Fajã Basin is a deep basin 

incised in the mountains on the northwestern side of the island 

(fig. 1). The basin is about the same size as the Ribeira Paul 

Basin, with an area of 16 km2. The basin’s vertical relief of 

1,300 m (fig. 9) is also similar to Ribeira Paul Basin, however, 

Ribeira Fajã Basin is longer than Ribeira Paul Basin, resulting 

in a smaller average topographic slope of about 23 degrees

(table 1). 

Like the other two study basins, the geology of the 

Ribeira Fajã Basin is volcanic. The geologic map (fig. 10) 

shows exposures of basalt and scoria along much of the main 

reach of the Ribeira Fajã stream channel (Hidroprojecto 

Engenharia Gestão, 2000). The side walls and head of the 

basin have exposed basalts with intercalated pyroclastic deposits, crosscut by near-vertical igneous dikes. The highland area 

along the southwestern margin of the basin has a thick outcropping layer of pyroclastic deposits, which have weathered 

to clays at the surface, resulting in a fairly low-permeability 

surficial soil that likely impedes infiltration of precipitation.

Annual precipitation in the Ribeira Fajã Basin is much 

less than that in the other two basins. The rain generally 

occurs between June and December, with the most intense 

storms during August and September (Langworthy and Finan, 

Figure 9. Location of wells, springs, and precipitation stations in the Ribeira Fajã Basin, São Nicolau, Cape Verde.

1000

400

600

200

400

800

400

200

1200

600

Galleria Tunnel

24°22' 24°20' 24°18'

16°38'

16°40'

Ribeira Fajã Basin

São Nicolau

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Cabo Verde National Grid Projection

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

Potential artificial 

 recharge

Water storage tank

Spring

Precipitation station

Town

Altitude, in meters

1,300

0

Basin boundary

200-meter topographic contour

100-meter topographic contour

Road

Stream

Well

EXPLANATION

FN41

Cha de Miguel de Jose

Cha de Monte

(dry spring)

FN12

Posto

Cachaco

Canto Faja

Monte Gordo

Queimada

Faja de Cima

Faja de Baixo

Cruz Roque

Monte Gordo

Lombo Pelado

FN21

FN41

FN39

Caldeirinha

Cha de Miguel de Jose

Cha de Monte

(dry spring)

FN12

Posto

Cachaco

Canto Faja

Monte Gordo

Queimada

Faja de Cima

Faja de Baixo

Cruz Roque

Monte Gordo

Lombo Pelado

FN21

FN41

FN39

Caldeirinha

Hydrogeologic Setting 15
16 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Figure 10. Geology of the Ribeira Fajã Basin, São Nicolau, Cape Verde.

FN12

Queimada

Fajã de Cima

Fajã de Baixo

FN21

FN41

FN39

Galleria Tunnel

FN12

Queimada

Fajã de Cima

Fajã de Baixo

FN21

FN41

FN39

Galleria Tunnel

Ribeira Fajã Basin

São Nicolau

Base from Instituto Nacional de Gestão dos Recursos Hídricos 

Digital Elevation Data, 2005, 1:2,400

Geologic data from Hídroprojecto Engenharia E Gestão (2000)

Cabo Verde National Grid Projection

24°22' 24°20' 24°18'

16°38'

16°40'

0 0.5 1 2 3 KILOMETERS

0 0.5 1 MILE

EXPLANATION

FF06

Town

Basin boundary

Road

Stream

Well

Basaltic slope deposits

Pyroclastic deposits

Phonolitic and basaltic chimney

Basalts with intercalated pyroclastic deposits

Vesicular basalt and scoria

1997). Seven-year (1996-2002) average annual precipitation 

was 293 mm at the Posto Fajã station (410-m altitude), 302 

mm at the Canto Fajã station (575-m altitude), and 415 mm 

at the Cachaço station (715-m altitude; fig. 9, table 2; Instituto Nacional de Meteorologia e Geofisica, written commun., 

2005). In contrast to Ribeira Paul, the Ribeira Fajã Basin follows the expected trend of higher precipitation rates at higher 

altitudes. 

A historical chart of precipitation at the Cachaço station 

(fig. 11) was constructed from data obtained from the Ministero de Agricultura e Ambiente (written commun., 2005), 

the Instituto Nacional de Meteorologia e Geofisica (written 

commun., 2005), Sabino (1986), and Olivry (1981). In order 

to estimate total precipitation at Cachaço for 2004, the ratio of 

the average precipitation for the Cachaço and Posto stations 

for 1978-85, 1995-2002, and 2005 was multiplied by the total 

2004 precipitation at Posto. No precipitation data are available 

within the Ribeira Fajã Basin for 2003. The histogram indicates that the most recent two decades (1984-2005) were wetter than the prior two decades (1961-1983). This illustrates the 

cyclical nature of precipitation in Cape Verde and is consistent 

with longer historical records that show periods of several successive extremely dry years (Haagsma, 1995; Langworthy and 

Finan, 1997). The driest year on record during the past four 

decades was 1972, with only 19 mm of rain; the wettest year 

was 2002 with 677 mm of rain (Olivry, 1981).

16 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa
Baseline Ground-Water Conditions

Mosteiros

Ground-Water Budget

The measurable sources of ground-water discharge in the 

Mosteiros Basin are from the Monte Vermelho spring (also 

referred to as Fajãzinha) and from pumping wells. Submarine 

discharge, particularly associated with the Monte Vermelho 

spring, is also known to occur and may be the dominant form 

of ground-water discharge from the basin. Discharge to the 

ocean below the spring has been observed during the spring 

and neap low tides (Jose Manuel Andrade Alves, Agua Brava, 

oral commun., 2005), but could not be quantified during 

this study. The two main sources of recharge to the basin are 

infiltration of precipitation and subsurface inflow from the 

adjacent Cha das Caldeiras closed basin. A previous report 

estimated a minimum infiltration rate within Cha das Caldeiras 

of 9,000 m3/d (Kallrén and Schreiber, 1988). Because of the 

island’s conical shape, however, it is likely that much of this 

infiltration would radiate outward toward other basins besides 

Mosteiros.

Monte Vermelho is the only large spring within the 

Mosteiros Basin and one of three large springs on the island of 

Fogo. It discharges at the base of the columnar basalts of the 

Principal Eruptive Complex. The spring’s location is believed 

to be controlled by the contact of the basalt with an underlying low-permeability clay layer. This clay layer is thought to 

dip toward the south beneath the island in a direction opposite 

to the topographic slope, thus acting as a hydraulic barrier 

and trapping freshwater above it. A discharge of 518 m3/d 

(190,000 m3/yr) was measured on December 2, 2005 (table 3). 

This is much higher than a previously measured discharge rate 

in 1986 of 200 m3/d (Kallrén and Schreiber, 1988) and likely 

represents increased flow associated with recent spring capture 

enhancements. During 2003-05, about 200 m3/d (80,000 

m3/yr) of the spring discharge was pumped into the municipal 

water system for public supply (fig. 12; Agua Brava L. da Delegacão dos Mosteiros, written commun., 2006). This is about 

twice as much as the average reported spring use of about 100 

m3/d (40,000 m3/yr) during 1998-2002. The rest of the water is 

lost as submarine discharge below the bottom of the catchment 

tank. 

Figure 11. Historical precipitation at Posto Cachaço, Ribeira Fajã Basin, 1961-2005.

0

100

200

300

400

500

600

700

800

1961 1966 1971 1976 1981 1986 1991 1996 2001 2005

PRECIPITATION, IN MILLIMETERS

PER YEAR 

missing

estimated

Figure 12. Total annual well withdrawal and the portion of 

Monte Vermelho spring discharge used for public supply in 

Mosteiros Basin, 1998-2005.

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

19981999200020012002200320042005

Monte Vermelho spring, public supply

Well withdrawal

 TOTAL ANNUAL WITHDRAWAL,

IN CUBIC METERS 

Baseline Ground-Water Conditions 17
18  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Table 3. Records of selected springs and streams in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde

[Altitude: m, meters; Discharge: m3/d, cubic meters per day; Specific conductance: μS/cm, microsiemens per centimeter at 25 degrees Celsius; Temperature: 

°C, degrees Celsius; Measurement Agency: INGRH, Instituto Nacional de Gestão dos Recursos Hídricos; MAAP, Ministero do Ambiente, Agricultura e 

Pescas; SAF, Servicio Autonimo Fajã; USGS, U.S. Geological Survey; —, no data]

Site name Basin

Latitude Longitude 

(degree/minute/ 

second)

Altitude 

(m)

Discharge 

measurement 

method

Date 

Springs 

Tabulera Paul 170654 0250328 830 Bucket & stopwatch 8/4/2005 

NA21 Paul 170834 0250234 440 Bucket & stopwatch 8/6/2005 

Dragueiro #1 Paul 170816 0250231 270 Bucket & stopwatch 12/8/2005 

Dragueiro #2 Paul 170823 0250231 250 Bucket & stopwatch 12/7/2005 

Seladinha Paul 170711 0250249 460 Bucket & stopwatch 12/5/2005 

Bucket & stopwatch 12/8/2005 

Passagem Paul 170756 0250232 300 Bucket & stopwatch 8/4/2005 

Monte Vermelho Mosteiros 150254 0242147 5 — 8/12/2005 

Bucket & stopwatch; tank level 12/2/2005 

Galleria Fajã (Total) Fajã 163845 0241927 250 Tank level, in-line flow meter 8/18/2005 

Tank level, in-line flow meter 12/13/2005 

G. Fajã Monte Gordo Irrigation Fajã 270 In-line flow meter & stopwatch 12/13/2005 

G. Fajã Monte Gordo Domestic Fajã 270 In-line flow meter & stopwatch 12/13/2005 

G. Fajã Cruz Roque Branch Fajã 267 Bucket & stopwatch 12/13/2005 

G. Fajã Lombo Pelado Branch Fajã 265 Bucket & stopwatch 12/13/2005 

Streams 

Ribeirinha Paul 170819 0250231 250 Bucket & stopwatch 12/6/2005 

Morcas Paul 170628 0250302 710 Bucket & stopwatch 12/5/2005 

Oio de Agua Paul 170624 0250320 955 Bucket & stopwatch 12/5/2005 

João Deaninha Paul 170648 0250227 740 Bucket & stopwatch 12/5/2005 

Covada Clara Paul 170646 0250239 550 Bucket & stopwatch 12/5/2005 

Cabouco Paul 170755 0250245 560 Bucket & stopwatch 12/5/2005 

Cringa stream channel Paul 170656 0250259 560 Bucket & stopwatch 12/8/2005 

Cringa - Cha de Padre Cima Paul 170654 0250256 480 Bucket & stopwatch 12/8/2005 

Zeagrios # 3 Paul 170629 0250234 680 Bucket & stopwatch 12/5/2005 

Zeagrios # 4 Paul 170629 0250232 700 Bucket & stopwatch 12/5/2005 

Bucket & stopwatch 12/8/2005 

Zeagrios # 5 Paul 170636 0250230 750 Bucket & stopwatch 12/8/2005 

Calistrinho Paul 170650 0250243 500 Estimate only 12/5/2005 

Calistrinho A Paul 170659 0250242 500 Bucket & stopwatch 12/8/2005 

Calistrinho B Paul 170659 0250242 500 Pygmy flow meter 12/8/2005 

Fidelo Diversion Paul 170715 0250258 430 Estimate only 12/5/2005 

Pygmy flow meter 12/9/2005 

Cabelão Diversion Paul 170714 0250256 430 Bucket & stopwatch 12/9/2005 

Ribeira Cabelão Paul 170702 0250258 425 Bucket & stopwatch 12/5/2005 

170717 0250256 420 Pygmy flow meter 12/9/2005 

Cha João Vaz A Paul 170734 0250248 360 Bucket & stopwatch 12/9/2005 

Cha João Vaz B Paul 170734 0250248 355 Bucket & stopwatch 12/9/2005 

Cha João Vaz C Paul 170734 0250248 350 Pygmy flow meter 12/9/2005 

Cha João Vaz D Paul 170734 0250248 355 Pygmy flow meter 12/9/2005 

1

Spring and stream sites measured by the INGRH on December 5-6, 2005, do not have specific-conductance measurements because of equipment failure.
Table 3. Records of selected springs and streams in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape 

Verde—Continued

Site name Discharge 

(m3/d)

Specific 

conductance 

(μS/cm)

Temperature 

(°C)

Measurement 

agency

Springs

Tabulera 95 220 18.9 INGRH, USGS

NA21 100 265 23.7 INGRH, USGS

Dragueiro #1 19 365 23.4 INGRH, USGS

Dragueiro #2 34 375 23.7 INGRH, USGS

Seladinha 35 1

— 24.1 INGRH

23 450 21.3 INGRH, USGS

Passagem 950 510 22.6 INGRH, USGS

Monte Vermelho — 200 — INGRH, USGS

518 215 22.2 INGRH, USGS

Galleria Fajã (Total) 430 — — SAF

430 — — SAF

G. Fajã Monte Gordo Irrigation 250 380 21.3 INGRH, MAAP, USGS

G. Fajã Monte Gordo Domestic 91 — — INGRH, MAAP, USGS

G. Fajã Cruz Roque Branch 56 440 — INGRH, MAAP, USGS

G. Fajã Lombo Pelado Branch 73 550 — INGRH, MAAP, USGS

Streams

Ribeirinha 35 1— 25.0 INGRH

Morcas 140 1— 18.3 INGRH

Oio de Agua 63 1— 21.2 INGRH

João Deaninha 95 1— 26.0 INGRH

Covada Clara 25 1— 23.3 INGRH

Cabouco 24 1— 24.0 INGRH

Cringa stream channel 170 440 22.4 INGRH, MAAP, USGS

Cringa - Cha de Padre Cima 230 420 20.2 INGRH, MAAP, USGS

Zeagrios # 3 86 1— 22.0 INGRH

Zeagrios # 4 320 1— 22.5 INGRH

270 200 19.1 INGRH, MAAP, USGS

Zeagrios # 5 260 190 19.0 INGRH, MAAP, USGS

Calistrinho 1,700 1— 23.5 INGRH

Calistrinho A 300 370 21.0 INGRH, MAAP, USGS

Calistrinho B 950 — — INGRH, MAAP, USGS

Fidelo Diversion 1,200 1— 23.2 INGRH

780 480 22.2 INGRH, USGS

Cabelão Diversion 42 590 22.0 MAAP, USGS

Ribeira Cabelão 19 1— 24.2 INGRH

420 570 21.4 INGRH, USGS

Cha João Vaz A 630 410 23.2 INGRH, USGS

Cha João Vaz B 860 520 23.0 INGRH, USGS

Cha João Vaz C 240 540 22.6 INGRH, USGS

Cha João Vaz D 2,100 580 22.3 INGRH, USGS

Baseline Ground-Water Conditions 19
20 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Table 4. Records of selected wells in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde

[Altitude, Depth of well, and Water level below land surface: m, meters; Pumping rate: m3/d, cubic meters per day; —, no data]

Well name Basin Latitude Longitude

 (degree/minute/second)

Altitude 

(m)

Year 

Drilled

Depth of 

well (m)

Water level 

below land 

surface (m)

Date

Pumping 

rate 

(m3/d)

FF06 Mosteiros 150133 0241908 60 70.6 63.50 12/02/05

FF07 Mosteiros 150221 0242014 20 35.8 17.61 12/02/05

17.57 12/03/05

17.73 03/31/06

FF21 Mosteiros 150246 0242104 40 54.3 — 08/11/05 180

39.46 12/03/05

FF22 Mosteiros 150246 0242104 40 53.0 39.55 03/11/98

39.55 08/11/05

39.25 12/03/05 470

FF29 Mosteiros 150229 0242043 60 2005 61.9 51.94 06/01/05

51.46 08/12/05

51.45 12/03/05

51.48 03/31/06

FA57 Paul 170820 0250140 80 1993 43.0 1.60 08/17/93

1.29 01/07/94

— 08/08/05 192

— 12/07/05

FA93 Paul 170849 0250104 20 2004 21.1 16.15 07/14/04

15.40 08/25/04

16.24 08/05/05

15.34 12/08/05

16.40 06/15/06

FN12 Fajã 163839 0241935 250 1978 146.0 111.10 10/14/78

FN21 Fajã 163813 0242014 310 1980 128.0 115.07 04/10/79

FN39 Fajã 163950 0241914 80 1980 104.3 51.54 01/10/80

52.58 10/30/82

72.43 11/02/98

72.81 02/18/02

73.12 12/13/05

73.11 03/08/06

FN41 Faja 163758 0242042 490 1980 189.2 173.40 03/10/80

163809 0242025 Dry 03/08/06
17.2

17.3

17.4

17.5

17.6

17.7

17.8

17.9

DEPTH TO WATER BELOW LAND SURFACE, IN METERS

JANUARY

2005 2006

DECEMBER FEBRUARY MARCH

Barometer-corrected

Barometer-corrected and tide-filtered

Figure 13. Water level in well FF07, Mosteiros Basin, December 2005 through March 2006.

Currently (2006) two production wells exist in the 

Mosteiros Basin: FF21 and FF22. Collectively, withdrawals 

from these wells averaged about 100 m3/d between 1998 and 

2005 (Agua Brava L. da Delegacão dos Mosteiros, written 

commun., 2006). Well FF21 had a measured instantaneous 

pumping rate of 180 m3/d on August 11, 2005 (table 4). Well 

FF22 had a measured instantaneous pumping rate of 470 m3/d 

on December 3, 2005. Combined total annual well withdrawal 

from the two wells from 1998 to 2005 has varied between 

about 28,000 and 48,000 m3 (fig. 12). One new well (FF29) 

was recently completed (2005) with plans to eventually install 

a submersible pump.

The average annual volume of precipitation falling within 

the Mosteiros Basin is estimated to be about 2.5 x 107 m3 (7 

x 104 m3/d). This is based on an average basin altitude of 816 

m, an interpolated annual precipitation value of 590 mm, and 

a basin area of about 42 km2. The interpolated 590 mm annual 

precipitation is based on a linear interpolation of the 1996-

2002 average precipitation for the three lower-altitude precipitation stations (75 m, 348 m, and 460 m) in the Mosteiros 

Basin, along with reported precipitation at Monte Velha (850 

m) and Cha das Caldeiras (1,650 m). Combining the Monte 

Vermelho spring discharge with withdrawals from the two 

production wells, the total spring and well discharge from the 

Mosteiros Basin is estimated to be about 620 m3/d or 230,000 

m3/yr. Subsurface outflow as submarine discharge is thought 

to be a large component of the ground-water budget, but could 

not be quantified during this study. By using the estimated 

average annual 2.5 x 107 m3 volume of precipitation falling 

within the basin, about 1 percent (2.3 x 105 m3 divided by 2.5 

x 107 m3) of this precipitation is calculated to discharge from 

the aquifer through wells and springs. 

Ground-Water Levels

Ground-water levels were measured with an electric 

tape at five wells in the Mosteiros Basin: FF06, FF07, FF21, 

FF22, and FF29 (table 4). These wells are all located along 

the coastal plain in the northeastern part of the basin (fig. 5). 

There are no wells located in the highlands part of the basin to 

evaluate ground-water levels or ground-water hydraulic gradients. The land-surface altitude of the five wells ranged from 

about 20 to 60 m and the depth to water ranged from 18 to 

64 m below land surface. Wells FF06 and FF07 are currently 

monitoring wells; both were drilled as production wells but 

subsequently abandoned. Well FF06 had a very low yield during well testing (Agua Brava L. da Delegacão dos Mosteiros, 

written commun., 2006) and well FF07 had only a thin lens of 

freshwater overlying brackish water.

A nonvented pressure transducer, barometer, and data 

logger were installed in well FF07 in December 2005 to 

continuously record water level and barometric pressure. 

Barometrically corrected water levels, both corrected and noncorrected for tidal fluctuations, are shown in figure 13. After 

removing the tidal effects, there is a gradual decline of about 

0.2 m between December 2005 and March 2006. Without 

additional information, it cannot be determined if this is a 

seasonal or longer-term decline. A seasonal decline in groundwater level may be caused by either decreased infiltration of 

precipitation at the end of the rainy season (August-November) or a delayed response to seasonal pumping. The combined 

monthly pumping of wells FF21 and FF22, located 1.6 km 

to the northwest, was 3,600 m3/month during May-October, 

2005, larger than the 4-year (2002-2005) average of about 

3,000 m3/month (Agua Brava L. da Delegacão dos Mosteiros, 

Baseline Ground-Water Conditions 21
22 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

written commun., 2006). Because precipitation data were not 

available beyond 2002, it cannot be determined if the small 

water-level decline is part of a longer-term climatic trend. 

Total annual pumping from wells FF21 and FF22 (fig. 12) also 

does not show any trend of increased pumping as a cause for 

long-term regional water-level declines.

Water Quality and Environmental Tracers

The specific-conductance value of ground water in the 

Mosteiros Basin varies from about 200 to 16,000 μS/cm (table 

5; fig. 14). The best-quality (lowest specific-conductance 

value) water was sampled at the Monte Vermelho spring. The 

poorest quality (highest specific-conductance value) water was 

sampled at depth in the FF07 observation well. Because of its 

proximity to the ocean, a thin lens of fresh ground water in 

the Mosteiros coastal plain overlies brackish water. Discrete 

depth sampling with a downhole water-quality meter at FF07 

illustrates this (fig. 14). The specific-conductance value of the 

shallowest 4 m of water was about 2,200 μS/cm, but gradually 

increased to more than 10,000 μS/cm at a depth of 7 m below 

the water table. Similarly, if either of the two production wells 

in the Mosteiros Basin (FF21 and FF22) pumps more than 

about 160 m3/d, they produce brackish water. This influx of 

brackish water was documented during environmental tracer 

sampling at FF21 on August 11 and 12, 2005. The specificconductance value of the well water at the beginning of the 

pumping period was 870 μS/cm and increased to about 1,500 

μS/cm after a 20-hour pumping period. Therefore, only one 

well is generally pumped at a time and for a maximum of 

about 8 hours per day. This is similar to salt-water intrusion 

problems reported by Kallrén and Schreiber (1988, fig. 4.2) at 

the Praia Ladrao spring located at a similar altitude (5 m) on 

the west coast of Fogo. The specific conductance of water discharging from this spring increased from about 400 to 1,500 

μS/cm during June 1987 and was attributed to pumping.

Ground-water pH and temperature ranges in the Mosteiros Basin are 6.8 to 7.8 and 20.3°C to 23.0°C, respectively 

(table 5). These values are similar to those of ground water 

from the other two study basins and do not indicate upwelling 

geothermal sources of water. The relatively high dissolvedoxygen concentration of 5.6 to 7.6 mg/L (61 to 88 percent 

saturation) indicates oxidized conditions within the aquifer 

and the lack of substantial bacterial decay of organic matter. 

Measured total dissolved-gas pressure values were within a 

very small range (747 to 766 mm Hg) and are nearly identical 

to barometric pressure values (754 to 758 mm Hg), indicating little or no excess gas in the aquifer from air entrapment, 

degassing of deep geothermal fluids, or anaerobic decay and 

gas production.

No source precipitation data were collected for stable-isotope analysis within Mosteiros Basin for this study. Stableisotope samples of two agricultural products (wine and juice) 

from the nearby Cha das Caldeiras were collected, but the 

analyses were not successful. Stable-isotope concentrations 

of Fogo precipitation were previously reported at Cha das 

Caldeiras (-7.0 permil for δ18O and -45 permil for δ2H) and 

at Achada Fora (-4.0 permil for δ18O and -30 permil for δ2H; 

Barmen and others, 1990). Except for well FF06, the stableisotope ratios of ground water from the Mosteiros Basin fall 

within a fairly tight cluster on the meteoric water line (fig. 

15), with values ranging from -3.4 to -3.8 permil for δ18O and 

from -15 to -21 permil for δ2H. The water sample from well 

FF06 is much more depleted (δ18O of -5.1 and δ2H of -28), 

indicating a higher-altitude precipitation source of recharge 

to this well. The well is located southeast of the other wells 

and Monte Vermelho spring, closer to the high-altitude Cha 

das Caldeiras caldera (fig. 5). The stable-isotope concentrations of the Monte Vermelho spring water do not seem to vary 

substantially with time: the concentration of δ18O and δ2H was 

-3.8 and -21 permil, respectively, on August 12, 2005, and -3.8 

and -18 permil, respectively, on December 2, 2005. A previous 

ground-water isotopic study by Kallrén and Schreiber (1988) 

reported similar concentrations on October 10, 1987, of -3.9 

and -22 permil for δ2H and δ18O, respectively. This indicates a 

steady, uniform recharge source.

The dissolved-gas concentration of N2, 40Ar, 84Kr, 20Ne, 4

He, and 129Xe in ground water is listed in table 5. Most of the 

dissolved gases in ground-water samples from the Mosteiros 

Basin are of atmospheric origin, as indicated by the close 

comparison between measured and theoretical atmospherically equilibrated concentrations of the individual gases (K. 

Solomon, written commun., 2006). Furthermore, the relatively 

low excess-air concentrations indicate that little air entrapment 

at the water table or gas generation from within or beneath 

the aquifer is occurring. Little excess air generally reduces 

uncertainty in the calculation of recharge temperatures based 

on dissolved-gas concentrations. Estimated most-probable 

recharge temperatures are 19°C for the Monte Vermelho 

spring and less than 7 to 19°C for wells FF07, 21, 22, and 

0

2

4

6

8

10

12

14

16

0 5,000 10,000 15,000 20,000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS

PER CENTIMETER

DEPTH BELOW WATER TABLE,

IN METERS

Figure 14. Depth profile of specific-conductance values at 

well FF07, Mosteiros Basin, December 2005.
29. Plotting Trmin/Trmax ranges for each sample against the 

temperature lapse curve (fig. 16) indicates a most-probable 

recharge altitude of 1,000 m for the Monte Vermelho spring. 

This is very similar to a previously reported estimate of 900 

m based on the relation between spring altitude and δ18O 

values (Kallrén and Schreiber, 1988). Estimated dissolved-gas 

recharge altitudes for wells FF07, 21, 22, and 29 range from 

1,000 to more than 2,000 m (table 5). These recharge altitudes 

are higher than the land-surface altitudes at all of these sites 

(5 to 60 m), indicating that recharge to these wells and springs 

does not occur locally, but rather higher up in the watershed. 

It also indicates that the water-table altitudes in the recharge 

zone of Mosteiros basin are much higher than sea level, either 

because of horizontal flow barriers and compartmentalization 

(fig. 2) or low permeability of the basalts.

Although no precipitation 3H samples were collected 

in Mosteiros Basin, two agricultural product samples were 

analyzed from Cha das Caldeiras: wine from grapes of the 

2004 growing season and juice from grapes and pomegranates grown during the 2005 season. These samples had 3

H 

concentrations of 3.0 and 1.0 TU, respectively (table 6). 

This indicates that precipitation 3H concentrations in Fogo 

have remained fairly constant since previous sampling in the 

1980s. Ground-water 3H concentrations from samples collected in Mosteiros Basin ranged from about 0.2 to 1.2 TU. 

As discussed in the “Methods” section, values less than 0.3 

TU indicate ground-water recharge that occurred prior to the 

early 1950s. Water from wells FF06, FF21, and FF29 all had 

3

H concentrations less than 0.3 TU. Water from well FF22 and 

the Monte Vermelho spring had 3H concentrations between 0.4 

and 0.8 TU, likely indicating a mixture of pre- and post-1950s 

recharge (a sample collected from Monte Vermelho spring on 

April 28, 1980, had a reported 3H concentration of less than 

2 TU (Barmen and others, 1990), which would be equivalent 

to a decay-corrected 2005 concentration of less than 0.5 TU). 

Water from well FF07 had a 3H concentration of 1.2 TU, 

indicating either uniform post-1960s recharge or a mixture of 

pre- and post-1950s recharge.

Elevated dissolved 4He concentrations and 3He:4He ratios 

were measured in water from all of the wells sampled in 

Mosteiros (fig. 17, table 6). This is likely caused by the occur-

-40

-35

-30

-25

-20

-15

-10

-5

0

-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

18O, IN PERMIL

2H, I

N PE

R

MIL

Precipitation (this study)

Ribeira Paul wells

Mosteiros wells

Ribeira Fajã wells

Global Meteoric Water Line (Craig, 1961)

Ribeira Paul springs

Mosteiros springs

Galleria Fajã

Precipitation on São Nicolau (Barmen and others, 1990)

Figure 15. Relation between δ18O and δ2H values in precipitation and ground water from the Mosteiros, Ribeira Paul, 

and Ribeira Fajã Basins, Cape Verde.

Baseline Ground-Water Conditions 23
24  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

5 7 9 11 13 15 17 19 21 23 25

TEMPERATURE, IN DEGREES CELSIUS 

LAND-SURFACE ALTITUDE, IN METERS

Dragueiro Spring

Galleria Passagem

Seladinha Spring

Tabulera Spring

Well FF07

Well FF21

Well FF22

Well FF29

Galleria Fajã

Average temperature lapse

Maximum temperature

Minimum temperature

Monte Vermelho Spring

Maximum

dissolved-gas

recharge

temperature

Minimum

dissolved-gas

recharge

temperature

Temperature lapse band

EXPLANATION

Well FA57

Well FA93

Well FN39

NA21 Spring

Figure 16. Temperature lapse band (from fig. 3) and the range of possible dissolved-gas ground-water recharge temperatures 

for sites in Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde.
Ribeira Paul Ribeira Fajã Mosteiros

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0.0E+00

5.0E-08

1.0E-07

1.5E-07

2.0E-07

2.5E-07

3.0E-07

3.5E-07

4.0E-07

4.5E-07

3

He:4He ratio

4

He concentration

RATIO OF 3HELIUM TO 4HELIUM IN SAMPLE DIVIDED 

BY RATIO OF ATMOSPHERIC 3HELIUM TO 4HELIUM 

Tabulera Spring

Seladinha Spring

NA 21 Spring

Galleria Passagem

Well FA93

Dragueiro #1 Spring

Well FA57

Well FN39

Galleria Fajã - August

Galleria Fajã - December

Monte Vermelho Spring

Well FF07

Well FF21

Well FF22

Well FF29

4HELIUM CONCENTRATION, IN CUBIC CENTIMETERS PER 

GRAM AT STANDARD TEMPERATURE AND PRESSURE 

Figure 17. 3Helium:4Helium ratios relative to the helium isotopic ratio of air and total 4Helium concentration in ground water from 

the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde.

rence of terrigenically derived helium in the aquifer, associated with recent volcanic activity of the island (last eruption in 

1995). 4He concentrations from these wells range as much as 

an order of magnitude higher than typical concentrations measured in atmospherically equilibrated ground water. Similarly, 

the dissolved 3He:4He ratios of samples from these wells are as 

much as eight times Ra, where Ra is the atmospheric 3He:4He 

ratio of 1.384 x 10-6. This is consistent with earlier studies of 

mantle-derived helium (Andrews, 1985). Because of its sensitivity to non-atmospheric sources of helium, 3

H/3He age dating 

of these samples was not possible. 

The 3He and 4He concentrations of these and other terrigenically influenced samples, however, are very useful for 

constraining the 3H/3He age dating of the Monte Vermelho 

spring and ground water from other basins not affected by terrigenically derived helium. The relation between the 3

He:4He 

ratio and the fraction of 4He derived from atmospheric gases 

in ground-water samples from Mosteiros and the other two 

basins is shown in figure 18. The samples not affected by 

terrigenic helium sources are those having a fraction of 4He 

derived from the atmosphere equal to about 1.0 and 3He:4He 

ratios of about 2 x 10-6. The samples that are affected by terrigenic helium plot along a linear trend of increasing 3

He:4He 

ratio with decreasing fraction of 4He derived from the atmosphere. The extrapolation of this straight line to a zero fraction 

of atmospherically derived 4He yields a y-intercept (3He:4He 

ratio) of about 1.3 x 10-5, very similar to mantle ratios 

(Andrews, 1985). This ratio was used as the end-member 

terrigenic helium value in the age-dating corrections for those 

ground-water samples not significantly affected by terrigenically derived helium. With this ratio, the Monte Vermelho 

spring sample collected on December 2, 2005, has an apparent 

age of 5 to 50 years, indicating that recharge occurred after the 

1950s (table 6). 

Ribeira Paul

Ground-Water Discharge

The measurable sources of ground-water discharge in 

the Ribeira Paul Basin are to springs, seepage to streams, 

and well withdrawals. Because of the shallow water table in 

the basin, ground-water discharge as evapotranspiration also 

likely occurs. As in Mosteiros Basin, submarine ground-water 

outflow may be the dominant form of ground-water discharge, 

but this could not be quantified during this study. 

About 100 springs and spring-fed streams are reported in 

the Ribeira Paul Basin. Measured discharge at five sites where 

springs discharge directly from the subsurface ranged from 19 

to 100 m3/d (table 3). Measured discharge at 21 stream sites 

Baseline Ground-Water Conditions 25
26 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Table 5. Selected field measurements, stable isotopes, dissolved-gas concentrations, and estimated recharge temperatures of 

samples collected from the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde

[Sample type: precip, precipitation; Altitude: m, meters; Barometric pressure and Total dissolved gas pressure: mm Hg, millimeters mercury; Dissolved 

oxygen: mg/L, milligrams per liter, %, percent saturation; Specific conductance: μS/cm, microsiemens per centimeter at 25 degrees Celsius; Temperature: °C, 

degrees Celsius; Dissolved gases: cm3STP/g, cubic centimeters per gram at standard temperature and pressure; E, exponent; Most probable recharge temperature and altitude: average of most probable range based on intersection of Trmin/Trmax line with temperature lapse band; —, no data; <, less than; ≥, greater than 

or equal to]

Name Sample 

type Basin

Latitude Longitude

Date Altitude 

(m)

(degree/minute/ second)

Monte Vermelho Spring Mosteiros 150254 242147 10/29/1987 5 

8/12/2005 

12/2/2005 

1

FF06 Well Mosteiros 150133 0241908 12/2/2005 60 

FF07 Well Mosteiros 150221 0242014 12/2/2005 20 

FF21 Well Mosteiros 150246 0242104 8/11/2005 40 

FF22 Well Mosteiros 150246 0242104 12/3/2005 40 

FF29 Well Mosteiros 150229 0242043 12/3/2005 60 

Cha das Caldeiras wine Precip. Mosteiros 145830 0242150 2004 1,650 

Cha das Caldeiras juice Precip. Mosteiros 145830 0242150 2005 1,650 

Dragueiro #1 Spring Paul 170816 0250231 12/8/2005 270 

NA21 Spring Paul 170834 0250234 8/6/2005 440 

Passagem Spring Paul 170756 0250232 8/4/2005 300 

Seladinha Spring Paul 170711 0250249 12/8/2005 460 

Tabulera Spring Paul 170654 0250328 8/4/2005 840 

FA57 Well Paul 170820 0250140 8/8/2005 80 

12/7/2005 80 

FA93 Well Paul 170849 0250104 8/5/2005 20 

Ribeira Grande Precip. Paul 171049 0250356 11/22/2005 18 

11/24/2005 

Vila das Pombas Precip. Paul 170851 0250048 11/23/2005 10 

G. Fajã - Monte Gordo Spring Fajã 163845 0241927 8/18/2005 270 

12/14/2005 

G. Fajã - Cruz Roque Spring Fajã 163845 0241927 12/15/2005 267 

G. Faja - Lombo Pelado Spring Fajã 163845 0241927 12/15/2005 265 

FN39 Well Fajã 163950 0241914 12/13/2005 90 

Posto Fajã Precip. Fajã 163829 0242012 9/16/2005 410 
Table 5. Selected field measurements, stable isotopes, dissolved-gas concentrations, and estimated recharge temperatures 

of samples collected from the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde—Continued

Field measurements Stable isotopes

Barometric 

pressure 

(mm Hg)

Total 

dissolved 

gas 

pressure 

(mm Hg)

Total 

dissolved 

gas 

pressure

 (atm)

Dissolved 

oxygen 

(mg/L and 

(percent 

saturation))

Specific 

conductance 

(μS/cm)

pH 

(standard 

units)

Temperature 

(°C)

δ2H 

(permil)

δ18O 

(permil)

— — — — — — — -22.3 -3.94

— — — — 200 7.2 — -21.0 -3.78

758 763 1.00 7.6 (88%) 210 7.3 22.2 -18.0 -3.76

755 758 1.00 5.6 (61%) 2,280 6.8 20.3 -27.7 -5.08

758 747 .98 7.5 (87%) 2,240 7.8 22.8 -14.8 -3.35

757 766 1.01 7.1 (82%) 870 7.0 23.0 -19.7 -3.74

758 756 .99 7.1 (82%) 700 7.4 22.4 -18.5 -3.83

754 758 1.00 6.4 (74%) 1,440 7.4 22.7 -17.7 -3.84

— — — — — — — — —

— — — — — — — — —

738 721 .95 3.0 (38%) 375 7.1 23.4 -15.9 -3.34

743 729 .96 4.6 (54%) 265 7.0 23.7 -17.0 -3.30

741 730 .96 5.7 (73%) 510 7.2 22.6 -18.3 -3.48

720 712 .94 6.5 (78%) 450 7.8 21.3 -17.2 -3.55

694 689 .91 7.5 (88%) 220 7.6 18.9 -29.1 -4.93

— — — — 580 7.1 — -15.7 -3.08

753 777 1.02 5.0 (61%) 580 7.4 24.0 -13.7 -3.19

762 745 .98 5.3 (60%) 710 7.2 23.0 -15.8 -2.97

— — — — — — — -16.3 -3.43

— — — — — — — -18.5 -3.55

— — — — — — — -32.1 -4.62

738 737 .97 7.2 (84%) 380 7.8 21.3 -21.6 -3.98

737 744 .98 7.9 (89%) 380 8.3 21.3 -21.3 -4.04

— — — — 440 — — -21.3 -3.95

— — — — 550 — — -19.4 -3.84

755 760 1.00 5.7 (68%) 500 7.6 23.9 -20.6 -3.93

— — — — — — — -5.0 -.63

Baseline Ground-Water Conditions 27
28  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

Table 5. Selected field measurements, stable isotopes, dissolved-gas concentrations, and estimated recharge temperatures of samples 

collected from the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde—Continued

Name

Dissolved gases

Excess air 

(cm3STP/g)

Most 

probable 

recharge 

temperature 

(°C)

Most 

probable 

recharge 

altitude 

(meters)

N2

(cm3STP/g)

40

Ar 

(cm3STP/g)

84Kr 

(cm3STP/g) 

20Ne 

(cm3STP/g)

4

He 

(cm3STP/g)

129

Xe

(cm3STP/g)

Monte Vermelho — — — — — — — — —

— — — — — — — — —

1.11E-2 2.98E-4 3.77E-8 1.71E-7 4.49E-8 2.37E-9 .000 19 1,000

1

FF06 — — — — — — — — —

FF07 1.25E-2 3.24E-4 3.98E-8 1.82E-7 5.43E-8 2.48E-9 .001 19 1,000

FF21 1.17E-2 3.08E-4 3.83E-8 1.69E-7 2.20E-7 2.33E-9 .000 19 1,000

FF22 1.28E-2 3.27E-4 4.37E-8 1.66E-7 1.71E-7 2.67E-9 .000 13 1,800

FF29 1.41E-2 3.72E-4 4.66E-8 1.93E-7 4.31E-7 2.93E-9 .001 <7 ≥2,000

Cha das Caldeiras wine — — — — — — — — —

Cha das Caldeiras juice — — — — — — — — —

Dragueiro #1 1.32E-2 3.42E-4 4.15E-8 1.91E-7 5.08E-8 2.65E-9 .002 13 1,300

NA21 1.14E-2 2.98E-4 3.79E-8 1.64E-7 4.47E-8 2.19E-9 .000 19 900

Passagem 1.16E-2 2.96E-4 3.59E-8 1.66E-7 4.51E-8 2.21E-9 .000 20 900

Seladinha 1.10E-2 2.86E-4 3.52E-8 1.58E-7 4.23E-8 2.13E-9 .000 21 900

Tabulera 1.16E-2 2.99E-4 3.86E-8 1.57E-7 4.19E-8 2.42E-9 .000 17 1,100

FA57 — — — — — — — — —

1.45E-2 3.24E-4 4.34E-8 1.80E-7 6.09E-8 2.57E-9 .001 13 1,300

FA93 1.33E-2 3.30E-4 3.91E-8 1.82E-7 4.98E-8 2.37E-9 .001 16 900

Ribeira Grande — — — — — — — — —

— — — — — — — — —

Vila das Pombas — — — — — — — — —

G. Fajã - Monte Gordo 1.21E-2 3.20E-4 3.94E-8 1.74E-7 6.04E-8 2.63E-9 .001 16 900

1.23E-2 3.05E-4 3.75E-8 1.54E-7 5.77E-8 2.51E-9 .000 219 —

G. Fajã - Cruz Roque — — — — — — — — —

G. Faja - Lombo Pelado — — — — — — — — —

FN39 1.08E-2 2.92E-4 3.68E-8 1.69E-7 4.44E-8 2.08E-9 .001 22 700

Posto Fajã — — — — — — — — —

1

The diffusion sampler for this site was improperly sealed; no dissolved gases were analyzed.

2

The chi squared error is too high, likely caused by natural or sample-collection gas stripping (causing artificially high recharge temperatures).
Table 6. Tritium and helium concentration and estimated age of samples collected from the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde

[3H and precision: TU, tritium units; 4He: cm3STP/g, cubic centimeters per gram at standard temperature and pressure; R/Ra: Measured 3He:4He isotopic ratio relative to the helium isotopic ratio of air; 4Heterr: terrigenic helium, cm3STP/g, cubic centimeters per gram at standard temperature and pressure; 3Hetrit: tritiogenic helium, TU, tritium units; 3Hetrit and Apparent 3H/3He age: ranges based on terrigenic 3He:4He ratios between 1.3 x 10-5 and 1 x 10-8; NMF, not meaningful; —, no data; ±, plus or minus; <, less than; >, greater than]

Name Sample type Basin Latitude Longitude Date 3H and precision (TU) 4He (cm3STP/g) R/Ra 4Heterr (cm3STP/g) 3Hetrit (TU) Apparent 3H/3He age (year) Apparent recharge year (degree/minute/ second)

Monte Vermelho Spring Mosteiros 150254 0242147 8/12/2005 0.8 ± 0.1 — — — — — 1post-1950s

12/2/2005 .4 ± .1 4.49E-8 1.028 <1.0E-10 0 to 6 5 to 50 post-1950s

FF06 Well Mosteiros 150133 0241908 12/2/2005 .2 ± .1 — — — — — 1pre-1950s

FF07 Well Mosteiros 150221 0242014 12/2/2005 1.2 ± .1 5.43E-8 1.893 5.02E-9 NMF NMF 1post-1950s

FF21 Well Mosteiros 150246 0242104 8/11/2005 .3 ± .1 2.20E-7 7.540 1.74E-7 NMF NMF 1pre-1950sFF22 Well Mosteiros 150246 0242104 12/3/2005 .9 ± .2 1.71E-7 7.002 1.26E-7 NMF NMF 1post-1950s

FF29 Well Mosteiros 150229 0242043 12/3/2005 .2 ± .1 4.31E-7 8.296 3.80E-7 NMF NMF 1pre-1950sCha das Caldeiras Wine Mosteiros 145830 0242150 2004 3.0 ± .1 — — — — — —

Cha das Caldeiras Juice Mosteiros 145830 0242150 2005 1.0 ± .1 — — — — — —

Dragueiro #1 Spring Paul 170816 0250231 12/8/2005 .5 ± .1 5.08E-8 1.039 <1.0E-10 1 to 5 15 to 45 post-1950sNA21 Spring Paul 170834 0250234 8/6/2005 .7 ± .1 4.47E-8 1.004 3.55E-10 0 to 0.3 0 to 10 post-1950s

Passagem Spring Paul 170756 0250232 8/4/2005 1.2 ± .1 4.51E-8 1.252 <1.0E-10 6 to 7 30 to 40 post-1950sSeladinha Spring Paul 170711 0250249 12/8/2005 2.1 ± .1 4.23E-8 .998 <1.0E-10 0 to 2 0 to 15 post-1950s

Tabulera Spring Paul 170654 0250328 8/4/2005 .2 ± .1 4.19E-8 1.004 4.89E-10 0 to 0.3 >50 pre-1950sFA57 Well Paul 170820 0250140 8/8/2005 1.7 ± .1 — — — — — —

12/7/2005 .9 ± .1 6.09E-8 2.645 1.11E-8 NMF NMF 1post-1950s

FA93 Well Paul 170849 0250104 8/5/2005 1.3 ± .1 4.98E-8 1.071 4.39E-10 0 to 2 0 to 20 post-1950sRibeira Grande Precip. Paul 171049 0250356 11/24/2005 1.1 ± .1 — — — — — —

Vila das Pombas Precip. Paul 170851 0250048 11/23/2005 1.4 ± .1 — — — — — —G. Fajã - Monte 

Gordo

Spring Fajã 163845 0241927 8/18/2005 .2 ± .1 6.04E-8 3.330 1.40E-8 NMF NMF 1pre-1950s

12/14/2005 .0 ± .1 5.77E-8 3.421 1.44E-8 NMF NMF 1pre-1950s

G. Fajã - Cruz Roque Spring Fajã 163845 0241927 12/15/2005 .0 ± .1 — — — — — 1pre-1950sG. Faja - Lombo 

Pelado

Spring Fajã 163845 0241927 12/15/2005 .0 ± .1 — — — — — 1pre-1950s

FN39 Well Fajã 163950 0241914 12/13/2005 .3 ± .1 4.44E-8 1.003 <1.0E-10 0 to 8 > 50 pre-1950s

1Apparent recharge year is only based on 3H concentration.

Baseline Ground-Water Conditions 29
30 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

ranged from 19 to about 2,100 m3/d. These streams include 

both high-altitude spring-fed streams and lower-altitude 

coalescing flow from tributary streams and irrigation canals. 

The 27 spring- and stream-discharge measurements made 

during this study provide a baseline for future evaluation of 

ground-water resources in the Ribeira Paul Basin.

A total spring and spring-fed stream discharge of about 

4,000 m3/d is estimated for the Ribeira Paul Basin. This 

quantity is based primarily on a cross section established at the 

Ribeira Cha João Vaz site along Ribeira Paul stream channel 

(fig. 7). The combined discharge of about 3,800 m3/d from the 

streambed and three diversions at this site (Ribeira Cha João 

Vaz A through D, table 3) represent all of the discharge above 

this point in the basin except for (1) the two black plastic pipes 

parallel to the stream carrying public-supply water, and (2) 

upgradient consumptive use of spring discharge by natural 

flora and irrigated agriculture. The public-supply pipes are 

small diameter and divert a relatively small proportion of the 

upper basin flow. The amount of consumptive use by natural 

flora and irrigated agriculture in the upper basin was not evaluated during this study. In addition to the Ribeira Cha João Vaz 

cross section, a total flow of about 200 m3/d from mid-basin 

springs (NA21, Dragueiro #1, Dragueiro #2) and spring-fed 

streams (Ribeirinha and Ribeira Cabouço) along the northwest 

wall of the valley is included in the total discharge estimate. 

Because the discharge at Ribeira Cha João Vaz was measured 

in stream channels soon after the end of the rainy season, it 

may include some surface-water flow and could overestimate 

the yearly average ground-water discharge above this point. In 

contrast, because the measured discharge at Ribeira Cha João 

Vaz does not include evapotranspiration or diverted publicsupply water, it may be an underestimate of actual groundwater discharge. Although there is much uncertainty in the 

4,000 m3/d spring-discharge estimate, it is generally consistent 

with the previous estimate of 7,500 m3/d of combined ground0.0E+00

2.0E-06

4.0E-06

6.0E-06

8.0E-06

1.0E-05

1.2E-05

1.4E-05

0.0 0.2 0.4 0.6 0.8 1.0 1.2

FRACTION OF 4HELIUM DERIVED FROM ATMOSPHERE, CORRECTED FOR EXCESS AIR

RATIO OF 3HELIUM TO 4HELIUM, CORRECTED 

FOR EXCESS AIR

Linear fit

Drageiro Spring

NA21 Spring

Galleria Passagem

Seladinha Spring

Tabulera Spring

Well FA93

Monte Vermelho Spring

Well FN39

Well FA57

Well FF07

Well FF21

Well FF22

Well FF29

Galleria Fajã - August

Galleria Fajã - December

Samples not

affected by

terrigenic

helium

End-member

terrigenic

helium ratio

EXPLANATION

Figure 18. Relation between the 3Helium:4Helium ratio and fraction of 4Helium derived from atmospheric gases in 

ground water from the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Cape Verde.
water infiltration made by Vailleux and Bourguet (1974) for 

both Ribeira Paul Basin and the adjacent Ribeira Mesa Basin.

Well FA57 is the only production well in the Ribeira 

Paul Basin. An instantaneous pumping rate of 190 m3/d was 

measured on August 8, 2005. It pumps about 4 hours per day 

to provide culinary water for the town of Vila das Pombas 

(Joãs Jose Andrade Sausa, Santo Antão Agua e Sanitación, 

oral commun., 2005), resulting in an average daily discharge 

of about 30 m3. Well withdrawal, therefore, is a relatively 

insignificant part of the total ground-water discharge from the 

Ribeira Paul Basin.

The average annual volume of precipitation falling within 

the Ribeira Paul Basin is estimated to be about 1.0 x 107 m3. 

This is based on an estimated average annual precipitation 

of 650 mm and a basin area of 15 km2. Unlike the other two 

basins, there is no clear trend of increasing precipitation with 

increasing altitude in Ribeira Paul. Therefore, rather than 

using a linear interpolation and average basin altitude, the 

estimated 650 mm annual precipitation is based on the average 

of the 1996-2002 average precipitation for the three precipitation stations. Combining estimated total spring discharge with 

withdrawals from the one production well, the total spring 

and well discharge from the Paul Basin is estimated to be 

about 4,000 m3/d or 1.5 x 106 m3 /yr. Subsurface outflow as 

submarine discharge is thought to be a large component of the 

ground-water budget but could not be quantified during this 

study. By using the estimated average annual 1.0 x 107 m3 volume of precipitation falling within the basin, about 15 percent 

of this precipitation is calculated to discharge from the aquifer 

through wells and springs.

Ground-Water Levels

During this study, ground-water levels in the Ribeira Paul 

Basin were measured only at well FA93. This well is currently 

the only open borehole suitable for water-level observations. 

It was drilled in 2004 and is located at an altitude of about 

20 m along the lower end of the Ribeira Paul stream channel. Although most of Ribeira Paul is perennial, this lowest 

reach is ephemeral and generally does not flow during the dry 

season. The well was drilled to a depth of 21 meters and is finished in streambed alluvium. Electric-tape and barometrically 

corrected transducer water-level measurements (fig. 19) indicate that the water level has varied between 14.6 and 16.4 m 

below land surface. Because of the relatively large water-level 

changes at this well, it was not necessary to correct for tidal 

effects. The highest level of 14.6 m was recorded on January 

29, 2006, shortly after the rainy season and when the nearby 

stream had flowing water. A vented transducer and data logger 

were installed in well FA93 in December 2005 to continuously 

record ground-water levels. In combination with electric-tape 

measurements in August 2005 and June 2006, the hydrograph 

shows a rise of about 0.8 m between August and January, 

followed by a decline of about 1.8 m between January and 

June. This is likely associated with the nearby stream, which 

only flows for a short time during and after the rainy season. 

In 2005, the major precipitation events occurred in November, 

with flow remaining in the stream channel through January. 

Because of the strong effects of the nearby stream, additional 

water-level measurements are needed to confirm that the 

changes are seasonal and not longer-term trends. Because this 

14.4

14.6

14.8

15.0

15.2

15.4

15.6

15.8

16.0

16.2

16.4

16.6

7/22/20058/21/20059/20/2005

10/20/200511/19/200512/19/2005

1/18/20062/17/20063/19/20064/18/20065/18/20066/17/2006

DEPTH TO WATER BELOW LAND SURFACE, IN METERS

Water level

Check measurement

Figure 19. Water level in well FA93, Ribeira Paul Basin, August 2005 through June 2006.

Baseline Ground-Water Conditions  31
32 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

well is so close to the stream, however, it may not be suitable 

for evaluating regional ground-water levels.

FA57 is the only other well in the Ribeira Paul Basin. 

It has an installed submersible pump and, thus, no access for 

water-level measurements. The original FA57 drillers’ log 

reported a depth to water of 1.60 m below land surface (80 m) 

on August 17, 1993. Assuming steady-state conditions in the 

basin (no substantial change in ground-water levels with time), 

a horizontal hydraulic gradient between the FA57 and FA93 

wells of 0.05 can be estimated on the basis of water-level 

altitudes of 78 m at FA57 and 4 m at FA93, and a distance of 

1,400 meters between the two sites. This is similar to groundwater hydraulic gradients reported for a basin in western 

Santiago having similar altitudes, geology, and precipitation 

(Barmen and others, 1984).

Water Quality and Environmental Tracers

The specific conductance of ground water in the Ribeira 

Paul Basin varies between 190 and 710 μS/cm (tables 3, 5). 

The lowest specific-conductance values (about 200 μS/cm) 

were measured at the Tabulera, Zeagrios #4 and Zeagrios 

#5 sites. At 840 m, Tabulera is the highest-altitude spring 

measured in the study (fig. 7) and discharges from the base of 

a thick sequence of basalt. Zeagrios #4 and #5 are sites along 

a spring-fed stream at an altitude of about 690 m (the actual 

spring discharge location is not accessible because of the steep 

terrain). The highest specific-conductance value (710 μS/cm) 

was measured at well FA93 at an altitude of 20 m, the lowestaltitude ground-water sampling point in the basin. In general, 

the specific-conductance value of ground water increases with 

decreasing altitude and proximity to the ocean. This is likely 

caused by (1) irrigation and wastewater return flow in the 

streams, (2) recharge to the lower-altitude springs from highersalinity precipitation and sea spray closer to the ocean, or (3) 

longer flow paths and residence times. 

Ground-water pH and temperature ranges in the Ribeira 

Paul Basin are 7.0 to 7.8 and 18.9°C to 24.0°C, respectively 

(table 5). These measurements were taken in water from wells 

and springs that discharge directly from the subsurface and do 

not include water from spring-fed streams where mixing with 

the atmosphere would affect these parameters. These values 

are similar to those of ground water from the other two study 

basins and do not indicate upwelling geothermal sources of 

water. As with specific conductance, both pH and temperature 

show altitude trends: pH generally increases with altitude and 

temperature generally decreases with altitude, as expected 

from the temperature lapse rate. As with the Mosteiros Basin, 

the relatively high dissolved-oxygen concentration of 3.0 to 

7.5 mg/L (38 to 88 percent saturation) indicate oxidized conditions within the aquifer and the lack of substantial bacterial 

decay of organic matter. Measured total dissolved-gas pressure values range from 689 to 777 mm Hg and are generally 

similar to barometric pressure measurements at each site (694 

to 762 mm Hg), indicating little entrapped (excess) air or gas 

production in the aquifer. The range of values for both total 

dissolved-gas pressure and barometric pressure in the Ribeira 

Paul Basin is larger than that of the other two basins because 

of the larger difference in altitude of the measured groundwater sites.

The stable-isotope ratios of ground water from the 

Ribeira Paul Basin generally follow the global meteoric water 

line (fig. 15), with the most-depleted δ18O and δ2H values of 

-29.1 and -4.93 permil, respectively, for the Tabulera spring 

(highest-altitude site) and the least-depleted δ18O and δ2H 

values of -13.7 and -3.19 permil, respectively, for well FA57 

(lower in the basin). Precipitation samples during a 3-day 

rain event in late November 2005 were collected by U.S. 

Peace Corps volunteers in and near the Ribeira Paul Basin for 

stable-isotope analyses. One sample was collected in Vila das 

Pombas at the mouth of Ribeira Paul and two others were collected in nearby Ribeira Grande, along the coast about 20 km 

to the northwest of Vila das Pombas. The two Ribeira Grande 

samples plot along the meteoric water line but are generally 

more enriched than the ground-water samples from Ribeira 

Paul, likely because of the lower altitude of the precipitation-collection site compared to the ground-water sites. The 

Vila das Pombas sample is very isotopically depleted (δ18O of 

-4.63 permil and δ2H of -32.1 permil; table 5, fig. 15) but also 

shows an evaporative shift off the meteoric water line, possibly 

caused by evaporation occurring between the precipitation 

event and sample placement in a sealed container. 

Like Mosteiros, most of the dissolved gases in groundwater samples from the Paul Basin are of atmospheric origin, 

as indicated by the close comparison of measured and theoretical concentrations of the individual gases and small amounts 

of excess air (≤0.0015 cm3/g). Calculated most-probable 

recharge temperatures range from 13-21°C for the springs and 

wells of Ribeira Paul Basin. The coolest recharge temperatures were 13°C, measured at the Dragueiro #1 spring and 

well FA57 (table 5). The warmest recharge temperatures were 

more than 20°C, measured at the Passagem and Seladinha 

springs. Plotting Trmin/Trmax ranges for each sample against the 

temperature lapse curve (fig. 16) indicates recharge altitudes 

from about 900 m for Passagem and Seladinha to 1,300 m 

for Dragueiro #1 and well FA57. These recharge altitudes are 

higher than the land-surface altitudes at all of these sites (20 

to 840 m), indicating that recharge to these wells and springs 

does not occur locally, but rather higher up in the watershed. The highest recharge altitude of 1,300 m indicates that 

recharge occurs either near the basin divide or in the La Cova 

caldera (fig. 7). It also indicates that the water-table altitudes 

in Paul basin are much higher than sea level, either as a result 

of horizontal flow barriers and compartmentalization or low 

permeability of the basalts. This is consistent with the numerous intrusive dikes visible in outcropping rocks, as well as the 

exposed tuffs and breccias in the center of the basin (fig. 8). 

Two precipitation 3H samples were collected in and near 

Ribeira Paul Basin during a rain event in late November 2005. 

These samples had 3H concentrations of 1.1 to 1.4 TU (table 

6). Ground-water 3H concentrations from samples collected in 

Paul Basin ranged from 0.2 to 1.7 TU. Tabulera was the only 
site having 3

H concentrations less than 0.3 TU, indicating that 

ground-water recharge to this spring occurred prior to the early 

1950s. The other springs in Paul Basin had 3H concentrations 

ranging from 0.5 to 1.2 TU. Water from wells FA57 and FA93 

had 3H concentrations of 0.9 to 1.7 TU. 

3

H /3He age dating was useful at five springs and one well 

in Ribeira Paul Basin: Tabulera, Dragueiro #1, NA21, Passagem, Seladinha, and well FA93 (table 6). These sites indicate that ground water in Ribeira Paul is a mixture of waters 

recharged both pre- and post-1950s. Water collected from 

Tabulera had an apparent age of more than 50 years, indicating 

recharge prior to the 1950s. Water collected from Dragueiro 

#1, NA21, Passagem, Seladinha, and FA93 all have apparent 

ages of less than 50 years, indicating recharge since the 1950s. 

Compared with Mosteiros Basin, there was a much lesspronounced influence of mantle-derived helium on ground 

water in Ribeira Paul Basin. This is likely because of the much 

longer time since there was active volcanism on the island of 

Santo Antão. Only one well (FA57) showed the influence of 

terrigenic helium (fig. 17, table 6). The 3He:4He ratio of this 

ground-water sample was about 2.5 times the atmospheric 

ratio and 4He was slightly above the atmospherically equilibrated concentration. Because of this terrigenic 4

He (fig. 18), 3

H/3He age dating was not possible at this one site. Water at 

the other well in Ribeira Paul Basin (FA93) is assumed to be 

coming from the adjacent stream and fell as recent precipitation. The sample had no tritiogenic helium and a 3

H concentration of 1.3 TU, within the range of current precipitation tritium concentrations measured during this study. Because this 

young water comes from a well screened in shallow alluvium 

(total depth = 21 m) and is adjacent to the stream, there is a 

high potential for anthropogenic contamination from upstream 

sources (nitrates, pathogenic bacteria). Although the water 

has low salinity, the measured specific-conductance value 

was higher than that measured at any other location in Ribeira 

Paul, possibly because of irrigation return flow or discharge of 

wastewater. 

Ribeira Fajã

Ground-Water Discharge

The main source of ground-water discharge in the Ribeira 

Fajã Basin is from the Galleria Fajã tunnel. No natural springs 

or ground-water evapotranspiration occurs within the basin 

and there is currently no well production. As in the other 

basins, submarine discharge may be the dominant form of 

ground-water discharge from the Ribeira Fajã Basin, but this 

could not be evaluated during the study.

The 1,800-m-long Galleria Fajã tunnel was constructed 

during the 1980s to act as a ground-water drain and supply a 

steady source of water for agriculture and public supply. The 

tunnel begins at an altitude of about 250 m near the town of 

Fajã de Baixo. It trends southwest toward the upland areas of 

the basin (fig. 9) at a slope of about 1 percent. Water enters 

the upper end of the tunnel at Lombo Pelado, Cruz Roque, and 

Monte Gordo, with altitudes between 265 and 270 m. Monte 

Gordo is the largest source, providing all of the culinary water 

and the majority of the irrigation water for the basin. 

Construction of the Galleria Fajã tunnel began in 1980 

under the supervision of the French government and was 

completed in 1986. The first discharge measurements were 

made in 1984 (fig. 20). Discharge rapidly increased to about 

1,500 m3/d by late 1984 (Geoffray, 1985). No discharge measurements are available from May 1985 to September 1986; 

discharge likely exceeded 1,500 m3/d during this interval. 

Discharge rapidly decreased from late 1986 to late 1997 and 

has remained fairly constant through 2005 at about 400 m3/d 

(measured by Servicio Autonimo Fajã (SAF) and reported by 

Ministero de Ambiente e Agricultura, 2005). The USGS evaluated the accuracy of the flow-meter measurements reported 

by SAF for the combined flow from the Lombo Pelado, Cruz 

Roque, and Monte Gorde irrigation spring by making independent bucket and stop-watch discharge measurements of 

the four separate springs on the same date as a combined SAF 

measurement (December 13, 2005). In order to compare the 

total flow measurements using both techniques (bucket and 

stopwatch versus in-line flow meter), 60 m3/d (the amount of 

public-supply water) was added to the metered irrigation pipe 

flow of 370 m3/d measured by the SAF. There is no separate 

in-line instantaneous flow meter for measuring the public-supply portion of the Monte Gordo spring, so the 60 m3

/d was 

calculated from a totalizing meter monthly flow calculation of 

the public-supply water reported by SAF (Ministero de Agricultura e Ambiente, written commun., 2005). This analysis 

results in a reported discharge of 430 m3/d for December 13, 

2005, compared with a total measurement of 470 m3/d for the 

four separate spring sources within the galleria measured by 

the USGS (table 3). The 10-percent difference is within the 

uncertainty generally associated with both in-line flow meters 

and the bucket-and-stopwatch method.

The annual precipitation at the Cachaço station shows 

that the decrease in Galleria Fajã discharge during 1986-97 

was not caused by drought. The average annual precipitation 

during this period (320 mm) was about the same as the average precipitation of 340 mm during the entire period (1984-

2005) of Galleria Fajã discharge measurements. The Galleria 

acts as a large ground-water drain for the Ribeira Fajã Basin 

and the large decline in discharge from 1986 through 1997 

was likely a result of dewatering of the aquifer in response to 

the construction of the tunnel. This is supported by historical spring discharge observations and water-level data from 

observation wells FN12, FN21, FN39, and FN41 (all drilled 

between 1978 and 1980; table 4) showing rapidly declining 

ground-water levels. According to local accounts (Armindo 

Basilio, Servicio Autonomo de Agua, Ribeira Brava, oral commun., 2005), a fairly large perennial spring discharged from 

the sea cliffs north of the town of Queimada (fig. 9) prior to 

construction of the Galleria, but went dry soon after completion of the tunnel. There are no accounts of any other surface 

Baseline Ground-Water Conditions  33
34  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

0

100

200

300

400

500

600

700

800

1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

 PRECIPITATION, IN MILLIMETERS 

PER YEAR

0

200

400

600

800

1,000

1,200

1,400

1,600

GALLERIA FAJA DISCHARGE, IN CUBIC

METERS PER DAY

(Estimated)

Vailleux (1985); Geoffray (1985); Ministerio do Ambiente, Agricultura e Pescas (written commun., 2005); Instituto Nacional de Meterologia e Geofisica (written commun., 2005). (Missing)

Discharge

Precipitation

Figure 20. Galleria Fajã discharge and annual precipitation at nearby Posto Cachaço, Ribeira Fajã Basin, 1984-2005.

water, besides ephemeral flow after large precipitation events, 

occurring in the Ribeira Fajã Basin prior to the construction of 

Galleria Fajã. 

Wells FN41 and FN21 are located about 820 m and 1,700 

m, respectively, from the main Monte Gordo spring at the head 

of the tunnel (fig. 9). The hydrographs of these wells (figs. 21, 

22) show ground-water levels declining rapidly beginning in 

1985. Well FN41 went dry in November 1986 and well FN21 

went dry in February 1987. Well FN39 is located about 4,000 

m from the Monte Gordo spring. Between 1982 and 1998, 

water levels in this well decreased by 20 m (from about 53 m 

to about 73 m below land surface; table 4). The loss of flow 

at the Queimada spring and the rapid water-level decline in 

these three observation wells is directly attributed to dewatering caused by the completion of the Galleria Fajã tunnel. Their 

response to the Galleria Fajã also indicates that, despite the 

fairly complex geology of the basin (fig. 10), the wells, spring, 

and tunnel are all hydraulically connected within the same 

aquifer system.

The average annual volume of precipitation falling 

within the Ribeira Fajã Basin is estimated to be about 5 x 106

m3. This is based on an average basin altitude of 483 m, an 

interpolated annual precipitation value of 300 mm, and a basin 

area of 16 km2. The interpolated 300-mm annual precipitation 

is based on a linear interpolation (r2 = 0.77) of the 1996-2002 

average precipitation for the three precipitation stations in the 

Ribeira Fajã Basin. For the water-budget estimate, Galleria 

Fajã is assumed to be the only measurable ground-water discharge from the basin, with a flow of 400 m3

/d (fig. 20). The 

total measurable annual discharge from the basin, not including submarine discharge, is estimated to be about 150,000 m3

per year. 

Ground-Water Levels

Ground-water levels were measured during this study in 

Ribeira Fajã Basin only at well FN39. An attempted waterlevel measurement by the INGRH at well FN41 on March 

8, 2006, confirmed that this well remains dry. A non-vented 

transducer, barometer, and data logger were installed in 

December 2005 to continuously record ground-water levels 

and barometric pressure in well FN39. Small water-level 

fluctuations of less than 0.1 m occurred between December 

2005 and March 2006 (fig. 23), much less than the 20-m 

decline during the 1980s and 1990s. Since 1998, water levels 

have remained fairly constant in well FN39 (table 4). Both the 

steady ground-water levels and the relatively constant tunnel 

discharge since 1998 indicate that a new ground-water equilibrium in the basin has been established in response to construction of the Galleria Fajã.

Because observation wells FN41 and FN39 are aligned 

along the northeast-trending axis of the Ribeira Fajã Basin, 

it is likely that they generally lie along a ground-water flow 

path. The pre-Galleria horizontal hydraulic gradient can be 

calculated by dividing the difference in the water-table altitude 

at these two wells by their distance apart. In 1980, the watertable altitudes in wells FN41 and FN39 were about 316 m 

and 39 m, respectively, yielding a water-level difference of 

277 m. The distance between the two wells is about 3,450 m, 

resulting in a horizontal hydraulic gradient of 0.08. Assuming 

the 270-m altitude of the Monte Gordo spring at the head of 

the Galleria Fajã represents the water table at that location, a 

similar calculation can be made of the post-Galleria horizontal 

hydraulic gradient. The recently measured water-level altitude 

at FN39 is 17 m, based on a land-surface altitude of 90 m and 

a depth to water of 73 m. Therefore the water-level difference 

between the Monte Gordo spring and FN39 is about 253 m. 

The distance between the spring and well is about 3,970 m, 

yielding a horizontal hydraulic gradient of 0.06. This indi-
cates that the dewatering associated with the Galleria Fajã has 

resulted in a less-steep hydraulic gradient in the lower twothirds of the Ribeira Fajã Basin. The opposite is probably true 

for the upper one-third of the basin – the hydraulic gradient 

in this upland area has likely increased since the construction 

of the Galleria. Both the pre- and post-Galleria horizontal 

hydraulic gradients are slightly higher than the gradient measured in the lower Ribeira Paul Basin.

Galleria Fajã discharge measurements and water-level 

drawdown in wells FN21 and FN41 during the mid-1980s 

were evaluated to estimate aquifer hydraulic properties in 

Ribeira Fajã. Two alternative methods were used. A transient 

numerical ground-water flow model of the aquifer yielded 

approximate values for transmissivity and storage of about 

20 m2/d and 0.01, respectively (K. Halford, U.S. Geological 

Survey, written commun., 2006). A multiple-well Theis curvematching method resulted in a transmissivity value of about 2 

m2/d and storage values between 0.003 and 0.009 (E. Weeks, 

U.S. Geological Survey, written commun., 2006). Assuming 

an aquifer thickness of 30 m, this range in transmissivity is 

equal to a hydraulic-conductivity range of 0.07 to 0.7 m/d, on 

the low end of the reported range of permeable basalts (0.1 

to 1,000 m/d; Freeze and Cherry, 1979) and similar to values 

reported for thick ponded Hawaiian basalts (Gingerich, 1999; 

Izuka and Gingerich, 1998). This indicates that the water table 

in Ribeira Fajã would have a steep gradient and be located at a 

substantial altitude above land surface. This is consistent with 

water-table altitudes, calculated from land surface and depthto-water measurements (table 4), of about 10 to 200 m.

Water Quality and Environmental Tracers

The measured specific-conductance value of ground 

water in the Galleria Fajã spring tunnel decreases with 

increasing distance from the tunnel entrance: 550 μS/cm at the 

Lombo Pelado spring, 440 μS/cm at the Cruz Roque spring, 

and 380 μS/cm at the Monte Gordo spring (table 5). Measured 

specific conductance in water from well FN39 was 500 μS/cm. 

Measured ground-water pH was 7.8 to 8.3 at the Monte Gordo 

spring and 7.6 at FN39. Ground-water temperature was 21.3°C 

at the Monte Gordo spring and 23.9°C at FN39. These values 

are similar to those of ground water from the other two study 

basins and do not indicate upwelling geothermal sources of 

water. Like the other two basins, relatively large dissolvedoxygen concentrations (5.7 to 7.9 mg/L; 68 to 89 percent 

saturation) indicate oxidized conditions within the aquifer and 

the lack of substantial bacterial decay of organic matter. Measured total dissolved-gas pressure values were 737 mm Hg at 

the Monte Gordo spring and 760 mm Hg in well FN39. These 

values are nearly identical to the barometric pressure measurements at each site (737 to 755 mm Hg), indicating little 

entrapped (excess) air or gas production in the aquifer.

172

174

176

178

180

182

184

186

188

190

May 1979 May 1980 May 1981 May 1982 May 1983 May 1984 May 1985 May 1986 May 1987

D

E

P

T

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RFA

C

E, IN

M

E

T

E

R

S

Construction of

Galleria Fajã

begins

Total depth of well (189.2 meters)

Galleria Fajã

completed

Initial Galleria Fajã

recorded

discharge

measurements

Figure 21. Water level in well FN41, Ribeira Fajã Basin, 1980-86.

Baseline Ground-Water Conditions  35
36 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

119

120

121

122

123

124

125

December 1983 December 1984 December 1985 December 1986 December 1987

D

E

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E, IN

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Total depth of well (124.2 meters)

Initial Galleria Fajã recorded

discharge measurements

Galleria Fajã

completed

Figure 22. Water level in well FN21, Ribeira Fajã Basin, 1984-87.

73.05

73.10

73.15

73.20

73.25

DEPTH TO WATER BELOW LAND SURFACE, IN METERS

JANUARY

2005 2006

DECEMBER FEBRUARY MARCH

Barometer-corrected

Barometer-corrected and tide-filtered

Figure 23. Water level in well FN39, Ribeira Fajã Basin, December 2005 through March 2006.
One precipitation sample was collected for stable-isotope 

analyses within Ribeira Fajã Basin for this study, with δ18O 

and δ2H values of -0.6 and -5.0 permil, respectively. Stableisotope concentrations for São Nicolau precipitation were 

previously reported for the 10 most important precipitation 

events of 1981, with δ18O values from -1.2 to -4.8 permil and 

δ2H values from -2 to -30 permil (Barmen and others, 1990). 

The stable isotope ratios of ground water from Galleria Fajã 

tunnel and well FN39 both plot along the meteoric water 

line within a very tight range, indicating a similar source of 

precipitation recharging the aquifer for these two locations 

(fig. 15). Values of δ18O range from -3.84 to -4.04 permil and 

values of δ2H range from -19.4 to -21.6 permil. Except for two 

samples (FF06 in the Mosteiros Basin and Tabulera Spring 

in the Ribeira Paul Basin), the stable-isotope ratios of ground 

water from the Ribeira Fajã Basin are more depleted than 

the other two basins, even though the average altitude of the 

Ribeira Fajã Basin is much lower than the average altitude of 

both the Mosteiros and the Ribeira Paul Basins (table 1). This 

indicates either that the majority of recharge in the Ribeira 

Fajã Basin is from higher-altitude source precipitation or the 

atmospheric patterns result in more isotopically depleted air 

masses entering the basin.

Like Mosteiros and Paul, most of the dissolved gases 

in ground-water samples from the Ribeira Fajã Basin are of 

atmospheric origin, as indicated by the close comparison of 

measured and theoretical concentrations of the individual 

gases and small quantities of excess air (≤0.001 cm3/g; table 

5). Calculated most-probable recharge temperatures are 16°C 

for the Galleria Fajã (Monte Gordo spring) and 22°C for well 

FN39. Plotting Trmin/Trmax ranges for each sample against the 

temperature lapse curve (fig. 16) indicates recharge altitudes 

of 900 m for the Galleria Fajã and 700 m for the FN39 well. 

These recharge altitudes are higher than the land-surface altitudes of the two sites (270 and 90 m, respectively), indicating 

that recharge does not occur locally, but rather higher up in 

the watershed (fig. 9). Like the other basins, this indicates that 

water-table altitudes in Ribeira Fajã Basin are much higher 

than sea level, likely caused by horizontal flow barriers and 

the low permeability of the basalts. This is consistent with the 

numerous intrusive dikes visible along the walls of the Galleria Fajã and the small values of hydraulic conductivity for the 

basalt determined by analyzing aquifer drawdown associated 

with completion of the tunnel. 

Ground-water 3H concentrations from samples collected 

in Ribeira Fajã Basin were all ≤ 0.3 TU, indicative of groundwater recharge occurring prior to the early 1950s. Both dissolved-gas samples collected in Galleria Fajã display evidence 

of terrigenic helium with R/Ra values greater than 3.0 (fig. 17, 

table 6). Because of this terrigenic helium (fig. 18), 3H/3He age 

dating of the Galleria Fajã samples was not possible. Water 

from well FN39, however, does not appear to be influenced by 

terrigenic helium; the estimated 3Hetrit concentration of 0 to 8 

TU results in an apparent age of greater than 50 years, indicating that recharge likely occurred prior to the 1950s.

Measured Ground-Water Discharge as 

a Percentage of Precipitation

As determined from measured ground-water discharge 

in the Mosteiros Basin, only about 1 percent of precipitation 

discharges from the aquifer as well withdrawals and spring 

flow. If a substantial part of the precipitation falling within the 

Cha das Caldeiras Cova caldera recharges the Mosteiros Basin 

ground-water system, this ratio of measured ground-water discharge to precipitation would be even less. Assuming steadystate conditions where ground-water recharge equals groundwater discharge, this infiltration ratio does not compare closely 

to values based on a previously published equation by Vailleux 

and Bourguet (1974) for estimation of ground-water infiltration in Cape Verde:

I = 0.25(P −300)	 (6)

where:

I is average annual infiltration in mm/yr, and 

P is average annual precipitation, in mm/yr. 

Applying equation 6 to Mosteiros Basin results in an 

infiltration rate of 73 mm/yr, or 12 percent of the estimated 

average annual precipitation of 590 mm/yr for the basin. The 

likely reason for this large discrepancy is that actual net infiltration and ground-water recharge rates are much larger than 

indicated by measured ground-water discharge, but much of 

this recharge is lost as unmeasured submarine outflow to the 

ocean. This prospect of large (and currently unexploited) submarine discharge was first proposed by Kallrén and Schreiber 

(1988). 

For the Ribeira Paul Basin, about 15 percent of the 

average annual 1.0 x 107 m3 volume of precipitation falling 

within the basin was measured as ground-water discharge. A 

similar infiltration rate of about 13 percent (88 mm/yr) of the 

annual precipitation for Ribeira Paul Basin was calculated by 

using equation 6. The actual percent of precipitation within 

the basin that recharges the aquifer and then discharges to 

wells and springs, however, may be lower than 15 percent 

if a substantial part of the precipitation falling within the La 

Cova caldera also flows into the aquifer beneath the Ribeira 

Paul Basin. Alternatively, if there is substantial ground-water 

evapotranspiration (agricultural or natural flora consumptive 

use) or submarine outflow of ground-water from the basin, 

the percentage of precipitation recharging the aquifer may be 

higher than 15 percent. 

For the Ribeira Fajã Basin, about 3 percent of the average 

annual 5 x 106 m3 volume of precipitation falling within the 

basin was measured as ground-water discharge. This estimated 

percent of precipitation is likely too low for two reasons: (1) 

the estimated basin-average precipitation value of 300 mm/yr 

may be higher than the long-term average because the 1961-

2005 average annual precipitation at the Cachaço station was 

294 mm, compared with the 1996-2002 average of 415 mm; 

Measured Ground-Water Discharge as a Percentage of Precipitation  37
38  Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

and (2) there likely is additional ground-water discharge as 

subsurface outflow directly to the ocean that could not be 

quantified during this study. The ground-water infiltration 

estimated for Ribeira Fajã Basin by using equation 6 (Vailleux 

and Bourguet, 1974), however, is even lower (0 mm/yr). Equation 6 assumes that no ground-water infiltration occurs in areas 

receiving less than 300 mm annual precipitation. However, if 

the Ribeira Fajã basin was divided into distinct precipitation 

zones rather than using one mean basin average, the equation 

would predict some infiltration in the higher-altitude parts of 

the basin where precipitation likely exceeds 300 mm/yr.

In summary, measured ground-water discharge as a percent of precipitation was about one percent for the Mosteiros 

Basin, three percent for the Ribeira Fajã Basin, and fifteen 

percent for the Ribeira Paul Basin. These are large differences, 

particularly between the Mosteiros and Ribeira Paul Basins, 

which receive similar average annual rainfall. Actual net 

infiltration and recharge as a percentage of precipitation may 

be smaller in Mosteiros Basin because of the smaller amount 

of soil cover, which results in less water retention and more 

runoff during storms. However, a large amount of unmeasured 

submarine ground-water discharge from the Mosteiros Basin 

also would cause this apparently small net-infiltration percentage. The large regional spring discharge near the high-tide 

mark at Monte Vermelho may be evidence for this alternative 

hypothesis. The steeper topographic gradients and occurrence 

of low permeability dikes, tuffs, and breccias in the Ribeira 

Paul Basin may force a higher percentage of ground water to 

discharge at springs, which may result in a smaller proportion 

of ground-water discharge as submarine outflow. Similarly, the 

smaller apparent net infiltration and recharge in the Mosteiros 

Basin as compared with Ribeira Fajã seems to be an enigma, 

particularly since the Mosteiros Basin receives about twice the 

amount of average annual precipitation and has a more northerly exposure. In order to determine if actual net infiltration 

rates are lower in Mosteiros because of its steeper topographic 

gradients and smaller amount of soil cover, further data collection and analysis is needed.

Challenges in Evaluating GroundWater Conditions in the Three Basins

Evaluating baseline ground-water conditions in the three 

Cape Verde watersheds has a certain degree of uncertainty 

because of various data limitations. This study and previous work have provided much information on precipitation, 

ground-water levels, some ground-water discharge fluxes 

(wells and springs), recharge source altitudes, and groundwater age. However, there are no simple methods for evaluating other ground-water budget components, such as net 

infiltration and recharge, changes in ground-water storage, 

plant evapotranspiration and consumptive use (Paul Basin 

only), and submarine ground-water discharge without substantial amounts of additional data and analysis. Contributing to 

additional uncertainty is the limited understanding within each 

basin of geology, soil permeability, and complex precipitation 

patterns caused by topography and wind patterns. Of particular concern are:

1. The scarcity of wells and historical water-level and 

precipitation information. Although dedicated monitoring wells were available in Mosteiros Basin, the 

Ribeira Paul and Ribeira Fajã Basins each only had 

one well temporarily available for monitoring.

2. The 3- to 6-month period of continuous water-level 

measurements made in monitoring wells during this 

study. This short time was insufficient for evaluation 

of longer-term responses to natural (seasonal, multiyear) climatic cycles. 

3. The spatial distribution of existing observation wells. 

These wells were not uniformly distributed within 

the basins, but were located only at low altitudes near 

the coast. Consequently, water levels from two of the 

three continuous recorder wells displayed strong tidal 

effects. 

4. The lack of precipitation data after 2002 for evaluating ground-water response to climatic change 

in Ribeira Fajã and the lack of data after 2004 for 

Mosteiros and Ribeira Paul. 

5. Spring and spring-fed stream-discharge measurements made in Ribeira Paul shortly after the rainy 

season. Total spring discharge for the Ribeira Paul 

Basin is an important value for evaluating baseline 

ground-water conditions. Yearly measurements during base-flow conditions (prior to the rainy season) 

would reduce the uncertainty in this ground-water 

budget component.

6. The presence of terrigenic helium in ground-water 

samples because of recent volcanic activity. This prevented 3

H/3He age-dating at some ground-water sites. 

At these sites, tritium could only be used as a general 

indicator of either modern (post-1950s) or older (pre1950s) recharge. Additional age-dating techniques, 

such as CFCs and 14C were outside the scope of this 

study. 

Summary and Conclusions

 This study assessed baseline ground-water conditions 

in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins of 

Cape Verde, on the basis of existing and new data collection. 

This information is needed to evaluate long-term changes in 

ground-water resources or water-quality associated with future 

development. The study initially focused on measuring current 

ground-water levels in each basin. Because of the scarcity of 

monitoring wells in all three basins, however, other hydrologic indicators also were examined to understand baseline 

ground-water conditions for all three basins. These indicators included the compilation of historical ground-water level 
information, the estimation of ground-water budgets on the 

basis of discharge measurements, field ground-water quality 

measurements to evaluate salt-water intrusion, and laboratory 

analysis and interpretation of environmental tracers to evaluate 

recharge altitude and ground-water age.

In the Mosteiros Basin, the measured discharge from 

wells and springs of about 600 m3/d represents about one 

percent of the precipitation. Therefore, unmeasured submarine 

outflow is likely an important form of ground-water discharge 

from the basin. Intercepting this discharge could provide an 

additional source of water for future development. Mount 

Vermelho spring accounts for most of the 600 m3/d of the 

measured discharge from the basin. Half of this is assumed 

to be lost to the sea. However, no historical records exist for 

evaluating the long-term effects of pumping or climate on 

discharge from this spring. The other source of discharge, well 

withdrawals along the narrow coastal plain, appears to have 

been fairly steady at about 100 m3/d. While demand for water 

has increased, well pumping from the two existing production wells has not increased because of salt-water intrusion 

problems. If the existing production wells are pumped continuously, the water becomes brackish (exceeding 1,000 mg/L 

dissolved solids). This is consistent with a specific-conductance depth profile of monitoring well FF06, which showed 

values exceeding 10,000 μS/cm (equivalent to about 5,500 to 

7,500 mg/L dissolved solids) at depths greater than 7 m below 

the water table. An important metric for evaluating the effects 

of additional pumping would be to monitor salinity in the 

observation and production wells of the coastal plain. Water 

levels monitored continuously in Mosteiros at well FF07 from 

December 2005 through March 2006 showed a 0.2 m decline. 

Reasons for this small decline could not be determined; longer 

periods of water-level records are needed to discern whether 

this is a climatic response or caused by nearby pumping. No 

historical water-level records exist for evaluating longerterm ground-water trends in Mosteiros Basin. Ground-water 

age dates, stable-isotope concentrations, and dissolved-gas 

recharge temperatures from Mosteiros Basin indicate that 

recharge to the wells in Mosteiros mostly occurred more than 

50 years ago and at much higher altitudes in the aquifer. This 

suggests that it may take many years for artificial recharge to 

result in a beneficial impact on the aquifer in areas where the 

agricultural projects are implemented.

Ribeira Paul Basin has more abundant ground-water 

resources than the other two basins. Measured ground-water 

discharge accounts for about 12 percent of the precipitation 

falling within the basin. The 4,000 m3/d of measured discharge 

(mostly from springs and spring-fed streams) is based on an 

inventory at the end of the rainy season and does not include 

ground-water evapotranspiration. Base flow spring/stream 

discharge and evapotranspiration measurements at the end 

of the dry season would provide more accurate estimates of 

available ground-water resources; the Ribeira Cha João Vaz 

constriction point and at the northern springs would be ideal 

locations for these comparative discharge measurements. Only 

a small amount of water (about 200 m3/d) is currently pumped 

out of wells. Water levels at the monitoring well appear to be 

strongly affected by streamflow in the nearby Ribeira Paul. 

The water level in this well rose by 1.3 m between August 

2005 and January 2006, then declined 1.8 m by June 2006. 

This seasonal variation is associated with flow in the nearby 

ephemeral stream channel. Because of this strong surfacewater influence on the water level, this well may not be suitable for evaluating long-term regional ground-water conditions. Ground-water age dates, stable-isotope concentrations, 

and dissolved-gas recharge temperatures from the Ribeira Paul 

Basin indicate a wide variation in recharge source altitudes, 

ground-water flow paths, and ground-water residence times 

Most-probable recharge altitudes estimated from dissolved-gas 

recharge temperatures range from 900 to 1,300 m, indicating 

that recharge generally occurs at altitudes much higher than 

the ground-water discharge points. Ground-water ages range 

from pre-1950s to modern, indicating both short, quick flow 

paths from nearby recharge and long, slow flow paths from 

distant recharge. This combination of differing flow paths 

is probably caused by the steeply incised topography of the 

basin, the relatively high precipitation rates, and the horizontal 

flow barriers associated with breccia deposits and intrusive 

dikes. Because of this complexity, detailed water budget and 

soil permeability investigations are needed for evaluating the 

feasibility of localized artificial recharge.

Ribeira Fajã is the driest basin, receiving only about half 

of the average annual precipitation of the other two basins. 

The only measurable ground-water discharge is from Galleria 

Fajã tunnel, currently flowing at about 400 m3/d. As with the 

Mosteiros Basin, it is likely that unmeasured submarine outflow is the dominant form of ground-water discharge from the 

basin. Historical water-level records from Ribeira Fajã indicate very large ground-water declines of tens of meters during 

the 1980s and early 1990s. This probably was associated 

with dewatering of the Galleria Fajã tunnel, whose discharge 

peaked at more than 1,400 m3/d in the mid-1980s. More-recent 

data indicate that ground-water levels in Ribeira Fajã have 

reached a new equilibrium, remaining fairly constant since the 

late 1990s. No substantial water-level changes were observed 

between December 2005 and March 2006 at the monitoring 

well in Ribeira Fajã. Ground-water age dates, stable-isotope 

concentrations, and dissolved-gas recharge temperatures all 

indicate that recharge to the Galleria occurred more than 

50 years ago and at much higher altitudes (about 900 m) in 

the aquifer. Analysis of water-level declines associated with 

the Galleria Fajã dewatering in the 1980s indicates that the 

fractured basalt aquifer is hydraulically connected throughout 

the lower basin but has low hydraulic conductivity (2 to 20 

m/d) and storage (0.003 to 0.01). The analyses of environmental tracers and hydraulic properties indicate that if the 

artificial recharge infrastructure is located above the upland 

part of the aquifer, it may eventually enhance discharge from 

the Galleria. However, the effect of such recharge structures 

on discharge from the Galleria may take decades or longer. 

Surface-water budget calculations and soil permeability testing 

for the planned artificial recharge locations would be helpSummary and Conclusions  39
40 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06

ful for evaluating the potential amount of recharge The only 

other potential source of ground water in Ribeira Fajã would 

be additional well pumping. Such ground-water development 

may intercept submarine outflow, but would need to be located 

both sufficiently far from the ocean to avoid salt-water intrusion and sufficiently downgradient from the galleria so as not 

to cause decreased flow from the tunnel.

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42 Evaluation of Baseline Ground-Water Conditions, Republic of Cape Verde, West Africa, 2005-06
V.M. Heilweil and others—Evaluation of Baseline Ground-Water Conditions in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, 

Republic of Cape Verde, West Africa, 2005-06—SIR 2006-5207