Evaluation of baseline ground-water conditions in the Mosteiros, Ribeira Paul, and Ribeira Fajã Basins, Republic of Cape Verde, West Africa, 2005-06

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 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 meters. 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. For the Ribeira Paul Basin, measured well discharge, spring discharge, and ground-water seepage to springs is about 1,600,000 cubic meters per year. 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 cubic meters per year. 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 meters. 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 except for one spring, Seladinha. This spring, in combination with generally younger ground-water ages in Paul, indicates 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. Although the Monte Vermelho spring in Mosteiros Basin also has very low salinity (200 microsiemens per centimeter at 25 degrees Celsius), water from the wells along the coastal plain has specific-conductance measurements of as much as 16,000 microsiemens per centimeter at 25 degrees Celsius. These higher values indicate some brackish water intrusion. Additional ground-water development of the Mosteiros coastal plain may exacerbate this situation.