geosciences Article Hydrochemistry and Evolution of Water Quality in a Context of Aridity and Increasing Agriculture in Three River Sub-Basins of Santiago Island (Cape Verde) Nemias Gonçalves, Teresa Valente * , Jorge Pamplona and Isabel Margarida Horta Ribeiro Antunes Campus de Gualtar, ICT, University of Minho, 4710-057 Braga, Portugal; [email protected] (N.G.); [email protected] (J.P.); [email protected] (I.M.H.R.A.) * Correspondence: [email protected] Abstract: In regions under development and facing recurrent droughts, increasing the area of irrigated agriculture may create additional disruption in water resources management. The present study was focused on three river sub-basins with the highest agricultural intensity (S. Miguel, Ribeira Seca and S. Domingos) in Santiago Island (Cape Verde). Sets of wells were selected to evaluate the influence of salinization and agriculture practices on the hydrochemistry. This assessment was performed by using data from the bibliography (2003) and a recent campaign (2016). The water chemistry indicates lower mineralization in the S. Miguel sub-basin. Nitrates and nitrites, typically associated with diffuse pollution, are present in all sub-basins, but with varying patterns. Additionally, sodium chloride waters occur in all the three sub-basins, especially those closest to the coastline. In turn, a bicarbonate-magnesium facies was identified in S. Domingos, at the furthest point Citation: Gonçalves, N.; Valente, T.; from the coast, indicating a geological control. The comparison between the two periods suggests a Pamplona, J.; Antunes, I.M.H.R. Hydrochemistry and Evolution of decrease in water quality. The rising extension of the irrigation area associated with aridity should Water Quality in a Context of Aridity intensify the already observed soil salinization. Thus, the present review highlights the strategic and Increasing Agriculture in Three importance of water monitoring at the basin level as a management tool for resources preservation in River Sub-Basins of Santiago Island insular arid and developing regions. (Cape Verde). Geosciences 2021, 11, 263. https://doi.org/10.3390/ Keywords: river basin; agriculture pressure; hydrochemistry; water quality; salinization risk geosciences11060263 Academic Editors: Dimitris Tigkas and Jesus Martinez-Frias 1. Introduction The sustainable use of natural resources, especially water, has assumed an increasing Received: 8 April 2021 relevance in unfavorable contexts, such as arid and semi-arid climates. The challenges to Accepted: 14 June 2021 adequate resource management are even greater in territories subject to anthropogenic Published: 21 June 2021 pressures associated with population growth and unregulated economic activities, as is the case in many developing countries. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Current scenarios of climate disruption, characterized by increasingly frequent and published maps and institutional affil- persistent droughts, make the preservation and sustainable management of the territory iations. and its resources even more pressing in regions such as the west coast of Africa. Water, a fundamental component of life and essential to a wide variety of economic activities, is one of the most abundant natural resources on the planet, but also one of the most susceptible to degradation. The need for its use for human consumption (e.g., drinking and cooking), and also for the development of activities such as agriculture, transport, Copyright: © 2021 by the authors. industry and recreation, highlights the critical nature of this resource. It is, however, a Licensee MDPI, Basel, Switzerland. This article is an open access article limited and limiting resource, as shown by the recurrent water scarcity problems in many distributed under the terms and parts of the world, sometimes with tragic consequences [1]. conditions of the Creative Commons The Cape Verde Archipelago, together with the archipelagos of the Canary Islands, the Attribution (CC BY) license (https:// Azores and Madeira, constitutes the region of Macaronesia, known for its biogeographical creativecommons.org/licenses/by/ specificities, including the floristic and faunistic richness [2]. The state of the art focused 4.0/). Geosciences 2021, 11, 263. https://doi.org/10.3390/geosciences11060263 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 22 Geosciences focused2021, 11, 263 on this territorial context also suggests similar problems related to water re-2 of 22 sources management. For example, the authors of [3–6] presented literature reviews and discussed problems associated with water quality, with reference to the main threats to supply in the Canaryon this Islands. territorial There context are also also suggests several similar works problems focusing related on to hydrogeochemi- water resources man- cal controls and salinizationagement. For of example, water thein several authors ofislands [3–6] presented from the literature Azores reviewsArchipelago and discussed [7– problems associated with water quality, with reference to the main threats to supply in the 10]. The present studyCanary follows Islands. these There areapproach also severales by works reviewing focusing the on hydrogeochemical water quality situation controls and in Santiago Islandsalinization (Cape Verde), of water a developing in several islands and fromsemi-arid the Azores territory Archipelago located [7– on10]. the The west present coast of Africa, subjectstudy follows to water these scarcity approaches and by anthropogenic reviewing the water activities, quality situation especially in Santiago agricul- Island ture. (Cape Verde), a developing and semi-arid territory located on the west coast of Africa, Cape Verde issubject a small to water country scarcity with and ten anthropogenic islands (Figure activities, 1). especially The focus agriculture. of this article is Cape Verde is a small country with ten islands (Figure1). The focus of this article is Santiago Island, whichSantiago is Island,the most which populated is the most island, populated since island, about since 55% about of 55% the ofCape the Cape Verde Verde population lives therepopulation [11]. livesThe therecapital [11 ].of The the capital country of the is country Praia City. is Praia City. Figure 1. GeographicFigure 1. Geographic location location of Cape of CapeVerde Verde and and Santiago Santiago Island Island (west (west coast coast of Africa).of Africa). Several technical and/or scientific studies can be cited (e.g., [12,13]). The first hy- Several technicaldrogeological and/or surveyscientific of the studies archipelago can was be carriedcited out(e.g., by [12,13]). [14], having The also first contributed hy- drogeological surveyto the of discussion the archipelago of the problem was carried of the water out supply,by [14], with having the objective also contributed of advocating to the discussion immediateof the problem solutions. of Additionally, the water the supply, project CVIwith 75/001, the objective financed by of the advocating United Nations immediate solutions.Development Additionally, Programme the (UNDP), project is CVI the source 75/001, of the financed greatest hydrogeological by the United knowledge Na- tions Developmentof theProgramme island. The reports(UNDP), prepared is the by [source15,16] should of the also greate be mentioned.st hydrogeological As a result of these projects, an extensive list of studies on the hydrogeological behavior knowledge of thein island. volcanic The terrains reports can beprepared cited, including by [15,16] [17–34 should]. also be mentioned. As a result of theseIn a moreprojects, or less an direct extensive way, these list works of studies put forward on inthe evidence hydrogeological the primordial be- strate- havior in volcanicgic terrains need for can protecting be cited, the including water, due to[17–34]. its scarcity and fragility. Indeed, groundwaters In a more orare less one direct of the mainway, sources these ofwork waters input regions forward with surfacein evidence water deficit the primordial or in territories without a centralized supply system. However, in many cases, the population consumes strategic need forgroundwater protecting inthe a disorganizedwater, due way to its and scarcity without qualityand fragility. control, with Indeed, consequent ground- risks to waters are one ofpublic the main health. sources of water in regions with surface water deficit or in territories without a centralized supply system. However, in many cases, the population consumes groundwater in a disorganized way and without quality control, with conse- quent risks to public health. In general, the literature review indicates a reduction in water quality, often linked to salinization in coastal areas, as a direct result of overexploitation of aquifers (e.g., [35]) and sand extraction on beaches that destroy natural barriers against saline intrusion (Figure 2). Geosciences 2021, 11, 263 3 of 22 In general, the literature review indicates a reduction in water quality, often linked to Geosciences 2021, 11, x FOR PEER REVIEWsalinization in coastal areas, as a direct result of overexploitation of aquifers (e.g., [35])3 and of 22 sand extraction on beaches that destroy natural barriers against saline intrusion (Figure2). FigureFigure 2.2. ImagesImages illustratingillustrating thethe exploitationexploitation ofof resourcesresources onon Santiago Santiago Island: Island: (A(A,B,B))