1. Introduction

Based on the porosity and permeability conditions of the geological units present in the study area, the following categories were classified: aquifer, aquitard or aquifuge, and they were assigned a hydrogeological potential: very high, high, medium, low or null. The areas obtained for each category were subsequently refined from the results of the inventory of water points, with which the information that fed all the elements of the conceptual hydrogeological model was gathered, laying the foundations that allowed the involvement of communities in the construction of knowledge and preparation for the subsequent definition and application of management measures.

In the regions associated with the Cauca River Canyon, in the department of Antioquia, groundwater has an important role as a main or secondary source of supply in the demands of consumption, economic and recreational activities, and their satisfaction. With the objective of deepening their knowledge and having the tools to protect the tropical dry forest ecosystem in the jurisdiction of Corantioquia, at the end of 2020, an exploration was launched into the hydrogeological potential of geological formations located in the area of direct and indirect influence of the tropical dry forest (bosque seco tropical or BS-T) associated with the Cauca River Canyon.

During the first months of development of the project, the collection of available secondary information was carried out, which could serve the initial purposes of groundwater exploration. The starting point was the recognition and determination of the area of influence, followed by the development of a sociodemographic evaluation that would allow the first approach to the fieldwork.

The geological conditions, determined both by the type of rocks and by the tecto-structural affectation on them, provided guidelines to identify and characterize the different types of hydrogeological units. For this, an assembly of the geological cartography at a scale of 1:100 000 and 1:400 000 of the Servicio Geológico Colombiano (SGC) was verified in the field.

Based on the porosity and permeability conditions of the geological units present in the study area, the following categories were classified: aquifer, aquitard or aquifuge, and they were assigned a hydrogeological potential: very high, high, medium, low or null. The areas obtained for each category were subsequently refined from the results of the inventory of water points, with which the information that fed all the elements of the conceptual hydrogeological model was gathered, laying the foundations that allowed the involvement of communities in the construction of knowledge and preparation for the subsequent definition and application of management measures.

2. Reference framework

The study area was delimited, considering the hydrographic subzones of the Cauca River basin which is a part of the BS-T ecosystem under the jurisdiction of the Regional Autonomous Corporation of the Center of Antioquia (Corantioquia). It has an area of approximately 10 662 km2 and runs through the municipalities associated with the Cauca River basin from Caramanta to Valdivia, with elevations ranging between 171 and 3609 meters above sea level (masl).

This ecosystem corresponds to a forest biome that develops at low altitudes from 0 to 1000 masl in intertropical areas with temperatures above 24 °C and annual rainfall between 700 and 2000 mm (IAVH, 1998). It is characterized by climate variability of one or two periods of very marked drought per year (rainfall less than 100 mm) of at least 5 to 6 months (Bullock et al., cited in Pizano and García, 2014). In the BS-T, the evapotranspiration process consumes most of the water, so the recharge and rate of change in the phreatic levels play an important role in the sustainability of vegetation because it becomes the only alternative to be used by plants in the processes of nutrient absorption and transpiration (Le Maitre et al., 1999).

In Colombia, this biome exists in at least six biogeographic regions: the Patía River Valley, the southern Cauca Valley, the Cauca River Valley, the upper and middle Magdalena River Valley, the departments of Santander and Norte de Santander and in the region of the Caribbean coast and Orinoquía (Figure 1). In the study of the tropical dry forest (TDF) ecosystem conducted by the Alexander von Humboldt Institute for Biological Resources Research (Pizano and García, 2014) and based on studies available since 1997 (IAVH, 1998; IAVH; 1997;Cabrera and Galindo, 2006), a loss close to 92% of the 9 million hectares covered by this ecosystem is evidenced in the map presented by Etter et al. (2008). In addition, it is mentioned that those territories that have been deforested and were BS-Ts currently present desertification scenarios; consequently, they are no longer productive for the development of agriculture or livestock, that is, they are unsustainable (Benítez et al., 2014). The state of this ecosystem in Colombia urgently needs measures to conserve the remnants that survive.

The Cauca River Basin is one of the most populated regions in Antioquia. Anthropic pressure on the territory has affected the natural balance of the ecosystem to the point that there is a scarcity of water for humans and other life forms in the region, including the BS-T, characteristic of the low-lying areas in the vicinity of the Cauca River Canyon. Thus, a project was proposed to deepen knowledge of the hydrogeological systems in the region, the opportunities offered by groundwater as the main or secondary sources of water and its relationship and impact on the sustainability of the BS-T ecosystem.

In the Earth’s crust, the nature of various geological units allows some features to conserve intergranular porosity or primary porosity, while others do not. However, porosity can be acquired thanks to subsequent events, such as dynamic effects or by the action of weathering, which results in secondary porosity.

Considering that a hydrogeological unit is a geological formation, part of it has similar hydraulic characteristics, which are related to the capacity to store water (porosity) and the possibility of flow between the pores (permeability or coefficient of hydraulic conductivity). Hydrogeological units are classified as aquifers, aquitards, aquicludes or aquifuges. An aquifer can store and transmit enough water in acceptable quantity conditions, and its extraction can be economically viable; an aquitard stores and transmits water very slowly, so its exploitation demands more energy than the previous one; an aquiclude stores but does not allow the transit of water between the pores;

Figure 1.
Distribution of the tropical dry forest in Colombia at a scale of 1:100 000

See legend in this kmz file (see in Google Earth). Source: Modified from Instituto de Investigación de Recursos Biológicos Alexander von Humboldt (2014).

and finally, an aquifuge does not store or let groundwater flow (Custodio and Llamas, 1996).

Those materials that make up units with hydrogeological potential, in order of importance, are the following: unconsolidated deposits of gravel and sand, karst formations, conglomerate sedimentary rocks or sandstone with low amounts of finer material, volcanic rock, and other rocks according to origin and evolution. Igneous rocks that have acquired secondary porosity and permeability, especially when their thicknesses are considerable in the case of deposits and sedimentary rocks. In contrast, they are not aquifers: clay and silt deposits, siltstone or claystone sedimentary rocks and igneous or metamorphic rocks without alteration.

Before 2021, several studies were conducted to understand hydrogeological systems in different regions of Antioquia, mostly focusing on hydrogeological units associated with sedimentary rocks and recent deposits. Two of these hydrogeological systems are located within the study area: the first is in the western subregion, with an extension of 103 km2, which includes the municipalities of Santa Fe de Antioquia, San Jerónimo, Sopetrán and Olaya; the second system integrates the municipalities of La Pintada and Valparaíso associated with the Cauca River with an area of 184 km2.

Table 1 shows the studies that were consulted, and Figure 2 shows the location of the hydrogeological systems that present progress in their knowledge.

The study area included approximately 40 geological units with formation ages between the Proterozoic and Cenozoic eras, including 7 metamorphic units, 24 igneous types and 9 sedimentary types.

Figure 3 shows the geological map of the area studied, which corresponds to an adaptation by Betancur Vargas (2021), modified from González (2001), and geological maps at scale 1:100 000 and 1:400 000 (Servicio Geológico Colombiano).

In any hydrogeological exploration project, one of the first activities developed is the inventory of groundwater points. Where a water point is a natural place, civil works allow direct or indirect access to the aquifer. In this sense, the previous definition includes all existing boreholes, whether or not exploited, abandoned or even destroyed, and the sources or springs that in principle should be considered natural spillways of the aquifers (Custodio and Llamas, 1996). The following were the objectives of this study: springs, filtering galleries, hand-dug wells, wells and piezometers.

Figure 2.
Location of the hydrogeological systems of Antioquia that present progress in their knowledge

See legend in this kmz file (see in Google Earth). Source: Modified from the Public Companies Foundation of Medellín, Government of Antioquia (2018).

Table 1.
Hydrogeological studies consulted as secondary information

Figure 3.
Geological map of the study area

See legend in this kmz file (see in Google Earth). Source: Betancur (2021), modified from González (2001)

3. Methods

In Colombia, the information collected in the inventory of water points is recorded following the parameters defined by the National Single Form of Groundwater (FUNIAS), proposed by the Institute of Hydrology, Meteorology and Environmental Studies (Ideam). This document contains a series of sections of various types, such as administrative location (municipality, village, neighborhood), technical (nature of the work, mode of drilling, diameter, materials, depth, etc.), hydrological (piezometric levels at different times, chemical quality), and uses. This activity can be carried out directly by touring the study area or indirectly by going to people or public and private entities that by their work can indicate the existence of water points and provide the information required for the inventory.

In the first instance, a review of secondary information was carried out to locate previously identified water points, such as records of groundwater concessions by the Environmental Corporation, information from existing FUNIAS, and the Water Resource Information System (Sistema de Información del Recurso Hídrico), Environmental Impact Studies (EIA), Aquifer Environmental Management Plans (PMAA), Watershed Management Plans (POMCA), Water Resources Management Plans (PORH), and a review of the databases provided by the corporation.

The collection and collection of primary information was carried out through field trips by professionals in hydrogeology, supported by a social, communication and relationship component, which was aimed at identifying the key actors, direct users and users of indirect resources of the underground water resource, as well as the organizations or entities that are part of the exercise of socialization and appropriation of the knowledge generated by the project in the territory.

For both primary and secondary information, the coding of the inventory of water points defined by Ideam was implemented, according to the FUNIAS; this structure allows, in a methodical way, the assignment of a unique code to each water point. This represents the possibility of ordering and unifying the registration information of the points from different sources, allowing a database with a common language between entities, which became a management tool for authorities and environmental entities in the administration. of water resources.

Figure 4 shows the methodological scheme that was used for the inventory of groundwater points.

Figure 4.
Methodological scheme of the groundwater inventory

For the characterization of the hydrogeological units, a geological-structural evaluation was carried out that focused on determining the relevant porosity and permeability conditions that promoted the storage and flow of groundwater. The deposits and clastic sedimentary rocks, given their primary porosity, were evaluated and verified in the field as potential aquifers. Likewise, the condition of the hydrogeological unit was determined for the hard rocks that have been or have not been affected structurally or by weathering with the help of professionals with experience in hydrogeology.

The fault systems with direct interference in the study area are the San Jerónimo System, the Espiritu Santo Fault and the Romeral System. The large fractures have, in general, a clear morphological expression and present fractured, sheared rocks with alteration and brecciation, while the smaller ones present only some of these characteristics and are preferentially marked by visible alignments in the aerial photographs.

Prior to the inventory and applying the Analytic Hierarchy Process (AHP) methodology based on the work of Saaty (2008), ten professionals in geology and hydrogeology, knowledgeable in the area of study, were consulted about the

Table 2.
Rating scale of hydrogeological conditions based on porosity and permeability

hydrogeological potential of the geological units in question (see annex 2). For this exercise, different levels of importance were considered, according to the porosity and permeability characteristics of each unit and the categories: aquifer, aquitard or aquifuge. This was done to assign hydrogeological potential based on the qualitative weighting matrix of Table 2. Analytical and hierarchical processes, such as Saaty, with a sufficiently large survey size and expert and independent evaluators, allow assigning weights and guarantee a reasonable reduction of the so-called “decision bias” (Universidad de Antioquia and Corpourabá, 2016).

According to the responses obtained, the initial mapping of hydrogeological potential was proposed (Figure 5, upper left: a), which was key for the planning of the field work, aimed at the development of the inventory of water points, one of the most suitable tools for the construction of preliminary knowledge of the hydrogeological conditions of the area. As a result of the matrix, the areas of potential were identified; then, the geology in the field and superposition of the inventoried water points corroborated or corrected the different lithological units of the study area.

4. Results

In the first stage of the project, 1172 water points were identified, distributed in 751 springs, 394 hand-dug wells, 13 piezometers, 12 wells and 2 filter galleries. The initial approach to the hydrogeological potential was corroborated by field reconnaissance work and the superposition of the inventoried water points, making it possible to confirm or redefine the hydrogeological character of the lithological units, which allowed us to complete the categorization exercise of the units from their benefits or limitations and as storage and facilitators of groundwater flow.

The results of the process of evaluating the hydrogeological potential of sites and subsequent additions to the inventory can

Figure 5.
Representation of the superposition of water points and hydrogeological potential adjustment

See legend in this kmz file (see in Google Earth). Source: Adapted from Corantioquia and Universidad de Antioquia (2021).

be visualized in Figure 5.Table 3 shows the results for some of the geological units of interest, according to the type of rock. In Table S1, detailed information regarding the total number of points inventoried is shown.

According to the results of synthesis and geological verification in the field, in the area of direct and indirect influence of the BS-T associated with the Cauca River in Corantioquia, 33% of deposits and sedimentary rocks are soft geological formations and 67% are hard rocks (igneous and metamorphic). Ninety-six percent of the soft rocks have a high or very high hydrogeological potential, and in the case of igneous and metamorphic rocks, which are mostly structurally affected, 24% have a medium hydrogeological potential, and 1.5% have a high hydrogeological potential. This distribution can be visualized in Figure 6.

As a result of this exercise, it was found that the quaternary deposits are the geological units with the highest concentration of water points, with the alluvial deposits and terraces having the greatest amount, a total of 358. The Amagá Formation, Upper Member, Combia Formation and Urrao Member of the Pendersco Formation are geological units that follow in importance, taking into account the number of water points associated with them. In the first unit, 109 water points were recorded, 146 points were identified in the Combia Formation, and 93 were found in the Urrao Member.

The aquifer potential is also evidenced in units such as the quartz-meritic schists of the Cajamarca Complex (62 water points inventoried); the Sabanalarga Batholith (13 points); Buriticá Andesite (21 points); Sheep Tonalite (17 points) and Pueblito Diorite (10 points).

Table 3.
Evaluation of the hydrogeological potential in geological units of interest

Figure 6.
Synthesis of the hydrogeological potential in the study area

5. Discussion

Of the 1172 groundwater points, 947 were identified in the collection of secondary information; although they represent a vast volume in the study area, it is necessary to complete missing data, such as water uses, volumes captured, properties; hydraulics, and subsoil records, with the objective that the information is useful to the inventory. The data obtained in the field work supplemented the primary information of the water point registry, added to the collection of secondary information and promoted dialog with the experts, providing the basis for an approximation to the hydrogeological potential of the study area.

From what was found in the field work, it was possible to corroborate the importance of deepening the study of groundwater and hydrogeological systems in the subregions that made up the study area, considering each area’s sociodemographic, economic and ecosystem characteristics, which affect the dynamics of the BS-T.

6. Conclusions

The inventory of water points is a fundamental tool in the classification and redefinition of hydrogeological units; from it, primary information is obtained that allows feeding most of the components of the conceptual hydrogeological model. The fieldwork of the inventory corroborates the importance of building knowledge from the implementation of participation, education and communication strategies that are supported in the dialog of knowledge and in the construction of citizen science.

The 1172 water points documented in the BS-T associated with the Cauca River Basin in Antioquia confirms the hydrogeological potential that exists in the region.

In Antioquia, the hydrogeological potential of igneous and metamorphic rocks that have acquired porosity and secondary permeability after being subjected to intense tectonic effects over time is significant, because 24% have a medium hydrogeological potential and 1.5% a high potential. In terms of groundwater availability, this is relevant data, since hard geological formations occupy a 67% study area.

This information serves as a reference point to deepen the knowledge of fractured environments and highlights the possibility of having alternative and complementary sources of water to satisfy human needs. In addition, it plays a fundamental role in terms of the sustainability of the tropical dry forest ecosystem.

7. Acknowledgments

We thank Corantioquia for financing exploration of the hydrogeological potential in the Cauca River Canyon in its the jurisdiction, making it was possible to carry out this research.

Thanks also to Teresita Betancur Vargas, geologist and project advisor; the engineers Cristina Martínez, Alba Nury Gallego, Breiner Dan Bastidas and Andrés Felipe Zapata; to the communicators Deisy Rivera and Liliana Monsalve; the technician Ángela Díaz; members of the GIGA Research Group of the Faculty of Engineering of the Universidad de Antioquia; and those who contributed their knowledge and work in the formulation and reality of the project.

We also appreciate those people who opened their doors to allow us to collect data about their use of groundwater and shared their perceptions and knowledge with us.

8. Supplementary data

Supplementary data for this article can be found online at https://doi.org/10.32685/0120-1425/bol.geol.49.1.2022.620

Available data

Table S1. Synthesis of information from the inventory of 1172 groundwater points in the study area

Data in kmz files

Figure 1. Distribution of the tropical dry forest in Colombia at a scale of 1:100 000

Figure 2. Location of the hydrogeological systems of Antioquia that present progress in their knowledge

Figure 3. Geological map of the study area

Figure 5. Representation of the superposition of water points and hydrogeological potential adjustment

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