Risk Assessment of Groundwater Contamination in the Gala, Tenguel, and Siete River Basins, Ponce Enriquez Mining Area—Ecuador

Abstract

Groundwater is a strategic resource, which is experiencing a growing threat of contamination worldwide. This study aimed to assess the vulnerability of the groundwaters in the basins of the rivers Gala, Tenguel, and Siete in the Ponce Enriquez mining area, considering as a hypothesis that anthropogenic activities (mainly mining and agricultural) conducted in the area generate a high risk of contamination. Vulnerability to contamination was quantified using the DRASTIC and GOD methods. In addition, the risk of contamination (Rc) was calculated considering the vulnerability index (I_V) and the danger index (I_P). Geographic information system (GIS) environment was used for the spatial analysis and the generation of vulnerability maps. The DRASTIC method showed that the negligible-to-low vulnerability category was predominant (50.7% of the area), followed by moderate vulnerability (25.6%), and high-to-extreme vulnerability (23.7%). Regarding the GOD method, the low and medium vulnerability categories were predominant in 41.4% and 27.5% of the area, respectively. Both methods agree that the center area, where the mining activities are located, has a vulnerability ranging from negligible to moderate. The I_P was low in the eastern zone (58.7 km²) and ranged from moderate (426.2 km²) to high in the central and western zones (371.1 km²), where mining activities predominate. The R_C revealed that 20% of the study area corresponded to a high contamination index; from which, 87% comes from agricultural activities, 9% from mining activities, and 4% due to shrimp farms and urban areas. This is the first study that evaluates the vulnerability to the contamination of groundwater in a mining area of Ecuador. The results of this research can serve as a baseline for future research, since the methodology used provides information on the priority areas for the protection of aquifers, considering the current land use in the study area.

1. Introduction

There is 1386 million km3 of water available worldwide, and only a small part (2.5%) is fresh water. From this fraction, 29.9% is stored in aquifers, turning it into one of the largest and most important freshwater reservoirs on the planet [1]. In Ecuador, 26.6% of the population does not have access to a drinking water source [2]. In addition, the main surface water sources in different country regions are deteriorating due to high-impact human activities such as agriculture [3], illegal mining [4,5] and domestic and industrial wastewater discharges [6]. Therefore, groundwater is an essential resource that may satisfy the water supply demand in areas where both surface water quality and quantity are deficient [7].

Groundwater protection has become a major challenge in many countries [8]. The population growth, anthropogenic activities, contaminant levels, and aquifers overexploitation have seriously affected the quality and quantity of this resource [9].

The aquifer vulnerability assessment has been widely used as a pollution detection and prevention tool, as well as a sustainable groundwater management means [9,10,11,12]. According to Ribeiro et al. [13], vulnerability is a specific or intrinsic property of an aquifer system that depends on hydrogeological factors and the system susceptibility to anthropogenic factors. The most common methods to determine the aquifers vulnerability are based on the lithological information of the area, the land use, and the existence of pollution loading [14].

In recent years, the depletion and contamination of large aquifers has led to the development of large-scale vulnerability assessment [15]. Around the world, there are many aquifer vulnerability studies, with some outstanding cases in Europe and North America; the Greater Cochin aquifer in India [16]; the Moulares-Reayef aquifer in Tunisia [17]; and the Melaka aquifer in Malaysia [18]. In South America, research about the vulnerability of aquifers has been conducted in Brazil, Argentina, Colombia, and Chile. In these countries, the vulnerability studies have led to the categorization of the areas with the highest contamination risk, providing enough information for the creation of contamination prevention strategies and groundwater resource sustainable management guidelines [7].

In Ecuador, there are few studies about the vulnerability of aquifers, which are focused on the impact of agricultural and oil activities. For example, Durango-Cordero et al. [19] evaluated the pollution risk of groundwater in oil areas in the north of the Ecuadorian Amazon; Loor-Bruno [20] assessed the aquifer agrochemical percolation vulnerability in Gral. Antonio Elizalde and Ribeiro et al. [13] assessed the vulnerability of the Daule aquifer—Guayas province, due to agriculture activities; Jarrín et al. [21] determined the pollution risk of aquifers as a consequence of oil activities in the Limoncocha Ecuadorian Amazon Reserve; and Sharadqah [22] evaluated the groundwater pollution risk in the Portoviejo—Manabí province, where agricultural activities prevailed.

Ponce Enriquez Mining District is one of the major gold mining fields in Ecuador. In addition to mining, there are important agricultural and shrimp activities in the area [23]. Various studies in Ponce Enriquez have reported the presence of potentially toxic elements, such as As, Cd, Cr, Cu, Ni, Pb, and Zn, in rivers, sediments, and soils [24,25,26,27,28,29], exceeding the maximum permissible limits according to Ecuadorian regulations. These studies diagnosed rivers such as Gala, Tenguel, and Siete basins to be strongly contaminated [30]. Nevertheless, there is no information about the conservation status of the aquifers, as well as their vulnerability degree due to anthropogenic activities conducted in this area. The objective of this research is to (a) evaluate the aquifers’ contamination risk due to anthropogenic activities in the Gala, Tenguel, and Siete basins using the DRASTIC and GOD methods; (b) calculate the danger index using the POSH method, where the pollutant loading depends on the land use; and (c) to assess the aquifer contamination risk based on vulnerability and danger index. The information provided in this study will allow decision-makers to propose protection and pollution control strategies for groundwater resource areas identified as vulnerable or at an elevated risk of contamination. In this manner, the quality of water could be protected, and, thus, the well-being of its consumers.

2. Materials and Methods

2.1. Study Area

The study area is in the southwestern region of Ecuador, between the Azuay and Guayas provinces, and has an area of 856 km². The area includes three basins: Gala (533 km²), Tenguel (176 km²), and Siete (147 km²), which occupy 55.76% of the surface of Camilo Ponce Enriquez canton (Figure 1). The main activities in the area are metallic mining and agriculture [31]. The topographic altitude in Ponce Enriquez varies between 43 and 3680 m.a.s.l. The climate is semi-humid, with temperatures ranging from 12 and 30 °C (average 21.0 °C) and annual rainfall between 1219–1240 mm (average 1229 mm). The months of intense rain are January and July. Evapotranspiration is between 695 and 1485 mm/year, and the average is 1090 mm/year [32,33].

Geological and Hydrogeological Settings

The study area is described as colluvial, alluvial, alluvial terrace, and undifferentiated Quaternary deposits, which overlie Cretaceous and pre-Senonian age formations and geological units (Figure 2a). The Pallatanga Unit (middle and late Cretaceous) is identified as the basement unit [34]. The Saraguro Group, from late middle Eocene to early Miocene age, rests and faults over the Pallatanga Unit, as well as the undifferentiated metamorphic rocks. Bordering the Saraguro Group, the Tarqui Formation (Upper Miocene) is outcropping, which covers the oldest geological units in the area. Several environments were recognized over the volcanic basement: fluvial, lacustrine, and deltaic. In Table S1 (Supplementary Materials), the lithology of the geological units is presented.

Alluvial deposits (QA) and alluvial terraces (QTA) have the highest permeability and porosity for a high-performance aquifer, followed by colluvial deposits (QCa) and the Tarqui Formation (MTq) with medium permeability and porosity, and with local and discontinuous aquifers of medium performance (Figure 2b). The Pallatanga Unit (KPa) and the Saraguro Group (E-Ms) have very low permeability and cracking porosity, with local or discontinuous aquifers, which are generally for spring uses. On the other hand, intrusive rocks (G) and undifferentiated metamorphic rocks (M) with zero permeability and porosity are generally identified as not aquifers. The lithopermeability map (Figure 2b) shows the areas with high, medium, and low permeability. The most hydrogeological interesting areas, where most of the wells are identified, are located at the west side of the study area. Finally, the creeks were identified in the lowest hydrogeological interest areas.

2.2. Data Sources

The information of water wells and rivers was retrieved from available databases of the Ministry of Water, Environment and Ecological Transition of Ecuador (MAATE) [35]. This study includes 58 monitoring wells. Additionally, information from the National Information System’s available spatial databases (http://sni.gob.ec/coberturas, accessed on 2 December 2020), the National Aeronautics and Space Administration (https://www.earthdata.nasa.gov, accessed on 2 December 2020), and the Military Geographic Institute (http://www.geoportaligm.gob.ec, accessed on 3 December 2020) were used. Information from databases and technical reports from the Ecuadorian government were useful [31,32,36]. The information of potential contamination sources was obtained from the Ministry of Agriculture and Cattle Raising of Ecuador as Land Use (LU) geographic information files [37]. A data summary is shown in Tables S2 and S3.

2.3. Groundwater Pollution Risk and Vulnerability Assessment

Figure 3 shows the applied methodology for this study. Contamination vulnerability (I_V) was quantified using the DRASTIC and GOD methods. The danger index (I_P) was evaluated using the POSH method. Finally, the groundwater contamination risk (R_C) was calculated. The analysis and processing of geospatial data were performed with the Spatial Analysis toolboxes from ArcGIS 10.8.2 software. This software allows the generation of overlay operations and a spatial analysis index. Weighed parameter maps were added spatially following the specific criteria according to the evaluation method applied [14].

2.3.1. DRASTIC Method

The DRASTIC method was applied for calculating the pollution vulnerability index (Iv), and it is based on the seven parameters overlapping to represent the hydrological conditions of an interest area [7]. This method was developed by Aller et al. [38] for the United States Environmental Protection Agency (USEPA). It is used to measure the aquifer intrinsic vulnerability on each i pixel (e.g., unit grid cell) (Equation (1)). Each parameter involved in the I_V calculation is rated and scaled from 1 (not vulnerable) to 10 (very vulnerable), and it is based on specific settings and location.

According to the methodology proposed by Aller et al. [38] (Table S2), a relative weight (w_i) was assigned to each parameter that ranges between 1 and 5. The I_V has seven categories: negligible (I_V < 100), very low (101 < I_V < 119), low (120 < I_V < 139), moderate (140 < I_V < 159), high (160 < I_V < 179), very high (180 < I_V < 199), and extreme (I_V > 200).

$${\mathrm{I}}_{\mathrm{V}}={ \mathrm{D}}_{\mathrm{R}}.{\mathrm{D}}_{\mathrm{w}}+{\mathrm{R}}_{\mathrm{R}}.{\mathrm{R}}_{\mathrm{w}}+{\mathrm{A}}_{\mathrm{R}}.{\mathrm{A}}_{\mathrm{w}}+{\mathrm{S}}_{\mathrm{R}}.{\mathrm{S}}_{\mathrm{w}}+{\mathrm{T}}_{\mathrm{R}}.{\mathrm{T}}_{\mathrm{w}}+{\mathrm{I}}_{\mathrm{R}}.{\mathrm{I}}_{\mathrm{w}}+{\mathrm{C}}_{\mathrm{R}}.{\mathrm{C}}_{\mathrm{w}}$$

(1)

where D_R is the depth to water table; R_R is the net recharge; A_R is aquifer media; S_R is soil media; T_R is the study area topographic slope; I_R is the impact of the vadose zone media; and C_R is hydraulic conductivity.

The D_R values are the well inventory information (n = 58) in the study area. R_R was calculated using the water balance of the Gala, Tenguel, and Siete river basins. The hydrological cycle parameters were calculated through the water balance using the Thornthwaite method and GIS with the data obtained from the LocClim software 1.0.

Finally, the recharge classification was performed according to Table S2. AR was obtained based on the relationship between permeability and porosity that predominate in the area rocks. S_R was established with the information of the taxonomic classification of soil in the study area. TR was obtained using an ALOS PALSAR radar sensor scene from the Japan Aerospace Exploration Agency (JAXA), using the L radar band, captured on 13th June 2007, and downloaded as high-res Terrain Corrected with 12.5 m of spatial resolution. I_R was classified through the available information on soil type. C_R was obtained from the lithology of the study area and their hydrological characterization.

2.3.2. GOD Method

This method was developed by Foster [39] and analyzes information regarding three specific physical media parameters: the groundwater occurrence (G), the overlying lithology of the aquifer (O), and the depth to groundwater (D) (Equation (2)). Each parameter has a numerical value assigned that varies between 0 (less vulnerable) and 1 (very vulnerable). The I_V is calculated by multiplying the assigned values of each parameter. This method considers five vulnerability classes: negligible (I_V < 0.1), low (0.1 < I_V < 0.3), medium (0.3 < I_V < 0.5), high (0.5 < I_V < 0.7), and extreme (0.7 < I_V < 1). This method is primarily used in data-limited regions that require a succinct evaluation of the groundwater status [14,40].

$${\mathrm{I}}_{\mathrm{V}}=\mathrm{G}.\mathrm{O}.\mathrm{D}$$

(2)

where G is the confinement of the aquifer grade, which is obtained from the study area geological information and hydrogeological characteristics such as porosity and permeability. O refers to the occurrence of the overlying substrate, which is defined according to the consolidation degree and the lithological characteristics of the aquifer’s unsaturated zone. D is the depth to groundwater, whose values are obtained from the database of 58 wells and springs registered by MAATE in the Gala, Tenguel, and Siete river basins.

2.3.3. POSH Method

The contamination danger index (I_P) was quantified based on the Pollutant Origin Surcharge Hydraulically (POSH) method, which is on the function of the pollutant load, being punctual, lineal, or diffuse. The inventory and the assessment of potential sources of contamination were constructed based on the main economic activities in Camilo Ponce Enriquez [36]. A value of 1 was assigned to areas with no contamination loading, 2 to areas with a reduced potential level of contamination, 3 to areas with a moderate potential level of contamination, and 4 to areas with a high potential level of contamination. Finally, using the GIS software, every potential contamination source was added and classified, getting as results: Reduce I_P < 7, 7 $\le$ Moderate I_P < 10, and Elevate I_P > 10.

The potential sources of diffuse contamination selected in this study were: urban areas, cultivated pasture areas, undifferentiated crop areas, short-cycle crops, banana crops, and shrimp farms. Data were obtained from the Land Use (LU) geographic information file by the Ministry of Agriculture and Livestock of Ecuador [37].

Environmental mining liabilities and mining concessions (artisanal and small-scale mining) were considered potential sources of punctual contamination; this information was acquired from the mining census and the socio-environmental diagnosis of the Tenguel study area [31]. The potential sources of lineal contamination correspond to the rivers identified as contaminated and the street network. The information was collected from the street network and the water system of Ecuador, belonging to the National Information System (SNI), as well as the water diagnosis carried out by the Mayor’s Office of Camilo Ponce Enriquez [36].

2.4. Risk Assessment (R_C)

The contamination risk assessment was calculated by map algebra between the vulnerability index (I_V) obtained from the DRASTIC method and the danger index (I_P) resulting from the POSH method. For this purpose, the vulnerability index from DRASTIC was modified as shown in Table S4. This established the ranges for evaluating the aquifer’s risk of contamination (R_C) (

Table 1. Aquifer’s risk of contamination (R_C).

RC		IV Mod INDEX
		Reduce	Low	Moderate	High
IP	Reduced—Null	Low	Low	Low	Moderate
	Moderate	Low	Moderate	Moderate	High
	Elevated	Moderate	High	High	High

).

3. Results and Discussion

3.1. Vulnerability Assessment: DRASTIC and GOD Methods

Figure 4 shows seven parameters used in the intrinsic vulnerability assessment of the aquifer using the DRASTIC method. The piezometric levels (D) recorded in the study area range between 0 and 20 m below ground level; the inventoried wells are in the plains of the coastal zone, where the alluvial deposits and terraces with medium-high permeability and intergranular porosity predominate. The different aquifer lithologies (A) inferred different aquifer types, prevailing local or discontinuous aquifers, since the Piñon and Saraguro Formations occupy the largest amount of area with volcanic origin rocks with very low permeability and secondary porosity. The aquifer recharge (R) varies in a range from 6% to 15% of precipitation; a recharge of 151 mm was reported in the western lower zone, whereas the maximum recharge was calculated at 191 mm in the study area’s upper basin. Regarding the soil media (S) and the vadose zone (I), sandy loam predominates in the recharge zones of the study area, as loam soil prevails in the discharge zones. Finally, the hydraulic conductivity (C) associated with local or discontinued aquifers was determined in a range from 10−2 to 10 m/day, whereas in the west and east of the study area, it varied in the range from 10−1 to 102 m/day.

Table 2. Vulnerability levels by DRASTIC and GOD methods.

IV	DRASTIC		GOD
	Area (km2)	%	Area (km2)	%
Negligible	208.6	24.4	4.1	0.5
Very low	57.5	6.7	-	-
Low	167.6	19.6	354.3	41.4
Moderate/Medium	219.4	25.6	235.6	27.5
High	123.3	14.4	137.0	16.0
Very High	79.3	9.2	-	-
Extreme	0.3	0.1	125.0	14.6

shows the vulnerability categorization results, and the percentage of the area obtained by the DRASTIC and GOD methods. The intrinsic vulnerability of the aquifer using the DRASTIC method is shown in Figure 5a. The I_V-DRASTIC results ranged between 80 and 204, with a vulnerability classification from negligible (I_V < 100) to extreme (I_V > 200). The category of vulnerability that prevails in the area is moderate (219.4 km²), followed by negligible vulnerability (208.6 km²), representing 25.6% and 24.4% of the area, respectively. The central zone of the study area is characterized by negligible-to-low vulnerability, but some specific sites of moderate vulnerability. On the other hand, the east zone shows moderate-to-high vulnerability values, whereas the west zone has moderate-to-very-high vulnerability values. Extreme vulnerability takes place in a very small area (0.3 km²), which is equivalent to less than 0.1% of the study area.

Regarding the GOD method results (Figure 5b), the range of I_V-GOD was between 0.32 and 0.72, predominating low vulnerability (354.3 km²; 41.4%), followed by medium vulnerability (235.6 km²; 27.5%) in the central and west study area, respectively. Conversely, a high vulnerability was identified in the western area (137 km²; 16.0%), whereas there is an extreme vulnerability to contamination in the east area (125 km²; 14.6%).

The eastern and western zone of the study area present a moderate-to-very-high pollution vulnerability using both methods, where the Alluvial Deposits and Terraces are predominant and considered as areas of high aquifer productivity. On the other hand, the central zone is dominated by the Pallatanga and Saraguro Groups, where local aquifer productivity is limited and represents the less vulnerable area. According to the DRASTIC method, this zone has a negligible–low vulnerability; however, the GOD method categorized the vulnerability between low and medium. Both methods concur that the central zone of the study area presents a vulnerability range from negligible to moderate, which benefits groundwater protection. This central zone possesses 99% of the mining concessions and environmental liabilities.

The results present some differences in the vulnerability categorization using both the DRASTIC and the GOD methods due to the involved parameters’ differences during evaluations. The D, I, and R parameters of the DRASTIC method have a major contribution in the DRASTRIC vulnerability quantification, with pondered weights of 5 and 4, respectively. For the GOD method, the O and D parameters have a major contribution to the vulnerability classes determination. Several studies agree that despite the possible differences found in the vulnerability results with the DRASTIC and GOD approaches, both methods have proven to be valid for acknowledging the general situation of groundwater quickly at low cost, easy data processing, and interpretation for decision-makers [7,22,40].

3.2. Aquifers Danger Index (I_P): POSH Method

The POSH method evaluated the groundwater pollution danger index based on the pollutant loading in the study area (Table S5). Six potential sources of diffuse pollution were identified: (i) urban areas, (ii) cultivated pasture areas, (iii) undifferentiated crop areas, (iv) short-cycle crops, (v) banana crops, (vi) and shrimp farms. There are around 101 potential contamination hotspots, which were assigned levels of contamination of reduced and elevated. Additionally, four pollution sources related to the mining areas and environmental mining liabilities were identified (83 contamination hotspots), which exhibited a high polluting potential. Moreover, two lineal pollution sources were identified: a load with reduced contamination potential related to the street network, and three loads with high contamination potential in the Gala, Tenguel, and Siete rivers. With these contamination data levels, a danger index map was elaborated (Figure 6), which shows a reduced I_P in the central-east zone (371.1 km²; 43%), whereas in the central-western zone, the I_P is moderate (426.2 km²; 50%) and high (58.7 km²; 7%).

The results of this study agree with those reported by Jarrín et al. [21], who determined a high hazard index (I_P) for the aquifers of the Limoncocha Ecuadorian Amazon Reserve, mainly due to unsustainable agricultural practices.

It is important to consider that the hazardous index (I_P) was quantified from the human activities that are currently carried out in the study area. However, these activities can vary both spatially and in time; therefore, the situation of the aquifer system may also be affected. Currently, approximately 50% of the study area presents a moderate I_P; however, there is the possibility of an increase and propagation of the I_P if management and control strategies of anthropogenic activities and land use in the area are not implemented in the study area.

3.3. Aquifer Risk of Contamination Assessment (R_C)

The results of the aquifer risk of contamination assessment are shown in Figure 7. The high R_C covers 20% of the study area (173.7 km²), the moderate R_C has 33% (279.3 km²), the low R_C to 35% (297.3 km²), and the null-reduced R_C has 12% of the study area (105.7 km²). The activities that contributed to compute the high R_C (in decreasing order) are short-cycle crops > banana crops > undifferentiated crops > cultivated grass > mining > shrimp farms > urban areas. The results show that agricultural activities point out high-risk contamination of the aquifers in 87% of the area, whereas mining activities represent a high risk in 9% of the area. The remaining 4% includes shrimp farms and urban areas. Additionally, 46% of the water samples are in high-risk areas, 29% are in moderate-risk areas, and 25% are in null-reduced risk areas. Therefore, it is recommended to evaluate the groundwater quality in the area, since these are highly exposed to the pollutant loading, which could harm the end-users´ health.

Land use and anthropogenic activities have a significant impact on groundwater vulnerability. Considering the land use scheme in Ponce Enriquez, mining and agricultural areas are the potential contamination hotspots. However, the potential contamination intensity has varied significantly amongst these activities. In general, the study reveals that agricultural activity areas were of high risk for aquifer contamination, whereas mining activity areas showed a moderate risk.

Similar results were reported in the Daule aquifer (Ecuador), where there is a high risk of contamination due to the agricultural activity carried out in the area [13]. In agreement, Huang et.al [41], Abera et al. [42], and Bera et al. [43] reported that agricultural activities were identified as potential sources of aquifer contamination in China, Ethiopia, and India, respectively.

Despite the fact that mining activity has been widely classified as a potential contaminant of aquifer systems [17,44,45], this study shows that areas with agricultural activity in the Ponce Enriquez area represent a high risk of contamination and, therefore, they must be monitored.

The unplanned activities and the lack of monitoring could create groundwater diffusion pollutants and deteriorate its quality. For this reason, assessing the contamination status of aquifers is recommended by monitoring contamination indicator parameters. Factors such as depth and water table play a fundamental role in vulnerability evaluations. Therefore, a future research line plans to get information about the water table levels in the eastern of the study area, as well as geological, lithological, and hydrogeological information on a smaller scale to contrast them with the results of this evaluation and produce a robust assessment. Other important aspects to consider are the spatial and temporal parameters variations in the DRASTIC and GOD vulnerability indexes, since changes in these parameters could significantly influence the vulnerability results [16]. In addition, the validation of the DRASTIC and GOD vulnerability maps in the study area must be carried out. Validation is important to avoid erroneous conclusions and subjective judgments [42,43]. A widely used tool in the vulnerability map validation process is the nitrate concentration [17,46], since nitrate does not come from natural sources. On the contrary, its presence is related to anthropogenic activities such as agriculture or other human activities; therefore, it is a good indicator of contamination [7]. This study includes a preliminary assessment of the groundwater vulnerability status in the Ponce Enriquez mining area and highlights the need to conduct thorough research on groundwater protection.

4. Conclusions

The aquifer vulnerability assessment is a detection tool to achieve groundwater protection and sustainable management. This is the first study that evaluated the groundwater vulnerability in intensive agricultural and mining areas in Ecuador. The vulnerability assessment was made using the DRASTIC and GOD methods. Furthermore, potential sources of contamination were evaluated using the POSH method, and finally, the groundwater contamination risk (R_C) was quantified. The DRASTIC method results show a null–low vulnerability in most of the area (433.7 km²), followed by a moderate vulnerability (219.4 km²) and a high-to-extreme vulnerability in the remaining area (202.9 km²). These results are related to the hydrogeological conditions that support the ability to attenuate contamination from the surface.

Regarding the GOD method, the low and medium vulnerability categories prevail in areas of 354.3 km² and 235.6 km², respectively. A high and extreme aquifer contamination vulnerability was identified in the west (137.0 km²) and east (125.4 km²) zone. Both methods showed that the central zone of the study area exhibited negligible to moderate vulnerability, where mining activities predominate. On the other hand, the area with a high risk of groundwater contamination is 173.7 km² (20% of the study area), turning it into the highest area with a contamination risk (152.1 km², 87%), corresponding to the zone dedicated to agricultural activities. The mining activity zones show a high risk in an area of 13.9 km² (9%), whereas shrimp farms and urban zones report a high risk in 7.7 km² (4%). As a future line of research, it is proposed to define the recharge zones of the aquifer system and carry out the validation of the vulnerability models used in this study, using contamination indicators. The information used in this study represents a baseline to provide input and establish strategies of sustainable land use to protect aquifers in zones of potentially polluting anthropogenic activities such as agriculture and mining.