Improving Groundwater Quality Through Biosphere Reserve Management: Insights from the Anaga Reserve, Tenerife

Abstract

The Canary Islands, an outermost Spanish territory in the Atlantic Ocean, are renowned for their subtropical climate and significant tourism. However, substantial areas are designated for environmental protection, notably the Anaga Rural Park in Tenerife, a UNESCO Biosphere Reserve, which is the focus of this study. This research investigates the influence of Biosphere Reserve designation on groundwater quality, a crucial resource for Tenerife’s population. We analysed the physicochemical properties of groundwater within the Anaga region over a decade (2007–2016). Our findings demonstrate that groundwater quality consistently meets regulatory standards, exhibiting no evidence of pollution. This high quality is attributed to several factors, including the low population density, limited tourism impact within the reserve, and crucially, the effective soil protection measures implemented within the Biosphere Reserve. The compact geology of the region further limits infiltration and potential pollution. The sustained high quality of groundwater, even in the absence of detectable pollution, highlights the importance of ongoing monitoring to maintain this valuable resource and support local biodiversity. This case study provides a valuable model for sustainable groundwater management and soil protection strategies in other areas of Tenerife and beyond.

1. Introduction

Groundwater is a vital natural resource for humanity, serving as a critical component in the provision of potable water, agricultural production, and industrial processes. The quality of groundwater is of paramount importance for the preservation of public health and the environment [1]. However, the mounting pressure exerted by human activities and natural phenomena has given rise to considerable challenges in terms of their preservation and management, particularly in geographically vulnerable contexts such as islands. It is therefore becoming increasingly necessary to plan the use, management and protection of this resource in a given territory [2].

Historically, the management of water resources in Spain has been characterised by a prioritisation of the increase in water supply [3], with the objective of meeting the demand derived from agricultural expansion and massive urbanisation, particularly in coastal areas with scarce water resources [4,5]. The conventional strategy of intensive freshwater production, encompassing desalination and the utilisation of abandoned wells, has resulted in a number of challenges, including marine intrusion [6] and a decline in water quality, in addition to public health concerns pertaining to human and animal pollution [7,8]. This approach has been subject to revision over time, with a paradigm shift towards demand-driven water management, environmental protection and cost recovery.

The management of groundwater on islands presents a number of unique challenges, largely due to the limited hydrogeological and geological characteristics of these environments and their exposure to extreme weather events [9,10,11]. In such environments, groundwater is of vital importance, and its protection from pollution is of the utmost importance to ensure the well-being of the population and the sustainability of economic activities [12]. This context emphasises the necessity for the implementation of efficacious measures to ensure the maintenance of water quality and the protection of island ecosystems [13,14].

A principal element of this protection is soil conservation, which represents a fundamental measure in the sustainable management of groundwater resources. The protection of soil not only preserves soil fertility and structure but also acts as a barrier to groundwater pollution [15]. Several studies conducted globally have demonstrated that the implementation of effective soil management strategies can effectively reduce the infiltration of pollutants, particularly in regions characterised by low population density and low levels of tourism and industrial activity [16,17,18,19,20,21,22]. In Europe, this integrated approach has become particularly important in protected areas, where the regulation of land use and the promotion of sustainable practices have been instrumental in maintaining the quality of aquifers and protecting biodiversity [23,24].

This context provides the backdrop for an investigation of the Anaga Rural Park in Tenerife, a UNESCO-designated Biosphere Reserve. The protected area has implemented land management strategies that limit pollution and promote the maintenance of groundwater quality, due to low population density and controlled tourism. The compact geology of the region, characterised by areas of low permeability interspersed with basaltic fissures and fractured volcanic rocks, impedes widespread infiltration and reinforces the aquifer’s natural protection against pollution. However, the aquifer is recharged mainly by the infiltration of atmospheric precipitation into zones of higher permeability. This case study presents an analysis of the impact of specific factors, including Biosphere Reserve status, low population density and low levels of agriculture, on water quality. The approach taken is aligned with the framework of the second and third cycles of the Tenerife Hydrological Plan.

Study Area

The Canary Islands are an archipelago located in the Atlantic Ocean, close to the Western Sahara. They are part of the Kingdom of Spain and are divided into two provinces: Tenerife and Las Palmas de Gran Canaria. In the province of Tenerife, from largest to smallest territories are Tenerife, La Palma, La Gomera and El Hierro. In the province of Las Palmas de Gran Canaria, from greater to lesser extent of territory are Fuerteventura, Gran Canaria, Lanzarote and La Graciosa [25]. The population of the region exceeds 2 million inhabitants. It is a region where the tourist industry predominates; therefore, the service sector but also the primary sector has some influence, with important agriculture exports, although they are in decline [26].

The average rainfall in the Canary Islands can be considered 400 mm per year, with great differences between islands and even within each island. Different areas must be considered, such as the south-facing area, with lower rainfall, compared to the north-facing area, with higher rainfall, influenced by the trade winds [27]. The humidity carried by the trade winds is used by the forest formations through a phenomenon known colloquially in the islands as the “sea of clouds”, which can manifest as horizontal precipitation or mist.

The forestry sector has the potential to become a vital contributor to the global effort to mitigate climate change. This is because it can play a role in carbon sequestration by increasing stocks in forests, provided that the forest in question is undergoing expansion or increasing organic mass. Furthermore, it can serve to mitigate carbon emissions by substituting materials that result in higher CO₂ emissions with more environmentally friendly alternatives. It is, however, important to note that a forest in an ecological steady state is carbon neutral [28,29,30]. Its role in regulating the hydrological cycle is also key in the Canary Islands, for example, by improving the recharge of aquifers, preventing the silting up of reservoirs, reducing erosion and damage to infrastructure, and controlling flooding due to torrential rainfall. The positive effect of the mountains in regulating the horizontal precipitation or fog that occurs in the Canary Islands’ forests is also worth noting [31,32].

The Anaga Biosphere Reserve (Figure 1) is located in the northwest of the island of Tenerife; it forms a well-differentiated area within the island due to its abrupt orography and because it is one of the oldest areas of the island, in geological terms [33]. The ancient edifices of Anaga, between 5 and 6 million years old (Middle Miocene-Lower Pliocene) [34] are large shield volcanic edifices, with deep ravines and cliffy coastlines. They are made up of different superimposed volcanostratigraphic sequences, mostly basaltic in composition. Likewise, the constituent materials are highly altered in those areas where later edifices have been superimposed and, in these areas, there may be intense tectonic fracturing/deformation induced by more recent volcanic activity [35].

The climate of Anaga, as with the remainder of the Canary Islands, is influenced by its location off the northwest coast of Africa, its proximity to the tropics and the cold ocean currents. The islands are subject to the influence of the East Atlantic high-pressure area, which remains stable for the majority of the year. The trade winds, which blow primarily from the north to northeast, bring cool, moist air to the islands. This layer of moist air is situated above a drier layer, forming a phenomenon known as a ‘sea of clouds’. This creates a stable atmosphere and limits convective movements.

Within the central portion of the Anaga mountain range, a wooded ridge extends over the elevated peaks of the massif, while on the northern slopes of the Anaga Massif, within the lower midlands, the terrain is characterised by a rugged valley that is exposed to the humid north winds. This environment is conducive to the development of a thermophilic forest of scattered junipers at intermediate altitudes and a dense scrubland at the upper edge, where orographic cloud formation is prevalent. The very humid atmosphere on the higher slopes and ridges of the massif, due to the orographic effect, manifests itself in the form of mist and fog on many days of the year.

This meteorological phenomenon, which includes dew and fog precipitation, plays a significant role in the annual water balance of these areas. Water occasionally flows through the ravines during autumn or winter, following heavy rainfall caused by the arrival of Atlantic depressions. Additionally, stratiform orographic clouds form in the high midlands and peaks of the massif, contributing to increased humidity in the region.

The small population residing within the Anaga Massif Biosphere Reserve is primarily concentrated in the transition zones. These zones consist of areas designated for moderate, special, and general use, as outlined in the Master Plan for the Use and Management of the Anaga Rural Park [33,36]. Anaga, the district with the largest area in the municipality of Santa Cruz de Tenerife, spans 119.55 km² and has a population of 11,851 inhabitants, resulting in a population density of 99.13 inhabitants/km². Situated within the Anaga Massif, the district’s mountainous and rural character has led to a dispersed settlement in small hamlets, with a coastal strip featuring urban centres and an industrial estate. The district is predominantly encompassed by the Anaga Massif Biosphere Reserve, a protected natural area of significant environmental value. In addition, the natural beauty of the park attracts tourists from all over the world. The local population heavily relies on the tourist industry for their livelihood due to the various services required, including restaurants, accommodation, and nature-related activities provided by the industry [26,36]. This dynamic has contributed to the growth of the local economy but has also posed challenges, such as environmental protection, land conservation, and sustainable management. The objective is to attain the proper equilibrium that endorses the responsible development of tourism whilst preserving the forests and the vulnerable natural environment of Anaga.

The geological material forming Anaga is highly impermeable due to its age (in comparison with other parts of the island), alteration, and degree of compaction. The absence of recent volcanic eruptions has contributed to the lack of fissures that facilitate water recharge of the aquifers [33,35]. The permeability resulting from micro-fissures is not very significant, as they have been sealed by materials eroded from adjacent rocks, preventing the free circulation of water. In general, this type of geology does not permit significant seepage, and its internal structure inhibits the formation of independent aquifers above the basement [37,38]. Occasionally, however, hanging aquifers form at the junctions with dykes and emerge to the exterior via springs. These properties generate a terrain that promotes surface runoff and gullies, resulting in most of the conventional, horizontal rainfall finding its way to the sea.

The objective of this study is to examine the relationship between the primary attributes of Anaga—such as its geology, soils, ecosystems, and land use patterns—and the quality of its water resources. Anaga is distinguished by its designation as a Biosphere Reserve, its unique soil features, its relatively low population density, and its agricultural practices that are less intensive than those observed in other regions of the island. The park’s status serves to restrict activities that could have a deleterious effect on the soil and aquifers, thereby reducing the risk of pollution. The following sections present a comprehensive analysis of the groundwater quality data for the Anaga Rural Park. In this context, the challenges, health risks and opportunities for sustainable water management in the area are discussed in detail.

2. Materials and Methods

The proposed methodology is aimed at establishing a relationship between the characteristics of the Anaga region and the quality of the water in the area. As previously stated, the area in question is situated in the northern region of the island of Tenerife and has been designated a Biosphere Reserve. This protected zone exemplifies the significance of land management in the preservation of water quality. Additionally, this region is distinguished both by a relatively low population density and a lack of industrial activity, while agricultural activity has become a supplementary activity. It is anticipated that these conditions will be demonstrated to be beneficial in terms of promoting good water quality.

The water samples analysed in the present study were exclusively sourced from groundwater, as the Anaga basin is entirely reliant on this resource for its water supply, with no alternative sources available. The region’s water supply is entirely dependent on groundwater, which is sufficient to meet the needs of the local population. Groundwater extraction in Anaga is facilitated by traditional horizontal galleries, a distinctive and unique feature in the Canary Islands. These galleries, which are horizontal tunnels dug into the mountainous terrain, intercept aquifers formed by basaltic fissures and fractures, thereby allowing the natural flow of groundwater without the need for active pumping. In addition to the galleries, small-scale boreholes are sometimes employed to supplement water collection from the galleries. The galleries are engineered to collect and convey water to storage tanks or directly into the distribution system. This method of groundwater management, adapted to the region’s geology and topography, is particularly efficient and helps prevent issues such as saline intrusion.

Samples were taken from two categories of reservoir: headwater and distribution, as well as from the water supply network following treatment. The six sampling points within the area of interest (Anaga) encompassed Almaciga; Ijuana I, II, III; Igueste de San Andrés; Pico Inglés; Roque Negro I–II and Taganana (approximate locations shown in Figure 2).

Empresa Mixta de Aguas de Santa Cruz de Tenerife SA (EMMASA) is responsible for the water supply in Santa Cruz de Tenerife, which includes the designated study area. The company is responsible for the disinfection and distribution of the water, which is subjected to rigorous quality controls.

In addition, EMMASA’s laboratory possesses the international accreditation UNE-EN ISO/IEC 17025, bestowed by the National Accreditation Entity (ENAC), which acknowledges the laboratory’s technical expertise in performing environmental tests. The laboratory has the necessary infrastructure, technology and personnel to analyse water throughout the entire cycle, facilitating efficient management.

The company made data from these monitoring analyses available for this research project for the 10-year period from 2007 to 2016.

The sampling points were selected in accordance with the guidelines laid down in Royal Decree 3/2023 of Ministerio de la Presidencia, [39]: (i) At least one sampling point has been installed at the outlet of the treatment plant. If this is not possible, the sampling point shall be located at the outlet of the collecting tank associated with the treatment plant. (ii) An additional sampling point has been located at the outlet of each regulating or distribution reservoir within each supply zone. (iii) At least one sampling point has been installed at each of the existing distribution points in the supply zone. (iv) One sampling point has been established in each distribution network, with an additional point being added for each 20,000 m³/day increase in network capacity. (v) Within each supply zone, as many distribution networks have been identified as there are regulating or distribution reservoirs. (vi) In cases where a tank supplies more than one population unit or nucleus, the sampling point was located in the unit or nucleus with the largest population.

In order to characterise the groundwater quality of the study area, a total of 406 samples were analysed. This dataset included 347 control analyses with 14 measured parameters: free residual chlorine, in-situ chlorine, coliform bacteria, colony count at 22 °C, Escherichia coli, laboratory turbidity, in-situ turbidity, ammonium, nitrate, pH, conductivity, colour, odour and taste. These parameters were selected based on their relevance to monitoring drinking water quality in accordance with the mandatory requirements established by Spanish legislation (Royal Decree 2/2023) and the European Union’s Drinking Water Directive. They are critical indicators for assessing water safety, pollution risks, and treatment efficiency. Furthermore, 60 complete analyses with 24 measured parameters, the 14 parameters mentioned above, and 10 additional ones, including residual combined chlorine, chloride, aluminium, iron, manganese, sodium, sulphate, oxidisability, Enterococcus and Clostridium perfringens. Although heavy metals were not included in the control analyses, some, such as aluminium and iron, were assessed as part of the complete analyses to provide a broader characterization of the water quality. This approach ensured that the study focused on key indicators of potability while also incorporating additional parameters to capture a more comprehensive understanding of water quality in specific cases.

The study analysed 347 water control samples collected between 2007 and 2016, except for a bacteriological control sample from Almáciga (5/2011). The analysis of the data revealed that 53% of the samples originated from the head tank (185), while the remaining 47% were drawn from the distribution network and reservoirs (162). For each location, between 28 and 76 data sets were collected over the period, with between 19 and 46 sets per year for all locations combined. The number of measurements per location ranged from 1 to 11 per year, with an average of 6 per year, except at Pico Inglés, where no measurements were taken in 2015.

This information serves to illustrate the temporal and spatial scope of the data, thereby underscoring its relevance as a baseline for future analysis and monitoring.

The distribution of the samples according to year and location is shown in the following table (

Table 1. Distribution of number of samples by year and location.

Sampling Point	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	∑
ALMACIGA	5	4	6	7	10	5	6	8	7	6	64
IJUANA	2	3	5	2	4	2	2	3	3	2	28
PICO INGLES	7	1	8	9	9	7	9	11	-	9	70
ROQUE NEGRO	3	3	3	4	5	2	5	6	4	4	39
SAN ANDRES	4	3	8	9	9	6	7	8	9	7	70
TAGANANA	5	5	6	8	9	7	9	10	9	8	76
∑	26	19	36	39	46	29	38	46	32	36	347

).

The data were then analysed, considering the maximum permissible values of the applicable regulations for the study area. Specifically, they are: (i) RD 3/2023 [39] and (ii) The Health Monitoring Programme for Water Intended for Human Consumption in the Autonomous Community of the Canary Islands [40].

Furthermore, it has been concluded that it is not necessary to calculate the Water Quality Index, since the limits applicable under Royal Decree 3/2023 are stricter than those established by the methodology used to calculate the WQI, after the preliminary results of the samples.

For the purpose of facilitating the analysis, the data were classified into three groups of parameters: (i) microbial parameters, including coliform bacteria, colony count at 22 °C, Escherichia coli, Enterococci and Clostridium perfringens, (ii) indicator parameters, including residual free chlorine, residual combined chlorine, in-situ chlorine, laboratory turbidity, in-situ turbidity, pH, conductivity, colour, odour, taste and oxidisability and (iii) chemical parameters, which include nitrate, ammonium, sulphate, chloride, sodium, aluminium, iron, and manganese.

The study has been constrained by the unavailability of more contemporary data. However, it has utilised all records available for the period 2007–2016, which represent the most robust evidence for the analysis of water quality conditions at that time. These limitations emphasise the necessity for the establishment of more consistent monitoring systems with open access to data but do not detract from the relevance of the findings as a basis for further research.

3. Results and Discussion

3.1. Microbial Parameters

Starting with the microbial parameters, the presence of coliform bacteria is the most commonly used indicator of disease-causing bacteria in water [41]. It is associated with improper maintenance of water distribution networks and indoor installation, as coliforms can survive and multiply within water distribution systems; water is only safe to drink if the test for coliform bacteria is negative. It is important to note that other sources of coliform bacteria, such as Escherichia coli, include animals, particularly through fecal pollution from livestock, wildlife, or domestic pets. Similarly, Enterococci species can originate from animal feces, serving as indicators of fecal pollution in water systems. Clostridium perfringens, a spore-forming bacterium, may also have diverse origins, including animals, decaying vegetation, and soil, thereby highlighting its natural presence in the environment and its potential to indicate pollution. Over a period of 10 years in the Anaga area, coliform bacteria were found in only two samples. All tests for Escherichia coli, Enterococci and Clostridium perfringens were negative. These parameters normally are indicative of wastewater pollution or contact with faeces in the system, so its presence would have posed a risk to public health [42,43]. Therefore, the absence of these indicators in the analyses suggests that the treatment system that comprises the Anaga Rural Park is adequately designed and dimensioned to prevent this type of pollution, thus guaranteeing water quality and safety for users. The colony count at 22 °C is the total number of culturable bacteria in a given sample and gives a general idea of how polluted the water is. The maximum allowable value is 100 ufc/1 mL and the highest value measured in any sample was 87 ufc/1 mL. In addition, 62% of the samples had a value of 0, 26% of the samples had values between 1 and 4 and 10% of the samples had values between 10 and 50 ufc/1 mL. After 2011, the colony count at 22 °C did not exceed 10 for all sampling sites. In the first part of the analysis, there is no reason to worry about the microbial parameters, as most of the results are positive in terms of the limit of detection values.

3.2. Indicator Parameters

Moving on to the second classification, statistical analysis of indicator parameters of all samples is shown in

Table 2. Statistical analysis of potable water quality indicator parameters and its coherence with RD 3/2023 and The Health Monitoring Programme for Water Intended for Human Consumption in the Autonomous Community of the Canary Islands regulations.

PARAMETER (Unit)	Min	Max	Range	Median	Mean	SD	CV%	Limit (RD 3/2023)	% Missing Values
pH	6.87	8.97	2.1	8.31	8.29	0.26	3	6.5–9.5	0.3
Conductivity (µs/cm)	277	963	686	401	421.6	97.89	23	2500	0.3
oxidisability (mg O2/L)	0.1 *	1.76	1.66	0.4	0.55	0.35	63	5	86
turbidity in-situ (FNU)	0.05 *	6.2	6.15	0.3	0.42	0.49	119	1	0
laboratory turbidity (NTU)	0.05 *	0.89	0.84	0.19	0.23	0.16	69	5	73
color (mg Pt-Co/L)	0.1 *	13.9	13.8	0.8	1.19	1.51	127	15	0.5
odour (in. dil)	0	1	1	-	-	-	-	3	0.3
flavour (in. dil.)	0	1	1	-	-	-	-	3	0.5
chlorine in-situ (mg/L)	0.2	1	0.8	0.8	0.73	0.14	20	0.2–1	3
residual free chlorine (mg/L)	0.05	0.99	0.94	0.68	0.64	0.18	28	0.2–1	70
residual combines chlorine (mg/L)	0	0.58	0.58	0.025	0.053	0.09	172	2	86

* 0.1 ≤ 0.2; 0.05 ≤ 0.1; [CV% of mean = 1 SD/mean × 100]; [% of missing values = number of samples without measured value/406 × 100].

.

Although pH does not normally have a direct effect on the consumer, it is one of the most important operational parameters of water quality [7]. Attention must be paid to pH control at all stages of water treatment to ensure satisfactory clarification and disinfection of water. It is known that pH can be affected by dissolved minerals and chemicals. All pH values measured were within acceptable limits, as can be seen in the following figure (Figure 3). In addition, it was observed that the Ijuana sampling location tended to have the lowest pH values. The samples with the highest pH values also showed the highest conductivity measurements [44,45]. Conductivity is one of the most sensitive indicators for detecting possible external pollution in the distribution network; comparing the conductivity at different points in the network can determine whether an indoor installation is well maintained. Conductivity measures the ability of water to conduct an electrical current, where dissolved salts and inorganic chemicals contribute to electrical conductivity. All conductivity values measured were within acceptable limits. Samples from the Pico Inglés tended to have the lowest values, while samples from San Andres tended to have the highest values. This trend may be due to the fact that Pico Inglés is at a higher altitude and San Andres is located on the coast. Surface and groundwater from precipitation have more time to dissolve the solids they come into contact with as they pass through terrestrial materials [46].

Turbidity is caused by the presence of suspended particles in the water. The particles responsible for turbidity vary in size from 1 nm to 1 mm and are mostly due to erosion of the soil surface [47]. High turbidity levels are commonly associated with poor drinking water quality and also interfere with the disinfection process, reducing its effectiveness [48]. Turbidity was measured both in situ and in the laboratory; the measurement can be seen in the next figure (Figure 4). In situ turbidity measurements were conducted using sensors installed at various sampling points, while laboratory measurements followed standardized analytical protocols. It is important to note that turbidity values obtained in the field are expressed in FNU (Formazin Nephelometric Unit), in accordance with the European ISO 7027 standard, whereas laboratory measurements are typically expressed in NTU (Nephelometric Turbidity Unit), as specified by the USEPA Method 180.1 or the Standard Methods for the Examination of Water and Wastewater. There were four samples with in situ turbidity levels exceeding the limit in 2007 and 2009. Of these, two samples were from the Ijuana location, which also tended to have the highest turbidity levels in the laboratory. The observed differences between field and laboratory measurements can be attributed to the distinct methodologies, equipment used and potential sample handling effects during laboratory analysis.

Ideally, drinking water should have no colour. The colour of the water is due to coloured organic matter (humic and fulvic acids) and the presence of iron or manganese. In drinking water, colour can be caused by the dissolution of iron or copper in indoor installations. If every value does not exceed the limitation, water quality can be considered to be good, in terms of this parameter. The variability is shown in the following figure (Figure 5).

The values obtained for odour and flavour were 0 or 1 (in dilution) for all samples. Therefore, no statistical parameters were calculated. An odour value of 1 was detected in 11% of the samples; a flavour value of 1 was also found in 11% of the samples. With the exception of one sample, all samples with a flavour value of 1 also had an odour value of 1. Most of the samples with odour and flavour values of 1 were collected at the Pico Inglés sampling location. The colour measurements with the highest values were mainly from the year 2009 and from the Ijuana sampling location. There are several factors that can cause a perceived change in the odour and/or taste of the water for the consumer. The most common causes are: (i) natural compounds related to the origin of the water; (ii) reagents used in the drinking water treatment process or by-products generated during the process; (iii) materials used in pipes, assemblies and installations; (iv) polluting discharge; and (v) long residence time of the water in the network.

In a water distribution system that uses chlorination as a disinfection treatment, it is necessary to measure residual chlorine to ensure that the system remains bacteria-free. A decrease in residual chlorine may indicate an increase in pollutants in the system that reacted with the residual chlorine. There were three samples with free residual chlorine values below the limit, all from 2007 (March–June) and located in Almaciga and Taganana. However, all the other parameters measured in these samples were within the permitted limits.

The presence of chloride in drinking water is due to natural causes, industrial effluents and marine intrusion, among others [49,50,51]. Excessive levels of chloride increase the corrosion of metals in pipes, depending on the alkalinity of the water. The WHO has not proposed any reference values for chloride in drinking water from a health point of view, but high concentrations of chloride can cause a detectable taste. However, the available data are insufficient to draw definitive conclusions regarding residual free chlorine and residual combined chlorine. Specifically, 349 samples were below the detection limit of the method and thus did not show detectable levels. This substantial number of non-detections is consistent with the characteristics of the water supply system in the study area, where chlorine levels dissipate rapidly due to factors such as the small size of the system, low storage volumes and limited distribution distances. These non-detections reflect the dynamics of chlorine behaviour in the system rather than sampling or analytical deficiencies. The values for these parameters can be seen in the following figure (Figure 6), specifically for each location, and a total of all the samples represented in yellow.

3.3. Chemical Parameters

The chemical parameters statistical analysis is shown in

Table 3. Statistical analysis of potable water quality chemical parameters and its coherence with RD 3/2023 and The Health Monitoring Programme for Water Intended for Human Consumption in the Autonomous Community of the Canary Islands regulations.

PARAMETER (Unit)	Min	Max	Range	Median	Mean	SD	CV%	Limit (RD 3/2023)	% Missing Values
NO3− (mg/L)	0.25 *	25.1	24.85	5.1	5.58	3.72	66.76	50	41
SO42− (mg/L)	2.5 *	31	28.5	14.5	15.47	6.08	39.30	250	86
Cl− (mg/L)	24.8	177.5	152.7	78.1	78.30	23.99	30.63	250	86
NH4+ (mg/L)	0.005 *	0.1	0.09	0.005 *	0.02	0.02	119.08	0.5	15
Na (mg/L)	26.25	113.3	87.05	44.02	51.01	20.13	39.46	200	86
Al (µg/L)	10 *	129	119	10	12.05	15.49	128.53	200	86
Fe (µg/L)	10 *	167	157	22.6	32.2	33.1	103	200	86
Mn (µg/L)	2.5 *	12.2	9.7	2.5 *	2.78	1.53	54.90	50	86

* 41 samples had a nitrate value <2 → transformed to 1 for statistics (2 samples had <0.5 → 0.25); 1 sample had sulfate value <5 → 2.5; 216 samples had ammonium value <0.1 → 0.005; 57 samples had aluminum value <20 →10; 28 samples had iron value <20 → 10; 56 samples had manganese value <5 → 2.5.

.

Ammonium is present in raw water as a result of agriculture, industry and chlorination. The presence of high levels of ammonium can compromise the effectiveness of disinfection or fail to remove manganese in filters, causing taste and odour problems. The presence of ammonium can be an indicator of faecal, agricultural or industrial pollution [52,53]. The analysis of this parameter can be seen in the next figure (Figure 7). It was observed that the ammonium measurements in the samples were generally higher in 2010 (April–August). In July 2009, a sample from Ijuana had the highest ammonium value (0.25 mg/L). However, five days later the ammonium value at this sampling site was below the detection limit (<0.01 mg/L). In these two samples, it can be observed that, as the ammonium value decreases, the in situ chlorine value also decreases and the pH value increases. The sample with the highest ammonium value had one of the lowest sulphate values.

Sulphates enter the water from industrial waste and atmospheric precipitation, but the highest concentrations are usually found in groundwater and come from natural sources [54,55]. The sulphate measurements in the samples were well below the allowable limit, shown in the following figure (Figure 8). The sample with the lowest sulphate value had one of the highest iron values. It should be noted that, in May 2010, the sulphate values were highest at different sites (Almaciga and Roque Negro), and these samples also had one of the highest nitrate values.

Nitrate is mainly used in inorganic fertilisers [56,57,58,59,60]. The toxicity of nitrate in humans is attributed to its reduction to nitrite. The main health risk is methaemoglobinaemia in infants, leading to cyanosis and, only at higher concentrations, asphyxia. Other risks from prolonged exposure have been associated with stomach cancer, although there is no evidence of a causal relationship. This is consistent with the conclusion of the IARC, which has classified nitrate and nitrite intakes under conditions leading to endogenous nitrosation in Group 2A (probably carcinogenic to humans) but not nitrate alone. Nitrate measurements in the samples were below the permitted limit and 18% of the samples had values below the detection limit, represented in the next figure (Figure 9). The lowest values were recorded in June 2010 and the highest values in April and May 2010 at all sampling sites. The highest value measured was in March 2012 in a sample from San Andres, which also had one of the highest pH and conductivity values. Samples from other sites (Pico Inglés and Taganana) had significantly higher nitrate values in March 2012 compared to April 2012, which were below the detection limit.

The presence of aluminium in drinking water is mainly due to the use of aluminium salts in drinking water treatment, in the flocculation-coagulation stage. A high residual concentration can give the water an undesirable colour and turbidity. Iron is one of the most abundant metals in the Earth’s crust. Iron can also be present in drinking water due to the use of iron coagulants or corrosion of steel or cast-iron pipes during water distribution [61]. The taste and appearance of drinking water may be affected by the presence of iron below 2 mg/L. The WHO has not proposed reference values for iron in drinking water. Manganese is a commonly occurring metal in the Earth’s crust, although it is less abundant than elements such as magnesium, calcium and potassium. In highly oxygenated water, deposits of manganese compounds can form, causing colour problems in the water. At levels above 0.1 mg/L, manganese in drinking water can cause an unpleasant taste and stains in laundry. Sodium is present in virtually all foods and this is the main source of exposure. The WHO has not proposed reference values for sodium in drinking water from a health point of view. However, concentrations above 200 mg/L may cause unacceptable taste. No firm conclusions can be drawn from these parameters, as we could not obtain further values to analyse from the samples collected. Nevertheless, none of the measurement values are in excess of the limit value in the regulations. The values for sodium and iron of all the samples are represented in the following figure (Figure 10).

Almost all samples analysed over this 10-year period had measured parameter values within the permitted limit, but there are two samples that stand out in terms of water quality, compared to the others.

In December 2007, a sample from Ijuana tested positive for coliform bacteria and had one of the highest colony counts at 22 °C (23 cfu/1 mL). The measurement of aluminium in this sample was 129 μg/L and was the only one not below the detection limit. This sample also had the highest or one of the highest values for colour (13.9 mg Pt-Co/L), in situ chlorine (1 mg/L), sodium (105.9 mg/L), oxidisability (1.68 mg O₂/L), in situ turbidity (4.7 UNF), and manganese (12.2 μg/L). It had one of the lowest pH values (7.25), although the odour and taste values in the dilution were 0.

In November 2009, a sample from Ijuana had the only other manganese measurement not below the detection limit (9.29 μg/L), the highest chloride (177.5 mg/L) and conductivity (809 μs/cm), and one of the highest in situ turbidity values (1.45 UNF), with odour and taste values of 0 in dilution.

As previously mentioned, soil conservation plays a vital role in preserving natural habitats and maintaining sustainable use of resources. This is particularly evident in the study area, where protecting soil has had a positive impact on various environmental and ecological factors that are essential for biodiversity preservation.

Furthermore, the implementation of soil conservation measures in this region has led to a noteworthy reduction in soil erosion, thus preventing the depletion of fertile soil and subsequent degradation of habitats. In addition, the maintenance of healthy soil serves a pivotal role in mitigating climate change by serving as a carbon sink. The deposition of organic carbon in soils helps decrease the concentration of atmospheric carbon dioxide, consequently reducing the impact of global warming.

Additionally, the forest sector and safeguarding soil in the Anaga Massif have favourable impacts on the region’s ability to maintain crops and vegetation over an extended period. It is crucial to preserve soil productivity for agricultural output and maintenance of terrestrial ecosystems. Moreover, this safeguard averts unsystematic urbanization, preserving natural scenery and verdant areas in an urban setting. This is crucial for developing ecotourism and fostering appreciation of nature for locals and visitors [12,62].

While the study highlights the positive outcomes of the reserve and its influence on water quality, it is important to acknowledge the significant challenges that the management of these areas is currently facing. Ecotourism, when managed effectively, can contribute to sustainability. However, it can also have adverse effects on ecosystems, such as habitat alteration or increased pollutants from tourism infrastructure. These factors have the potential to undermine the conservation achievements and water quality gains that have been made in the long term [60,63].

Primarily, soils are crucial for water quality, functioning as innate filters as water seeps through them, purifying water by detaching impurities and pollutants. This process is vital for maintaining water quality in streams and aquifers within the Anaga Biosphere Reserve. This, in turn, benefits the aquatic ecosystems and availability of freshwater for local flora, fauna and people. The study results confirm that the water was in a good state during this period. Protection of the soil, the hydrogeology of the area, and the remarkable maintenance of facilities are responsible for these commendable results.

4. Conclusions

In conclusion, this study provides a comprehensive overview of groundwater quality in the Anaga Rural Park and its importance for the sustainable management of this exceptional ecosystem. The designation of the area as a Biosphere Reserve has been pivotal in the protection of water quality, as it has enabled the implementation of rigorous regulations that, although restrictive, have proven to be effective. The distinctive characteristics of the soil, in conjunction with the reserve status and the associated safeguards, have been a significant factor in the conservation of this natural area. This has led to the preservation of biodiversity, an enhancement of water quality, climate change mitigation, the maintenance of agricultural sustainability and the preservation of natural landscapes. These outcomes have been achieved within the context of sustainable development and environmental preservation.

During the course of the research, the overwhelming majority of the samples analysed were found to comply with the rigorous limits and quality standards prescribed by the regulations currently in force in the region. The analysis of microbial parameters indicates that the maintenance practices in the distribution networks are adequate, as no coliform bacteria or signs of sewage pollution were found. This ensures that there is no threat to public health.

The evaluation of water quality using the indicator parameters revealed that the pH values were within the acceptable range in all samples, with a slight tendency towards lower values at the Ijuana site. Furthermore, the conductivity values were within the acceptable range, although the samples from Pico Inglés exhibited lower values and those from San Andrés demonstrated higher values. The presence of suspended particles resulted in turbidity levels exceeding the permitted threshold, with the highest concentrations observed at the Ijuana site. However, the water colour remained within the acceptable range, indicating that the water quality was satisfactory. While the majority of samples exhibited satisfactory levels of odor and taste, some, particularly those from Pico Inglés, displayed aberrant characteristics.

Three samples exhibited a chlorine residual below the established threshold, indicating a potential reduction in water disinfection efficacy. However, no other noteworthy quality issues were identified. The presence of chloride was deemed to be of natural origin and did not indicate concentrations that would pose a public health concern. However, the lack of sufficient data precludes definitive conclusions regarding free and combined chlorine parameters.

The Anaga Rural Park attracts tourists from across the globe and relies on the local population to provide tourist services. Preserving the environment in this context of increasing tourism is a challenge, as the influx can negatively affect regional culture and biodiversity. Therefore, effective sustainable management, especially through constant monitoring, is crucial to safeguard the park’s essential natural and cultural heritage.

The present study provides a robust foundation for future research on sustainable groundwater management in protected areas. The results demonstrate the efficacy of conservation measures and geological characteristics in maintaining water quality, while also underscoring the complexities associated with the management of ecotourism areas.

The findings will serve as a reference point for monitoring long-term changes and for developing strategies to balance the benefits of ecotourism with the need to preserve vulnerable ecosystems, thereby ensuring water and environmental sustainability in the region.