European Geologist Journal 54

Exploring low enthalpy geothermal energy in Costa Rica: A pilot project to improve tourism energy matrix efficiency 

 

by  Agustín F. Solano-Arguedas^1,2 *, Ingrid Vargas-Azofeifa³, Kenia Barrantes-Jiménez⁴, Geovanni Carmona-Villalobos⁵, Kattia Solís-Ramírez⁵, Karina Rodríguez-Mora¹, Yoselyn Álvarez-Saborío³, Pedro Casanova-Treto^1,5

¹ Forest Resources Unit (Reforesta), Engineering Research Institute (INII), Universidad de Costa Rica, San José, 11501-2260, Costa Rica

² School of Chemistry, Universidad de Costa Rica, San José, 11501-2260, Costa Rica

³ Central American School of Geology, Universidad de Costa Rica, San José, 11501-2260, Costa Rica

⁴ Health Research Institute (INISA), Universidad de Costa Rica, San José, 11501-2260, Costa Rica

⁵ School of Biosystems Engineering, Universidad de Costa Rica, San José, 11501-2260, Costa Rica

Contact: agustin.solano@ucr.ac.cr

Abstract

Geothermal energy has been an important and stable part of the renewal energy matrix of Costa Rica for more than 25 years. However, the exploitation has been centred in high enthalpy energy for electric power generation, relegating the low enthalpy range to touristic uses like thermal spas. This pilot project is pioneering in the country, aiming to improve the energy efficiency of a local thermal resort by directly using the geothermal resource available. First, a geochemical and geological characterization of hydrothermal wells, springs and surrounding river was done to study local geothermal potential. Based on local geothermal capacity, direct-use engineering applications of low enthalpy geothermal resource were proposed to meet some energetic necessities of the hotel and enhance its efficiency.

This work is licensed under a Creative Commons Attribution 4.0 International License.

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1. Introduction

Geothermal energy systems are considered hydrothermal convection systems and can be classified according to temperature into high, medium, or low enthalpy resources. High enthalpy systems have high pressure and temperature above 150 °C. These systems are commonly used for electric power generation, and their study began worldwide after the 1970s crisis [1]. Medium enthalpy systems are between 100-150 °C, whilst low enthalpy ones are below 100 °C. These systems are the most abundant and are located at shallower depths, thus resulting in great potential to develop direct-use technology. Different applications of low and medium enthalpy systems have been studied worldwide and include aquaculture, balneology, cattle breeding, urban heating, domestic water, greenhouses, fish farming, spas and resorts, and growing vegetables and treating wood products, among others [2]. Currently, the touristic sector of different countries benefits from direct uses of low-enthalpy energy, although the Central American region is still behind compared to them, mostly because of the lack of knowledge of appropriate technologies or the absence of a legal framework to implement it.

Lund et al. [3] show the annual energy consumption (TJ/yr) of direct uses of geothermal energy in multiple countries including Costa Rica, with data recorded between 1995 and 2020. The data indicate an important growth between 2015–2019 (52%) in the installed capacity for direct uses of geothermal energy, increasing 7% compared to the previous period (45% from 2010 to 2014). From all possible applications described by Lindal [4], and depending on each regional context, several applications have been particularly successful and have shown larger growth in the last two decades. Figure 1 shows the most important applications: geothermal heat pumps, bathing and swimming, space heating, greenhouse heating, aquaculture pond heating and industrial uses, with geothermal heat pumps having the largest growth, from 125,000 TJ/yr between 2010–2015 and 273,133 TJ/yr between 2012–2020 [5].

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Costa Rica is the second smallest country in Central America but has great geothermal potential due to the local geotectonic environment, resulting in a sustainable and environmentally low-impact source of autochthonous energy. In the 1970s, in response to the world energetic crisis, the Instituto Costarricense de Electricidad (ICE; Costa Rican Institute of Electricity) promoted geothermal exploration in areas of potential interest that were previously highlighted by United Nations experts. This first effort covered 500 km², and included basal superficial exploration using geological, geochemical, and geophysical methods [6]. Currently, Costa Rica has several potential geothermal areas dispersed along the central range of the country, although the most important are in the north-western part of the Guanacaste Province, where two geothermal projects are operating and producing electric power at present. These areas exploit high enthalpy hydrothermal systems and generate around 13% of the Costa Rican energy matrix [7], placing the country as the leader in Central America and third within the American continent in geothermal capacity installed.

The entire geothermal energy potential of Costa Rica has not been explored, low-enthalpy areas are yet to be exploited and the applications are yet to be implemented. The applications of direct uses of geothermal energy in Costa Rica represent 21 TJ/yr, less than half of other countries in the Central American region such as Guatemala and El Salvador with 56 TJ/yr [3]. The low-enthalpy geothermal resource is normally found within tourism facilities across the country, where the local hydrothermal resource is mainly used for spas and balneology, evidencing the scarce development of other direct-use applications.

Thus, here we hypothesise that local low-enthalpy geothermal resources can be used to cope with the energetic necessities of a small tourism industry in Costa Rica. With this research we aim to improve the energy efficiency of a local thermal resort by directly using the geothermal resource available there. To achieve this, we first explored the geothermal potential within the hotel complex with a comprehensive multidisciplinary approach, considering the geology, geochemistry and microbiology of groundwater and superficial water resources. Then, we propose a pilot project with potential engineering solutions according to the resort’s specific necessities and the characteristics of local water sources. This pilot project will set the foundations for the exploration of low-enthalpy geothermal resources and will pioneer the development of appropriate technology to maximise the potential of this energetic resource.

2. Materials and Methods

Surface and groundwater sampling was performed to establish the predominant geochemical conditions to build a conceptual hydrogeological and geothermic model that will support the selection of geothermal industrial water uses. Mineral depositions in the hydrothermal water pipeline system were sampled, too. Also, due to the social importance of the geothermal resource for tourism, a microbiological profile of hot springs was obtained, including public health indicators (faecal coliforms or FC, total coliforms or TC, Escherichia coli or EC and coliphages), pathogen detection (Pseudomonas aeruginosa or PA, Enterococcus faecalis or EF) and the aerobic plate microbial count (APMC).

Geological and hydrogeochemical context

The study area is in the district of Venecia, San Carlos, in the Alajuela province, in the community of Marsella, specifically in Recreo Verde Hot Springs & Spa. The site is located about 5 km to the SE of the town of Marsella, on the left side bank of the Toro Amarillo River and to the NW of Hule Lake (Figure 2). Regionally, the area of interest is in the northern section of the Central Volcanic Mountain ridge of Costa Rica, to the north of the Poás Volcano, which is an active stratovolcano.

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The present study focuses on a local scale, where understanding the geological and groundwater chemical conditions of Recreo Verde hotel property and surrounding areas is imperative to assess the local low enthalpy geothermal energy potential. According to Ruiz et al. [8] the following geological formations can be found in the area (Figure 3):

1. Temporal phase Paleo Poás: Río Cuarto lavas unit (201 ± 30ka), is composed of basaltic andesitic lava flows and underlays the Congo Member. The origin is related to volcanic focus of the Late Paleo Poás phase or Early Neo Poás.
2. Congo Member (35.6 ± 0.6 ka): this belongs to the Poás Formation and is composed of kalco alkaline basalts to andesites with low to normal K content, these lava flows are covered by pyroclastic deposits. This geological member has an age of Late Pleistocene.
3. Bosque Alegre (6.1 ka): this is composed of pyroclastics and a few basalts with olivine lava flows; these materials were produced by Hule maar.
4. Laguna Kooper Unit (3-4 ka): these materials were grouped in three layers of pyroclastic deposits, associated with Río Cuarto o Kopper maar.
5. Fluvial deposits and tephras: this is one of the youngest geological deposits in the area, belonging to distal phases of Congo and foothill deposits of the San Miguel escarpment.

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Alvarado and Carr [9] indicate that different geochemical and geothermal studies have resulted in a high potential for geothermal energy in areas surrounding the study site; the authors highlight the hot springs of La Marina, the Toro River valley and the extinct fumarolic area of ​​Cerro Viejo, which have several fault systems with NNW-SSE, NNE-SSW and NE-SW directions.

The Platanar-Porvenir volcanic complex is associated with a regional thermic anomaly controlled by deep geological structures [10]. This complex was classified as an A1 reservoir with an optimal vocation for heating extraction, presenting temperatures above 200 °C at three kilometres depth. This report also mentions the existence of manifestations of thermal waters in the N sector of Cerro Congo, characterised by low temperatures, moderate conductivity, and neutral pH, as well as being mostly Bicarbonate-alkaline-type with a lesser extent as Chloride-sodium type.

Soto [11] indicates that there are emanations of thermal waters along the trace of the San Miguel fault, as well as small fractures with CO₂ emanations. Also, in the vicinity of the Hotel Recreo Verde, near the Toro III Hydroelectric Project, ICE [12] identified a faulting pattern with a NW-SE and N-S direction, the product of local effects associated with the activity of the Poás Volcano and regional strengths due to the subduction process. Moreover, Vargas [13] studied and classified the nearby Poco Sol Geothermal Field as a liquid dominant high temperature geothermal field and pointed it out as one of the most promising geothermal prospects in Costa Rica.

ICE [14] carried out a series of hydrogeochemical analyses upstream and downstream the Toro Amarillo River and its main tributaries, as well as in hot springs located on both banks of the Toro Amarillo River, especially in hot springs located in the vicinity of the Recreo Verde Hotel. As a result of this study, hot springs were classified as hyperthermal due to their temperature, and as practically neutral magnesium bicarbonate waters, associated with the dissolution of CO₂ in the water at greater depths. In terms of the hydrogeological conditions, this highlights that most of the thermal springs are associated with perched aquifers facilitated by the highly permeable geological materials, especially the Congo Distal and Bajo del Toro Units. The study also indicates that the hot springs located in this sector tend to be pH neutral. In addition, it is mentioned that the deep reservoir presents temperatures between 282 and 295 °C, and that there is the presence of two aquifers, one confined and one superior, the first presenting slightly alkaline waters and the second slightly acidic [14].

ICE [14] determined the presence of several geological structures (Figure 3), that included the NW, W and SW sectors of the study area. As part of the field work carried out, the presence of the San Miguel inverse fault was corroborated, characterised by having an escarpment in the N72°W direction. In addition, the study determined a series of alignments parallel to the trace of the San Miguel fault, with dextral displacement components, as well as the presence of the Toro fault, which has a sinistral displacement component and is indicated by the alignment of the Toro Amarillo River in a NE direction, running through a boxed channel. Finally, downstream of the main intake, at the Hotel Recreo Verde, there is a system of inverse faults trending NE, parallel to the Toro fault [14].

Location and geochemical sampling

Samples were collected in Recreo Verde Hot Springs & Spa in Marsella, San Carlos, Costa Rica, during the rainy seasons of 2020 (October) and 2021 (June). Sample locations considered two hydrothermal wells, one spring water well and surface water from three points at the north margin of the Toro River that surrounds the resort property (Table 1, Figure 4). Those river points were sampled considering a horizontal transect starting at the boundary of the river next to the hotel property limit (R-1), where an area of hydrothermal discharge seemed to be present. A sample was collected every 3 m from this point towards the inner section of the river (R-3) (Figure 4).

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Table 1: Summary of the locations sampled within the Recreo Verde hotel property.

Sample	Water resource type	Latitude	Longitude
MAN-2	Cold Spring	10°19’24.0’’N	84°14’49.6’’W
P2-2	Hydrothermal well	10°19’17.6’’N	84°14’34.1’’W
P3-2	Hydrothermal well	10°19’13.4’’N	84°14’37.8’’W
R-1	Horizontal transect, point 1, Toro River	10°19’18.5’’N	84°14’35.0’’W
R-2	Horizontal transect, point 2, Toro River	10°19’18.7’’N	84°14’35.8’’W
R-3	Horizontal transect, point 3, Toro River	10°19’19.3’’N	84°14’35.5’’W

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Water samples were collected from each location to analyse water geochemistry, dissolved gases and microbial composition. Rinsed bottles were used to collect water samples, tedlar bags to sample gases and sterile recipients to analyse microorganisms. Major cations (ICP-AES), minor cations (ICP-MS) and principal anions were analysed at Bureau Veritas laboratory, Canada.

Field parameters were also measured at each sampling site using a multiparameter probe (Hannah Instruments, HI9829): temperature, pH, ORP, dissolved oxygen, conductivity, total dissolved solids, turbidity and resistivity. Additional points of the northern margin of the river surrounding the hotel facilities were analysed for physicochemical parameters, too.

Mineral depositions within the thermal water pipeline of the hotel were sampled. The minerals were analysed by X-ray diffraction spectroscopy (XRD, PANanalytical Empyrean, analysed at the Instituto Tecnológico de Costa Rica), Fourier-transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum 1000) and Energy-dispersive spectroscopy (EDS, IXRF Systems) coupled to a Scanning Electron Microscope (SEM, Hitachi S-3700N).

Microbiology

Total coliforms (TC), faecal coliforms (FC) and Enterococcus faecalis (FC). Microorganisms were enumerated using the most probable number technique (9221E and 9230B) [15]. The sample was inoculated within less than 30 h of collection into lauryl tryptose broth (Oxoid) for TC and FC and into azide dextrose broth (Oxoid) for Enterococcus, according to APHA protocol. After an incubation period of 48 hours, tubes with a positive reaction were checked and reported.

Aerobic Plate Count (APC). APC was performed according to the pour plate method 9215B [15]. After incubation, colony forming units (CFU) were examined and counted.

Somatic coliphage quantification. With modifications, somatic coliphage concentrations were determined according to methods 9924B (somatic coliphage assay) [15]. Two hundred fifty mL of water was collected using a sterile recipient. The sample was first filtered using an 80-μm glass fiber filter (Sartorius Stedim Biotech, Goettingen, Germany) pre-treated with beef extract pH 7.2 (Oxoid) to prevent losses associated with viruses sticking to the filter. Then, a second sample filtration using 0.2 μm cellulose acetate filters (Sartorius Stedim) was pre-treated with beef extract pH 7.2. Then it was mixed with 10 ml of a fresh culture of E. coli ATCC 13706 (O.D.¼0.300 nm), CaCl₂, to a final concentration of 0.2 M with 2% trypticase soy agar (TSA). The mix was spread into Petri dishes and incubated at 35 °C overnight. After incubation, samples were analysed to count for plaque-forming units (PFU).

Pseudomonas aeruginosa (PA). PA concentrations were detected using multiple tube techniques according to 9213F [15].

3. Results and discussion

Geochemistry and mineralogy

Samples were classified according to main ion concentration using a Piper diagram (Figure 5a). Two samples from Toro River (R-1 and R-2) are Bicarbonate Na-K type, as well as samples from two thermal groundwater wells (P2-2 and P3-2) located in the hotel; these waters are peripheric waters according to the Ternary graph shown in Figure 5b. R-1 and R-2 were the samples from the transect closer to the hotel property (Figure 4), and the water chemistry similarities found with the thermal groundwater confirmed that R-1 and R-2 were located within a discharge area of the thermal aquifer to the Toro river. Sample R–3, at the end of the transect within the Toro River, was classified as Sulphate-Na type, which is characteristic of mature waters; this sample also could be influenced by volcanic sulphur dissolution from Poás Volcano deposits. The different geochemical classification of R-3 confirmed that the geological origin of the hydrothermal groundwater found at the hotel is distinct from that of the surrounding Toro River. Geological conditions as well as the presence of some faults and alignments promote the presence of thermal groundwater at the study site. Finally, the sample from the cold spring (MAN-2) is Bicarbonate Ca water type, which corresponds to meteoric water.

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The main physicochemical characteristics of the water resources in the hotel supported the geochemical differentiation of three water sources (Figure 6a, Table 2). Thermal groundwater from wells P2 and P3 was confirmed as low-enthalpy geothermal water due to temperature range between 38-44 °C, irrespective of sampling campaigns. Additionally, these wells had low dissolved oxygen values (<2 ppm) and negative Eh values (Table 2), indicating anoxic conditions. Those wells also had the highest values of TDS and conductivity (Figure 6a, Table 2), as was expected due to the high concentrations of dissolved cations, both major and trace elements (Table 3). pH data showed circumneutral conditions for the wells in contrast to river water, where pH was close to 4. Also, R-1 and R-2, despite being in the Toro River riverbank, had values more like those measured for the thermal wells than to the last sample of the transect located inside the river (R-3) for all the field parameters measured (Table 2). A similar trend can be observed with the elemental composition of the water sources (Table 3). These results confirmed that the points of the transect that are closer to the hotel property are within an area of hydrothermal discharge to the river.

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Table 2: Field parameters measured in each water source of the six sampling locations within the hotel property.

Sample	T (°C)	pH	ORP (mV)	OD (ppm)	DO (%)	Conductivity (mS/cm)	SDT (ppm)	Salinity (PSU)
MAN-2	22.99	7.66	221.3	4.29	51.4	0.147	73	0.07
P2-2	38.64	7.13	-6.8	0.77	13.3	4.945	2453	2.51
P3-2	43.27	7.27	-58.2	1.12	18.1	6.325	3150	3.34
R-1	36.5	7.05	49.6	1.08	17.4	4.534	2264	2.37
R-2	32.16	7.07	21	2.58	40	3.016	1419	1.34
R-3	21.75	4.54	148.5	10.98	133	0.499	255	0.24

Details of measurements done at additional points of the Toro River can be found in the supplementary material.

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The mineral depositions extracted from the hotel thermal water pipeline (Figure 6b) were confirmed as aragonite (CaCO₃) after XRD and FTIR analyses (Figures S1 and S2), as expected. However, the EDS analysis (Figure 6c) showed a large proportion of Fe (20–25%) within the carbonate mineralogy, even in similar proportions to calcium in some cases (20–25%), suggesting that other mineral phases such as siderite (FeCO₃) could be present. Si was also present but to a lesser extent (4–5%).

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Table 3: Major cations and main minor cations analysed in each water source of the six sampling locations within the hotel property.

Sample	Al	As	B	Ba	Bi	Br	Ca	Cl
	(ppb)	(ppb)	(ppb)	(ppb)	(ppb)	(ppb)	(ppm)	(ppm)
MAN-2	<1	0.5	27	14.00	<0.05	18	12.91	2
P2-2	<600	<3000	1653	<80	<3000	*	250.0	*
P3-2	<600	<3000	1908	<80	<3000	*	75.2	*
R-1	<600	<3000	1364	<80	<3000	*	227.2	*
R-2	700	<3000	1253	<80	<3000	*	210.0	*
R-3	11939	1.0	74	14.02	<0.05	113	26.41	43
Sample	Co	Cr	Cu	Fe	K	Li	Mg	Mn
	(ppb)	(ppb)	(ppb)	(ppb)	(ppm)	(ppb)	(ppm)	(ppb)
MAN-2	<0.02	4.5	<0.1	<10	2.54	0.6	6.36	0.05
P2-2	<300	<300	<80	<10000	86	<1000	360	253
P3-2	<300	<300	<80	<10000	102	<1000	450	170
R-1	<300	<300	<80	<10000	74	<1000	311	337
R-2	<300	<300	<80	<10000	67	<1000	280	331
R-3	1.18	0.6	2.6	440	4.45	7.3	15.34	271.27
Sample	Na	Ni	P	Pb	S	Si	Sr	Zn
	(ppm)	(ppb)	(ppb)	(ppb)	(ppm)	(ppb)	(ppb)	(ppb)
MAN-2	6.85	<0.2	63	<0.2	<1	20917	122.82	0.6
P2-2	386	<300	<4000	<4000	212	*	1901	<300
P3-2	459	<300	<4000	<4000	272	*	440	<300
R-1	336	<300	<4000	<4000	194	*	1687	<300
R-2	307	<300	<4000	<4000	174	*	1584	<300
R-3	18.77	0.3	<10	<0.2	46	19453	226.02	18.6

*: not reported. / Samples with values presented as “< number” were not quantified and are presented as a value below the upper detection limit of the ICPMS. The upper limit of each element corresponds to the number after the “ < ” symbol. / Complete cations data can be found in the supplementary material.

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Microbiology and geomicrobiological potential

The microbiological parameters provided information about the quality and the presence of pathogens in the thermal spring waters. These indicators have already been referenced worldwide, given their usefulness even in identifying the source of contamination [16, 17]. Parameters such as FC, EC, and EF are useful as indicators of faecal contamination, whether of human or animal origin. They should not be naturally present in hot springs. Similarly, APMC is a general indicator of a load of aerobic microorganisms in a water sample.

Regarding Table 4, most of the bacteriological parameters (FC, EC, EF) were negative or non-detectable in the water samples, which indicates there was no faecal source for contamination at the moment of sampling. Also, TC as a general guide of environmental contamination indicated a low count of total aerobic microorganisms in water samples. Note that there is no regulation in the country to establish a quantification limit in the case of thermal water.

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Table 4: Microbiological results of each water source of the six locations within the hotel property.

Parameter	MAN	P2	P3	Toro River
Aerobic plate count (CFU/ml)	26	1	ND	220
Total coliforms (MPN/100 m)	1.1	ND	ND	350
Faecal coliforms (MPN/ 100 ml)	ND	ND	ND	39
E. coli (MPN/ 100 ml)	ND	ND	ND	4
E. faecalis (MPN/ 100 ml)	ND	ND	ND	17
P. aeruginosa (MPN/ 100 ml)	4	ND	ND	4
Somatic coliphages (PFU/ 100 ml)	ND	100	360	ND

ND= Non detectable

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PA is an interesting parameter to analyse in hot springs as it refers to a Gram-negative bacterium that is highly resistant to disinfection treatments and is considered a pathogen for humans. This pathogen was positive in a low concentration in the non-hydrothermal samples in this study.

Interestingly, somatic coliphages were positive in both thermal water samples (P2 and P3), with 100 and 360 PFU/100 ml. The other samples were undetectable. These are useful indicators of enteric virus contamination. Coliphages detected in wastewater reveal a significant relationship between the detection of at least one of the five human enteric viruses such are Norovirus I and II, Rotavirus, Enterovirus and Hepatitis A. Its usefulness for thermal waters could also refer to the presence of enterovirus pathogenic for humans, however, more analysis should be performed to establish an association of this parameter in hot spring waters [18].

Additionally, the anoxic and warm settings found in the hydrothermal wells (Table 2), with their high concentration of metals such as iron, manganese, or sulphur (Table 3), suggest a potential geomicrobiological environment of interest. The biogeochemical cycles of Fe, Mn or S are well studied, highlighting crucial steps of those cycles that are developed by microorganisms via specific redox reactions, both in oxic and anoxic conditions [19, 20, 21]. Different geomicrobiological processes could be happening in these thermal environments: redox transformations of elements, biomineralisation and bioweathering of minerals; these are processes that must be further studied.

Potential uses of local low-enthalpy geothermal resource in a pilot project

Results of the geothermal potential of Recreo Verde hotel provided valuable inputs to decision- making about which direct uses can be applied at the site. Water composition, temperature and fluxes of groundwater, water springs and the surrounding river facilitated the engineering design suitable for each specific application.

The hotel is a small touristic industry, resulting in an excellent facility to implement a demonstrative pilot project to give decision-makers a starting point in the Central American region in terms of the regional low-enthalpy geothermal energy exploitation. From the study to understand the performance and the energetic necessities of the hotel, we found that several work lines can guarantee the operation of the hotel by directly using the geothermal resource and with the addition of geothermal heat pumps.

Recreo Verde hotel has 10 bungalow rooms built with a wooden structure and zinc roofing. The analysis determined that the thermal comfort of the rooms can be guaranteed using a geothermal heat pump, resulting in considerable financial savings compared to traditional acclimatising systems. Also, modifications in the rooms are recommended to avoid thermal losses and to diminish the visual impact of the future project. In terms of the heating source, the study identified local water springs that can be used as the cold focus source for the geothermal heating pump that will be using the low-enthalpy geothermal resource to the room climatization. The surrounding river can be considered as a cold focus too, but it was discarded for this purpose due to its geochemical composition and because an additional pumping system would have to be included in the final design.

Also, the hotel demands sanitary hot water for showers and for the hotel restaurant. The area has three thermal groundwater wells that can cope with this solution, but only one of them is not employed for bathing and swimming uses, so it could be exclusively used as the hot focus source for this other direct use. However, the geochemical composition of these hydrothermal systems prevents the use of that water in open circuits for heating pumps as minerals could precipitate inside the system proposed. These mineral depositions, such as the CaCO₃ found inside the pipelines (Figure 6b), could impact the effectiveness negatively or even reduce the life span of the design. Thus, a closed system design is recommended to transfer the water heat to the heating pump, and then to the accumulation reservoir that will guarantee the hot water flux according to the demand from the rooms and the restaurant.

Finally, the hotel administration is interested in producing its own food in the hotel for local consumption. The results obtained in this study and the examination of the property lead us to conclude that it is possible to design a greenhouse with an aquaponic system. The design will guarantee a circular economy system where the heat from the low-enthalpy geothermal resource will provide thermal comfort for fish and plants growing there. The system would be integrated by a geothermal heating pump for the greenhouse and a heat-exchange system for the fish farm.

Lund et al. [22] presents a detailed analysis of different methods to exploit the heat from low-enthalpy geothermal resources according to different applications. The proposals discussed here, based on the local geothermal potential that was determined in this study, are aligned with the applications described by Lund et al. [22], but some adaptations would be mandatory according to the specific site where every single application would be implemented.

4. Conclusions

Geological and structural conditions such as faults and alignments favour the presence of thermal groundwater at the study site. The geothermal resources studied are within the low-enthalpy category according to the temperature of each water sampled.

Three different water sources were identified from chemical analysis which correspond to the geological setting. Groundwater from the wells is Bicarbonate Na-K waters and in the geothermal context are peripheric waters. The cold spring receives meteoric water, and it has a characteristic HCO₃-Ca classification, while the water from the centre of the Toro River was classified as Sulphate-Na coming from deep water (mature water).

Three important applications can be implemented based on the low-enthalpy geothermal potential found, the geochemical composition of each water source in the hotel property and the operational demands of the hotel. The applications that would be developed in the pilot project are room acclimatisation, sanitary hot water supply for showers and restaurant needs, and a geothermal greenhouse coupled to an aquaponic production system. All of them will directly use the local low-enthalpy geothermal resource, thus contributing to improving the energetic matrix efficiency of the hotel. Moreover, the results of this pilot project could be replicated and expanded among the thriving Costa Rican touristic sector, contributing to the overall efficiency of renewable energy of the country.

The datasets supporting the conclusions of this article are available at https://doi.org/10.17632/6sh963jm64.1 with a CC BY 4.0 license.

Acknowledgements

The authors acknowledge the Universidad de Costa Rica (UCR), German Agency for International Cooperation (GIZ) and Recreo Verde Hot Springs & Spa, for financial support granted to the research project where this work was done. We thank Pedro Rojas Camacho from the Forest Resource Unit for logistics support and fieldwork assistance and extend our gratitude to Recreo Verde personnel and to all the assistants that helped in each field campaign. Thanks to Dr. Olman Arias for his collaboration with geological discussion, to Eduardo Valerio from the School of Chemistry (UCR) for technical support with FTIR and to CIEMIC research centre (UCR) where KRM performed the EDS analyses.

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This article has been published in European Geologist Journal 54 – Geothermal energy – A geological contribution to the energy transition

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