Abstract
Access to safe drinking water remains a major public health issue in low- and middle-income countries. Local geological materials offer an affordable alternative to costly conventional treatments. This study evaluates the effectiveness of basalt, quartzite, and granitic weathered rock in purifying surface water in western Cameroon. Materials collected in Dschang were crushed, sieved (0.3 mm), washed, and sterilized, then used to make gravity filters. Filtered river water was analyzed before and after treatment for physicochemical and bacteriological parameters according to APHA methods. Data were compared using the Kruskal-Wallis test (α = 5%). All materials reduced bacterial load and turbidity. Granitic alterationite performed best, with total elimination of fecal streptococci and Salmonella spp., and greater than 99% for Vibrio spp. A significant decrease in turbidity, conductivity, TDS, and nitrates was observed, while basalt showed a tendency to release ions. Granitic alterationite is the most effective material for making surface water drinkable. These results confirm the potential of local geological materials as simple and sustainable solutions for access to drinking water in resource-limited environments.
Introduction
Access to safe drinking water, recognized as a human right by the United Nations General Assembly and the Human Rights Council in 2010, remains a major public health challenge, particularly in low- and middle-income countries, where water treatment infrastructure is often inadequate or non-existent (Petculescu et al., 2022; United Nations, 2023). Globally, despite progress over the past decade, approximately 2.1 billion people, or one in four, still lack access to safely managed drinking water services (i.e., accessible at home, free from contamination, and available in sufficient quantities), including 106 million who drink directly from untreated surface sources such as rivers or lakes (UNICEF & WHO, 2025). In addition, 3.4 billion people do not have access to safely managed sanitation, and 1.7 billion do not have basic hygiene facilities at home, highlighting the scale of the challenges related to water, sanitation, and hygiene in today’s world (UNICEF, 2025).
This issue is particularly worrying in rural and peri-urban areas, as well as in isolated living environments where access to conventional water purification systems is limited (Perveen & Amar-Ul-Haque, 2023; Pokam Djoko et al., 2024). Surface waters, such as rivers and streams, are an essential source of water supply for many communities. However, they are frequently exposed to anthropogenic and natural contamination, including domestic, agricultural, and industrial discharges, as well as soil runoff (Das, 2024; Mkilima et al., 2022; Rafi et al., 2022; Shaheen et al., 2023). These contaminations result in a deterioration of the physical, chemical, and bacteriological quality of the water, marked by high turbidity, a significant microbial load, and the presence of enteric pathogens such as Escherichia coli, Salmonella sp., Shigella sp., and Vibrio sp., but also by chemical pollutants including heavy metals, nitrates, phosphates, pesticides, and other organic or inorganic compounds, thus posing a major health risk to exposed populations (Ashanka et al., 2022; Das, 2025).
There is growing interest in the development of simple, inexpensive, sustainable water treatment solutions that are adapted to local contexts. Filtration systems using natural materials represent a promising alternative to conventional technologies, which are often costly and energy-intensive. Several studies have shown that certain local geological materials, such as sand, gravel, volcanic rock, and granite residue, have physical and chemical properties that are conducive to retaining particles and microorganisms, as well as improving certain physicochemical parameters of water (Compaoré et al., 2022; Muto et al., 1996; Samah & Mohammed, 2020). Among these materials, natural rocks such as basalt, quartzite, and granite are of particular interest due to their abundance, low cost, and specific mineralogical characteristics. Basalt, a volcanic rock rich in mafic minerals, can influence the ionic composition of water through water-rock interactions (Romano et al., 2024). Quartzite, a predominantly siliceous metamorphic rock, is distinguished by its hardness and chemical stability, which favor physical filtration processes (Ramanamane & Pita, 2025). Granite altérite, resulting from the alteration of granitic rocks, has high porosity, heterogeneous grain size, and is rich in clay minerals and metal oxides, which can enhance the adsorption and trapping of microbial contaminants (Anovitz et al., 2021).
In the Cameroonian context, and more particularly in the West region, the use of local materials for water treatment is a relevant and sustainable approach. Recent studies have shown the potential of certain local filters to significantly improve drinking water quality (Tsafack et al., 2024). However, data on the effectiveness of several types of natural rocks, including basalt, quartzite, and granite altérite, remain limited, particularly about their ability to simultaneously retain bacteriological and physicochemical contaminants. This study, therefore, aims to assess water quality before and after filtration using three types of natural rock (basalt, quartzite, and granite altérite) and to determine their respective effectiveness in retaining bacteriological and physicochemical contaminants. With this in mind, this work aims to contribute to the search for simple, accessible water purification solutions that are adapted to resource-limited contexts, while making use of locally available geological materials.
Methodology
Type and Period of the Study
This is a comparative experimental study aimed at evaluating the effectiveness of three types of local rock (basalt, quartzite, and weathered granite) in improving the physicochemical and bacteriological quality of surface water. The study was conducted over a 6-month period, from June to November 2025, in the town of Dschang, in the West region of Cameroon. This period was chosen to cover different climatic and hydrological conditions that could influence water quality and the performance of the filter materials.
Rock Sample Collection and Packaging
Three types of rock were collected in the town of Dschang: basalt, granite altérite, and quartzite. The basalt (Figure 1) was collected from a quarry located in the Foréké district, at geographical coordinates 5°26.1560 N latitude, 10°2.4210 E longitude, and at an altitude of 1,353.4 m. The granite altérite (Figure 2) and quartzite (Figure 3) were collected on a mountain in the Foto district, at coordinates 5°26.1010 N, 10°3.8880 E, at an altitude of 1,410.6 m. Each sample was carefully packaged in labeled bags to preserve the integrity of the material and facilitate subsequent analysis. The samples were then transported to the Earth Sciences Laboratory at the University of Dschang for confirmation of their identity based on morphological observations.
Basalt before and after treatment.
Granite altérite before and after treatment.
Quartzite before and after treatment.
Rock Processing
The rocks were processed by crushing them using a rock crusher. Once crushed, the rocks were sieved using 0.3 mm mesh sieves. The fractions obtained were then washed with tap water until the wash water ran clear. After washing, the retained fractions were disinfected by heating in boiling water for about 3 hours, in accordance with the recommendations of Varlikli et al. (2009). The samples were dried in the sun for 3 days, allowing the removal of organic matter, colloids, and certain soluble elements. Finally, the samples obtained were also autoclaved to ensure complete sterilization before use.
Collection of Water to be Used for the Experiments
The water used in this study was collected from a river crossing the campus of the University of Dschang, at coordinates 5°26.7110 North, 10°4.0400 East, and at an altitude of 1,402.5 m. This site was selected because of its high level of anthropogenic pollution, characterized by high turbidity, a significant microbial load including bacteria indicative of fecal contamination (Escherichia coli, total coliforms) as well as potential enteric pathogens such as Salmonella sp., Shigella sp., and Vibrio sp., in addition to the likely presence of chemical pollutants including nutrients (nitrates, phosphates), dissolved organic matter, and other compounds from surrounding domestic activities (Tsafack et al., 2024). The site’s proximity to the laboratory also motivated this choice (Figure 4). The physicochemical and bacteriological characterization of this water was carried out three times in order to obtain a reliable and representative estimate of the average level of contamination of this watercourse.
Water sampling site.
Experimental Device for Evaluating the Water Purification Capacity of Different Types of Rock
As part of the assessment of the potability of different types of rock, a device was designed and assembled in a sterile environment to ensure the integrity of the samples. Sterile 1.5-L plastic bottles were used as containers. Each bottle was carefully cut with a sterile blade to create an opening at its base. After being emptied of their contents (mineral water), the bottles were turned upside down to allow the rocks to be introduced. The rocks were then introduced up to a volume of 1.25 L, corresponding to 83.33% of the container’s capacity, with respective masses of 1.62 kg for quartzite, 1.61 kg for basalt, and 1.60 kg for granite altérite. The lid of each bottle was perforated using a perfuser fitted with a PVC tube to collect the filtered water. Finally, for each type of rock, three identical filtration units were set up in order to obtain a reliable and representative average of the filtration performance (Figure 5).
Device for evaluating different types of rock in water purification.
The filtration system was designed to optimize the passage of water through the different rocks. When the water was introduced into the bottles, it passed through the filter materials, allowing particles to be retained according to their size and nature. The flow rate was set, using the perfuser regulator, at a speed of one drop per second to ensure sufficient contact time between the water and the rocks. The filtered water was immediately collected in a sealed glass container that had been autoclaved beforehand, ensuring a contamination-free sample.
Analysis of Water Quality Before and After Filtration
The analysis of water quality before and after filtration focused on both physicochemical and bacteriological parameters in accordance with the standard protocols and methods of the American Public Health Association (APHA, 1998) and the American Society for Testing and Materials (ASTM; Institute of Medicine (US) Council on Health Care Technology & Goodman, 1988). For each filter, four water samples were taken: one for bacteriological analysis, one for turbidity measurement, and two for evaluation of physicochemical parameters.
For the physicochemical parameters, pH, total dissolved solids (TDS), and electrical conductivity were measured using a multiparameter meter, by direct reading after immersion of the electrodes, which had been rinsed and dried beforehand. Turbidity was determined using a Palintest turbidimeter, by direct reading after filtration of the samples (Bin Omar & Bin MatJafri, 2009). The concentrations of iron, free chlorine, nitrate, potassium, phosphate, and sulfate were quantified using Palintest analysis kits (iron, free chlorine, nitrate, potassium, phosphate, and sulfate), and readings were taken with a photometer in accordance with the manufacturer’s recommendations.
The bacteriological characterization focused mainly on indicators of fecal contamination. The analyses were performed on raw water and filtered water using membrane filtration. Thus, 100 mL of water was filtered through a sterile cellulose nitrate membrane with a porosity of 0.45 µm and a diameter of 47 mm, then deposited on a specific culture medium previously poured into a Petri dish, allowing the development of the microorganisms sought. The bacteria targeted as indicators of fecal contamination were: total coliforms, fecal coliforms, Escherichia coli, fecal streptococci, Shigella sp., Salmonella sp., and Vibrio sp. (Bontoux, 1983).
Statistical Analysis of Data
Once the field phase was complete, the data were entered by the principal investigator into Excel 2016 and then imported into R Studio version 4.4.2 for analysis. During the analysis of this study, descriptive statistics (mean and standard deviation) were generated for all variables taken into account. The data for the different rock categories were compared for their effectiveness in improving water quality using the Kruskal-Wallis test, and post hoc paired comparisons were made using Dunn’s test. In addition to statistical significance tests, effect sizes were estimated in order to assess the actual magnitude of the differences observed between the filter materials. The significance threshold was set at α = .05. Each analysis was performed in triplicate to improve the reliability of the results, reduce the influence of experimental errors, and take into account the variability inherent in physicochemical and bacteriological measurements. The effectiveness of the filters in improving water quality was determined using the equation used by Kiagho et al. (2016): E = (C0−Cf / C0) × 100. Where: Co is the initial concentration of the parameter mentioned in the initial water sample before filtration, Cf is the final concentration of the parameter mentioned after filtration, and E is the efficiency of the water filter.
Results
Water Quality Before and After Filtration on Rocks
Table 1 shows the bacteriological quality of the water before and after filtration through different types of rock. In general, a reduction in bacterial load was observed after water filtration. However, this decrease was not always statistically significant for fecal coliforms, total coliforms, E. coli, and Shigella sp. On the other hand, significant differences were observed for fecal streptococci (p = .037), Salmonella spp (p = .016), and Vibrio spp (p = .042). A posteriori comparisons showed that filtration through granite altérite resulted in a significant reduction in these bacteria compared to river water and the other rocks tested.
Bacteriological Quality of Water Before and After Filtration Through Rocks.
Note. Guideline value: 0 CFU in 100 ml. CFU = colony forming unit; SD = standard deviation; Mean = average; p-value = significance level.
p < .05.
Table 2 shows the physicochemical quality of the water before and after filtration through different types of rock. This table shows that filtration resulted in significant variations in electrical conductivity, TDS, turbidity, and nitrate and potassium concentrations (p < .05). Conductivity was lower with granite altérite (0.05 ms/cm) and higher with basalt (0.19 ms/cm), while pH, at around 7, remained within the recommended range, with no significant difference between rocks (p = .072). Turbidity decreased significantly after filtration on all rocks, reaching compliant values (<5 UNT). The concentrations of iron, sulfate, free chlorine, and phosphate did not vary significantly between rock types, remaining below regulatory thresholds. Filtration on basalt showed the highest values for TDS (0.20 ppt) and potassium (3.0 mg/l), while nitrate was lower after filtration on quartzite. A posteriori comparison also confirmed that the differences between rock types were statistically significant for several key parameters, indicating that certain materials, such as granite altérite, may be better suited depending on the target parameter.
Physicochemical Quality of the Water Before and After Filtration Through Rocks.
Note. UNT = Nephelometric Turbidity Unit; SD = standard deviation; p-value = significance level.
p < .05.
Contaminant Retention Rates by Rocks
Table 3 evaluates the retention rate of bacteriological contaminants by different categories of rocks. The results reveal that granite altérite demonstrated particularly high retention rates for several bacteria, with 100% efficiency for fecal streptococci and Salmonella spp. In addition, very high rates were observed for Shigella spp (97.85%) and total coliforms (93.11%). Basalt, although it performed less well than granite altérite, achieved a retention rate of 91.18% for fecal streptococci and 80% for Salmonella sp. In contrast, quartzite showed varying retention rates, with a peak of 96.37% for Salmonella spp, but less effective performance for other bacteria, notably with 14.68% for Vibrio spp.
Bacteriological Retention Rates of Different Rock Categories.
Table 4 shows the retention efficiency of the main physicochemical parameters of water after filtration through different rocks. The results showed that granite altérite had the highest efficiency on turbidity (82.22%). Basalt showed high retention of sulfates (79.54%) and iron (52.63%). Quartzite, on the other hand, performed better in reducing nitrates (63.23%), while showing moderate efficiency on other parameters.
Retention Rates of Physicochemical Parameters of Water by Rocks.
Discussion
The objective of this study was to evaluate the effectiveness of three local rocks in purifying polluted surface water. The results provide key information on the effectiveness of different rocks as natural filters and highlight some important aspects of water quality.
In terms of microbiological parameters, granite altérite achieved total elimination of fecal streptococci (0 CFU/100 mL), corresponding to a retention rate of 100%, while basalt (91.18%) and quartzite (85.3%) showed lower but still appreciable effectiveness. Similarly, for Salmonella spp., filtration through granite altérite resulted in total retention (100%), significantly higher than that obtained with basalt (80%) and quartzite (96.37%). These results are consistent with those obtained by Compaoré et al. (2022), who reported a retention rate of 99.42% with 1 to 2 mm gravel, and are higher than those of Muto et al. (1996), who found a rate of 84.4% after filtration on granitic porphyritic materials. This result can be explained by the fact that granite altérite has a more developed porous structure, a high specific surface area, and a particle size distribution that promotes better adsorption and more effective trapping of bacteria (De Muynck et al., 2011). In addition, its mineralogical composition, rich in clays and metal oxides, can enhance electrostatic interactions between filter particles and microorganisms, thereby improving retention (Hendiani et al., 2024).
Furthermore, particularly striking differences were observed with regard to the retention of Vibrio Spp. Granite altérite showed exceptional performance, with a retention rate of 99.55%, significantly higher than those obtained with basalt
With regard to physicochemical parameters, electrical conductivity and TDS showed statistically significant variations after filtration, depending on the type of rock, reflecting changes in the ionic charge of the water (p = .022 for conductivity; p = .035 for TDS). The conductivity of raw water, initially 0.09 ± 0.01 mS/cm, decreased significantly after filtration through granite altérite (0.05 ± 0.01 mS/cm) and quartzite (0.08 ± 0.005 mS/cm), while it increased sharply with basalt (0.19 ± 0.01 mS/cm), approaching the lower limit of WHO/EU standards (0.2–1 mS/cm). This trend was confirmed by TDS, which decreased from 0.06 ± 0.02 ppt in raw water to 0.05 ± 0.01 ppt with granite altérite and 0.08 ± 0.01 ppt with quartzite, but increased to 0.20 ± 0.01 ppt after filtration through basalt. These results are consistent with those reported by Ashanka et al. (2022), who observed a conductivity of 0.4 mS/cm and TDS of 0.05 ppt after filtration through gravel. This variation can be explained by the fact that the mineral composition and porous structure of rocks directly influence the transfer of dissolved ions. Granite altérite and quartzite, with their high surface density and suitable porosity, promote the retention or adsorption of ions, which leads to a decrease in conductivity and TDS. On the other hand, basalt, which is rich in soluble minerals and has lower porosity but more open flow channels, can release certain ions into the filtered water, thereby increasing conductivity and total dissolved solids values (Fouillac et al., 1989).
The results also showed that, for turbidity, granite altérite achieved the highest retention rate (82.22%), followed by basalt (77.71%) and quartzite (73.43%). Although satisfactory, these performances remain lower than those reported by Nambiar et al. (2023) in India (96%) and Meiramkulova et al. (2023) in Kazakhstan (100%) after filtration on zeolite. This difference can be explained by the fact that zeolite has a highly organized microporous crystalline structure with an extremely high specific surface area and strong ion exchange capacity, which optimizes the retention of colloidal particles responsible for turbidity. However, among the materials tested in this study, granite altérite remains the most effective. This superior performance results from its irregular particle size, greater porosity, and mineralogical composition rich in metal oxides and clay minerals.
Nitrate concentrations decreased significantly after filtration (p = .034), from 1.36 ± 0.36 mg/L in raw water to 0.82 mg/L with granite altérite and basalt, and to 0.50 mg/L with quartzite.
Quartzite showed the best efficiency (63.23%) compared to granite altérite and basalt (≈39%), which can be explained by its compact structure combined with a reactive mineral surface and favorable grain size, offering more adsorption sites for nitrate ions. In all cases, the values remain well within WHO/EU standards. However, this result is lower than that obtained by Meiramkulova et al. (2023) in Kazakhstan, who found a retention rate of 73.09% after filtration on zeolite. This difference can be explained by the fact that zeolite has a highly developed crystalline microporous structure and a high ion exchange capacity, optimizing nitrate adsorption.
In addition, potassium showed a significant variation after filtration. Granite altérite slightly reduced potassium to 1.80 ± 0.10 mg/L, while basalt caused a more marked increase to 3.0 ± 0.20 mg/L, and quartzite showed an intermediate value of 2.43 ± 0.20 mg/L. This difference suggested that basalt released potassium ions into the water, probably due to its mineralogical composition rich in feldspars and mafic minerals likely to release cations upon contact with water (Zhang et al., 2023). Conversely, granite altérite, a more siliceous and weathered material, promoted slight potassium adsorption due to its reactive surfaces and increased porosity (Awgchew et al., 2024). Quartzite, which is less chemically reactive, had an intermediate effect on potassium concentration.
The use of these rocks for water treatment can significantly reduce pathogens, thereby lowering the risk of waterborne diseases such as diarrhea and salmonellosis. However, the ion release observed with certain rocks, particularly basalt, and the lack of data on long-term clogging, suggest that regular monitoring of water quality would be necessary to avoid any potential risks associated with prolonged use of these materials.
The results show that, although filtration through different types of rock generally improves the physical and chemical quality of the water, certain residual concentrations must be interpreted in light of WHO recommendations. The iron content after filtration varies between approximately 0.36 and 0.53 mg/L, which is higher than the generally recommended range for acceptable drinking water (0.1–0.2 mg/L). Although iron does not pose a major toxic risk at the concentrations observed, these levels can cause organoleptic problems such as a metallic taste, brownish discoloration due to iron oxidation, and the formation of deposits in containers or distribution systems, sometimes promoting the growth of iron-loving bacteria. On the other hand, sulfate concentrations after filtration (approximately 2.66 to 3.66 mg/L) remain well below the WHO guideline value (⩽500 mg/L), indicating no significant health risk. At these levels, sulfates do not affect the taste of the water or the health of consumers, as adverse effects such as laxative effects generally only occur at much higher concentrations. Thus, rock filtration improves water quality, but further reduction of iron could be considered to optimize the acceptability of water for consumption.
The increase in electrical conductivity, total dissolved solids (TDS), and potassium observed after filtration through basalt can be explained by water-rock interaction mechanisms. Basalt contains silicate minerals such as feldspars, pyroxenes, and olivine, which undergo chemical alteration when in contact with water, leading to the dissolution and leaching of ions such as K+, Ca2+, Mg2+, and Na+. This release of ions increases the ionic charge of the water, resulting in an increase in conductivity and TDS. However, the concentrations observed remain well below WHO guideline values, indicating a low immediate health risk. In the long term, however, continued leaching could lead to slight mineralization of the water and affect its organoleptic characteristics, which justifies regular monitoring of physico-chemical parameters.
Although significant reductions in bacterial loads were observed after filtration, the decreases in fecal coliforms, total coliforms, and Escherichia coli were not statistically significant (p = .141, p = .147, and p = .152). Furthermore, residual concentrations remain well above the WHO guideline value, which requires 0 CFU/100 mL for indicators of fecal contamination in drinking water. These results indicate that, although rocks particularly weathered granite significantly improve the microbiological quality of water and allow for a substantial reduction in pathogens, filtration alone does not guarantee complete microbiological potability. Therefore, these materials should be considered as a pre-treatment or clarification step, capable of reducing microbial load and turbidity, but requiring additional disinfection (boiling, chlorination, or solar disinfection) to ensure full compliance with potability standards. This interpretation reinforces the relevance of these natural filters in resource-limited settings, while highlighting the need for final treatment to ensure the safety of water intended for human consumption.
Although this study evaluated the effectiveness of three types of local rocks in improving water quality, several limitations should be noted. First, the experiments were conducted in the laboratory on small volumes of water, which may not fully reflect the complexity of domestic or community water supply systems. In addition, the short duration of the experiments did not allow for an assessment of long-term performance, durability, or potential clogging of the filter materials, factors that could influence filtration efficiency during repeated use.
Furthermore, the study did not assess the risk of ions or dissolved elements being released from the rocks into the treated water, which could alter the chemical composition of the water and pose an additional risk in the event of prolonged use. Due to the lack of access to adequate technical equipment, no detailed geological, mineralogical, or textural characterization of the materials (e.g., XRD, SEM, particle size distribution) could be performed. Interpretations regarding porosity, adsorption capacity, and filtration mechanisms, therefore, remain hypothetical, based on literature rather than direct analysis. Finally, due to limited resources, the study relied on a single sample of surface water collected at a single location and during a single event, which does not allow for spatial and temporal variability in water quality to be taken into account. The results should therefore be interpreted as reflecting filtration performance under specific experimental conditions, and not as representative of general water quality scenarios.
Conclusion
This study evaluated the comparative effectiveness of basalt, quartzite, and granite altérite in improving the physicochemical and bacteriological quality of surface water in Dschang, western Cameroon. The results show that all materials contributed to a reduction in bacterial load and turbidity, but with varying degrees of effectiveness. Granite altérite stood out as the most effective material, completely retaining fecal streptococci and Salmonella spp., as well as significantly reducing Vibrio spp. and total coliforms. In terms of physicochemical properties, it also significantly reduced turbidity, conductivity, total dissolved solids, and certain ions, while basalt showed partial ion release, and quartzite was more effective on nitrates. These observations confirm that the high porosity, heterogeneous grain size, and mineralogical composition rich in metal oxides of granite altérite promote the adsorption and trapping of contaminants. These results highlight the potential of local geological materials as simple, sustainable, and economically accessible solutions for improving access to safe drinking water in resource-limited settings. They pave the way for the use of granite altérite-based domestic filters for the prevention of waterborne diseases in rural and peri-urban communities.
Footnotes
Acknowledgements
We would like to express our sincere gratitude to all those who contributed to this study. We would particularly like to thank the University of Dschang for the resources made available to us, as well as the Earth Sciences Laboratory for its technical support throughout our research. We would also like to thank Professor Télesphore Benoît Nguelefack of the Department of Animal Biology at the University of Dschang for their technical and logistical support throughout this study. Special thanks go to those who participated in sample collection and data analysis, whose dedication and hard work were essential to the success of this project.
Author Contributions
Godfroy Rostant Pokam Djoko designed and coordinated the study, collected the data, wrote the article, and analyzed the results as the corresponding author. Honorine Ntangmo Tsafack and Emile Temgoua both contributed to the methodology while supervising the project and providing critical revisions of the manuscript.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was carried out with funds from the DAAD (German Academic Exchange Service) program for Sub-Saharan Africa, granted to the first and corresponding author.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Data Availability Statement
Data generated and analyzed during this study are available on request from the authors. We encourage transparency and are willing to share data to support future research. Data will be provided subject to requests respecting the ethical and confidentiality conditions established during the study.