1 Introduction and background

A "dhungedhara" is a traditional stone drinking fountain in Nepal, forming a crucial part of the drinking water system, especially during the rainy season when other sources are dry. Dhungedhara is also regarded as open ground water locally. Supported by ponds and canals, it mitigates water pressure from monsoon rains. In the Kathmandu Valley, Kathmandu Upatyaka Khanepani Limited (KUKL) offers sanitation and water delivery services, tackling issues like pollution and scarcity. The Melamchi Water Supply Project diverts water to alleviate scarcity, facing challenges. Sundarijal, a vital water source, balances the supply for Kathmandu's densely populated area. Challenges to water supply and tankers persist, necessitating chlorination. Among 400 stone spouts, 45 vanished, 68 dried, and 43 linked to city supply due to declining natural flow [1]. WHO guidelines recommend maintaining chlorine levels for safe water discharge [2]. Jar water commonly denotes water stored or packaged in jars for distribution, prevalent in areas lacking affordable bottled water. It can refer to water stored in large containers like plastic jars. Distilled water results from distillation, a process purifying water by heating and condensing, removing impurities and minerals. Useful in labs and industries, it lacks minerals and impurities found in tap water. Mineral water, sourced from springs or aquifers, contains minerals and trace elements. Its benefits vary based on geological sources, with regulations ensuring accurate labeling. Moderate consumption is recommended due to its potential health benefits and natural carbonation. The temperature and pressure dependence of hydrogen bond length of water [3] were studied but not optical properties of water with impurities.

Li et al. studied the properties of water molecules at different temperatures and found the temperature increases, the hydrogen bond length becomes slightly longer, the hydrogen bond angle becomes smaller, and the hydrogen bond coordination number becomes less [4]. Therefore, as the temperature increases, the number of hydrogen bonds in water molecules decreases, and the number of weak hydrogen bonds increases. This also means that the average bond length of high temperature water molecules is longer, which is in line with the discussion of the average length of the hydrogen bond but they haven’t studied the optical properties of water with impurities.

In liquid water, the energy of attraction between water molecules (hydrogen bond enthalpy) is optimally about 23.3 kJ mol−1 and almost five times the average thermal collision fluctuation at 25 °C. This is the energy required for breaking and completely separating the bond, and equals about half the enthalpy of vaporization (44 kJ mol−1 at 25 °C), as an average of just under two hydrogen bonds per molecule are broken when water evaporates. Hydrogen bond lifetimes are 1–20 ps, whereas broken bond lifetimes are about 0.1 ps [5, 6] but they don’t study the optical properties during such short time.

1.1 Problem of statement

The water supply system in Kathmandu Valley relies on diverse water sources, including surface water from KUKL and Melamchi, and ground water from Tanker water, Distill, mineral, and Jar water suppliers. With these sources catering to the daily water needs of the valley's inhabitants for various purposes, ensuring the quality and safety of the supplied water is paramount for public health. The objective of this research is to investigate the optical properties of commercial water in the Kathmandu Valley during the summer season. Literature show, temperature and pressure effects on water is studied but doesn’t study the optical properties and their links with volatile impurities present in water. By examining parameters such as transmittance, absorbance, refractive index, total molecular cross-section, density, and absorption coefficient. The hypothesis suggests that the optical properties of water may be influenced by temperature and dissolved particles or contaminants present in it. In addition, the nature of contaminated particles with temperature dependents also affects optical properties. The understanding of water's optical properties is crucial for assessing the water quality in the Kathmandu Valley. So, it is important to study the optical properties of water with heat because the heating effect shows the presence of volatile substances by changing the optical properties. Correlating properties such as molecular cross-section with transmittance aids in identifying impurities size present in the water which necessitate the development of new filtration technologies for various daily water uses.

1.2 Significant of research

This research hypothesizes that contaminated water may exhibit lower transmittance compared to uncontaminated water. By understanding these optical properties, the study aims to assess the quality of water from different suppliers in the valley. This knowledge is crucial for ensuring safe and clean drinking water, protecting public health, and identifying potential health risks associated with waterborne contaminants. Moreover, the research can contribute to the development of efficient monitoring systems for water quality assessment, aid in water resource management decisions, and support policymakers in setting water quality standards and regulations. In addition, the technique used in this research to test the quality of water is chemical free, cost effective and time saving.

2 Methods and materials

For this study, we gathered a total of 9 samples from various sources in the Kathmandu Valley. These samples comprised three from Dhungedhara, three from commercially available water sources, and three from natural underground water. The Dhungedhara water is transported from the reservoir to human use through an underground brick-lined canal. The distinction between open ground water and natural underground water lies in the fact that open ground water comes from a reservoir without any covering, leaving it exposed to atmospheric pollution. On the other hand, natural underground water is sourced from beneath the earth's surface. We have studied the transmittance for each sample at different temperature (8 °C to 48 °C) because most of the temperature variations in the Kathmandu valley are in this temperature range. Notations for sample are Melamchi water (MW), Kathmandu Upatyaka Khanepani Limited water (KUKLW), Sundarijal Water (SW), Tanker Water (TW), Dhungedhara Patan Water (DDPW), Dhungedhara Kirtipur Water (DDKW), Mineral water (MiW), Distill water (DW), Jar water (JW), SE = Standard Error. For all samples we took transmittance of water using a UV–Vis Spectrophotometer (Cary 60, Agilent Technologies) at various temperatures.

2.1 Measurement the density of water

Water's density, measured by mass per unit volume, shifts with temperature. At 4 °C, its density is the highest. Cold water, denser due to molecules' proximity, contrasts with warmer water's lower density as molecules disperse. This impacts ecosystems, creating stratification in water bodies and affecting nutrient and gas dispersion. In engineering, temperature-driven density affects heat transfer and buoyancy. Density is influenced by temperature, pressure, and dissolved substances, altering molecular arrangement and spacing. Table 1 display the densities of various samples of water at different temperature and measurement of density is taken pycnometer technique. The pycnometer technique involves using a specialized glass container called a pycnometer to measure the density of a substance. This is done by weighing the empty pycnometer, adding the substance, weighing the combined mass, and then calculating density based on known or determined volumes.

Table 1 Average density of water at different temperature
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2.2 Transmittance and absorbance

The absorption coefficient μ can be derived from the Lambert–Beer law

$$I={I}_{0}{e}^{-\mu t}$$
(1)

where \({I}_{0}\) is the incident intensity when measured without sample, I is intensity transmitted through the sample, and t is the sample thickness (cm). The absorption coefficient \((\mu )\) is calcauted by using [7].

$$\mu =\frac{1}{t}{\text{ln}}\left(\frac{{I}_{0}}{I}\right)$$
(2)

2.3 Mass attenuation coefficient

The experimental mass-absorption coefficient (μ/ρ) of elements is given by

$$\frac{\mu }{\rho }=\frac{1}{\rho t}{\text{ln}}\left(\frac{{I}_{0}}{I}\right)$$
(3)

where ρ is the material density (g/cm3). The theoretical mass-absorption coefficient μ/ρ (cm2/g) for any chemical compound or mixture of elements [8].

2.4 Total molecular cross section

The values of the mass attenuation coefficients can be used to determine the total molecular cross section, \({\sigma }_{t,m}\) by the following relation [9]:

$${\sigma }_{t,m}=\left(\frac{\mu }{\rho }\right)\frac{M}{{N}_{A}}$$
(4)

where M is the molecular weight and \({N}_{A}\) is Avogadro’s number.

3 Result and discussion

3.1 Density of different sample water

The impact of temperature on water density has been extensively investigated. Darros‐Barbosa et al. observed that the density of pure water varies from 0.9832 to 0.9997 g/cm3 across the temperature range of 60 °C to 10 °C [10] and more detail is shown in Table 1. In our study, water density exhibited a range of 1.1725 to 1.0015 g/cm3 within the temperature interval of 8 °C to 48 °C. Thormahlen et al. reported a wider range of water density, spanning from \(2.8\times {10}^{-6}\) g/cm3 to 1.0450 g/cm3 [11]. Similarly, Mallamace et al. found water densities ranging between 0.9000 g/cm3 and 1.0200 g/cm3 [12]. Additionally, Prezioso et al. documented density variations at 25 °C, ranking doubly distilled water \(<\) acqua rocchetta \(<\) mineral water \(<\) tap water [13]. Our findings align with the established trends in temperature-dependent water density behavior.

3.2 Transmittance of water at different temperature

The UV–Vis spectrometer was employed to analyze the transmittance of water samples from diverse sources within the Kathmandu Valley. Notably, the results revealed an upward trend in transmittance as the wavelength increased. In the UV wavelength range, transmittance demonstrated lower values, characterized by a slender band for all samples. In contrast, the visible wavelength range showcased distinct behavior with broader variations in transmittance among the samples. This distinction arises from water's propensity to absorb shorter wavelength energies, particularly within the UV range, due to impurities. In contrast, longer wavelengths experience weaker energy absorption by water molecules. The process of evaporation, both in natural and industrial settings, is well-documented. Commonly, "thermal evaporation" prevails, emphasizing the heat-induced energy input. This is attributed to water's limited absorption of visible light energy [14]. This weak absorption causes transmittance the high in visible region as shown in Fig. 1.

Fig. 1
figure 1figure 1

Transmittance a MW, b KUKLW, c SW, d TW, e DDPW, f DDKW, g MiW, h DW, i JW

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Figure 1a shows the transmittance of MW at different temperatures and it was found that transmittance increases with increasing wavelength. In shorter wavelength regions, the transmittance is low, while in longer wavelengths it is high. This is because in shorter regions, water absorbs short wavelength photons while in higher wavelength regions the absorption is lower/weaker. In addition, the presence of soluble impurities and small fine particles also play an important role in transmittance. The transmittance of MW at 8 °C is higher than another considered wavelengths as shown in Fig. 1a. This indicates the possibility of soluble impurities and particles having a particle size smaller than their wavelength. Therefore, the interaction between them is much lower and hence, photons get easily passed through the sample with high transmittance. But with the increasing temperatures the transmittance goes decrease. This is because the size of impurities and water molecules increases and hence photons get more interaction. This causes lower transmittance with temperature. In addition, the impurities of particle in MW are such types that follow a linear expansion relationship with temperature. Figure 1b shows the nature of transmittance of KUKLW. Nature shows in short wavelength region transmittance are lower but the difference in transmittance with temperature is very narrow but in visible region the transmittance at 28 °C is broader than other. This shows the presence of such soluble impurities and particles in water that evaporate at 28 °C and the evaporate impurities concentration is high, while the others impurities doesn’t change at this point and are not effected by heat. At 28 °C the transmittance is almost the same because the dissolved impurities are the same in nature with a uniform distribution.

Transmittance increases with wavelength but does not uniformly rise with temperature as seen in Fig. 1c. This is a result of soluble contaminants in the SW water sample. Because of the contaminants, Fig. 1c displays three distinct transmittance levels. Because there are soluble contaminants in the sample, there is less interaction between the photon and the transmittance at 8 °C than at other temperatures. Furthermore, transmittance decreases with temperature as a result of a strong photon-particle interaction caused by the kinetic energy of contaminants in the sample. However, when we compare the transmittance at higher temperatures, we find that the transmittance at 48 °C is higher. This is because soluble impurities evaporate at this temperature, reducing the amount of interaction that occurs between incident photons and residual contaminants. With the aid of transmittance, we can conclude that the impurity type in SW and tanker water is the same because Fig. 1c and d have the same nature. Transmittance increases with wavelength but does not uniformly rise with temperature as seen in Fig. 1e. This is a result of soluble contaminants in the DDPW water sample. Because of the two distinct sizes of impurities present, Fig. 1e displays two different transmittance levels. At 18 °C and 48 °C, the transmittance is higher than at other temperatures due to the presence of soluble contaminants in the sample, which reduces photon interaction. When temperature increases to 18 °C, one type of impurities evaporates, and when temperature increases again to 48 °C, another type of impurities evaporates. However, at 28 °C and 38 °C, the impurities present in the sample do not evaporate. At 8 °C, the transmittance is lower and the impurities do not evaporate. As a result, the incidence photon interacts more with contaminants in the samples at 8 °C, 28 °C, and 38 °C, resulting in poorer transmittance than at 18 °C and 48 °C.

The transmittance increases with wavelength in Fig. 1f, but it does not rise uniformly with temperature. This is due to soluble contaminants in the DDKW sample. Because of the two distinct sizes of impurities present, Fig. 1f displays two different transmittance levels. At 8 °C and 48 °C, the transmittance is higher than at other temperatures because the soluble contaminants in the sample interact with photons less (the impurities don't affect the transmittance at 8 °C, but at 48 °C, they evaporate). When the temperature rises to 38 °C, contaminants do not evaporate and the transmittance is reduced to 18 °C. As a result, the incidence photon interacts more with contaminants in the samples at 18 °C, 28 °C, and 38 °C, resulting in poorer transmittance than at 8 °C and 48 °C. The transmittance increases with wavelength in Fig. 1g, but it does not rise uniformly with temperature. This is due to the water sample from MW having a distinct chemical makeup. The two distinct transmittance levels shown in Fig. 1g are caused by differing particle sizes. Because there is less particle interaction with the photon in the sample at 8 °C, 28 °C, and 38 °C, the transmittance is higher at these temperatures than at others. Because the particles present at these temperatures have two different types of bond stretch and randomness when they absorb energy as a source of heat, the transmittance is lower at 18 °C and 48 °C. Since the water in this sample has been cleaned using advanced technology, there is no chance of volatile substances present, or the authors have chosen to ignore them.

Figure 1h illustrates how transmittance increases with wavelength but does not uniformly rise with temperature, this is because many constituents present in DiW. The two distinct transmittance levels shown in Fig. 1h are caused by differing particle sizes. The transmittance is higher at 8 °C and 18 °C than at other temperatures because the existence of particles in the sample reduces their interaction with photons, and temperature has very little effect on them. The transmittance decreases at 28 °C, 38 °C, and 48 °C because the existence of particles at these temperatures is temperature-dependent, leading to the conclusion that two types of particles are present. Since the water in this sample has been cleaned using advanced technology, there is no chance of volatile substances present, or the authors chose to ignore them. The transmittance increases with wavelength in Fig. 1i, but it does not rise uniformly with temperature. This is due to the water sample's chemical composition, which consists of several elements. The transmittance of the jar sample is almost the same, as seen in Fig. 1i, due to the nearly homogeneous particle size and uniform distribution in water of the same nature. The transmittance overlaps at about 300 nm in the short wavelength range, but as the wavelength increases, the transmittance at various temperatures approaches unity.

In their study, Nachabé et al. examine the absorption spectrum above visible wavelengths at varying temperatures. However, they discover that in a specific wavelength range, the absorption exhibits a mixed nature, meaning that it either increases or decreases with temperature. In the remaining region, however, the absorption remains constant across all temperatures examined [15]. However, practically every sample in this type of experiment was collected at a different temperature. According to a study done by Kou et al. and published by Cumming in 2023, water absorption at 25 °C, 8 °C, and 25 °C exhibits non-uniform features, meaning that it either increases or decreases with temperature [16]. In our instance, this kind of result was shown in every sample when observations were made between 8 and 48 °C. The various vibrational modes of the water molecule are excited, which gives absorption by water a complex structure. The areas between 740 and 800 nm have the biggest temperature effects on absorption [17]. The influence of temperature is observed in this study in the 700–800 nm region at the temperature that is taken into consideration in every sample.

3.3 Transmittance of water of different sources

Figure 2a–e illustrate two distinct levels of transmittance nature at 48 °C, 38 °C, 28 °C, 18 °C and 8 °C, respectively. This is a result of the sample's soluble particles and contaminants. Instead of this, it is quite difficult to identify species contaminants and particles because nine samples are gathered from various sources. We can determine the type and size of contaminants present in the sample based on transmittance. Because different substances evaporate at different temperatures, the presence of contaminants is random. As a result, the transmittance varies with temperature. The transmittance varies but is lower at shorter wavelengths, and for all samples, it is nearly constant beyond 500 nm. This indicates that while the impurity sizes present in the sample are all about identical, the influence of temperature on transmittance varies, as does the interaction between the particles and the incidence particles. At 28 °C and 48 °C, the transmittance of MW, MiW, and DW is higher, while that of the others is lower. At 38 °C, mineral water has a higher transmittance than other types. At 18 °C, the transmittance of MW and DW is higher than that of the other. At 8 °C, SW's transmittance was determined to be higher than others. At 8 °C, the transmittances are divided into three distinct levels.

Fig. 2
figure 2

Transmittance of different sample at a 48 °C, b 38 °C, c 28 °C, d 18 °C and e 8 °C

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3.4 Absorption coefficient at 28 °C

Figure 3 illustrates how the absorption coefficient decreases with increasing wavelength. In the UV region, the dispersed nature was observed, but not in the visible zone. In the visible spectrum, a unique absorption coefficient level with a narrow band is seen. This demonstrates that the samples include two distinct particles. Because the particles present in the water are almost the same diameter, the DW, JW, and MiW absorption coefficients are quite close in the visible region of the spectrum. As a result, incident photon interactions with sample particles are nearly identical. However, the impurities in this sample are smaller in size when compared in the visible region, but the sizes differ in the UV region, thus the absorption of other samples falls into a distinct group. The dispersion graph is absorbed as a result. Furthermore, we may state that treated water has a lower particle content than natural source water (DW, MiW, and JW). Consequently, the visible region is where the narrow band gap is seen. Furthermore, natural source water absorbs more UV radiation than treated water does. This further demonstrates that treated water has a lower particle content than untreated water. Room temperature (28 °C) was used to make this observation.

Fig. 3
figure 3

Absorption coefficient at 28 °C of different sample

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The nature of the absorption coefficient with various contaminants (such as metaldehyde, clothianidin, tryptophan, thiamethoxam, uric acid, and potassium hydrogen phthalate) present in the water study [18] reveals that while the absorption is uniform with narrow width in the 300–360 nm region, it is not uniform in the UV region. In contrast, the current work shows that the absorption is uniform from 250 nm to longer wavelengths. This is due to the uniformity of particle presence in samples as compared to Spangenberg et al.

3.5 Mass attenuation coefficient (MAC)

Figure 4 illustrates how the mass attenuation coefficient (MAC) of water falls as wavelength increases. In radiation physics and radiology, a physical quantity known as the mass attenuation coefficient—often written as μ/ρ—is employed. It depicts the likelihood that a photon will interact with a substance during its passage through it, accounting for both scattering and absorption effects. Figiure 4 illustrates how the mass attenuation coefficient drops with wavelength. This is due to the fact that water loses its ability to block radiation. This suggests that there will be fewer interactions between the radiation and the water, resulting in less absorption and higher penetration. In this instance, the material permits more radiation to pass through and the transmitted radiation intensity is higher.

Fig. 4
figure 4

MAC of different sample at 28 °C

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Mass attenuation coefficients tend to be lower in materials with lower atomic numbers or densities. All samples showed a drop in MAC with wavelength, with MiW, DW, and KUKLW exhibiting lower MAC values than the others. This suggests that compared to other samples, radiation passing through MiW, DW, and KUKLW interacts more.

3.6 Molecular cross section area (MCA)

The MCA diminishes with wavelength, as Fig. 5 illustrates. MiW, TW, and KULKW were found to have lower MCAs than the other samples. This indicates that the incident photon interacts strongly with the particles in the mineral, tanker, and KUKLW samples, but less so in the other sample. This suggests that there is screening-type particle repulsion in addition to larger energy impurity sizes in the sample. The approximate size of the particles is visible from MCA.

Fig. 5
figure 5

MCA of different sample at 28 °C

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From MCA it is observed that the particle sizes are of order 10−24cm2. The particle size is categorized into two ranges: 3.5 to 5.25, while the other is 1.25 to 3.25. This particle is very fine in size and can’t be identified even from existing nano device like Nanofiltration membrane whose limitation is only can filter 0.001 to 0.1 µm size impurities present in water sample [19,20,21]. Therefore, to identify the size of a particle is the best option to calculate the size of particle, which is simple, feasible, and easy. As the size of water molecules is about 2.75 Angstrom and if we assume sphere, the cross section area is about \(2.375\times {10}^{-15}\)cm2. From these two data, we can conclude that the particle present in water sample is smaller than water molecules. So, in such a case, fine particle needs nano-filtration technology.

4 Conclusion

In summary, the investigation into the optical properties of water revealed trends such as an increase in transmittance and a decrease in absorption, MAC, and MCS with wavelength. These trends are influenced by both temperature and the presence of impurities in samples taken from various water sources. Analyzing the molecular cross-section with transmittance allows for the determination of impurity sizes in different water samples, indicating a need for the development of nano-particle filtration technology. While transmittance is a valuable indicator of water purity, it has limitations, particularly when dealing with small-sized impurity particles. In such cases, high transmittance may occur due to limited interaction between the particles and the incident light spectrum. Therefore, the optical properties offer a means to ascertain impurity sizes and guide the development of new nano-filtration technology. Furthermore, considering transmittance, MW, KUKLW, SW, TW, DDPW, and DDKW are suitable for daily use but not for drinking due to the presence of temperature-dependent volatile impurities.