Seasonal variation and chemical composition of total precipitation and throughfall in an urban forest fragment in the Central Brazilian Amazon

Variação sazonal e composição química da precipitação total e interna em fragmento florestal urbano na Amazônia Central Brasileira

ABSTRACT

The objective of the research was to determine the total precipitation and throughfall of an urban forest fragment in Manaus and to quantify the concentration of nutrients present in rainwater. The study was developed in two seasonal periods (dry and rainy) between 2020 and 2022. The pluviometers were installed in an urban forest fragment, in an open area (total precipitation), and inside the forest (throughfall). The water samples were sent to the laboratory for analysis of physical and chemical parameters. Precipitation showed a significant difference between periods, where the highest volumes of rain were observed in the second half of each year. The concentrations of nutrients in the water samples were higher in the dry period, and the throughfall showed high concentrations of nutrients in both seasonal periods due to washing the canopy, trunks, and leaves. In addition, the seasonality and volume of rainfall play important roles in the chemistry of rainwater in preserved and urban areas. Therefore, it is a key part of several biological processes in the ecosystem, including in streams and Amazonian rivers. Therefore, studies of this nature are scarce in the region, and the data provided are important for establishing long-term experiments.

Keywords:
Rain; Water quality; Urbanization; Pollution; Manaus

RESUMO

O objetivo da pesquisa foi determinar a precipitação total e interna da área um fragmento florestal urbano em Manaus, e quantificar a concentração de nutrientes presentes nas águas das chuvas. O estudo foi desenvolvido nos dois períodos sazonais (estiagem e chuvoso), entre 2020 e 2022. Os pluviômetros foram instalados em um fragmento florestal urbano, em área aberta (precipitação total) e dentro da floresta (precipitação interna), e as amostras de água foram encaminhadas para análise dos parâmetros físico-químicos em laboratório. A precipitação apresentou diferença significativa entre os períodos, onde os maiores volumes de chuva foram observados no segundo semestre de cada ano. As concentrações de nutrientes presentes nas amostras de água foram maiores no período seco, e a precipitação interna apresentou concentrações altas de nutrientes em ambos os períodos sazonais, devido a lavagem do dossel, troncos e folhas. Além disso, a sazonalidade e o volume da precipitação exercem papéis importantes na química das águas das chuvas tanto em áreas preservadas quanto em áreas urbanas, sendo peça chave para diversos processos biológicos no ecossistema, inclusive nos igarapés e rios amazônicos. Portanto, estudos dessa natureza são escassos na região e os dados indicados são importantes para o estabelecimento de experimentos de longo prazo.

Palavras-chave:
Chuva; Qualidade da água; Urbanização; Poluição; Manaus

INTRODUCTION

Forest fragments are important and essential components of the urban and rural landscape, benefiting the microclimate and improving human thermal comfort levels. In addition, they redistribute rainwater, dampening, directing, and retaining raindrops that reach the ground, reducing erosive effects and direct runoff (Vital et al., 2021). In addition to their representativeness, there is a growing interest and need to study their hydrological behavior (Vital et al., 2024).

Precipitation in the state of Amazonas has unique and particular characteristics, as it is located in the tropics and close to the equator, with a large flow of solar radiation, high temperature, and water vapor (Monteiro et al., 2015). The region has the world's largest expanse of tropical rainforest, with a wide river network that moves the largest volume of fresh water available on Earth. As a result, the evaporation of part of the water and mainly the transpiration of the forest are responsible for the abundant rainfall in the region (Honório, 2007).

Of the total precipitation that reaches the forest cover, part returns to the atmosphere in the form of evaporation and transpiration from the plants (evapotranspiration) and the other part reaches the forest floor after washing off the upper canopy, understorey trees, and runoff through the tree trunks (effective rainfall). The amount and spatial distribution of rain that reaches the forest floor (throughfall) depends on the type and shape of openings in the upper canopy, the total leaf area, the number of vegetation layers, and the intensity of precipitation (Vital et al., 2003).

The atmosphere in the Amazon has been impacted by significant changes in its structure and composition due to anthropogenic activities in some areas of the region. These changes can occur on a micro and macro scale, also influencing the hydrological cycle by altering cloud condensation and its microphysical properties (Artaxo et al., 2005). The city of Manaus has undergone a process of intense and unplanned urbanization, causing major impacts on the local environment and climate. Allied to the creation of the Zona Franca de Manaus, population growth has increased considerably, being one of the main factors responsible for the increase in deforestation and the development of the urban area (Candido et al., 2021).

Forest cover is of great importance in the context of the local water balance, and precipitation plays a fundamental role in the nutrient cycling process by bringing these elements into the atmosphere through rainfall. In addition, vegetation contributes to the retention of these particles that will be removed by the rains (Diniz et al., 2013), and directly and indirectly influences biological processes in the Amazon (Arcos et al., 2021). In the city, episodes of intense rainfall increase erosion processes, flooding, and overflows, causing various inconveniences, including deaths (Aleixo & Paula, 2023).

The chemical composition of precipitation is the result of several factors, with dynamic and complex atmospheric processes involving the emission, transportation, dilution, chemical transformation, and emission of pollutants. A wide variety of pollutants can be present in the atmosphere, from natural sources (volcanoes) or anthropogenic sources (industrial processes), which can cause adverse effects on human and environmental health, such as contamination of water resources, food, and absorption through the skin (Fornaro, 2006). In addition, it has been shown that air pollution from forest fires significantly increases the number of hospital admissions in the eastern Amazon (Moura et al., 2021).

According to Silva & Vieira (2017) there must be continuous attention to the levels of contaminants present in the air of Brazilian cities, presenting adequate minimum conditions for a good quality of life, reducing the impact on the population's health. In large cities, the emission of pollutants can come from a variety of sources (fires, industries, fuel burning), including nitrogen oxide, which plays an important role in the chemical composition of the atmosphere. The reaction of these oxides in the atmosphere forms other acids that contribute to the formation of acid rain.

Studies show that the composition of rainwater is affected by chemicals in the atmosphere, and also depends on local emission sources, pollutant transport, and precipitation itself (Baron & Denning, 1993; Chidambaram et al., 2014; Liu et al., 2015). Knowing the chemistry of atmospheric water helps us to understand the relative contributions of different sources of atmospheric pollutants (Zhang et al., 2012; Rao et al., 2015). We hypothesized that throughfall likely exhibits higher nutrient concentrations due to canopy “wash-off” and particle deposition and that the chemical composition of rainwater changes with the local season. Therefore, the study aimed to determine the total precipitation and throughfall of an urban forest fragment in Manaus, and to quantify the concentration of nutrients present in rainwater in the local seasonal periods.

MATERIAL AND METHODS

Study area

The study was carried out at the Instituto Nacional de Pesquisas da Amazônia - INPA, Campus I and II, with an area of 256,736.48m² in Manaus - AM, at an altitude of 40.33m above sea level (Figure 1). The experimental area is located in a stretch of forest called Bosque da Ciência, at coordinates: 03°08'7''S and 60°01'34''W Greenwick (Vital et al., 2021), and the rainfall volume was collected between January 2020 and December 2022, covering the region's seasonal periods (transition to rainy, rainy, transition to drought, drought).

Figure 1
Study area located in an urban forest fragment in the city of Manaus, Amazonas.

The green area comprises secondary vegetation growing on clay soil, but there are some small stretches with fragments of the original vegetation. The forest fragment studied, although young, has a floristic composition in a stage of natural succession because it has the same diversity in some families and floristic composition as primary forests (Erazo & Ferreira, 2001).

Collection procedures

This study used a PALMEX rain collector (Gröning et al., 2012), developed to simplify the rainfall sampling procedure and whose design prevents evaporation. Total precipitation was determined using two pluviometers installed in an open area (INPA 1 and INPA 2). To determine throughfall, 25 rain pluviometers were installed inside the vegetation, leveled, and placed at 0.90m from the ground, following a systematic distribution in a 30 x 20m grid (Vital et al., 2024). All the weekly rainfall measurements in milliliters (mL) were accumulated each month and converted into millimeters in pre-formatted tables for further analysis (Vital et al., 2024).

The rainwater samples collected weekly from the pluviometers between January 2021 and December 2022 were sent to the Laboratório de Química Ambiental (LQA/INPA) for physicochemical analysis, including pH, electrical conductivity, nitrite, nitrate, ammonia, chloride, and phosphate, as shown in Table 1 (American Public Health Association, 2017). For physicochemical and chemical characterization, 100mL of water from each sample were filtered using Whatman Glass Microfiber Filters: type GF/F (47mm diameter; 0.5μm porosity), and sent for analysis using the 1800-Schimutz UV-visible spectrophotometry model with the aid of the FIA (Flow Injection Analysis) system and the VARIAN GBC-906 atomic absorption model. The pH, temperature, and conductivity were measured with portable equipment in the field (American Public Health Association, 2017).

Table 1
Variables analyzed in the field and laboratory and methodology applied.

Variable	Unid	Ab. *	Measurement methods, device
Potential of hydrogen	pH	pH	Direct measurement, S230 Seven Compact™
Electrical conductivity	μS/cm-1	EC	Direct measurement, Mettler Toledo™
Chloride concentration	mg/L-1	Cl-	Spectrophotometry, Shimadzu UV-1800
Ammonium concentration	mg/L-1	NH4+	Spectrophotometry, Shimadzu UV-1800
Nitrite concentration	mg/L-1	NO2-	Spectrophotometry, Shimadzu UV-1800
Nitrate concentration	mg/L-1	NO3-	Spectrophotometry, Shimadzu UV-1800
Phosphate concentration	mg/L-1	PO4-3	Spectrophotometry, Shimadzu UV-1800

• ^*
  Ab. =Abbreviation.

The Shapiro-Wilk normality test was performed to select an appropriate analysis. Based on the non-normality of the data, we used the ANOVA and Kruskal-Wallis analysis of variance (considering p=<0.05). These tests were carried out to compare the volume between the seasonal periods and between the total precipitation and throughfallin the seasonal periods using Past software version 4.0 (Hammer et al., 2001). The data collected every week was aggregated to be analyzed monthly. Principal component analysis (PCA) was carried out on the physico-chemical variables and seasonal periods in the areas studied, using the statistical software PAST version 4.0 (Hammer et al., 2001).

RESULTS AND DISCUSSIONS

Total rainfall varied from 4mm to 322mm during the period with the lowest rainfall and from 106mm to 441mm during the period with the highest rainfall, with the highest rainfall volumes in March, April and May, which corresponds to the rainy season in the region. Throughfall varied from 3mm to 265mm during the dry season and from 63mm to 482mm during the rainy season, with the highest volumes in December. Overall, the volume of total precipitation was greater than that of throughfall (Figure 2). A significant difference was identified in the volume of total precipitation and throughfall between the seasonal periods respectively (p=0.000 and p=0.000) and between total precipitation and throughfall in the rainy period (p=0.05). No difference was identified between total precipitation and throughfall in the dry season (p=0.29).

Figure 2
Seasonal and temporal variation of total precipitation and throughfall in an urban forest fragment in Manaus, Amazonas, Brazil.

According to D’Avila Junior & Vieira (2019), the rainiest months in the city of Manaus are from November to June (April is the rainiest month), and the least rainy are July, August (least rainy), September, and October. In addition, some daily rains are responsible for the entire month's rainfall, especially during the dry season. A similar rainfall profile was identified during the study, with higher rainfall volumes in the first half of each year. This seasonality is well-defined in the region, but it is influenced by climatic events. Climate phenomena such as El Niño have already been linked to an increase in carbon monoxide concentration in the Manaus region (Signori et al., 2023), and also affect climate dynamics in the Amazon, causing changes in the daily maximum rainfall regime, as well as increasing or decreasing the volume of precipitation (Moreira et al., 2018). According to Lopes et al. (2021), El Niño and La Niña events in the region directly influence the flooding and ebbing of rivers, with a direct effect on rainfall in the state, which can cause social problems. The rainy season also plays an important role in the recovery of surface and subsurface water levels in the region (Bastos et al., 2023).

Overall, total precipitation was higher than throughfall (Figure 2). The loss of rainfall volume in throughfall is expected and has been observed in studies in the Amazon region (Oliveira et al., 2011; Vital et al., 2021, 2024). In addition, throughfall contributes most of the water that reaches the soil surface in the forest, and is directly related to total precipitation (Oliveira et al., 2008). The volume of total precipitation and throughfall in this study is associated with the seasonal period in the region, with lower volumes of throughfall being observed in the dry season. According to Oliveira et al. (2011), throughfall is a variable dependent on total precipitation above the canopy, where both follow similar trends of variation.

Inter-annual variations in rainfall are also associated with tree diameter growth in the Amazon region (Higuchi et al., 2011), thus indicating the great importance of the hydrological regime for the region. Therefore, according to Arcova et al. (2003), forest cover plays a fundamental role in the water balance, influencing the amount of rain that will reach the soil surface. Extreme rainfall events in Manaus can cause serious problems throughout the city due to impermeability, such as flooding and social problems (Santos et al., 2012).

The electrical conductivity of the total rainfall during the rainy season ranged from 1.1μS/cm^-1 to 8.5μS/cm^-1 (mean = 4.1 ± 1.5 μS/cm^-1 standard deviation). During the dry season, total rainfall varied between 2.0μS/cm^-1 and 14.7μS/cm^-1 (7.7 ± 3.3μS/cm^-1) (Figure 3a). The conductivity for the throughfall samples showed higher values, varying in the rainy season between 2.5μS/cm^-1 and 48.1μS/cm^-1 (20.0 ± 11.6μS/cm^-1). In the dry season, the conductivity of throughfall ranged from 10.0μS/cm^-1 to 76.8μS/cm^-1 (34.0 ± 16.2μS/cm^-1) (Figure 3b).

Figure 3
Temporal variation in the electrical conductivity of water from total precipitation (a) and throughfall (b) in the seasonal periods between 2020 and 2022.

High electrical conductivity values were observed mainly during the dry season in both areas (total precipitation and throughfall). When comparing the two types of precipitation, in general, total precipitation showed low conductivity (Figure 3). Studies in the Amazon region show a similar profile in the difference in electrical conductivity between the two types of precipitation, and between seasonal periods (Silva et al., 2021; Vital et al., 2021). Therefore, rainy events reduce the electrical conductivity values in rainwater (greater quantity of water, greater dilution), and during drought, they coincide with an increase in electrical conductivity in surface and groundwater (Löbler et al., 2015; Arcos et al., 2020).

Higher electrical conductivity values are also associated with materials in the form of dissolved solids deposited on the surfaces through which the rainwater passes. In addition, samples collected directly from rainwater in the atmosphere show a decrease in conductivity values compared to samples collected in contact with some surfaces (Hagemann & Gastaldini, 2016). According to Monteiro et al. (2014), high volumes of rain also increase the flow of surface water, resulting in high peaks of electrical conductivity in the water. In the present study, high electrical conductivity values were observed in samples from throughfall (Figure 3). This result is strongly influenced by rainwater washing off trees, leaves, and trunks, which are rich in nutrients and end up falling into the collectors inside the forest, thus increasing the amount of ions coming from the atmosphere and accumulating on the plant surface.

The variation in the pH of total precipitation during the rainy season was between 4.0 and 6.1 (mean = 5.2 ± 0.5 standard deviation). In the dry season, the pH of total precipitation varied between 4.3 and 6.4 (5.6 ± 0.6) (Figure 4a). The pH of throughfall ranged from 5.5 to 6.9 (6.3 ± 0.4) in the rainy season. During the dry season, the pH of throughfall varied between 5.5 and 6.6 (6.1 ± 0.3) (Figure 4b). The pH values during the months of collection in the two seasonal periods ranged from acidic 4.1 to neutral 6.9, with more acidic values for total precipitation and neutral values for throughfall.

Figure 4
Temporal variation in water pH from total precipitation (a) and throughfall (b) in the seasonal periods between 2020 and 2022.

The pH of rainwater is around 5, however, high ammonium emissions can lead to the neutralization of the acidity of this water (Moreira-Nordemann et al., 1997). A study carried out by Honório et al. (2010) in cities in Amazonas observed that in the Manaus the pH varied between 3.7 and 4.5 in forested areas, and between 3.4 and 4.9 in open areas. The high pH values may also be related to the varied intensity of rainfall and low wind speed, which favors the resuspension of basic soil particles into the atmosphere. Similar values were identified in the present study in the INPA forest fragment, with the pH ranging from slightly acidic to neutral. We should also point out that an important factor that increases pH values in the Amazon region is burning (deposition of nutrients), especially during the dry season (between July and October) (Artaxo et al., 2006; Torres-Filho et al., 2014).

According to Pereira et al. (2022), another factor that can influence the pH values of rainwater in a given area are local “polluting” sources, such as ceramics factories which, in their production process, release sulphur dioxide and nitrite, which in the atmosphere is converted to nitrate. Rainfall is also indicated as one of the main influencers of river water quality in the Amazon, with environmental variables (e.g. pH, conductivity) correlating with the rainfall regime in the region (Silva et al., 2008).

During the rainy season, the mean nitrate concentration for total precipitation ranged from 0.02mg/L^-1 to 0.03mg/L^-1, and during the dry season, the mean ranged from 0.01mg/L^-1 to 0.06mg/L^-1. For throughfall, the mean nitrate in the rainy season was between 0.24mg/L^-1 and 0.63mg/L^-1, and during the dry season was between 0.88mg/L^-1 and 4.18mg/L^-1 (Table 2). Nitrite during the rainy season showed means for total precipitation between 0.001mg/L^-1 and 0.006mg/L^-1 and during the dry season between 0.004mg/L^-1 and 0.007mg/L^-1. For throughfall, nitrite mean values ranged from 0.004mg/L^-1 to 0.044mg/L^-1 during the rainy season, and from 0.031mg/L^-1 to 0.198mg/L^-1 during the dry season (Table 2).

Table 2
Concentrations of the chemical variables of total precipitation and throughfallin an urban forest fragment in the city of Manaus, Amazonas.

Variables				2021		2022
				Rainy	Dry	Rainy	Dry
Nitrate mg/L-1	Total		Min/Max	<DL - 0.06	<DL - 0.28	<DL - 0.04	<DL - 0.01
			Mean/S.D.	0.03 ± 0.03	0.06 ± 0.06	0.02 ± 0.01	0.01 ± 0.005
	throughfall		Min/Max	<DL - 0.52	0.26 - 1.79	<DL - 1.74	0.01 - 10.32
			Mean/S.D.	0.24 ± 0.26	0.88 ± 0.47	0.63 ± 0.54	4.18 ± 3.80
Nitrite mg/L-1	Total		Min/Max	0.005 - 0.007	<DL - 0.020	<DL - 0.005	<DL - 0.008
			Mean/S.D.	0.006 ± 0.001	0.007 ± 0.004	0.001 ± 0.002	0.004 ± 0.005
	throughfall		Min/Max	<DL - 0.008	0.009 - 0.836	<DL - 0.298	0.005 - 0.082
			Mean/S.D.	0.004 ± 0.004	0.198 ± 0.241	0.044 ± 0.096	0.031 ± 0.031
Ammonium mg/L-1	Total		Min/Max	0.06 - 0.14	0.03 - 0.2	<DL - 0.1	0.1 - 0.2
			Mean/S.D.	0.09 ± 0.03	0.11 ± 0.03	0.08 ± 0.04	0.16 ± 0.05
	throughfall		Min/Max	0.2 - 0.4	0.08 - 1.4	0.1 - 0.9	0.6 - 5.8
			Mean/S.D.	0.30 ± 0.10	0.59 ± 0.42	0.32 ± 0.25	2.8 ± 2.22
Phosphate mg/L-1	Total		Min/Max	0.006 - 0.023	<DL - 0.112	0.001 - 0.017	<DL - 0.01
			Mean/S.D.	0.014 ± 0.008	0.017 ± 0.022	0.008 ± 0.005	0.006 ± 0.005
	throughfall		Min/Max	0.012 - 0.093	<DL - 0.805	0.004 - 0.373	<DL - 1.99
			Mean/S.D.	0.044 ± 0.043	0.221 ± 0.231	0.179 ± 0.149	0.832 ± 0.813
Chloride mg/L-1	Total		Min/Max	1.1 - 1.55	<DL - 9.05	0.5 - 2.75	1.65 - 6.67
			Mean/S.D.	1.33 ± 0.23	1.78 ± 1.65	1.31 ± 0.57	3.62 ± 2.21
	throughfall		Min/Max	1.1 - 2.99	0.02 - 6.63	0.9 - 4.88	3.01 - 7.98
			Mean/S.D.	1.91 ± 0.97	2.12 ± 1.53	2.25 ± 1.21	5.05 ± 2.14

• Legend: Mn: minimum, Max: maximum, S.D.: standard deviation; <DL: less than the detection limit.

Nitrite and nitrate showed higher concentrations in the dry season, especially during throughfall, but showed low concentrations in both periods when compared to other studies in urban centers in Brazil (Carvalho-Junior, 2004; Migliavacca et al., 2005; Martins et al., 2016). In the city of Humaitá, located in the southern mesoregion of Amazonas, mean nitrate values of between 0.310mg/L^-1 and 0.418mg/L^-1 were found, incriminating the thermoelectric plant and the gases eliminated by cars in the region as the source of these concentrations in rainwater samples (Torres-Filho et al., 2014). In the present study carried out within the urban fragment in Manaus, lower concentrations were identified in the rainy season compared to the dry season.

Sanusi et al. (1996) indicate that higher concentrations of nitrate and sulphate are found in urban areas, linking their origin to large urban centers and polluting sources. High concentrations of nitrate and ammonium in urban areas are indicative of anthropogenic origin, such as thermoelectric plants, steel mills, and vehicle emissions. And these ions are common in rainwater and can contribute to its acidity (Migliavacca et al., 2012).

The mean value of ammonium during the rainy season was 0.08mg/L^-1 for total precipitation and 0.13mg/L^-1 during the dry season. For throughfall, the mean ammonium in the rainy season was 0.31mg/L^-1, and 1.69mg/L^-1 in the dry season (Table 2). According to Carvalho-Junior (2004), in general, the likely sources of ammonium in rainwater come from animal husbandry (80%), fertilizer use (17%), and industrial processes (1%), which contribute to the generation of the high NH₄⁺ concentrations found in rainwater. The highest concentrations of this nutrient were found in the throughfallsamples, which naturally accumulate higher levels of these elements that are adhered to plant surfaces, in addition to the contribution of animals that inhabit the environment.

The mean phosphate concentration in total precipitation ranged from 0.008mg/L^-1 to 0.014mg/L^-1 in the rainy season, and between 0.006mg/L^-1 and 0.017mg/L^-1 in the dry season. For throughfall, the mean phosphate varied between 0.044mg/L^-1 and 0.179mg/L^-1 during the rainy season, and between 0.832mg/L^-1 and 0.221mg/L^-1 during the dry season (Table 2). Chloride during the rainy season had a mean value of 1.32mg/L^-1 for total precipitation, and 2.70mg/L^-1 during the dry season. For throughfall, the mean chloride during the rainy season was 2.08mg/L^-1, and 3.58mg/L^-1 during the dry season (Table 2). In both areas, the highest concentrations were observed during the dry season, with high values in throughfall. Precipitation is a determining factor for the leaching of phosphate and other nutrients in the soil, where high volumes of precipitation tend to accelerate the leaching process of this nutrient (Pinheiro et al., 2013).

In Brazil, there is no specific resolution for assessing rainwater quality, however, the limits are derived from existing standards. For particular uses, such as non-potable reuse, it is recommended to follow the parameters of CONAMA 357/2005 and local guidelines. In industrial areas, contaminated rainwater must comply with CONAMA 430/2011 before disposal (Brasil, 2005, 2011a). Some states and cities have specific rules for using rainwater, e.g., São Paulo Law nº 12.526/2007 (São Paulo, 2007). The Brazilian Ministry of Health's Ordinance nº 2.914 of 2011 sets out the procedures for controlling and monitoring the quality of water for human consumption and its standard of potability. This ordinance was revoked by Consolidation Ordinance nº 5 of September 28, 2017 (Brasil, 2011b, 2017). In addition, there are some guidelines for using rainwater, such as ABNT NBR 15.527/2007 (use of rainwater in urban areas), which recommends minimum treatments (filtration and chlorination) and suggests quality parameters based on the WHO and CONAMA 357/2005 (Associação Brasileira de Normas Técnicas, 2007). Heavy metals, turbidity and bacteria are the main parameters monitored.

Figure 5 shows the results of the principal component analysis (PCA) for total precipitation and throughfall in the different seasonal periods (Figure 5). The PCA of total precipitation explained 95.9% of the variation in the data (axis 1 explained 83.1% and axis 2 explained 12.8% of the total variation) according to Figure 5a. Electrical conductivity (EC) was associated with axis 1 in the rainy season (r=0.982). pH was associated with axis 2 in the rainy season (r=0.777), and chloride in the dry season (r=-0.559) (Figure 5a).

Figure 5
Ordination diagram of the principal component analysis (PCA) of the physicochemical and chemical variables of total precipitation (a) and throughfall (b) precipitation in the two seasonal periods. Nitrate (NO₃^-), Nitrite (NO₂^-), Ammonium (NH₄⁺), Phosphate (PO₄^-3), Chloride (Cl^-), Potential of hydrogen (pH), Electrical conductivity (EC).

The PCA of throughfallexplained 99.3% of the variation in the data (axis 1 explained 98.4% and axis 2 explained 0.99% of the total variation) according to Figure 5b. Electrical conductivity (EC) was associated with axis 1 in the dry season (r=0.997), and the chloride and ammonium variables were associated with axis 2 in the dry season respectively (r=0.870, r=0.454) (Figure 5b). These variables were more explanatory for the variation in data on axes 1 and 2 of the PCA for total precipitation and throughfall, clearly showing the influence of precipitation on the dynamics of rainwater quality (Figure 5, Table 3).

Table 3
Loadings for the principal components and total variance (%) of total precipitation and throughfall.

	Variables	PC 1	PC 2
Total precipitation	Nitrate (NO3-)	0.004	-0.007
	Nitrite (NO2-)	-0.006	0.024
	Ammonium (NH4+)	-0.029	0.174
	Phosphate (PO4-3)	0.024	-0.133
	Chloride (Cl-)	0.074	-0.559
	Potential of hydrogen (pH)	-0.165	0.777
	Electrical conductivity (EC)	0.982	0.182
	% variance	83.1	12.8
Throughfall	Nitrate (NO3-)	0.014	0.130
	Nitrite (NO2-)	-0.000	-0.020
	Ammonium (NH4+)	0.049	0.454
	Phosphate (PO4-3)	0.011	0.113
	Chloride (Cl-)	0.051	0.870
	Potential of hydrogen (pH)	0.002	-0.013
	Electrical conductivity (EC)	0.997	-0.070
	% variance	98.4	0.99

It was observed that the nutrients present in rainwater are strongly influenced by the area in which the collectors are installed and the seasonal periods. This result was strongly influenced by rainwater washing off trees, leaves, and trunks, which are rich in nutrients and end up falling into the collectors inside the forest, thus increasing the amount of nutrients that reach the soil. For Carvalho-Junior (2004), the study of the chemical composition of rainwater becomes an important tool, as it allows the quantification of biogeochemical processes in natural ecosystems, where water flows in the environment are dependent on the spatio-temporal distribution of rainfall (Carvalho-Junior, 2004). In addition, this water provides information on the spatial and temporal variability of the atmospheric chemical composition, also indicating its influences (anthropogenic or natural origin).

The total rainfall washes the forest canopy and consequently, from the throughfall, the concentration of macronutrients from the intercepted water increases. This pattern was also observed in the present study carried out in the urban forest fragment, where the highest concentrations of nutrients were obtained in the samples of throughfall. According to Diniz et al. (2013) when it precipitates, rainwater carries mineral and organic elements suspended in the atmosphere with it, and when it passes through the forest they can be absorbed, thus contributing to its forest nutrition.

High concentrations of NH₄⁺ and Cl^- ions also directly influence the increase in electrical conductivity in rainwater, especially during periods of low precipitation, as these elements tend to accumulate for longer in the atmosphere and adhere to the surfaces of leaves and trunks. In addition, a study by Lima et al. (2009) shows that high concentrations of ammonium in rainwater increase pH values. According to Leal et al. (2004) in large cities the potential sources of NH₄⁺ are linked to urbanization processes, such as the burning of biomass and fuels, the decomposition of organic matter and urban waste. Couto (2011) points out that air masses carried in from the ocean and gaseous HCl emissions from companies in large industrial centers are potential sources of chloride ions in rainwater. In this sense, for our study, the influence of seasonality and the effects of urbanization play an important role in the chemical composition of rainwater.

A study carried out by Lima et al. (2024a) identified a higher concentration of ions present in the stemflow (order: Ca > Cl > K > P > S > Na > Mg) compared to rainwater (K > Ca > Cl > S > Na > P > Mg), corroborating our results which indicated a higher concentration of nutrients in throughfall. The same study showed that the flow of chemical variables in urban environments is influenced by interactions between factors, including environmental conditions (e.g., anthropogenic influences and atmospheric deposition). According to Lima et al. (2024b), stemflow is an important route for transporting nutrients to the soil, as rainwater is enriched through contact with the tree's surface. In general, the highest concentration of nutrients is found in the stemflow concerning total precipitation (Tonello et al., 2021).

When we talk about rain, according to Tadiello et al. (2014) the high concentrations of nutrients present in the atmosphere are indications of strong anthropogenic influence (e.g. Zinc, Copper, Manganese, and Nitrate), also pointing to the use of bioindicators to assess atmospheric pollution. Large volumes of rain maintain a balance between the pollutant particles that enter the atmosphere and, consequently, are precipitated onto the ground. In addition, some air pollution indicator variables tend to decrease in concentration and content during rainfall (Taguchi et al., 2012). Therefore, the quantity and quality of rain depends on various factors that influence the final uptake of this water in the soil. A recent study by Aleixo & Paula (2023) points out that extreme events have been increasing in recent decades in Manaus city, and the eastern zone is the region most affected by disasters caused by intense rainfall, interfering with the population's quality of life.

When assessing the environmental quality of a given environment, it is necessary to identify whether these sites have preserved their characteristics, otherwise, it is important to investigate the possible causes of these changes. In addition, anthropogenic actions can favor the emergence of pathogens that cause diseases for animals and humans (Arcos et al., 2020). According to Arcos & Cunha (2021, 2022), the process of environmental degradation has been increasing in many locations in the Amazon, so there is a need for a monitoring system that takes into account all stages of the hydrological cycle (rainfall, surface water, groundwater). Seasonality and urbanization are important elements in hydrological processes, directly and indirectly affecting water quality (Arcos et al., 2021; Falcão et al., 2021), therefore, changes in the hydrological/environmental regime can directly affect aquatic organisms, biogeochemical cycles, and human-environment interactions.

The effect of urbanization has also been observed in primary forest areas far from urban centers, such as the Cueiras Biological Reserve, which is 60 km from Manaus. In this study, a heterogeneous chemical composition of rainwater was observed, and in the dry season there was an increase in the concentration of these chemical elements, due to the direction of the winds coming from urban regions combined with the fires in the region (De La Cruz et al., 2024). In addition to monitoring the chemical variables of water, Bastos et al. (2024) emphasize the study of isotopic composition to investigate the origin of the main contaminants in various aquatic environments. This highlights the importance of large urban centers in the quality of rainwater that runs through urban, peri-urban, and forest regions.

CONCLUSIONS

During the study, there was a considerable oscillation in the amount of rainfall between the seasonal periods in the region, with a significant difference between them. These results show behavior similar to that found in studies carried out in natural environments throughout the region's hydrological periods.

The concentration of nutrients was higher in the throughfall precipitation compared to the total precipitation, and this result is directly influenced by the washing away of nutrients in the canopy, trunks, and leaves, which leads to an increase in nutrients in the rain gauges installed inside the forest. In addition, higher concentrations were observed during the dry season. These results show that the area has good rainwater quality, preserving the physical and chemical characteristics of less altered environments with little interference from urbanization. However, throughfall rainwater has the characteristic of aggregating higher concentrations of nutrients, which will be absorbed by the soil in a natural process.

Knowledge about seasonal variation and quality of rainwater is fundamental for the management of urban forest fragments, as it allows us to learn more about the optimization of infiltration and the resilience of vegetation at different times of the year. This information also helps to mitigate flooding since urban forests regulate surface runoff, absorbing rainfall peaks and reducing flooding. In addition, rainwater quality influences aquifer recharge and soil health, while forest fragments filter pollutants and support essential ecosystem services such as climate regulation, water purification and biodiversity maintenance, ensuring water sustainability, especially in urban areas.

This study points to a focal reality but with great indications for medium and long-term studies in larger areas. Among the new applications is the association with studies on stable isotopes, which will help to understand the processes of origin, transportation and traceability, helping to better understand the hydrological cycle and potential sources of water pollution.

ACKNOWLEDGEMENTS

The authors would like to thank the Instituto Nacional de Pesquisas da Amazônia (INPA), the Programa de Grande Escala da Biosfera‑Atmosfera na Amazônia (LBA) and the Instituto Federal de Educação, Ciência e Tecnologia do Amazonas (IFAM) for the opportunity for scientific qualification, logistical support, technical material and consumables for the development of the research. This article is the result of the Research and Development (P&DI) project “IETÉ”, which has been funded by Samsung, using resources from the Information Technology Law for the Western Amazon (Federal Law N^o 8.387/1991), and its disclosure is in accordance with Article 39 of Decree N^o 10.521/2020”. This funding is an INPA/SAMSUNG partnership.

DATA AVAILABILITY STATEMENT

Research data is available in the body of the article.

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Edited by

• Editor in-Chief:
  Adilson Pinheiro

• Associated Editor:
  Stephan Fuchs

Publication Dates

• Publication in this collection
  29 Sept 2025
• Date of issue
  2025

History

• Received
  20 Jan 2025
• Reviewed
  13 May 2025
• Accepted
  26 June 2025