Headnote
ABSTRACT
The present study aims to evaluate the water quality in the hydrographic sub-region of the Acará River, in the state of Pará, through the Water Quality Index (WQI) and the Trophic State Index (TSI), using physicochemical and biological parameters. The theoretical framework is based on environmental concepts that address the impacts of anthropogenic activities on water sources, with an emphasis on the WQI and TSI as adapted by the Environmental Company of the State of São Paulo (CETESB) and the standards established by CONAMA Resolution No. 357/2005. The adopted methodology involved the analysis of secondary data from the monitoring of the National Water Quality Assessment Program (PNQA) of ANA, with samples collected at six points along the Acará River and its tributaries, during rainy and dry periods. The results indicated that water quality ranged from "good" to "fair", with trophic state values falling between the oligotrophic and mesotrophic ranges, and with increases in turbidity and thermotolerant coliforms during the rainy season, suggesting the influence of untreated sewage discharge and the runoff of sediments and nutrients. The implications of this research highlight the need for investments in basic sanitation and continuous monitoring strategies, contributing to the sustainable management of water resources. This study presents originality by integrating historical data and environmental indicators to support decision-making in water management, demonstrating its potential impact on the development of public policies.
Objective: The objective of this study is to investigate the water quality in the hydrographic sub-region of the Acará River, aiming to evaluate physicochemical and biological parameters through the Water Quality Index (WQI) and the Trophic State Index (TSI), identify possible sources of contamination, and propose environmental management measures for the preservation of water resources.
Theoretical Framework: This study is based on theories related to water quality, the impact of anthropogenic activities, and water resource management. It highlights the concepts of WQI and TSI, as adapted by CETESB, and the regulatory guidelines of CONAMA Resolution No. 357/2005, which provide a robust foundation for the assessment of water sources.
Method: The research adopts a quantitative approach using data from the PNQA by ANA. Seasonal sampling was conducted at six points distributed along the Acará River and its tributaries, analyzing nine parameters (dissolved oxygen, thermotolerant coliforms, pH, turbidity, BOD, total nitrogen, total phosphorus, and total solids). Sampling followed the procedures described in the Standard Methods for the Examination of Water and Wastewater and ABNT standards (NBR 9898). The WQI and TSI were calculated using a weighted product of parameter values and statistical formulas, applying the weights defined by CETESB.
Results and Discussion: The results showed that water quality in the sub-region ranged from "good" to "fair," with lower indices in areas close to urban zones and higher sewage discharge, while the trophic state remained within the oligotrophic to mesotrophic ranges. Seasonality played a significant role, with higher turbidity and thermotolerant coliform concentrations observed during the rainy season.
Research Implications: The implications of this research are significant for water resource management, as the findings can support public sanitation policies and environmental monitoring strategies. The data indicate the urgent need for investments in wastewater infrastructure and the implementation of sustainable management practices, with positive impacts on environmental management, urban planning, and public health.
Originality/Value: This study contributes to the literature by integrating historical environmental monitoring data with the application of the WQI and TSI, offering an innovative analysis of water quality in the Acará River subregion. The adopted approach allows for the identification of seasonal trends and sources of contamination, adding value to the development of water management strategies and the formulation of environmental policies.
Keywords: Water Quality, Water Quality Index (IQA), Trophic State Index (TSI), Acará River, Environmental Monitoring, Basic Sanitation, Water Resource Management.
RESUMO
O presente estudo tem como objetivo avaliar a qualidade da água na sub-região hidrográfica do Rio Acará, no estado do Pará, por meio do Índice de Qualidade da Água (IQA) e Índice de Estado Trófico (IET), utilizando parâmetros físico-químicos e biológicos. O referencial teórico baseia-se em conceitos ambientais que abordam os impactos das atividades antrópicas nos mananciais, com ênfase no IQA e IET conforme adaptado pela Companhia Estadual de Tecnologia de Saneamento Básico e de Controle de Poluição das Águas (CETESB) e na normatização estabelecida pela Resolução CONAMA n 357/2005. A metodologia adotada compreendeu a análise de dados secundários do monitoramento do Programa Nacional de Avaliação da Qualidade das Águas (PNQA) da ANA, com coletas realizadas em seis pontos ao longo do Rio Acará e seus afluentes, em períodos chuvosos e menos chuvosos. Os resultados indicaram que a qualidade da água variram entre "boa" e "regular", com os valores de estado trófico se concentrando nas faixas oligotrófica a mesotrófica, com elevações de turbidez e coliformes termotolerantes em períodos de maior chuva, sugerindo a influência do despejo de esgoto não tratado e do carreamento de sedimentos e nutrientes. As implicações desta pesquisa apontam para a necessidade de investimentos em saneamento básico e em estratégias de monitoramento contínuo, contribuindo para a gestão sustentável dos recursos hídricos. Este estudo apresenta originalidade ao integrar dados históricos e indicadores ambientais para fundamentar a tomada de decisão em gestão hídrica, evidenciando seu potencial impacto na elaboração de políticas públicas.
Objetivo: O objetivo deste estudo é investigar a qualidade da água na sub-região hidrográfica do Rio Acará, com o intuito de avaliar os parâmetros físico-químicos e biológicos por meio do Índice de Qualidade da Água (IQA) e o Índice de Estado Trófico (IET), identificar possíveis fontes de contaminação e propor medidas de gestão ambiental para a preservação dos recursos hídricos.
Referencial Teórico: Neste trabalho, fundamenta-se a pesquisa em teorias sobre qualidade da água, impacto das atividades antrópicas e gestão de recursos hídricos. Destacam-se os conceitos do IQA e IET, adaptado pela CETESB, e as diretrizes normativas da Resolução CONAMA n 357/2005, que fornecem uma base robusta para a avaliação dos mananciais.
Método: A pesquisa adota uma abordagem quantitativa, utilizando dados do PNQA da ANA. Foram realizadas coletas sazonais em seis pontos distribuídos ao longo do Rio Acará e seus afluentes, com a análise de nove parâmetros (oxigênio dissolvido, coliformes termotolerantes, pH, turbidez, DBO, nitrogênio total, fósforo total e sólidos totais). A coleta seguiu os procedimentos descritos no Standard Methods for the Examination of Water and Wastewater e as normas ABNT (NBR 9898). O IQA e o IET foram calculados por meio de um produtório ponderado dos valores dos parâmetros e fórmulas matemáticas e estatísisticas, utilizando os pesos definidos pela CETESB.
Resultados e Discussão: Os resultados obtidos revelaram que a qualidade da água na sub-região varia entre "boa" e "regular", com índices mais baixos em pontos próximos a áreas urbanizadas e com maior produção de esgoto enquanto o estado trófico se concentrando nas faixas oligotrófica a mesotrófica. A sazonalidade teve papel relevante, evidenciando maiores concentrações de turbidez e coliformes termotolerantes no período chuvoso.
Implicações da Pesquisa: As implicações desta pesquisa são significativas para a gestão de recursos hídricos, pois os resultados podem subsidiar políticas públicas de saneamento e estratégias de monitoramento ambiental. Os dados indicam a necessidade de intervenções em infraestrutura de esgoto e práticas de manejo sustentável, impactando positivamente setores de gestão ambiental, planejamento urbano e saúde pública.
Originalidade/Valor: Este estudo contribui para a literatura ao integrar dados históricos de monitoramento ambiental com a aplicação do IQA e IET, oferecendo uma análise inovadora da qualidade da água na sub-região do Rio Acará. A abordagem adotada permite identificar tendências sazonais e fontes de contaminação, agregando valor ao desenvolvimento de estratégias de gestão hídrica e à formulação de políticas ambientais.
Palavras-chave: Qualidade da Água, Índice de Qualidade da Água (IQA), Índice de Estado Trófico (IET), Rio Acará, Monitoramento Ambiental, Saneamento Básico, Gestão de Recursos Hídricos.
RESUMEN
El presente estudio tiene como objetivo evaluar la calidad del agua en la subregión hidrográfica del río Acará, en el estado de Pará, mediante el Índice de Calidad del Agua (ICA) y el Índice del Estado Trófico (IET), utilizando parámetros fisicoquímicos y biológicos. El marco teórico se basa en conceptos ambientales que abordan los impactos de las actividades antrópicas en los manantiales, con énfasis en el ICA y el IET, conforme a la adaptación realizada por la Compañía Estatal de Tecnología de Saneamiento Básico y Control de la Contaminación del Agua (CETESB) y a la normativa establecida por la Resolución CONAMA n. 357/2005. La metodología adoptada comprendió el análisis de datos secundarios del monitoreo del Programa Nacional de Evaluación de la Calidad del Agua (PNQA) de la ANA, con muestreos realizados en seis puntos a lo largo del río Acará y sus afluentes, en períodos lluviosos y de menor precipitación. Los resultados indicaron que la calidad del agua varió entre "buena" y "regular", con los valores del estado trófico concentrados en las categorías oligotrófica a mesotrófica, con aumentos en la turbidez y en los coliformes termotolerantes durante los períodos más lluviosos, lo que sugiere la influencia de vertidos de aguas residuales sin tratamiento y del arrastre de sedimentos y nutrientes. Las implicaciones de esta investigación apuntan a la necesidad de invertir en saneamiento básico y en estrategias de monitoreo continuo, contribuyendo así a la gestión sostenible de los recursos hídricos. Este estudio presenta originalidad al integrar datos históricos e indicadores ambientales para fundamentar la toma de decisiones en la gestión hídrica, evidenciando su potencial impacto en la elaboración de políticas públicas.
Objetivo: El objetivo de este estudio es investigar la calidad del agua en la subregión hidrográfica del río Acará, con el fin de evaluar los parámetros fisicoquímicos y biológicos mediante el Índice de Calidad del Agua (ICA) y el Índice del Estado Trófico (IET), identificar posibles fuentes de contaminación y proponer medidas de gestión ambiental para la preservación de los recursos hídricos.
Marco Teórico: Esta investigación se fundamenta en teorías relacionadas con la calidad del agua, los impactos de las actividades antrópicas y la gestión de los recursos hídricos. Se destacan los conceptos del ICA y del IET, adaptados por la CETESB, así como las directrices normativas de la Resolución CONAMA n. 357/2005, que proporcionan una base sólida para la evaluación de los cuerpos de agua.
Método: La investigación adopta un enfoque cuantitativo, utilizando datos del PNQA de la ANA. Se realizaron muestreos estacionales en seis puntos distribuidos a lo largo del río Acará y sus afluentes, analizando nueve parámetros (oxígeno disuelto, coliformes termotolerantes, pH, turbidez, DBO, nitrógeno total, fósforo total y sólidos totales). La recolección de muestras siguió los procedimientos descritos en el Standard Methods for the Examination of Water and Wastewater y las normas de la ABNT (NBR 9898). El ICA y el IET fueron calculados mediante un producto ponderado de los valores de los parámetros y fórmulas matemáticas y estadísticas, utilizando los pesos definidos por la CETESB.
Resultados y Discusión: Los resultados obtenidos revelaron que la calidad del agua en la subregión varía entre "buena" y "regular", con índices más bajos en puntos cercanos a zonas urbanizadas y con mayor generación de aguas residuales, mientras que el estado trófico se concentró en las categorías oligotrófica a mesotrófica. La estacionalidad tuvo un papel relevante, evidenciando mayores concentraciones de turbidez y coliformes termotolerantes durante el período lluvioso.
Implicaciones de la investigación: Las implicaciones de esta investigación son significativas para la gestión de los recursos hídricos, ya que los resultados pueden servir de base para el diseño de políticas públicas de saneamiento y estrategias de monitoreo ambiental. Los datos indican la necesidad urgente de intervenciones en la infraestructura de saneamiento y la implementación de prácticas de manejo sostenible, con impactos positivos en la gestión ambiental, la planificación urbana y la salud pública.
Originalidad/Valor: Este estudio contribuye a la literatura al integrar datos históricos de monitoreo ambiental con la aplicación del ICA y del IET, ofreciendo un análisis innovador de la calidad del agua en la subregión del río Acará. El enfoque adoptado permite identificar tendencias estacionales y fuentes de contaminación, agregando valor al desarrollo de estrategias de gestión hídrica y a la formulación de políticas ambientales.
Palabras clave: Calidad del Agua, Índice de Calidad del Agua (IQA), Índice del Estado Trófico (IET), Río Acará, Monitoreo Ambiental, Saneamiento Básico, Gestión de Recursos Hídricos.
(ProQuest: ... denotes formulae omitted.)
1 INTRODUCTION
Water is a natural resource essential for sustaining life and human activities, being fundamental for agriculture, industry, energy generation, and human consumption (Melodia & Oliveira, 2022). In the Brazilian context, water sources play an even more important role, considering the country's extensive hydrographic network and dependence on water resources for various economic and social activities. However, the state of water has been compromised by natural factors and, mainly, by anthropogenic actions, such as the discharge of domestic and industrial effluents, in addition to the inappropriate use of soil and natural resources (Zhang et al., 2021; Uddin et al., 2023).
Good water parameters are also an important factor for maintaining aquatic ecosystems and human health. In this sense, water quality assessment becomes an essential tool for the sustainable management of this resource, allowing the identification of sources of contamination and the proposal of mitigation and preservation measures (Maranhão et al., 2017; Siddique et al., 2022). In Brazil, environmental legislation establishes guidelines for the classification of water bodies and their standards of excellence, with CONAMA Resolution No. 357/2005 being one of the main regulatory references for the verification and classification of surface waters in the country (BRAZIL, 2005).
The Acará River hydrographic sub-region, located in the state of Pará, plays a fundamental role in supplying local communities covering municipalities such as Acará, Tailândia, and Tomé- Açu. In addition to the municipal headquarters, several rural communities also depend on this natural resource for their daily activities (FAPESPA, 2023). To assess these conditions, environmental indicators such as the Water Quality Index (WQI) and Trophic State Index (TSI) have been widely used in environmental studies, allowing for a quantitative analysis of water status based on physical-chemical and biological parameters (CETESB, 2020).
The WQI is an internationally consolidated index, originally developed by the National Sanitation Foundation (NSF) and later adapted to Brazilian conditions by the São Paulo State Environmental Company (CETESB). It is an indicator that assesses the condition of a water body based on nine physical-chemical and biological parameters, such as dissolved oxygen, pH, and coliforms, generating a value between 0 and 100. it was developed to facilitate understanding of water quality, especially for public supply purposes, while the Trophic State Index (TSI) measures the degree of eutrophication of water bodies, that is, the level of enrichment by nutrients, mainly phosphorus, and its effects, such as excessive algae growth. The higher the TSI, the greater the environmental risk associated with the loss of water quality (CETESB, 2019).
Given this context, this study aims to assess water quality in the Acará River hydrographic sub-region, using the Water Quality Index (WQI) and the Trophic Status Index (TSI) based on physical-chemical and biological parameters by consulting records from the National Water Quality Assessment Program (PNQA) of the National Water Agency (ANA) for the year 2024. In addition, it seeks to identify possible sources of contamination and propose environmental management measures for the preservation of water resources in the region.
2 THEORETICAL REFERENCE
2.1 STUDY AREA
The Acará River hydrographic sub-region is located in the state of Pará, in northern Brazil, within the Western Northeast Atlantic Hydrographic Region. The area covers the municipalities of Acará, Aurora do Pará, Bujaru, Concórdia do Pará, Ipixuna do Pará, Moju, São Domingos do Capim, Tailândia, and Tomé-Açu (ANA, 2018; IBGE, 2015). The sub-region has an area of 13,495.24 km2, corresponding to 1.08% of the territory of the state of Pará (SEMAS, 2020).
The region is characterized by a diversity of economic activities, with emphasis on agriculture, livestock, and extractivism. According to data from IBGE (2018), cassava is the main temporary crop in the region, with a production of 373,809 tons/year in the municipality of Acará, while corn stands out in Tailândia, with 54,756 tons/year. In permanent crops, açaí and bananas are the main products, with production of 42,469 tons/year and 6,000 tons/year, respectively. Livestock farming is also important, particularly cattle farming in Tomé-Açu, which has a herd of 99,534 head/year, and poultry farming in Acará, with 21,413 head/year (IBGE, 2018).
The climate of the sub-region is classified as Af, according to the Köppen climate classification, corresponding to a humid tropical climate, with an average annual temperature of 26°C and average annual rainfall of 2,990.15 mm. The rainy season occurs between December and May, with an accumulated rainfall of 1,823.61 mm, while the least rainy period occurs from June to November, with 465.36 mm of rainfall (Dias, 2019). The region has a welldefined seasonality, with intense rains in the wet season and a shorter dry season, which directly influences the dynamics of water resources and water quality (SEMAS, 2020).
2.2 SOILS AND HYDROGRAPHY
The soils of the sub-region are mainly of the Yellow Latosol type, representing 67.34% of the total area. This soil is characterized by being very weathered, with low base saturation and high aluminum saturation, presenting varied textures between very clayey, clayey, and medium. Other types of soils present include Haplic Gleissolo, found in floodplain areas subject to periodic flooding, representing 4.74% of the area, and areas of continental water bodies, which correspond to 27.92% (SEMAS, 2020).
The soil cover in the sub-region is dominated by secondary vegetation, resulting from the natural regeneration process after agricultural and livestock activities, covering 37.4% of the area. Agriculture occupies about 3.31%, while livestock farming corresponds to 8.21%. Dense Lowland Rainforests represent 25.22% of the area, highlighting the importance of preserving primary forest areas (SEMAS, 2020).
The Acará hydrographic sub-region borders the Capim, Guamá, and Moju hydrographic subregions and is drained by the Acará River, which runs approximately 400 km from its source in Ipixuna do Pará to its mouth on the Guamá River. The main tributaries of the Acará River include the Acará-mirim River, the Urucuré River, the Aju-Açu River, and the Turiaçu stream (SEMAS, 2020). The land cover in the region is predominantly composed of secondary vegetation and continental water bodies, with areas of agriculture and livestock distributed throughout the basin (SEMAS, 2020).
The hydrographic network of the sub-region is mainly composed of the Acará River, which is about 400 km long, originating in the municipality of Ipixuna do Pará and flowing into the Moju River. Other relevant water bodies include the Acará-Mirim River, the Urucu É River, the Aju-Açu River, and the Turiaçu stream. Hydrography plays an essential role in the drainage and maintenance of local ecosystems, directly influencing water quality and land use in the region (SEMAS, 2020).
3 METHODOLOGY
3.1 WATER QUALITY INDEX (WQI)
The Water Quality Index (WQI) is a widely used indicator for assessing the quality of surface and groundwater intended for human consumption, public supply, and environmental conservation. This index was originally developed by the National Sanitation Foundation (NSF) in the United States and later adapted to Brazilian conditions by the São Paulo State Environmental Company (CETESB) (CETESB, 2020). The IQA is based on the analysis of nine physical-chemical and microbiological parameters, which are weighted according to their relevance to water quality.
The nine parameters used in the calculation of the IQA are:
a) Dissolved oxygen (DO): Indicator of the capacity to support aquatic life and the presence of organic pollutants;
b) Hydrogen ion potential (pH): Measures the degree of acidity or alkalinity of the water.
c) Thermotolerant coliforms: Represent fecal contamination and the risk of waterborne diseases;
d) Biochemical oxygen demand (BOD): Measures the amount of biodegradable organic matter present in the water.
e) Water temperature: Influences the solubility of gases and biological activity in the aquatic environment.
f) Total nitrogen: Related to pollution from sewage and agricultural activities.
g) Total phosphorus: Indicates the presence of nutrients that can stimulate eutrophication.
h) Turbidity: Measures the presence of suspended particles that can affect water transparency.
i) Total solids: Represents the amount of substances dissolved or suspended in water.
Each of these parameters is given a specific weight in the calculation of the IQA, according to the CETESB methodology (Table 1).
The IQA is calculated using the weighted product equation of the individual parameter values, expressed as follows:
... (1)
where:
IQA = Water Quality Index, a number between 0 and 100;
qi = quality of the i-th parameter, a number between 0 and 100, obtained from the respective "average quality variation curve," depending on its concentration or measurement;
wi = weight corresponding to the i-th parameter, a number between 0 and 1, assigned according to its importance to the overall quality conformance;
n = number of variables included in the IQA calculation;
After applying the equation, the final IQA value is interpreted according to the scale in Table 2:
3.2 TROPHIC STATE INDEX (TSI)
The Trophic Status Index (TSI) is a widely used indicator to classify water bodies according to their trophic level, i.e., the degree of nutrient enrichment and its impact on the excessive growth of algae and cyanobacteria. This index is based on two main parameters: total phosphorus (P-total) and chlorophyll-a (CL), whereas water transparency, measured by the Secchi disk, is not considered in lotic environments due to the interference of turbidity caused by suspended mineral material.
The two parameters used in the calculation of the IET are:
a) Total phosphorus (µg/L): Considered the main limiting agent in the eutrophication process;
b) Chlorophyll-a (µg/L): Indicator of phytoplankton biomass present in the environment.
The IET is calculated using specific logarithmic equations for total phosphorus and chlorophyll-a. The equations adopted for rivers are as follows:
...
where:
TSI = Trophic State Index;
CL = chlorophyll-a concentration (µg/L);
PT = total phosphorus concentration (µg/L);
ln = natural logarithm.
After applying the equations, the final TSI is obtained by the arithmetic mean of the values of the Total Phosphorus and Chlorophyll-a Trophic State Index; it can be interpreted according to the scale in Table 3:
3.3 COLLECTION AND ANALYSIS
Sample collection and analysis follow the guidelines established in the 23rd edition of Standard Methods for the Examination of Water and Wastewater (APHA, 2017) and the standards of the Brazilian Association of Technical Standards (ABNT, 1987). The action plan adopted comprises:
a) Seasonal collections: Quarterly monitoring campaigns were carried out during the rainy season (January to April) and the less rainy season (July to November).
b) Sampling points: Representative points were selected along the Acará River, covering different environmental conditions and levels of anthropic impact.
c) Sample preservation: Samples were stored in sterilized bottles, kept refrigerated, and analyzed in an accredited laboratory.
d) Instrumentation: Use of a YSI EXO1 multiparametric probe for in situ measurement of parameters such as DO, pH, turbidity, and temperature.
The selection of collection points was based on the environmental representativeness of the Acará River hydrographic sub-region, considering areas of greater anthropic influence, such as urban areas and regions with agricultural and industrial activity, as well as areas of lesser impact for comparative purposes. According to the Report on Surface Water Quality Monitoring in the State of Pará, prepared by the State Secretariat for the Environment and Sustainability (SEMAS), six monitoring points distributed along the Acará River and the Acará- Mirim River were used for retrospective assessment, covering the municipalities of Tailândia, Acará, and Tomé-Açu (SEMAS, 2020).
The sampling points for past data are georeferenced and identified through the QUALIÁGUA Program, using the National Water Quality Monitoring Network (RNQA) of the National Water Agency (ANA). The geographic coordinates of the collection points are detailed in Table 4.
The methodology for applying the WQI in the Acará River hydrographic sub-region followed the same criteria established by CETESB, as well as CONAMA guidelines, resolution No. 357/2005 (present in Table 5).
4 RESULTS AND DISCUSSIONS
The results of the study reveal that water quality in the Acará River hydrographic subregion varies significantly between different collection points and between rainy and dry periods, especially at points located in more populated areas near the cities of Tomé-açu and Acará, namely Campina and Porto da Balsa, respectively.
4.1 DISSOLVED OXYGEN (DO)
Dissolved oxygen (DO) values ranged from 6.2 mg/l to 6.8 mg/l, indicating that the oxygen concentration in the water is adequate for sustaining aquatic life. The absence of significant variations between the rainiest and driest periods suggests that water oxygenation is not strongly influenced by rainfall (shown in graph 1).
4.2 HYDROGEN ION POTENTIAL (PH)
The pH, in turn, varied between 6.1 and 6.8, remaining within the range considered neutral to slightly acidic, which is suitable for most aquatic organisms. These results indicate that, in terms of oxygenation and acidity, the water in the region is in satisfactory condition (shown in graph 2).
4.3 BIOLOGICAL OXYGEN DEMAND (BOD)
Biochemical oxygen demand (BOD) was low, ranging from 2.0 mg/l to 5.0 mg/l, indicating a low load of biodegradable organic matter in the water. However, at some points, such as Campina, there was a significant increase in the less rainy period (5.0 mg/l), suggesting possible sources of organic contamination during this period (shown in graph 3).
4.4 TOTAL NITROGEN
Total nitrogen ranged from 2.0 mg/l to 3.6 mg/l, with increases observed in locations such as Itinga II and Porto Da Balsa during the rainiest period. These increases may be related to the transport of nutrients from agricultural activities or domestic sewage, highlighting the need for more sustainable management practices (shown in graph 4).
4.5 TOTAL PHOSPHORUS
Total phosphorus values were low, ranging from 0.001 mg/l to 0.028 mg/l. However, in locations such as Ponte do Rio Acará, there was an increase during the least rainy period, indicating possible point sources of phosphorus contamination (shown in graph 5).
4.6 TURBIDITY
Turbidity varied significantly between the rainiest and driest periods, with much higher values in locations such as Campina and Itinga II during the rainiest period, 63.4 NTU and 67.7 NTU, respectively (shown in graph 6). These increases are likely the result of sediment transport during rainfall, which can be mitigated with soil and riparian forest conservation practices.
4.7 TOTAL SOLIDS
Total solids values were relatively low, ranging from 50.0 mg/l to 128.0 mg/l. However, at the Acará River Bridge, there was a significant increase in the less rainy period (128.0 mg/l), suggesting a greater disposal of solids at this location. (shown in graph 7).
4.8 THERMOTOLERANT COLIFORMS
Thermotolerant coliforms showed wide variations, with some points, such as Campina and Porto Da Balsa, showing significant fecal contamination during the rainiest period, respectively, 24,000.0 NMP/100ml and 1,900.0 NMP/100ml (shown in graph 8). These high levels indicate possible discharges of untreated sewage or contamination by animal waste, reinforcing the need for improvements in basic sanitation. This reflects the sanitary conditions of the municipalities of Tomé-Açu, Tailândia, and Acará, which face significant challenges with regard to basic sanitation, especially in sewage treatment.
In Tomé-Açu, only 3.67% of the population has access to the general network or a septic tank connected to the network, while the majority depends on rudimentary septic tanks or holes, without an established Municipal Basic Sanitation Plan (PMSB). The situation in Acará is equally critical, with no record of a public sewage system and only 13.32% of the population served by water supply. In Tailândia, although specific data on sewage systems are limited, the regional reality tes similar challenges. These indicators highlight the need for investment in sanitation infrastructure to reduce environmental impacts and improve the quality of life of the population (ÁGUA E SANEAMENTO, 2024; PGE-PA, 2024).
4.9 WATER QUALITY INDEX (WQI)
The WQI ranged from 50.0 to 84.0, indicating water quality ranging from "fair" to "good." The highest values were observed at Fazenda São Paulo (84.0 in the least rainy period) and Travessia Do Alto Acará (81.0 in the least rainy period), suggesting that these locations have better water quality conditions (shown in graph 9). On the other hand, Campina had the lowest IQA (50.0 in the rainiest period), indicating "fair" water quality, possibly due to high turbidity and contamination by thermotolerant coliforms. At Ponte do Rio Acará, the WQI fell significantly in the less rainy period (52.0), probably due to an increase in total solids and thermotolerant coliforms.
4.10 TROPHIC STATE INDEX (TSI)
The analysis of the Trophic Status Index (TSI) in the Acará River hydrographic subregion, in the state of Pará, shows moderate spatial and seasonal variation in the degree of trophic status of the monitored water bodies. In general, the TSI values are concentrated in the oligotrophic to mesotrophic ranges, with the exception of the Ponte do Rio Acará point, which presented a supereutrophic level (64.1) in the least rainy period (shown in graph 10).
The results indicate that, at many points, eutrophication is enhanced in the less rainy period, as observed at Travessia do Alto Acará, Porto da Balsa, and especially Ponte do Rio Acará. This may be the result of nutrient accumulation from specific sources, such as domestic sewage and agricultural effluents, whose influence is more evident when water flow is reduced.
On the other hand, locations such as Fazenda São Paulo follow the expected behavior, with higher IET in the rainiest period, suggesting that surface runoff carries nutrients to water bodies, intensifying the eutrophication process. The Campina and Itinga II points, on the other hand, showed little variation between periods, which may indicate a situation of relative ecological balance or constant anthropic pressure.
5 CONCLUSION
The results obtained in this study demonstrate that water quality in the Acará River hydrographic sub-region exhibits spatial and temporal dynamics influenced by natural factors, such as rainfall patterns, and by anthropogenic pressures related, above all, to the absence of adequate sanitary infrastructure and agricultural activities. The integrated application of the Water Quality Index (WQI) and the Trophic Status Index (TSI) allowed not only to diagnose the current situation of water bodies, but also to highlight the main sources of contamination, distinguishing areas with satisfactory conditions from those that require corrective actions.
It was found that points such as Fazenda São Paulo and Travessia do Alto Acará presented good water quality conditions, while Campina and Ponte do Rio Acará recorded parameters with potential aggravating factors, especially during the rainy season, reflecting the impact of untreated effluent discharge and sediment and nutrient transport. The high levels of thermotolerant coliforms in some samples highlight the urgency of basic sanitation, especially in locations that lack a sewage network and municipal sanitation management plan to reduce the load of contaminants reaching water bodies.
In summary, this study demonstrates that, although the Acará River hydrographic subregion has areas with good water quality, there are significant challenges to be overcome, especially in relation to sewage contamination and inadequate soil management. The implementation of measures to control point and diffuse sources of contamination, combined with continuous monitoring and active community participation, will be crucial to ensuring the preservation of water resources and the environmental sustainability of the region. In this sense, protecting water quality is not only an environmental issue, but also an essential condition for public health and socioeconomic development in the Acará River sub-region.
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