The quality of surface water depends on many factors, and may undergo hydroclimatic influences such as natural weathering and environmental changes caused by anthropic actions1. The various environmental factors that modify the water quality of a given region are related to the geological structure, the mineralogy of the watersheds and the way in which the use and occupation of the soil occurs in the surroundings of these areas 23456789101112.

Household and industrial effluents generated from human activities may contain hazardous chemicals that can cause major impacts on aquatic ecosystems. These substances can last a long time in the environment and have the ability to percolate in the soil and reach the lower layers of the subterranean aquifers, for this reason, soil geochemistry has a great influence on water pollution/contamination13.

In recent decades there has been a great increase in the levels of toxic chemicals in the environment, this event is a consequence of the lack of planning by the municipalities and industries installed in relation to the way the effluents are released in the water bodies generating immense release of compounds Harmful to animal and plant life 141516.

The Barcarena City in the Pará State-Brazil has in its territory an important industrial pole that works in the processing and export of kaolin, alumina, aluminum, cables for transmission of electric energy among other activities. Approximately 40,000 hectares were used for the installation of industries and the local population that previously used natural resources for their livelihoods, such as hunting and fishing, had to adopt other productive bases for their economic survival17. The local economy has a traditional base in agriculture, but also advances with tourism beyond industrial activities, generating economic growth18. The presence of these industries causes the rivers to become targets of recorrent environmental accidents, for example, the spillage of red mud in the springs of the Murucupí river in April 2009, a product from the beneficiation of bauxite that caused great environmental damages and to the local population19.

The launch of untreated sanitary sewage and the recorrent environmental accidents have damaged the quality of the waters of that region. In this sense, the Murucupí River was chosen, which runs through the industrial area of Vila do Conde and urban agglomerates and the Arienga River, which is located in the territorial boundary between the cities of Barcarena and Abaetetuba, Pará State-Brazil, a little further away from industrial area.

This study, the water temperature of the Arienga and Murucupi rivers ranged from 25 to 31 °C and from 27 to 32 °C, respectively, with average variations from 26 to 30 °C. The pH values in this study ranged from 4.75 to 7.33 in the Arienga river and from 5.51 to 7.3 in the Murucupi river, with means ranging from 5.55 to 6.71.

The electrical conductivity and total dissolved solids (TDS) determined in the rivers Arienga and Murucupí indicated a difference in its characteristics physicochemical, with values ranging from 4 to 49 µS.cm^-1 and 25 to 122 µS.cm^-1 e from 2 to 26 mg.L^-1 and 12 to 57 mg.L^-1 respectively, the mean electrical conductivity and TDS in the Arienga river were 21 to 25 µS.cm^-1 and 11 to 13 mg.L^-1 and Murucupi River from 38 to 83 µS.cm^-1 and 19 to 38 mg.L^-1.

The DO concentrations in the Arienga and Murucupi rivers ranged from 4.40 to 7.98 mg.L^-1 and from 1.17 to 6.55 mg.L^-1 respectively, with averages from 5.21 to 6.29 mg.L^-1 in the Arienga River and from 3.86 to 4.70 mg.L^-1 in the Murucupi River. The BOD concentrations in the Arienga and Murucupi rivers ranged from 5 to 19 mg.L^-1 and from 6 to 26 mg.L^-1 respectively, with averages ranging from 9 to 15 mg.L^-1 in the two rivers.

The ammonia nitrogen, nitrite and nitrate concentrations in the Arienga and Murucupi rivers ranged from 0.014 to 6.336 mg.L^-1, 0.003 to 0.334 mg.L^-1 and 0.009 to 4.818 mg.L^-1 and from 0.021 to 2.021 mg.L^-1, 0.001 to 0.406 mg.L^-1 and 0.025 to 4.201 mg.L^-1 respectively, with averages from 0.062 a 1.496 mg.L^-1 (ammonia nitrogen), 0.019 to 0.080 mg.L^-1 (nitrite) and 0.075 to 0.679 mg.L^-1 (nitrate) in the Arienga River and from 0.248 to 0.392 mg.L^-1 (ammonia nitrogen), 0.047 to 0.121 mg.L^-1 (nitrite) and 0.791 to 1.247 mg.L^-1 (nitrate) in the Murucupi River.

The phosphate concentrations in the Arienga and Murucupi rivers ranged from 0.014 to 0.239 mg.L^-1 and from 0.056 to 0.313 mg.L^-1 respectively, with averages from 0.064 to 0.117 mg.L^-1 in the Arienga River and from 0.109 to 0.187 mg.L^-1 in the Murucupi River. The sulfate concentrations in the Arienga and Murucupi rivers ranged from 0.011 to 4.886 mg.L^-1 and from 0.897 to 5.367 mg.L^-1 respectively, with averages from 0.812 to 1.347 mg.L^-1 in the Arienga River and from 1.743 to 2.474 mg.L^-1 in the Murucupi River. The highest concentrations of total hardness in the Arienga and Murucupí rivers occurred in the dry period, with variations from 0.232 to 14.389 mg.L^-1 and from 0.112 to 18.568 mg.L^-1 respectively, with averages from 2.823 to 5.824 mg.L^-1 in the Arienga River and from 3.101 to 8.769 mg.L^-1 in the Murucupi River.

From the analytical data obtained in this study it was possible to compare water quality of the Arienga and Murucupí rivers through groups formed by principal components in correlation matrices by degree of similarity.

The physicochemical results of the rainy season for ebbing and flooding tide conditions provided the formation of 3 well defined groups as observed in Figure 2A, that is, in PC1 (43.5%), there was an optimal separation between groups 1 and 3 , in which most of the sampling points of the rivers Arienga and Murucupí are observed. Similar characteristics were observed in relation to group 2 formed in PC2 (20.4%) by sampling points close to the mouths of the Arienga and Murucupí rivers. In general, there is a transition in the physical-chemical characteristics of these waters between the sampling points in those of the extensions evaluated.

In Figure 2B, referring to factorial load analyzes for two components, it was evidenced that the levels of dissolved oxygen and BOD represented the main factors for the formation of group 1 in the river Arienga, in this river a homogeneous characteristic was observed for BOD with higher values At the ends (springs and mouth) of the evaluated and heterogeneous extension to DO with increasing levels from the sources to its mouth in the Pará river, this behavior is probably related to several factors such as the presence and decomposition of organic matter introduced in the extension evaluated by other tributary water bodies, miscegenation and contribution of leachate materials in these waters due to the higher rainfall intensity in the seasonal period in question and the larger aeration area in its mouth.

In group 2, the physicochemical variables that had the same behavior were pH, temperature, DO, BOD and Hardness, therefore, the two rivers presented the same characteristics in their mouths for these variables. The variables that contributed most to the formation of group 3 were: electrical conductivity, total dissolved solids, ammoniacal nitrogen, nitrite, nitrate, sulfate and phosphate, showing that there is interference in the water quality of the river Murucupí by pollutants characteristic of domestic sewage originating from the communities located around this water body, especially at the sampling points closest to its sources. The environmental impacts caused by the spillage of chemical wastes in these waters over the years, can also have their degree of contribution in the water quality conditions of the Murucupí River from the release gradual of residues from the bottom sediment.

Similar to the rainy season, the PC1 (43.4%) of Figure 3A (dry season) shows the formation of three groups (1, 2 and 3). Groups 1 and 2 presented similarity of 39.79% and were formed by most of the sampling points of the Arienga river, this time, in group 2, the sampling points located at the mouth of the river Arienga (AE6, AF5 and AF6) (ME1 and MF1), evidencing well heterogeneous characteristics in the waters of the Murucupí River, probably due to the lower influence of the rainfall in this seasonal period and consequently the greater concentration and dispersion of the variables analyzed.

In the analyzes of factorial loads for two components on the data of the dry season (Figure 3B), it was observed that there was greater influence of the variables OD, BOD and ammonia nitrogen for the formation of group 1. In group 2, there was a greater influence of the BOD variable in the sampling points of the mouth of the river Arienga (AE5, AE6, AF4, AF5 and AF5) and of the nitrate and phosphate variables at the point of the Murucupí sources (ME1 and MF1). Group 3 repeated the same behavior of rainy season, where it was evidenced changes in concentration of variables, most likely by the influence of domestic sewage, a situation observed in field work through the visualization of emissaries (pipes) of wastewater located on the banks of the Murucupí River.

Table 2 and Table 3 are the descriptive statistics (mean, standard deviation, maximum values and minimum values) applied to the values of the physicochemical variables determined in the rivers Arienga and Murucupí respectively, in the rainy (February and May) and dry (August and November) in ebbing and flooding tide conditions in 2011.

The more acidic pH values in surface water are due to the influence of soil leaching and riparian forest, which through organic decomposition releases humic and fulvic acids to these aquatic environments 35363738. In the Caeté (Bragança) and Arari (Marajó Island) rivers in the Pará State-Brazil and other Amazonian rivers such as Solimões and Purus, in the Amazonas State, the values of pH and temperature were similar to those recorded in the rivers Arienga and Murucupí, with variations of 3.8 to 7.0 and 25 to 34 °C respectively 30394038. The temperature of the surface waters in the Amazonian environments oscillates according to the thermal amplitude of daily and seasonal, being able to vary according in the depth of the river and time of day, with possibility of affecting the characteristics physical, chemical and biological of the aquatic ecosystem38(Alves et al., 2012). The study in the water bodies of Malawi in East Africa pointed to the influence of industrial effluent discharges, with pH values between 6.3 and 7.5 40, similar to the surface waters of the Murucupí River, which were also impacted by domestic and industrial sewage 41.

The highest values of these variables were determined in the Murucupí River, indicating a higher presence of dissolved solids, possibly due to greater anthropic influence through the launching of sewage and deforestation in areas near to its banks. The transport of solid materials in different aquatic environments influences in the rates and types of chemical, biological processes and geomorphic transformations4243. As example of these different characteristics, it is observed the values of electrical conductivity in the waters of the Solimões and Purus rivers and their tributaries which had variations from 0.02 to 26.4 μS.cm^-1 in the black waters and from 47 to 122.4 μS.cm^-1 in the white waters31. In the Maracanã River, northeast Pará State-Brazil, TDS values ranged from 10 to 28 mg.L^-1 in distinct seasonal periods and in the Madeirinha and Acará rivers on the left bank of the Madeira River in Amazonas State, this variation was 14.2 to 74.2 mg.L^-1 4445.

The concentrations of DO were higher in the waters of the Arienga River, while the Murucupí River presented less oxygenated water, a fact that can be explained by the greater amount of organic matter in this river due to the release of domestic sewage, biochemical decomposition of organic matter and respiration by the aquatic ecosystem contribute to the reduction of dissolved oxygen in water 46. Other studies corroborate this information, for example, DO values in water bodies located in the island of Marajó-PA ranged from 2.98 to 5.29 mg.L^-1 in the rainy season and 2.59 to 5.88 mg.L^-1 in the dry season, the authors consider that the dynamics of this gas in aquatic ecosystems is related to its non-conservative behavior, for example, the decrease of water levels of the rivers in the dry season and the introduction of semidiurnal tides favors the increase of the concentration of dissolved oxygen in surface waters38. Similar values of DO were also recorded in other rivers in Brazil, such as the case of the Cachoeira River in the Bahia State, with values between 4.1 and 7.7 mg.L^-1 and higher values in the Turvo Limpo River in Minas Gerais State, that oscillated between 7.7 and 9.1 mg.L^-1 4748.

The Arienga and Murucupí rivers had a similar distribution in relation to the BOD concentration, the highest records occurred in the rainy season, this fact may be related to the greater amount of organic matter introduced through resuspension processes or by action of the high tides that flooding the river banks, as well as the discharge of domestic sewage in the region 38. The concentrations of BOD in the rivers Arienga and Murucupí had similar behavior to the waters of the Parauapebas River, southeastern Pará State-Brazil, the BOD values varied 2.2 and 9.79 mg.L-1 with largest records at points closest to the urban center where the discharges of effluent are more intense 49. Turvo Limpo River in Minas Gerais State presented higher values with ranging from 13.3 to 411 mg.L-1, for the authors of this study, high BOD values may be related to the presence of large amounts of biodegradable biomass and discharge of domestic effluents in the hydrographic basin 48.

The Arienga River presented a distinct behavior in relation to the maximum values of ammonia nitrogen, nitrite and nitrate, with higher concentrations observed in the dry season, while in the Murucupí river, the maximum values were similar between the seasonal periods. The Arienga River may have been influenced by diffuse sources of natural or anthropogenic origin and the Murucupí River most likely suffered greater influence of domestic sewage besides the particulate material leached through rains. In aquatic ecosystems of the Brazil is perceived great anthropic influence, mainly in the dissolved inorganic forms, with emphasis of ammonia nitrogen and nitrate, a reality observed in this study. Table 4 shows the values of the nitrogen forms determined in the aquatic ecosystems of different regions of Brazil.

Phosphate concentrations were similar in the Arienga and Murucupí rivers, with higher values in the dry period, but without great variations. Values close to those recorded in the present study were observed in other rivers in Brazil, such as the phosphate values recorded in the Tibagi River between 0.02 and 0.35 mg.L^-1, Paraná State51. The concentration of phosphate near the discharge of industrial effluents in the Igarapé Curuperê River in the municipality of Barcarena-Pará reached 0.220 mg.L^-1, with values recorded in the margins of this water body varying between 0.050 mg.L^-1 and 0.175 mg.L^{- 1}. In all evaluated extension of this water body, the variations were from 0.062 mg.L^-1 to 0.430 mg.L^-1 at ebbing tide and from 0.124 mg.L^-1 to 0.290 mg.L^-1 in the flood tide. In the same area of study, in the Igarapé Dendê, values ranging from 0.010 mg.L^-1 to 0.182 mg.L^-1 were recorded at ebb tide and from 0.050 to 0.110 mg.L^-1 in the flooding tide52. Among the main factors responsible for the increase of phosphate in surface water are the releases of domestic and industrial waste, therefore, the mainly responsible for levels of phosphate in aquatic environments47^,51.

The Arienga and Murucupí rivers presented similar concentrations of sulfate in their waters, however, their highest values were recorded in different periods. The highest concentrations in the Arienga river occurred in the dry period, while in the Murucupí river the maximum values were found in the rainy season. Sulphate concentrations in the two rivers were close to those reported by other authors in studies in Brazil, in most cases the presence of sulfate is related to the geology of the sites, such as those observed in rivers affluent to the rivers Solimões and Purus, with averages of 5.2 mg.L^-1 and 7.7 mg.L^-1, while in the Solimões and Purus rivers the averages were 7.1 mg.L^-1 and 5.8 mg.L^{-1 (31)}. Other authors reported variations from 1.30 mg.L^-1 to 26.5 mg.L^-1 in the Guarapiranga system in São Paulo and from 0.00 mg.L^-1 to 12.60 mg.L^-1 in two rivers of the Federal District. Brazil 53^,54.

In another study on the Murucupí river, hardness ranged from 11 mg.L^-1 to 21 mg.L^-1 and according to the author, hardness concentrations are typical of the region and the fact that the highest concentrations are found in the period Dry is related to the intrusion of brackish waters of the Marajó Bay in the Barcarena estuarine system55. In the Água Branca stream, mean values of hardness of 0.40 mg.L^-1 were recorded in the rainy season and of 0.29 mg.L^-1 in the dry period and in the Puraquequara stream, the average found was 0.46 mg.L^{-1 (30)}.

In study in the Ocoí River basin it was used the Principal Component Analysis (PCA), starting from obtained results found that diffuse sources of pollution were more evident and predominant in the rainy season56. The PCA also was used to evaluate the water quality of the Tibagi River in Ponta Grossa, allowed to observe the level of deterioration through point and diffuse sources near the industrial district of the city57. Other study carried in 2007 used this statistical tool to verify if the waters of the Tarumã-Açú River was polluted with the residues of the sanitary landfill near of this hydrographic basin and found that a differentiation in water quality occurred between the points sampled, well as they said that the water body was still able to reduce the impact of the sanitary landfill58. In aquatic systems studied in northern Greece, multivariate analysis helped identify four groups with different physicochemical characteristics and pollution levels59. Likewise, PCA significantly helped in the identification of similar and different quality groups in the extensions of the water bodies evaluated in this study.

The variables studied in these water bodies have their concentrations with well-defined trends in relation to rainfall and variation in physicochemical characteristics in the extensions evaluated, either by the action of natural or anthropogenic agents. This environmental behavior is clearly observed for the variables temperature, pH and dissolved oxygen, which presented very similar characteristics in the two rivers, that is, lower values in their sources and increase in the mouths.

From the multivariate analysis, it can be said that there is a physicochemical difference between the Arienga and Murucupí rivers, where it was possible to observe that the Murucupí River presented higher concentrations for most of the physical-chemical variables analyzed in relation to the Arienga river, indicating that the Arienga River may be more influenced by local geology, while in the Murucupí river both geology and anthropic factors are responsible for its physicochemical characteristics was visualized lower anthropic influence in surrounding of Arienga River, however, demand of the tourists in leisure areas during the dry season may have influenced the levels of nitrogen matter introduced in this water body. The information generated in this study allows us to say that the Murucupí river suffered a greater anthropogenic influence, mainly due to the domestic sewage released in this water body without due treatment, as well as other adjacent contributions that provided different physicochemical characteristics for these waters. The industrial activities developed in this region and other anthropogenic contributions have already caused great damage to certain environmental compartments, among them, the water bodies, through recurrent industrial waste leaks and untreated sewage launching. This information on the physicochemical characteristics of the rivers studied will serve for population, competent authorities and the scientific community in order to take more detailed measures and studies to evaluate the risk factors environmental and population health in region.

Thanks to IEC/FIDESA/MPE-PA Project (Process 001/2007), Evandro Chagas Institute and Federal Rural University of Amazonia for funding, laboratory support, and academic contributions in the surveys. Thanks Dr. Ricardo de Deus for help with the map elaboration.