Application of a shallow water model to analyze environmental effects in the Amazon Estuary Region: a case study of the Guajará Bay (Pará – Brazil)

Increasing urbanization in Brazilian cities and the absence of a proper sanitary infrastructure have contributed to the degradation of the water quality in rivers, lakes, ponds and estuaries that are used as domestic and industrial wastewater receptors as a short-term solution that has been adopted by public agencies for basic sanitation. This action has compromised the water quality not only at the site of sewage disposal but also over the entire hydrographic basin. Although developed countries have addressed this issue with a sustainable solution, given their available sanitary infrastructure and more developed environmental laws, there are numerous current studies on global pollutant dispersion.

For instance, Lopes et al. (2005) modeled the hydrodynamics and water quality of the Ria de Aveiro Lagoon in the Northwest of Portugal. The hydrodynamic pattern consisted of semi-diurnal tides. The following parameters were used to analyze the water quality: dissolved oxygen (DO), biochemical oxygen demand (BOD), ammonia, nitrate, nitrite and phosphate. Cunha et al. (2006) used the coupled model for shallow waters, 2DH, to describe the hydrodynamics and water quality of the Sepetiba Bay in the state of Rio de Janeiro. The DO and BOD were used to simulate the water quality. Lee & Seo (2007) studied pollutant transport in the Han River (South Korea). These authors used a two-dimensional (2D) advection–diffusion model. The velocity fields that were required for the pollutant transport simulation were obtained from a 2D hydrodynamic model. Montaño-Ley et al. (2007) investigated the tidal hydrodynamics of the coastal lagoon system of Topolobampo on the Northwest coast of Mexico. The shallow waters hydrodynamic model was used, and the 2D equations were solved using the finite difference method. The advective–diffusive process was simulated by releasing a hypothetical pollutant in the coastal lagoon. Barros et al. (2011) used computational modeling to analyze the hydrodynamics of the estuary system of the Guajará Bay. The 2D longitudinal-transversal shallow waters model was applied. The results successfully described the flow patterns in the Guajará Bay, and the calculated water levels were comparable to those observed in experimental studies. Periáñez (2012) developed a 2D numerical model to study the behavior of pollutants in the Algeciras Bay in the South of Spain in the Strait of Gibraltar. Hydrodynamic and sediment transport and pollutant dispersion moduli were considered in the model. The results of the hydrodynamic model were combined with tidal current data to evaluate the interaction between water and sediments and the behavior of pollutants in the bay.

Industries and important residential areas in the metropolitan region of Belém are concentrated along the Guajará Bay waterfront. This area contains highly ecologically significant environments, including floodplains and rivers that are used by the local population as economic and food subsistence sources. These water resources are supplied to the city of Belém via a potable water collection system. The long-standing but slow environmental degradation processes in Guajará Bay have been accelerated in recent decades by urban expansion and industrial activities. The disposal of often untreated domestic and industrial wastewater has affected the water quality of the bay, especially the bathing water quality. It has been speculated that the beaches surrounding the city are being polluted by effluent disposal from the municipality of Belém (Barros 2005). A few studies have been conducted on the Guajará Bay area to understand the discharge standards for and the behavior of the constituents in the estuary water bodies in an integrated manner: we cite studies by Pinheiro (1987), Batista (2005), Barros et al. (2011) and Blanco et al. (2013) among these studies.

Further details on the derivation of these equations can be found in Rosman (2001).

The flowrate data of the Belém channels are not well known being difficult to acquire, primarily because the tidal action that reaches its median point or even its head during the high tide. Thus, the flow values were estimated for some of the primary hydrographic basins of the channels in the municipality of Belém. The results for the flowrate and the organic load estimations were obtained from Barbosa & Silva (2002) and are presented in Table 2.

The characteristically high velocities of the Onças Island Channel can be attributed to the acceleration of the flow because of the peculiar bathymetry of the channel. The channel has much deeper regions (Figure 2) than the other regions of the Guajará Bay, thus becoming a preferred flow path. A low-velocity zone originates close to the Port of Belém and advances in the Northern direction, ending at the coastal zone of the city. As the Guamá River flows into the Guajará Bay, the low-velocity stream tends to flow to the East, where it is obstructed by Onças Island and continues a course to the North. Hence, the city waterfront area is protected from the influence of this low-velocity stream.

These figures demonstrate that the pollutant plumes flow from Belém towards the beaches in the North, particularly those located in the Icoaraci and Outeiro districts, thereby affecting the bathing water quality. Thus, the pollution in these areas is not localized as Braz (2003) has suggested. The pollutant plumes also flow very close to the city waterfront because of the low velocities in this region, as was discussed for the hydrodynamics results. Hence, the pollutants tend to remain confined longer in the regions around the waterfront. In these regions, the effects of turbulent diffusion are less intense, reducing pollutant dispersion, because dispersion depends strongly on the local flow velocities. One of the possible applications of the model is to determine the optimum locations and times for pollutant disposal that promote local turbulence action. The model could be used to project future disposal schemes through channels and submarine outfalls to decrease the pollution at the Belém waterfront.

The simulated FC concentrations were comparable to results in the literature. For instance, Ribeiro (2002) presented quantitative data for FCs in the Paracuri stream that were obtained in the wet and dry periods, with average values of 2.5 × 10⁵ and 2.8× 10⁵ in MPN/100 ml, respectively. Berredo et al. (2000) verified that the FC values were on the order of 10⁶ MPN/100 ml at sites in the Guajará Bay (Figure 8).

We simulated the hydrodynamic circulation for a typical neap tide scenario with a flood cycle. These results were used to simulate FC plumes as a function of the time after disposal in the Guajará Bay. The simulated FC concentrations were in satisfactory agreement with values found in the literature. The analysis of the simulated plumes demonstrated that the FCs that are produced in the metropolitan region of Belém flow towards the beaches in the North, especially those in the Icoaraci and Outeiro districts, thereby affecting the bathing water quality. The dispersion of only one species was simulated in the present study. However, hydrodynamic and pollutant dispersion models of the Guajará Bay are available and can be used to simulate other species. The methodology is sufficiently robust and can be applied to other waterbodies to improve water resource management.