Modeling of Coastal Water Contamination in Fortaleza ( Northeast Of Brazil )

An important tool in environmental management projects and studies due to the complexity of environmental systems, environmental modeling makes it possible to integrate many variables and processes, thereby providing a dynamic view of systems. In this study the bacteriological quality of the coastal waters of Fortaleza (Brazil) was modeled considering multiple contamination sources. Using the software SisBaHiA, the dispersion of thermotolerant coliforms and Escherichia coli from three sources of contamination (rivers, storm drains and submarine outfall) was analyzed. The models took into account variations in bacterial decay due to solar radiation and other environmental factors. Fecal pollution discharged from rivers and storm drains is transported westward by coastal currents, contaminating strips of beach water to the left of each storm drain or river. Exception to this condition only occurs on beaches protected by the breakwater of the harbor, where counterclockwise vortexes reverse this behavior. The results of the models were consistent with field measurements taken during the dry and the rainy season. Our results show that the submarine outfall plume was over 2 km from the nearest beach. The storm drains and the Maceió stream are the main factors responsible for the poor water quality on the waterfront of Fortaleza. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Modeling of coastal water contamination in Fortaleza (Northeastern Brazil) S.P. Pereira, P.C.C. Rosman, C. Alvarez, C.A.F. Schetini, R.O. Souza, R.H.S.F. Vieira 1 Company of Water and Wastewater of Ceará (CAGECE) / Federal University of Ceará (UFC), Rua Tomás Lopes 85, Fortaleza, Ceará, Brazil (silvanopereira@terra.com.br). 2 Federal University of Rio de Janeiro (UFRJ), Brazil (pccrosman@ufrj.br). 3 Environmental Hydraulics Institute (IHCantabria), University of Cantabria, Spain 4 Federal University of Pernambuco (UFPE), Brazil 5 Federal University of Ceará (UFC), Brazil Abstract An important tool in environmental management projects and studies due to the complexity of environmental systems,An important tool in environmental management projects and studies due to the complexity of environmental systems, environmental modeling makes it possible to integrate many variables and processes, thereby providing a dynamic view of systems. In this study the bacteriological quality of the coastal waters of Fortaleza (a state capital in Northeastern Brazil) was modeled considering multiple contamination sources. Using the software SisBaHiA, the dispersion of thermotolerant coliforms and Escherichia coli from three sources of contamination (local rivers, storm drains and submarine outfall) was analyzed. The models took into account variations in bacterial decay due to solar radiation and other environmental factors. Fecal pollution discharged from rivers and storm drains is transported westward by coastal currents, contaminating strips of beach water to the left of each storm drain or river. Exception to this condition only occurs on beaches protected by the breakwater of the harbor, where counterclockwise vortexes reverse this behavior. The results of the models were consistent with field measurements taken during the dry and the rainy season. Our results show that the submarine outfall plume was over 2 km from the nearest beach. The storm drains and the Maceió stream are the main factors responsible for the poor water quality on the waterfront of Fortaleza. The depollution of these sources would generate considerable social, health and economic gains for the region.


INTRODUCTION
Coastal waters receive pollution from many sources, including rivers, storm drains, effluent outfall, sewer overflow and diffuse source inputs (EPA, 2013). This may lead to the formation of a visible sewage field near the discharge points, depletion of dissolved oxygen, algae blooms and microbial pollution of bathing water. Pathogenic bacteria and viruses discharged into the sea constitute a potential health risk for bathers, especially in densely populated areas. Uncontrolled and excessive waste disposal tend to create unacceptable levels of seawater pollution, compromising local economic activities and the ecological balance of coastal waters (Esen et al., 2011).
Over the past decades, population and tourism have grown extraordinarily along Brazil's 8,000 km of coastline. With approximately 2.5 million inhabitants, Fortaleza is the fifth-largest city in the country and one of the most important economic and recreational hubs in Northeastern Brazil. During the 1970s, a submarine outfall with a flow capacity of 4.8 m 3 /s was built to protect urban beaches from pollution with untreated sewage, although less than half this capacity is currently attained. Two rivers in the metropolitan region (the Ceará River to the west and the Cocó River to the east) flow into the sea near beaches used for bathing. Thirty-two storm drains along the city's 25 km of waterfront discharge stormwater mixed with untreated domestic sewage and drain a 35 km 2 basin formed by the marine slope of Fortaleza.
In order to protect the marine environment and public health, the impact of the discharge from rivers and storm drains on the quality of bathing water is monitored weekly by the local environmental agency (SEMACE), while the impact of the discharge from the submarine outfall on the quality of water, sediment and biota is monitored biannually by the local water company (CAGECE).  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 Environmental modeling is not only a helpful tool in the monitoring of coastal environments, but may be used to design projects and studies involving environmental management of complex environmental systems. For example, the use of mathematical models makes it possible to obtain the individual effect of a set of sources of bacteriological pollution and identify cause and effect relationships (López et al., 2013), integrating a large number of variables and processes into a dynamic whole and evaluating present and future conditions. Based on hydrodynamics and data on water quality variation, environmental modeling can also be used to optimize sewage disinfection dosages thereby minimizing the impact of undesirable chlorinated disinfection by-products on the marine environment and meeting beach water quality standards (Chan, 2013). In addition, environmental modeling has been successfully used in emergency response situations (Ibid.).
In bacterial modeling, the decay rates of indicator microorganisms are critical for quantifying biological hazards and predicting the distribution of bacterial concentrations. Bacterial decay is affected by a number of environmental factors such as solar radiation, temperature, salinity, adsorption, sedimentation, pH and nutrient deficiency (Yalcin and Muhammetoglu, 2011;Muhammetoglu et al., 2012;Thoe et al., 2012;Feitosa et al., 2013;López et al., 2013;Boye et al., 2014). Solar radiation has been found to be of particular importance in the assessment of the impact of sewage discharged in marine waters (Roberts et al., 2010;Chan et al., 2013;Feitosa et al., 201;Chan et al., 2014). The significance of factors affecting bacterial decay (e.g. solar intensity and temperature) may be expressed in empirical ratios, such as the time required to reduce a bacterial concentration by 90% (T90) (Feitosa et al., 2013b).
The purpose of this study was to make an integrated assessment of the quality of the bathing water along the waterfront of Fortaleza. The distribution of bacterial indicators (thermotolerant coliforms) discharged from three major sources (submarine outfall, rivers and storm drains) was predicted with a depth-averaged 2-D integrated hydro-environmental model, considering local physical parameters and processes of bacterial decay. The model was calibrated with field data on currents and coliform concentrations gathered locally by government monitoring agencies.

METHODS
The resolution of equations involving hydrodynamics and transport of substances requires the establishment of initial and boundary conditions, including realistic values of bathymetry and geometry (Rosman, 2011). The initial conditions used in the hydrodynamic models were zero velocity and free surface elevation, corresponding to the elevation at the initial moment of each model. The boundary conditions were the affluence of rivers to the boundary, free surface elevation and differences in phase and angle.
Oceanographic monitoring over the last 10 years and surveys performed by Occhipinti (1976) prior to the building of the outfall both indicate very small density gradients and no thermohaline stratification in the area, justifying the use of a 2-D model in the present simulation.

Computing Tool
The software SisBaHiA ® (Basic System of Environmental Hydrodynamics) was used to model the hydrodynamics, initial dilution, plume dispersion and bacterial decay in the study area. The tool can simulate hydrodynamic, eulerian or lagrangian transport processes of solutes and sediments in estuarine and coastal waters, model water quality (with up to 11 parameters), wave generation and propagation and to make analyses and predictions of tides. In this study only hydrodynamic and lagrangian transport models were used, the latter combined with the near-field model proposed by Roberts (1979) and Roberts et al. (1989) and the bacterial decay model developed by Mancini (1978). The software and the models were described in detail by Rosman (2011) and Feitosa et al. (2013a). Figure 1 shows the bathymetry of the region covered by the hydrodynamic and transport models. The region encompasses 283 km 2 of sea, with 43 km of coastline of which about 29 km are beaches used for bathing (19 km in Fortaleza and 10 km in Caucaia, an adjacent municipality). The grid contains 1,783 quadratic elements and 7,564 nodes and was designed so as to allow for the highest possible level of detail in the main areas of interest, i.e. discharge points and areas characterized by complex circulation patterns (breakwaters and the harbor). Bathymetric and surface roughness data were retrieved from charts DHN #701 (1:13.0000) and DHN #710 (1:50.000) produced and updated in 2011 by the department of hydrography and navigation of the Brazilian Navy.

Area of study
The bathymetric map in Figure 1 was based on 2,395 depth points of the nautical charts, producing a 30x30m grid for interpolation with the Kriging method (Andriotti, 2004). The depth at each node was subsequently entered into the SisBaHiA database. The roughness chart was based on 143 points comprising different soil types converted into rugosity.

Choice of scenarios
At three degrees south of Equator, Fortaleza has a tropical, semiarid climate. Rainfalls are practically restricted to the rainy season (essentially from February to May) (Appendix, Figure A1). In contrast, during the dry season, when skies are mostly cloudfree, wind speeds ( Figure A1) and solar radiation levels increase. To represent these two scenarios, the months of March and October were selected for simulation.
The bacterial loads of the two other sources of discharge (storm drains and rivers) were based on data from a 2009 monitoring program by the Ceará State University. An overview of the bacterial concentrations and loads used in each scenario is given in the Appendix (Table A2).

Bacterial decay rates
The model assumed a variable cloud cover calculated from the incidence of solar radiation in order to obtain thermotolerant coliform and E. coli decay rates. This information and wind data were obtained from a weather station installed on the coast near the submarine outfall. Light attenuation in the water column was estimated based on Secchi depths for October (2.5 m) and March (4 m) and using the methodology described by Feitosa et al. (2013a). The wind was measured simultaneously near the outfall diffusers (ocean wind) and at the weather station (land wind) to help calibrate the hydrodynamic model. In addition, gust wind speeds were measured on land.

Currents
An acoustic Doppler current profiler was anchored near the outfall diffusers in October 2011 at a depth of ~15m in order to obtain a vertical profile of current speeds. The device also recorded changes in pressure, temperature, conductivity and sea level. Some of these parameters were used to calibrate the hydrodynamic model.

Bathing water quality
The results produced by the models were compared with data on coastal water quality provided by a state environmental agency (SEMACE) conducting weekly samplings at thirty points along the waterfront of Fortaleza. The percentage of samples exceeding maximum acceptable concentrations of thermotolerant coliforms (1,000 MPN/100mL) in 2009 is shown in the Appendix (Table A3).

RESULTS
The correlation between observed and modeled sea levels was satisfactory in terms of both amplitude and phase ( Figure A3), suggesting the model is a reliable tool for the prediction of this parameter.
The modeling results and measurements of currents show that the hydrodynamics off Fortaleza are primarily determined by wind patterns, with tides playing a minor role. Figure 2 shows the mean current intensities in the water column modeled for three different wind types and measured with a current profiler anchored near the outfall diffusers. The results obtained with the model using gust winds were closest to actual values. Therefore, gust winds were adopted in the hydrodynamic and transport models analyzed below.  Figure 3 shows the behavior of the currents off Fortaleza at low and high spring tide. The main direction of the currents is westward, but a more complex pattern is observed near the shore, especially along the urban beaches closest to the long breakwater protecting the harbor. Several smaller breakwaters have been installed in this area to reduce the erosive action of waves and coastal currents. Note the formation of counter-clockwise vortexes in the harbor and around the breakwaters on the central and eastern beaches, reducing current speeds and water renewal rates.

Dispersion of contaminants
The outfall plume was shorter in March (when the currents are less intense) than in October ( Figure  3), but the lateral dispersion and the concentration were greater. This trend was confirmed by an analysis of the probability of thermotolerant coliform concentrations exceeding maximum limits, as   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 shown in Figure 6 which illustrates the percentage of time each month in which the water was unsuitable for bathing (>800 MPN/100mL for E. coli). The collected water quality data were similar to the results of the model with regard to the frequency with which thermotolerant coliform concentrations exceeded maximum limits, but the values obtained with the model were slightly lower because not all sources of discharge were considered.
As shown in Figure 6, the plume was over 2 km from the nearest beach-a very favorable situation for recreation and bathing. Brazilian legislation does not specify the minimum distance required to protect areas of human recreation from marine sources of pollution, but in the Mediterranean the reference value is 300 m (UNEP 2004). Thus, the water in the recreational areas along the waterfront of Fortaleza is not contaminated by the outfall. However, the situation is less favorable with regard to the impact produced by other sources. Discharge from the Cocó river in the eastern sector, from five storm drains (#7, 9, 10 and 14) and the Maceió stream in the central sector, and from all the storm drains in the western sector was found to have a significant impact on the bathing water quality in Fortaleza regardless of the season. Wind, tide and currents spread fecal coliforms from these sources to recreational areas. Low radiation levels during the night favor the persistence of plumes, extending their influence along the urban waterfront .   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62 63 64

CONCLUSION
The local hydrodynamic and environmental conditions (including intense sunlight) protect the beaches of Fortaleza from the influence of the submarine outfall. As shown by the model, the outfall plume remains at least 2 km from the recreational areas on the shore.
Bacterial decay is much slower at low levels of solar radiation, such as in the early morning hours. At this time of day, the beach is often used by bathers looking to avoid exposure to harmful UV rays. Thus, unfortunately, the protection against skin cancer afforded by bathing in the early morning hours is offset by an increased exposure to fecal contaminants.