The monitoring of water quality in large coastal regions demands great analytical efforts through the collection of many samples, over long periods. Remote sensing is a reliable tool that can provide valuable information on the spatial and temporal variations of environmental parameters, particularly turbidity and chlorophyll a. The aim of the present research was to evaluate the spatial and temporal distribution of water quality from 2005 to 2017 along the north coast of S ão Paulo and its responses to the implementation of industrial developments and to variations in rainfall. Fifty-two MODIS images were used, showing concentrations of chlorophyll a and turbidity, in the dry season and wet season, from 2005 to 2017. The results showed that dilution processes (due to rainfall) control chlorophyll a concentrations. However, a notable increase in concentrations could be identified after the installation of some of the developments in the region, particularly roads and ports. Turbidity was also shown to be affected by dilution processes, and during the wet season this parameter presented lower values. No effect in the results of turbidity could be identified from the installation of roads or ports, showing that vegetation cover exerts an important control on the erosional processes.
Long-term monitoring programs can provide information on water-quality changes caused by population growth, large infrastructure projects, and increased industrial activity [
The application of remote sensing to evaluate water quality began in the early 1970s, and was based on the concept that light backscattering changes as a function of the presence of different substances in the water [
The validation of remote sensing data has been the subject of various research programs (e.g.: Hakvoort, de Haan [
Yet, aside from the importance of producing extended series of temporal and spatial data, what matters most is the interpretation of these various images of water quality. The relationship between anthropogenic inputs and water quality is not as direct as one might expect. Local water currents and rainfall patterns can significantly affect the behavior of the plumes of rivers draining populated areas [
An earlier study [
In the present research, the evolution of the water quality along the northern coast of São Paulo State, Brazil, was evaluated based on satellite (Acqua) images (MODIS) from 2005 to 2017, taking into account the additional influence of large infrastructure projects recently constructed in the region (since 2005), population increases, and rainfall events. The association of these parameters as explaining elements for the water quality determined by satellite images is a new approach for a better understanding of the factors that control concentrations of chlorophyll a and turbidity. Regardless the fact that images provide 35 spectral bands that are capable of displaying a number of water quality parameters, these two elements were chosen as indicators, because they are good indicators and, because they are reliably mapped from satellite images.
The north coast of São Paulo State is located some 170 km ESE from the city of São Paulo and comprises the municipalities of São Sebastião, Ilhabela, Caraguatatuba and Ubatuba (
Because of access limitations, human occupation of the north coast of São Paulo was not prioritized during the nineteenth or twentieth centuries. However, near the end of the 1960s, the construction of the maritime terminal Tebar in the city of São Sebastião led to fast-paced industrial development in that city. In the early 1980s the SP-55 road (presently BR-101) was constructed, creating easy land transportation access to the area [
The petroleum industry in the region is mainly associated with offshore pre-salt exploration and may generate significant impacts on this very ecologically sensitive territory—similar to what occurred in the region of Macaé (North of Rio de Janeiro State), where degradation of the socio-environmental structure was reported by Binsztok and colleagues [
Development | Description | Commencement of activity |
---|---|---|
Platform and underwater pipeline from Mexilhão Field | Offshore—Gas and condensed gas production from Mexilhão field, Santos Basin | February 2007 |
TLDs Guará | Offshore—Long-term testing in the offshore petroleum fields of Guará, Carioca, Tupi and Iracema, Santos Basin | September 2009 |
Pilot production test in the field of Tupi/Lula | Offshore—Pilot production and outflow of petroleum and gas from the field of Tupi, Santos Basin. | September 2008 |
Exploration and development projects of the pre-salt in the Santos Basin | Offshore/onshore—integrated projects of production and outflow of petroleum and natural gas from the pre-salt | November 2011 |
UTGCA | Onshore—Caraguatatuba gas treatment unit | April 2006 |
GASTAU | Onshore—Gas pipeline Caraguatatuba-Taubaté | April 2006 |
New pier—TEBAR | Terminals and ports—Enlargement of a petroleum pier in the terminal of São Sebastião | September 2011 |
Enlargement of the São Sebastião Port | Terminals and ports—Enlargement of the São Sebastião Port and its integration with the city | October 2009 |
Nova Tamoios Highway sub-track upland | Transportation facilities—duplication of the Tamoios Highway in the upland region | August 2011 |
Nova Tamoios: sub-track southern contours | Transportation facilities—Construction of the southern contours of the Tamoios Highway linking Caraguatatuba and São Sebastião | February 2010 |
Nova Tamoios: sub-track northern contours | Transportation facilities—Construction of the northern contours of the Tamoios Highway north of Caraguatatuba | December 2011 |
might be expected. Regional population growth was significant in the decades of the 1990s and 2000s, reaching 44% in the former period and 22% in the second [
Rainfall data were obtained from various meteorological stations within the region (stars in
Satellite images were obtained every year, from 2005 until 2018, in the summer (December-March; wet season) and in the winter (June-September; dry season) from the sensor MODIS (Moderate Resolution Imaging Spectroradiometer), installed in the satellite Aqua (originally known as EOS PM-1). The acquired images provide 35 spectral bands that can be treated to evaluate many aspects of the surface of the water, including concentrations of chlorophyll a and water turbidity. These images were obtained from the site “Ocean Color Browse” (https://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=am). Images displaying chlorophyll a concentrations and water turbidity in a day with little cloud cover were chosen for each period, downloaded and analyzed using the software “SeaDAS7.4”. The final maps were prepared in the environment “ArcMap” using the software “ARCGIS 10.2”. In the summers of 2007 and 2012, the images were inadequate (due to cloud cover) and could not be used.
De Souza Rolim, Paes de Camargo [
The results of monthly rainfall data from 2005 to 2017 are represented in the graphs of
strong rainfall activity during the summers of 2011 and 2013 in the stations CEEPAM (Caraguatatuba) and Ubatuba. During the spring of 2009 and the summer of 2011 very high values were registered in Ubatuba. High values were also observed in CEEPAM (Caraguatatuba) in October 2008. The highest monthly values for the municipalities of Caraguatatuba and Ubatuba reached 500 mm, while in São Sebastião, the highest values never exceeded 400 mm. The years 2009, 2010, 2011 and 2013 also registered excessive rainfall in other areas of southeast Brazil; these were probably associated with more intense El Niño phenomena [
In the present work 26 images (dry and wet seasons from 2005 to 2017 inclusive) were prepared, displaying the parameter chlorophyll a, with another 26 images displaying the parameter turbidity. A complementary sample set of images is presented in the appendix A (A1 to A9).
Among the parameters that may control intensity of chlorophyll a production, the provision of nutrients is relevant and associated with wet-season (summer) tourism activity in the region [
For chlorophyll a, the analysis of the whole image sequence (since 2005) indicates that in the wet season primary production is less intense than in the dry season (
the results of turbidity (also for the year 2010) did not show the expected higher values during the wet season, indicating that dilution from higher rainfall is more important than other influences. This trend was similar for all the other years.
For a temporal evolution of the concentrations of chlorophyll a and turbidity (2005-2017), besides rainfall, we considered the major development projects in the region and their impacts.
Development | Aspects/Impacts [ | Intensity of impact | Commencement of activity |
---|---|---|---|
Platform and underwater pipeline from Mexilhão Field | Dredging and underwater movements/offshore resuspension of sediments | Irrelevant | February 2007 |
Long-Term Testing Guará | Dredging and underwater movements/offshore resuspension of sediments | Irrelevant | September 2009 |
Pilot production test in the field of Tupi/Lula | Dredging and underwater movements/offshore resuspension of sediments; intensification of naval activity along the coast | Light | September 2008 |
Exploration and development projects of the pre-salt in the Santos Basin | Dredging and underwater movements/offshore resuspension of sediments | Irrelevant | November 2011 |
UTGCA—Gas treatment unit | Vegetation removal, landfilling, population increase/degradation of the landscape | Mild | April 2006 |
GASTAU—Gas pipeline | Vegetation removal, landfilling, population increase/degradation of the landscape | Mild | April 2006 |
New pier—TEBAR | Dredging and underwater movements; intensification of naval activity along the coast, population increase/sediment resuspension, noise, emissions, degradation of water quality | Mild | September 2011 |
Enlargement of the São Sebastião Port | Dredging and underwater movements; intensification of naval activity along the coast, population increase/sediment resuspension, noise, emissions, degradation of water quality | Strong | October 2009 |
Nova Tamoios Highway sub-track upland | Vegetation removal, landfilling, population increase (distant from the coast)/degradation of the landscape; degradation of the water quality. | Strong | August 2011 |
Nova Tamoios: sub-track Southern Contours | Vegetation removal, landfilling, population increase (near the coast)/degradation of the landscape; degradation of water quality. | Very strong | February 2010 |
Nova Tamoios: sub-track Northern Contours | Vegetation removal, landfilling, population increase (near the coast)/degradation of the landscape; degradation of water quality. | Very strong | December 2011 |
production of chlorophyll a is clearly verifiable from the images in the dry seasons of 2007 and 2012 (
Turbidity did not show the same trends as chlorophyll a; there was no significant evolution in the concentrations, with the development along the north coast of São Paulo. Some years, such as 2005, presented high turbidity during the dry season, but in the dry season of 2013 (
the most impacting development, the turbidity was low. It is probable that, regardless of whether the most impacting development promoted the resuspension of sediments and the intensification of erosion in the continent, the region presented a well-preserved vegetation cover, particularly in the eastern portion [
Various parameters seem to control chlorophyll a in the water column of the north coast of São Paulo including provision of nutrients (associated with population increase and tourism). The transparency (depending on turbidity) of the water also contributes to the primary production by increasing the intensity of sunlight incidence also depending on the season. The volume of rainwater entering the system that promotes dilution of the concentrations also seems to be important. The occurrence of higher concentrations of chlorophyll a in the dry season (winter) is unexpected, because this period has less tourism activity, less incidence of sunlight and higher turbidity. Therefore, the dilution promoted by intense rainfall during the wet season (summer) is more effective in reducing primary production than all the other parameters, along the north coast of São Paulo.
The temporal evolution of the concentrations of chlorophyll a (2005-2017) shows that impacting developments like the construction and maintenance of roads and the installation or upgrade of ports promote population growth and increases in maritime activity. These impacting events, which were intensified after 2009, resulted in increasing chlorophyll a concentrations along the north coast of São Paulo. To mitigate the impacts, developments like these should be accompanied by extensive sanitation works, to reduce the load of nutrients.
Images of turbidity along the north coast of São Paulo show that dilution from heavy rainfall in the wet season (summer) also controls this parameter in the water column. Most of the images show a higher turbidity in the dry season (winter) than in the wet season (summer), except for the years 2005 and 2009. Nevertheless, no temporal trends in turbidity could be observed from 2005 to 2017. After the installation of the most impacting developments during 2009-2011 no consistent or relevant increase in turbidity could be identified. Apparently, the preservation of the vegetation cover along the north Coast of São Paulo, and particularly in the eastern portion of the region, hinders increases in turbidity due to impacting developments like dredging and landfilling.
Finally, it is interesting to highlight some limitations of the research. First, although satellite images of the chlorophyll a and turbidity have been largely calibrated in many studies, the fact that we did not calibrate the results with real samples is a limitation, because the color response of the images may be different in distinct locations. Second, the interpretation of the results with other parameters like position of oceanic waters may be important, as discussed on the results of the year 2017. Unfortunately, the temporal behavior of oceanic currents in the southeast Brazil is poorly known and further studies are necessary. Third, it would be very helpful to compare the results of images with other biogeochemical parameters, like salinity, pH, redox potential, dissolved oxygen and freshwater discharges. Some of these parameters like salinity can also be estimated by satellite imaging, while others have to be measured in situ.
The authors are grateful to Nemus Ltd. for financial support. JCW is also grateful to the Brazilian Council of Scientific and Technological Development (CNPq) for a research grant (grant # 306714/2013-2). PB is also grateful to CAPES for financial support (grant # 001). These financial supports do not imply any sort of bias in the results or their interpretation.
The authors declare no conflicts of interest regarding the publication of this paper.
Bettencourt, P., Wasserman, J.C., Dias, F.F., Alves, P.R., Bezerra, D.B., Santos, C.A., Zotes, L.P. and Barros, S.R. (2019) Remote Sensing Applied to the Evaluation of Spatial and Temporal Variation of Water Quality in a Coastal Environment, Southeast Brazil. Journal of Geographic Information System, 11, 500-521. https://doi.org/10.4236/jgis.2019.115031