Risks of microbial contamination in recreational waters

Tele-epidemiology uses spatial technologies such as satellite imagery in combination with an epidemiological approach to monitor and assess the distribution of animal and human illnesses linked to climatic and environmental variations ^{Footnote 5}). Satellite imagery can provide reliable information on land characteristics over large territories and, as such, represents a powerful asset in the global assessment of fecal contamination of recreational waters ^{Footnote 6}. Given that Canada hosts thousands of lakes that are easily accessible by large segments of the population and are being more frequently used for recreational purposes, we aimed to assess the added value of tele-epidemiology to efficiently monitor the microbial risk of these waters in a Canadian context.

Indices derived from satellite data were included in statistical models and compared with the average contamination level of several beaches in southern Quebec. In addition to evaluating the performance of this approach, we also compared various types of satellite imagery from an operational perspective. Technical details pertaining to this study can be found in a more comprehensive report ^{Footnote 7}.

Method

Study area

The study area included three watersheds in southern Quebec, namely the Yamaska, Saint-François and l'Assomption river watersheds. Public beaches adjacent to a lake and monitored by the Quebec recreational water monitoring program for at least three summers between 2004 and 2011 were included for analysis (n=78) (Figure 1) ^{Footnote 8}.

Figure 1: Distribution of beaches studied on the three watersheds in southern Quebec

Text Equivalent - Figure 1

Figure 1 is a map showing the distribution of beaches studied on the three watersheds in southern Quebec. The watersheds include the region of Assomption, north of the St. Lawrence River and the regions of Yamaska and Saint Francois that are both south of the St. Lawrence River. There are over 70 beaches identified by red dots in these three watersheds.

Measure of fecal contamination

Fecal contamination measures were taken from the Quebec recreational water monitoring program (Environnement-Plage). According to this program, water samples were taken in June, July and August every summer during the study period (from 2004 to 2011). A detailed description of this program is available ^{Footnote 9}. For this study, the geometric mean of fecal coliform concentration among all samples taken during the study period was used as an index measure representing the average contamination level for each beach. This measure was meant to represent a general contamination indicator for a given beach, with minimal influence of extreme values which may arise following specific circumstances, such as heavy rains or heat waves.

Environmental factors

Two broad types of factors are known to influence the fecal contamination of natural recreational waters and thus their quality: meteorological conditions (e.g. rainfall and temperature) which may vary depending on the time of sampling and environmental characteristics (e.g.topography and land use) which are relatively stable over time for a beach ^{Footnote 10 Footnote 11 Footnote 12 Footnote 13 Footnote 14}. This project addressed environmental factors associated with fecal contamination of recreational waters that remain stable over time ^{Footnote 15}.

Four factors related to the land use and land cover were extracted from satellite images (agricultural lands, impervious surfaces, forest and wetlands). Agriculture lands include cultivated areas and pastures. These lands are known to influence water quality downstream since they can be a source of fecal microorganisms from spread manure or grazing animals ^{Footnote 16 Footnote 17}. Impervious surface were used as a proxy of urban areas. These areas can be linked to fecal water pollution through wastewater treatment plan discharges and urban runoff ^{Footnote 13}. Conversely, forest and wetlands have been associated with a better water quality by acting as a filter of microorganisms ^{Footnote 18 Footnote 19 Footnote 20}. Four additional factors associated with specific geohydrological characteristics of the beaches and not extracted from satellite imagery were also added to the analysis. These were: land topography, number of tributaries, the lake area and an index of plant hardiness by Environment Canada, the latter being used to represent the region's average climatic conditions ^{Footnote 21 Footnote 22}.

Estimate of environmental factors by satellite image classification

Various satellite sensors were used to extract land use data: SPOT-5, Landsat 5, Meris FRS L1, Moderate-resolution Imaging Spectroradiometer (MODIS), Advanced Very High Resolution Radiometer (AVHRR), GeoEye-1 and WorldView-2. Figure 2 illustrates the steps involved in transforming raw satellite images into classified images that can be used for analyses. Every sensor offers specific features that can affect their capacity to assess environmental characteristics associated with a higher level of fecal contamination of recreational waters. In the context of possible use in a monitoring program, we also looked at operational criteria including the cost of images, hours of work, level of expertise and material required to process images as well as criteria relating to the fit and performance of statistical analyses representing the association between contamination levels and environmental factors.

Figure 2: Diagram of general methodology for processing various satellite imagery used in the assessment

From the image acquisition, this process includes the correction and the calibration of the images and the selection of training sites for the classification of images. Classified images representing various land covers are used for data extraction and then epidemiological analyses.

Correction and calibration of the satellite image

a. Unclassified satellite image

+

b. Sites selected for identifying and characterizing surfaces to be classified

c. Classified image

Text Equivalent - Figure 2

Figure 2 is a diagram with three images. The first image shows what an unclassified satellite image looks like – showing the earth's terrain in browns, greens and blues- after correction and calibration. There are then purple, green and yellow dots added to identify the sites selected for identifying and characterizing surfaces to be classified.  This then leads to a classified image that shows the following by color coding: impervious surfaces are red, wetland is purple, forest is dark green, water is blue, agriculture is light green and small areas with no data are black.

Statistical Analyses

For each sensor, we performed logistic regressions to assess the relationship between mean coliform concentration and environmental factors.

Outcomes

Performance of satellite imagery in characterizing the territory

Satellite imagery classification performed well for the study area but the estimates of surfaces corresponding to environmental determinants studied were highly influenced by the type of satellite images used.

Statistical analyses

Using logistic regression analyses, we identified two land uses associated with a greater risk of having a higher level of fecal coliforms: 1. farming activities and 2. urban activities. Among the sensors studied, Landsat-5 performed best in terms of goodness-of-fit of the model and in terms of the model capacity to classify the beaches in the right category.

Comparison of earth observation imagery

Several types of satellite imagery used in this study were compared according to the various operational criteria previously mentioned. In light of these comparisons, Landsat-5 presented the best compromise between all criteria, especially because if offered the best statistical performances and provided free images.

Discussion

This project showed that data extracted from satellite imagery can be used to characterize regional areas and identify environmental factors associated with a higher risk of fecal contamination of recreational waters. Identifying beaches with a higher risk of fecal contamination is an important component of a global assessment of water microbiological quality, integrating both risk and current level of fecal pollution. By pre-identifying high-risk locations and specifying the possible environmental susceptibility of beaches to fecal contamination, this approach could contribute to better resource allocations for risk assessment activities, especially over large territories where hundreds of beaches need to be monitored.

Other sources of data can be used in addition to satellite imagery to characterize the environment. Census, surveys and field records have been used many times and have proven their reliability. Nonetheless, in comparison, satellite imagery may provide greater benefits. Some limitations must be also taken into consideration before a broader application such as this one in recreational water monitoring programs. Table 1 summarizes the advantages and disadvantages of using satellite imagery to detect recreational waters more at risk of fecal contamination.

Table 1: Advantages and disadvantages of using satellite imagery to detect recreational waters at risk of fecal contamination

Advantages	Disadvantages
Provides considerable coverage including hard-to-reach areas	Requires technical training to gather, process and analyze data
Reproducibility	Limited awareness within public health
Constancy	Cost for some images
Precision	Some sensors unable to capture data under cloudy conditions

This project illustrates one of the many contributions that satellite technologies can make to improve public health. Indeed, the current technical performance and operational reliability of available remote-sensing satellites make them relevant and efficient when addressing various public health issues, including vulnerable populations in remote areas, public health emergencies, health security issues (e.g. a natural disaster or an outbreak) and environmentally dependant diseases like those transmitted by insects or wildlife. Overall, space technologies are now considered to be mature and versatile solutions which can effectively enhance our current capacity to deliver key public health objectives including health surveillance, risk assessment and emergency preparedness and response.

Acknowledgements

The authors acknowledge the people involved in the Quebec provincial recreational water monitoring program (Environnement-Plage) and all individuals involved in the project's report ^{Footnote 7}. The authors also acknowledge Guy Aubé and Paul Brian of the Canadian Space Agency for their support throughout this project.

Conflict of interest

No conflicts of interests to declare.

Funding

This project was funded by the Canadian Space Agency and the Public Health Agency of Canada.