A Spatial Analysis of Private Well Water Escherichia Coli Contamination in Southern Ontario
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Geospatial Health 8(1), 2013, pp. 65-75 A spatial analysis of private well water Escherichia coli contamination in southern Ontario Julia Krolik1,2, Allison Maier1, Gerald Evans2,3,4, Paul Belanger5,6,7, Geoffrey Hall8, Alan Joyce1, Anna Majury1,2,4,7 1Public Health Ontario, Regional Ontario, Canada; 2Department of Pathology and Molecular Medicine, Queen’s University, Kingston, Ontario, Canada; 3Division of Infectious Diseases, Department of Medicine, Queen’s University and Kingston General Hospital, Kingston, Ontario, Canada; 4Department of Biomedical and Molecular Sciences, Queen’s University, Kingston, Ontario, Canada; 5Department of Geography, Queen’s University, Kingston, Ontario Canada; 6Kingston Frontenac Lennox and Addington Health Unit, Kingston, Ontario, Canada; 7Department of Public Health Sciences, Queen’s University, Kingston, Ontario, Canada; 8Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada Abstract. Research to date has provided limited insight into the complexity of water-borne pathogen transmission. Private well water supplies have been identified as a significant pathway in infectious disease transmission in both the industri- alised and the developing world. Using over 90,000 private well water submission records representing approximately 30,000 unique well locations in south-eastern Ontario, Canada, a spatial analysis was performed in order to delineate clus- ters with elevated risk of E. coli contamination using 5 years of data (2008-2012). Analyses were performed for all years independently and subsequently compared to each other. Numerous statistically significant clusters were identified and both geographic stability and variation over time were examined. Through the identification of spatial and temporal pat- terns, this study provides the basis for future investigations into the underlying causes of bacterial groundwater contami- nation, while identifying geographic regions that merit particular attention to public health interventions and improvement of water quality. Keywords: spatial cluster analysis, private well water, Escherichia coli contamination, public health, Canada. Introduction Specifically, faecal contamination of well water is a serious public health issue, which has been linked to Any compromise to drinking water integrity poses a outbreaks of various water-borne infections (Furtado serious threat to human, animal and environmental et al., 1998; Raina et al., 1999; Macler and Merkle, health and is a major health issue in developed nations 2000; Corkal et al., 2004). There were 288 con- to this day. Water quality is jeopardised by microbio- firmed documented outbreaks of infectious enteric logical and/or chemical agents, water-borne bacteria, diseases in Canadian drinking water with the most viruses and protozoa posing the greatest threat common pathogens being Giardia intestinalis, (Charrois, 2010). In North America, contaminated Campylobacter sp., Salmonella and rotavirus over a drinking water is conservatively estimated to cause 27-year period (Schuster et al., 2005). In 2000, the one thousand deaths and one million illnesses per year municipal water supply in Walkerton, Ontario was (Sierra Legal Defense Fund, 2006) and Europe shows contaminated with Escherichia coli O157 from similar rates (Charrois, 2010). Thus, safety of drinking manure runoff originating in a nearby farm. It is to water is a fundamental public health priority, with date, the largest municipal water-borne outbreak of drinking water quality management being a crucial E. coli O157 in Canadian history and it resulted in at component in the prevention and control of water- least seven deaths and 2,300 cases of illness borne disease (WHO, 2010). (Salvadori et al., 2009). Since the outbreak in Walkerton, numerous policies and public health Corresponding author: actions have been implemented to prevent future out- Anna Majury breaks in municipal water supplies (Ontario Ministry Public Health Ontario Laboratories of the Environment, 2009). However, the quality and 181 Barrie Street, Kingston, ON K7L 4V6, Canada Tel. +1 613 548 6630; Fax +1 613 547 1185 maintenance of private well water systems remains E-mail: [email protected] the responsibility of the owner (Kreutzwiser et al., 66 J. Krolik et al. - Geospatial Health 8(1), 2013, pp. 65-75 2011). In Canada, approximately three to four mil- Material and methods lion, or 1 in 10 persons rely on private well water sources (Statistics Canada, 2011), which remain vul- Data prepration and GIS nerable to contamination and should be properly monitored (Charrois, 2010). In England and Wales, Private well water quality data was compiled from the incidence rate of intestinal infectious outbreaks ongoing bacteriological testing at Public Health in recipients of private water supplies was 35 times Ontario Laboratories (PHOLs), resulting in the use of that of those receiving public water supplies (Smith a convenience sample (i.e. routinely collected data that et al., 2006). Within Canada, a study in British is subsequently used for data analysis not intended at Columbia found that the risk of enteric disease was collection). In this case, the submitter is responsible for 5.2 times higher for individuals living on land serv- collection and transportation prior to bacteriological iced by private wells than those who relied upon testing. All such data are stored in the Water Testing municipal groundwater systems (Uhlmann et al., Information System (WTIS) database owned by PHO. 2009). For the present study, all records from samples submit- Understanding of the endemic water-borne risks ted to the Kingston PHOL (PHOL-K) between January associated with private wells and groundwater is cur- 1, 2008 and December 31, 2012 were retrieved from rently lacking (Uhlmann et al., 2009). Surveillance of the database. A 5-year study period was chosen to limit contaminated wells and identification of clusters with spatial sampling bias, as the convenience sample dif- a high risk of contamination would allow public fered by year. Records that were rejected due to health practitioners to better assess and design inter- improper sample handling were not included. The ventions and aid in the identification of contributing resulting dataset contained 107,547 records, however factors leading to contamination. This can be 3,723 had insufficient address information for geospa- achieved using geographical information systems tial analysis and were removed. The remaining records (GIS) and other spatial analytical tools. For example, were geocoded as summarised in Fig. 1 using Google spatial clustering can be employed to detect foci of Earth™ (Google Inc., Google Earth™ version 5.1, well water contamination. Although many studies http://www.google.com/earth/index.html) to obtain have employed the spatial scan statistic to detect clus- geocoordinates (geocodes). Google Earth™ only ters of various pathogens in specific geographic loca- allows input of street address, city and postal code and tions (Brownstein et al., 2002; Odoi et al., 2004; thus, for the primary stage, city was defined as Coleman et al., 2009; Szonyi et al., 2010), few have “city/town/municipality”. A secondary stage was per- used this method to investigate clusters pertaining to formed utilising “county” for the city field. Given the water-borne infections or water contamination in rural nature of the wells, the provided information Canada. E. coli is the principal bacteriological indica- consisted of hamlets too small to be recognised by tor used in Ontario to assess faecal contamination of Google Earth™ or outdated place names resulting drinking water (Ontario Ministry of the from boundary redefinitions or municipal amalgama- Environment, 1994) and Public Health Ontario tions. Thus, geocoding was difficult and the two-step, (PHO) provides bacteriological testing of private well rather than single-step, Google Earth™ search allowed water samples (as a free service). A previous study of for an additional 10% of records to be included in the 235 rural wells in Ontario found that 9.5% of house- study. This two-step process was then repeated on any holds had at least one E. coli - positive water sample non-geocoded records using LIOcoder, a service pro- (Strauss et al., 2001). vided by the Ontario Ministry of Natural Resources As the first study to conduct spatial analysis of bac- (http://www.lio.ontario.ca/liocoder/index.jsp) and then teriological contamination in private well water, we ArcGIS version 10.0 (North America Geocode Service, aimed to provide a geospatial description of the CAN_RoofTop locator, 2010) (ESRI Inc.; Redlands, prevalence and risk of E. coli contamination in south- USA). To ensure similarity in the geocoordinates pro- eastern Ontario for a 5-year period (2008-2012). vided by the different systems, a pilot comparison of Furthermore, possible geographical trends were geocoordinates was performed (data not shown). explored on a temporal scale and, as private well Between each geocoding software stage, manual water contamination is widespread, cluster analysis geocoding was done by matching multiple submissions was performed at the provincial level for 2012 to from the same address. Due to address quality (e.g. investigate other regions with elevated risk of E. coli spelling or data entry mistakes or incorrect informa- contamination. tion), the automated systems did not always assign all J. Krolik et al. - Geospatial Health 8(1), 2013, pp. 65-75 67 Fig. 1. Summary of the geocoding process.