RESUMO
A foodborne Escherichia coli O157:H7 outbreak in December 2006 included 77 illnesses reported in Iowa and Minnesota. Epidemiologic investigations by health departments in those states and the U.S. Centers for Disease Control and Prevention (CDC) identified shredded iceberg lettuce (Lactuca sativa L.) as the vehicle of transmission. The U.S. Food and Drug Administration (FDA) and Minnesota and California public health agencies traced the lettuce to several growing regions in California based on information from a lettuce processor in Minnesota. Samples from an environmental investigation initiated by the California Food Emergency Response Team (CalFERT) revealed a genetic match between the outbreak strain and environmental samples from a single farm, leading to an in-depth systems-based analysis of the irrigation water system on that farm. This paper presents findings from that systems-based analysis, which assessed conditions on the farm potentially contributing to contamination of the lettuce. The farm had three sources of irrigation water: groundwater from onsite wells, surface water delivered by a water management agency and effluent from wastewater lagoons on nearby dairy farms. Wastewater effluent was blended with the other sources and used only to irrigate animal feed crops. However, water management on the farm, including control of wastewater blending, appeared to create potential for cross-contamination. Pressure gradients and lack of backflow measures in the irrigation system might have created conditions for cross-contamination of water used to irrigate lettuce. The irrigation network on the farm had evolved over time to meet various needs, without an overall analysis of how that evolution potentially created vulnerabilities to contamination of irrigation water. The type of systems analysis described here is one method for helping to ensure that such vulnerabilities are identified and addressed. A preventive, risk-based management approach, such as the Water Safety Plan process for drinking water, may also be useful in managing irrigation water quality.
RESUMO
This study assesses and characterizes the vulnerability of unregulated groundwater systems to microbial contamination in 18 counties in the state of Georgia using a contamination risk screening strategy based on watershed characteristics and elements of the Safe Drinking Water Act's Wellhead Protection program. Environmental data sources analyzed include septic systems, elevation, land use and land cover data, soil, vegetation coverage, demographics, and livestock. A geospatial overlay/index modeling approach was developed to identify areas of higher vulnerability for groundwater pollution by taking into consideration watershed land use, hydrology, and topography (LHT). Sensitivity analysis was used to evaluate the effectiveness of model variables. The results of the model were validated by using field data and output from U.S. EPA's DRASTIC model, a widely used intrinsic vulnerability assessment tool. The validation showed a higher risk of microbial contamination for wells located in a high to medium LHT vulnerability zones. LHT provided a clear distribution of satisfactory and unsatisfactory wells in the three vulnerability zones; however, the majority of wells (>75%), with both satisfactory and unsatisfactory test results, are located in medium DRASTIC vulnerability zone. This difference between LHT and DRASTIC can be attributed to the microbial contamination specific factors incorporated into LHT index. It is concluded that although inclusion of potential contamination sources on adjacent land uses in the vulnerability assessment framework adds to the complexity of the processes involved in a vulnerability assessment, such inclusion provides a meaningful perspective to groundwater protection efforts as an effective screening tool.