Total Coliform Excursions: Mitigation Strategies Learned from the 2004 Hurricane Season in Florida

摘要

The distribution system for the City of Clearwater, Florida provides 13 MGD of chloraminated water through approximately 588 miles of pipeline. Since the inception of the total coliform rule (TCR), routine monitoring of 110 distribution system samples per month has rarely yielded positive samples. However, an increased incidence of positive total coliform samples occurred during the height of Florida's 2004 hurricane season. In response to the positive TCR sampling results, the City of Clearwater embarked on an extensive sampling campaign and expanded water quality monitoring efforts throughout the distribution system. In conjunction with the additional monitoring, a comprehensive audit was conducted to identify potential reasons for the coliform excursions and to develop measures to prevent future occurrences. The audit included GIS mapping of water quality patterns, hydraulic modeling of the variable flows and pressures associated with hurricane preparation and evacuation activities, operational simulations, field inspection of sampling sites, and a detailed analysis of water quality and SCADA data. An extensive GIS database was developed to allow for mapping of sampling sites, water treatment plant and storage tank locations, interconnects, customer complaints, and the location of construction and flushing activities. A hydraulic model was integrated with the GIS database and used to assess flow and pressure variations throughout the distribution system under normal and abnormal operating conditions. System dynamics were examined further with a review of SCADA data, simulations of abnormal operations, and field testing. In addition, comprehensive reviews were conducted of historical water quality data, treatment system operations, disinfection residuals, sampling and testing procedures, cross connection control practices, flushing activities, and construction activities in an effort to simulate the conditions associated with the positive TCR results. While the increased incidence of positive total coliform samples could not be traced to a single cause, it is likely that the storm-induced shutdowns and post-storm start-up operations resulted in short-term localized perturbations in water velocities and flow directions. Collectively, changes in flow direction and velocity may have mobilized bacteria from biofilms or introduced bacteria through intrusion or other mechanisms. In addition, some samples may have been contaminated due to flooding of sampling ports. Based on the findings of the audit, the City of Clearwater has modified its hurricane response plans for management of water treatment and pumping facilities, updated procedures and operational controls, implemented above-grade dedicated sampling taps, and modified sampling protocols. The lessons learned from this experience will be presented in the paper and should be of interest to other utilities in hurricane-prone or flood-prone regions.