Stig Regli

RISK ASSESSMENT AND CONTROL OF WATERBORNE GIARDIASIS

BACKGROUND Waterborne giardiasis has been increasing in the United States with 95 outbreaks reported over the last 25 years. The Safe Drinking Water Act has mandated control of this pathogen. METHODS A risk assessment model was developed to estimate risk of infection after exposure to treated waters containing varying levels of Giardia cysts. The model was defined by a dose-response curve developed from human feeding studies for Giardia and assumed 2L of water consumption per day. Data on concentrations and distribution of the organism in source waters were used to assess exposure after varying reductions achieved through treatment. RESULTS In surveys reporting prevalence and levels of Giardia cyst contamination, average levels of cysts in surface waters ranged from 0.33 to 104/100L; from pristine watersheds (protected from all human activity) 0.6 to 5/100L. Yearly risks were 4.8 x 10(-3) for systems using polluted waters and 1.3 x 10(-4) for pristine waters with a 10(-3) treatment reduction. CONCLUSION Public Health officials will need to work with the water industry to ensure a risk of less than 1/10,000 for source waters with 0.7 to 70 cysts per 100 liters through treatment achieving reduction of 10(-3) to 10(-5), respectively, of Giardia cysts.

Use of microbial risk assessment in setting US drinking water standards

The paper outlines the U.S. EPAs general strategy for using microbial risk assessment to support the development of the National Primary Drinking Water Regulations. It discusses specifically the use of such risk assessment in the development of upcoming regulations for disinfection of groundwater and for control of disinfectants and their chemical byproducts, and possible amendments to the current Surface Water Treatment Rule.

Assessing the public health risk of microbial intrusion events in distribution systems: Conceptual model, available data, and challenges

Low and negative pressure events in drinking water distribution systems have the potential to result in intrusion of pathogenic microorganisms if an external source of contamination is present (e.g., nearby leaking sewer main) and there is a pathway for contaminant entry (e.g., leaks in drinking water main). While the public health risk associated with such events is not well understood, quantitative microbial risk assessment can be used to estimate such risk. A conceptual model is provided and the state of knowledge, current assumptions, and challenges associated with the conceptual model parameters are presented. This review provides a characterization of the causes, magnitudes, durations and frequencies of low/negative pressure events; pathways for pathogen entry; pathogen occurrence in external sources of contamination; volumes of water that may enter through the different pathways; fate and transport of pathogens from the pathways of entry to customer taps; pathogen exposure to populations consuming the drinking water; and risk associated with pathogen exposure.

Estimating Potential Increased Bladder Cancer Risk Due to Increased Bromide Concentrations in Sources of Disinfected Drinking Waters

Public water systems are increasingly facing higher bromide levels in their source waters from anthropogenic contamination through coal-fired power plants, conventional oil and gas extraction, textile mills, and hydraulic fracturing. Climate change is likely to exacerbate this in coming years. We estimate bladder cancer risk from potential increased bromide levels in source waters of disinfecting public drinking water systems in the United States. Bladder cancer is the health end point used by the United States Environmental Protection Agency (EPA) in its benefits analysis for regulating disinfection byproducts in drinking water. We use estimated increases in the mass of the four regulated trihalomethanes (THM4) concentrations (due to increased bromide incorporation) as the surrogate disinfection byproduct (DBP) occurrence metric for informing potential bladder cancer risk. We estimate potential increased excess lifetime bladder cancer risk as a function of increased source water bromide levels. Results based on data from 201 drinking water treatment plants indicate that a bromide increase of 50 μg/L could result in a potential increase of between 10(-3) and 10(-4) excess lifetime bladder cancer risk in populations served by roughly 90% of these plants.

Development of Giardia C · t values for the surface water treatment rule

Abstract As a consequence of the 1986 Amendments to the Safe Drinking Water Act (SDWA) the U.S. EPA has issued a Surface Water Treatment Rule (SWTR) for systems using surface and ground waters under the direct influence of surface water. In the Guidance Manual to the SWTR, the EPA recommends C·t values (product of disinfection concentration in milligrams per liter and disinfectant contact time in minutes) for different disinfectants to achieve required levels of inactivation for Giardia lamblia. This paper describes the procedure by which C· values were calculated for Giardia lamblia by chlorine disinfection in the SWTR. A model has been developed which can be used to approximate the C·t values that are embodied in the SWTR. It was found that C·t values increased due to higher pH, the level of inactivation required, and chlorine concentration, and were inversely related to temperature.

PATHOGEN INTRUSION IN DISTRIBUTION SYSTEMS: MODEL TO ASSESS THE POTENTIAL HEALTH RISKS

A model for estimating the probability of infection from intrusion events associated with low/negative pressure occurrences in distribution system is presented. The modeling approach, based on the principle of quantitative microbial risk assessment, predicts infection rates as a function of several parameters: the orifice equation (for calculation of intrusion flow rate), the external contaminant concentration, the starting time, the duration of low/negative pressures, the location and extent of intrusion area, the hydraulic and operational conditions in the distribution system, consumption events at fixed-times and dose-response information for specific microorganisms. The approach combines the use of a probabilistic model to determine the possible range of contaminant mass rates that could be encountered and the use of a hydraulic model to determine population exposure to contaminated water once an intrusion event has taken place. Using a model distribution system (EPANET Example Network 2), the effects of intrusion event characteristics (starting time, duration, location, contaminant mass rate) on the probability for an healthy adult of being infected by Cryptosporidium from sewage contamination of the distribution system were investigated. Based on the current model assumptions, results show that the risk of infection may vary over several orders of magnitude depending upon where the water is consumed and the intrusion event characteristics.