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Pathogens (bacteria, viruses, and protozoa) pose problems for public waters and beaches. These organisms can cause illness, ranging in severity from rashes, swimmer's itch, throat infections, stomach cramps and nausea to more extreme conditions like cholera, salmonella, typhoid fever, or hepatitis A. Economic impacts of closed beaches can be considerable.
Obvious sources of pathogens are sewage outfalls, but since these sources are frequently discharged well offshore and are designed to achieve high dilutions of effluents, they are not necessarily the major sources of beach contamination. Studies show that urban runoff, rivers, and upstream sources frequently account for the majority of the observed pathogen load. Consequently, wet weather is associated with the highest observed concentrations and combined sewer outflows represent major sources. During periods of high precipitation, fecal material deposited during dry weather washes into water bodies.
Pathogens are difficult to detect and measure directly, and their presence frequently cannot be quantified accurately. Indicator organisms are commonly measured to indicate the likely presence of pathogens. Coliform bacteria are used as indicators because, like human pathogens, they are found in the digestive systems of warm-blooded animals. Total coliforms are found in feces, but also in decaying matter and soil. Increasingly, indicator criteria focus on fecal coliform and enterococcus, which inhabit the digestive systems of warm-blooded animals. Enterococcus is a preferred indicator in saltwater due to its uniform decay rate.
An obvious solution to the problem of beach contamination is control of the sources. Inland publically owned treatment plants (POTW's) are required to treat wastewater before discharging it. Some coastal POTWs are granted waivers from secondary treatment through the federal 301(h) program, but only when they discharge using well-designed offshore outfalls, as well as meeting other requirements. However, many sources of pathogens have non-human origins and cannot be practically controlled.
Prediction is an alternate approach to prevention. In many cases, beach closure based on monitoring is overly protective, one reason being that quantitative measurement of bacterial concentrations frequently take 24 to 48 hours to complete. Public health warnings based on measurement are reactive; conditions that lead to the closure are dated and may no longer apply. Warnings are likely to be open-ended and span periods during which bacterial concentrations do not exceed water quality criteria. A predictive approach may help identify safe periods, as well as critical times to target and intensify monitoring. Such an approach will hopefully lead to accurate public announcements on the status of beaches.
The Plumes Computer Model
Since 1979, the EPA has developed and disseminated models that predict the physical properties and dilution of plumes, most recently one called Visual Plumes. Visual Plumes is able to use time-series file input to analzye long sequences of conditions to determine critical dilutions. Visual Plumes also has the ability to model the buildup of background pollution in one-dimensional tidal channels. Finally, Visual Plumes has a sophisticated bacterial decay model sensitive to salinity, temperature, solar radiation, and water depth (absorption of radiation). Visual Plumes is evolving into Visual Beach, which will be generalized to accommodate additional sources, representing combined sewer overflows and other main sources. Another task is to generalize a far-field transport algorithm to carry the pollutant from source to the point of concern (beach). An initial approach will enable the use of real-time data, as from current meters, to update the predicted position of contaminated water parcels that were released at earlier times. Knowledge of water parcel location, environmental conditions (movement), and time would be used to estimate the present position and concentration of all of the contaminated water parcels. In the future, the model may be linked to numerical hydrodynamical modules to make the model fully predictive.
Windows from VISUAL PLUMES
To assess the effect of the equation-of-state assumption on model accuracy, Visual Plumes was used to predict dilutions over a wide range of salinities and temperatures using both linear and the more accurate non-linear equations of state.