A predictive water quality model is a tool that may work wonderfully for predicting bacteria levels at your beach while both REDUCING the number of bacteria samples you need to collect during the swim season and STILL protecting public safety. However, a predictive model is not an appropriate tool for every beach. Using predictive water quality model for a beach has both PROS and CONS.
-The model may predict bacteria levels at your beach more accurately than yesterday’s bacteria sample results.
-Sampling costs may be reduced in the long run.
-For beaches on the Great Lakes, additional weather and stream flow data may be available to allow for models to be created for use on non-sampling days so that predictions of bacteria levels at your beach could be made every day during the swim season.
-The model building process can be time and money intensive.
-Explaining to the public that a beach is under advisory because of a model result is often more complicated than just stating a bacteria result is over a standard.
-Models are not perfect and sometimes advisories will be issued when bacteria levels were low or not issued when bacteria levels are elevated.
Work through the following questions, adapted from the US EPA publication “Six Key Steps for Developing and Using Predictive Tools at Your Beach”, to help determine if a predictive model would be a good fit for your beach.
1. Are bacteria levels are known to stay the same at your beach for more than 24 hours? If so, water sampling and laboratory analysis alone will most likely protect swimmer health adequately.
2. Are rapid bacteria sampling methods, such as qPCR, used daily at your beach? If so, the results will come back quickly enough to most likely protect swimmer health adequately.
3. Are the bacteria results from your beach always (or almost always) below or above recreational water quality standards? If bacteria sampling results exceed standards less than 20% of the time, a model would probably not predict the very few elevated bacteria events based on previous sampling. If bacteria sampling results exceed standards more than 80% of the time, a model would probably not predict the very few situations when the water at the beach does not exceed standards based on previous sampling.
If it appears that modeling would work for your beach, there are still some practical questions to be answered before embarking on creating a beach model.
1. Are long term conditions at your beach consistent? If your beach has random, unpredictable events that impact it – such as frequent, but unpredictable, sewage spills or irregular visits from water fowl - bacteria levels may not be easy to predict.
2. Are historical bacteria data available for your beach? At least 50 sampling events over 3 seasons are recommended for creating models.
3. Do you have enough resources to develop and maintain a model? You will need personnel and technical experts for data collection and model creation, equipment for monitoring, and funds for model upkeep.
If creating a predictive model for your beach sounds like a good fit for your beach, this website will help you learn how to use one software package, Virtual Beach, to develop models for your beach. This is a resources for learning about the software, trouble-shooting issues as they arise, and connecting with other beach managers who use Virtual Beach to make decisions about beach advisories.