Municipal Guide To Clean Water: Conducting Sanitary Surveys to Improve Coastal Water Quality
Wet weather and storms typically result in the “worse-case scenario” for water quality, leading to advi- sories/closures of valued coastal beaches and shellfish growing ar- eas due to increased fecal bacteria concentrations.34, 52 Studies have demonstrated that swimmers have a higher risk of contracting a water- borne illness if they swim in coastal waters receiving stormwater runoff, especially swimming next to storm drain outfalls.7
Where does the water go when it rains?
Urban and suburban development changes the path of rain water through structures such as imper- vious surfaces (parking lots, roofs, etc.), catch basins, culverts, storm sewers, ditches, etc. Impervious surfaces do not allow water to per- colate into the ground; instead, water flows across the land surface and is conveyed directly to streams and rivers at a more rapid rate and at higher volumes. The volume of runoff and the amount of pollution reaching receiving waters increases as human development and the amount of impervious surfaces in- crease in the watershed.7, 33, 60 In Maine, stormwater collection sys- tems are generally separated from sanitary wastewater collection sys- tems, allowing untreated runoff to enter receiving waters.
Determine the general size, age, and condition of the stormwater infrastructure; some municipalities have mapped their stormwater sys- tems, although many areas in Maine rely on ditches and swales rather than enclosed storm sewer pipes. If the path of stormwater is not well docu- mented, spend time in the field during wet weather to observe the flow of water, and record it using a marker, map, and GPS unit. Collecting bacteria samples from storm drains, ditches, catch basins, etc. during wet weather can help determine whether stormwa- ter is negatively impacting the area of concern.
What is the relationship between rainfall and bacteria levels?
Local climate influences the fre- quency and duration of wet weather events, which in turn influences rainfall’s impact on receiving wa- ters. Rapid influx (i.e., pulse effects) of stormwater can create highly contaminated “plumes” leaving storm drains and river mouths.The extent of wet weather influence on bacteria concentrations is linked to transport dynamics such as mixing, dispersion, and bacteria die-off, and can vary from several hours in well-flushed areas to several days in areas with limited circulation.72 Studies have shown that bacte- ria levels are greatest within 24 hours following rainfall and de- cline steadily over the next three days.8 Runoff can re-suspend sedi- ments and further impact surface waters by flooding beach sand and sediments collected in storm sew- ers during dry periods, which can contain large quantities of fecal indicator bacteria.
Statistical analysis can help determine whether rainfall events are linked to high bac- teria levels. Compile rainfall statistics for the area. A sim- ple rain gauge can be used to measure local rainfall data. Al- ternatively, rainfall information may be obtained from a local weather station managed by the National Weather Service. It is important that rainfall data be measured in close proxim- ity to the beach and/or shellfish growing area.
For example, analyses of data from Ogunquit Beach determined that rainfall levels exceeding one inch resulted in the highest recorded levels of fecal indicator bacteria, especially at the beach monitor- ing site closest to the mouth of the Ogunquit River, and that a potential lag time (up to five days) exists between rain events and elevat- ed bacteria levels recorded on the beach. Findings from this study also showed that the Ogunquit Riv- er is the primary source of bacterial pollution impacting the nearby beach area.73
Stormwater pollution is not just a rainy-day issue. Dry weather flows from storm sewers can be a symptom of systematic problems, including sanitary sewer cross- connections and groundwater infiltration. Standing water remain- ing in storm drains between wet weather events can be rich with nutrients, causing substantial re-
growth of bacteria.72
Tr a n s i e n t ,
illegal dumping of sewage and other materials into storm drains (e.g., dog waste, cat litter, and sew- age from recreational vehicles) also can cause problems during dry weather.