Study: Rehoboth Ave. stormwater, sewer cross-contamination

A new stormwater study in Rehoboth Beach has uncovered cross-contamination with a sewer line on Rehoboth Avenue. It also found Wilmington Avenue stormwater filters are breeding grounds for bacteria.

The report is Phase III of stormwater sampling conducted by city contractor GHD, an international engineering firm. Stormwater testing was required by Delaware Department of Natural Resources and Environmental Control when it approved Rehoboth’s ocean outfall permits.

During Phase I, GHD studied theoretical modeling, which predicted high levels of bacteria in Rehoboth’s ocean in the hours immediately following a big storm.

During Phase II, GHD took samples from the stormwater drains, which confirmed higher-than-recommended levels of bacteria – but they were lower than predicted. During Phase II, extremely high levels of bacteria were found in the stormwater pipes on Rehoboth, Wilmington, Baltimore and Maryland avenues. As a result of the sampling, the city found a stormwater vault on Rehoboth Avenue wasn’t working because pumps designed to move the most contaminated stormwater into the city’s sewer system failed.

Recommendations from GHD following this phase included regular maintenance and cleaning; it recommended infrastructure improvements including more permeable pavement, engineered sand filters, flushing of pipes and extending stormwater outfalls in the ocean.

During a commissioner workshop Dec. 9, Leita Bennett, GHD associate, noted the city has repaired the Rehoboth Avenue pump, replaced the sand filter on Wilmington Avenue, cleaned portions of the stormwater pipe and begun videoing stormwater pipes. A whole lot of maintenance has been done, she said.

Bennett said as a result of the city’s maintenance, problem areas have shown significant decreases in bacteria. She said typically, stormwater has 8,000 colony-forming units per liter. According to a table provided as part of her presentation, the majority of locations tested were under or within small percentages of that threshold.

“It’s amazing how much it’s gone down,” Bennett said.

Testing for human waste was also part of Phase III. Bennett said Baltimore and Wilmington avenues came back clean, but three different samples on Rehoboth Avenue detected human waste.

The study results show the custom-made sand filters on Wilmington Avenue continue to have high levels of bacteria. Bennett recommended more regular cleaning of the sand filters to start, but she also suggested the city consider replacing them.

In a follow-up email Dec. 11, Rehoboth Beach Public Works Director Kevin Williams said the city will continue to identify and eliminate the causes of high bacteria. GHD will continue to be a valuable partner in this effort, he said.

Williams said the city is now analyzing the method to be used to identify the potential cross-contamination with the sewer system. He said the work should take place in the coming months.

Related to the Wilmington Avenue filters, Williams said the first thing the city is going to do is replace each of the components within the filters and then retest. He said there were significant problems with tree roots in that stormwater line, so the city wants to make sure that wasn’t a contributing factor to the high levels of bacteria in the filter.

Bennett said in addition to stormwater pipe sampling, GHD and the city conducted public outreach to restaurants to inform them of expectations for cleaning grease traps. A town hall was held for businesses, but only one person showed up, Bennett said.

Bennett said the city’s current fats, oils and grease program for restaurants is simple – have a grease trap and empty it.

Bennett said the biggest issues related to restaurants were trash containers and outdoor cleaning. Trash attracts a lot of birds, and cleaning outside flushes waste down the drain and into the ocean, she said.

To better control waste from restaurants, Bennett recommended the city create a grease trap ordinance that can be enforced, standardize inspections and inspect at random intervals. People within the industry talk, she said, and if one restaurant gets inspected they’ll let the others know.