Research links English Creek contamination to a Plant City sewer break, shows sharp drop after repair

Dr. Valerie Harwood, an environmental microbiologist and associate dean at the University of South Florida, told the Hillsborough County Environment Protection Commission on June 18 that DNA-based microbial source-tracking identified a sewage source on English Creek near Plant City and that levels fell sharply after utilities repaired a sewer main break.

Harwood described microbial source tracking as a “who-done-it” forensic approach that uses host-specific DNA markers. “So the red dots along the creek indicate where we are seeing E. coli concentrations in excess of regulatory guidelines,” she said, and noted the team uses two markers together: HF183 (a sewage marker) and H8 (a viable E. coli marker). “If we see H8 along with HF183, then we know, okay, this is really a sewage source,” Harwood said.

In field sampling at a site labeled EC8, Harwood reported extremely high pre-repair numbers from February: “we see 1,000,000 E. coli over … almost almost 10,000,000 HF183 at that site,” she said. After Plant City Utilities located and repaired a large sewer main break, follow-up sampling in March showed E. coli levels near 100 and HF183 falling below 10,000 — “three to four orders of magnitude,” Harwood said, demonstrating a substantial reduction after the repair.

Commissioners asked how sites are selected for sampling and how officials distinguish treated reclaimed water from sewage. Harwood said studies typically begin at an EPC hot spot and proceed upstream to identify where a contamination plume originates. She noted reclaimed water can carry sewage DNA (HF183) even when treatment removes living E. coli, which is why the H8 marker is useful: “If we only see HF183, we’re like, this is some treated-water source like reclaimed water; if we see H8 along with HF183, then this is really a sewage source,” she said.

Harwood credited the partnership among researchers, EPC and utilities for locating the break and for improving responses. She also reviewed funding sources that historically supported microbial source-tracking research, naming federal agencies (EPA, USDA, National Science Foundation) alongside local partners.

The commission did not take formal action on the presentation. Harwood said she is available to answer follow-up questions and encouraged continued monitoring and collaboration between researchers and utilities to locate and remediate sewage leaks.

The presentation and related data were part of agenda item 9a and were introduced by EPC staff prior to Harwood’s talk.