For some time, the Town of Bedford, Massachusetts had been receiving complaints about odors coming from the sewer system in neighborhoods downstream of its Middlesex Turnpike Pumping Station. The odor complaints initiated an investigation of the sanitary collection system upstream of the neighborhoods. The layout of the collection system indicated two potential problem areas. The Middlesex Turnpike Pumping Station pumped sewage through a 2,500-foot force main, which eventually fed a 21-inch reinforced concrete pipe with a very shallow slope. Both of these areas had the potential to produce hydrogen sulfide (H2S); thus the Town decided to undertake an H2S monitoring program to further assess the problem.