A comparison of watershed contamination between a sewered and an unsewered community

Graduation date: 1980In 1968, a study conducted by the sanitarians of the State Health Division and Benton County Health Department revealed a high failure rate of on-site sewage disposal systems in Southwest Corvallis. The survey team concluded that sewage disposal in this area was inadequate, and a significant health hazard existed due to contamination of the watershed. Following these findings, a part of Southwest Corvallis was annexed to the city of Corvallis and sewered. The rest of the community remained unsewered and continued to use on-site sewage disposal systems. This follow-up study, therefore, was designed to quantitatively determine and compare the bacteriological levels of watershed contamination in the sewered and the unsewered area. Specifically, this investigation was to determine if the replacement of on-site sewage disposal systems by municipal sewer service in the sewered area significantly reduced surface water contamination and subsequently eliminated the sewage related health hazards. Major hypotheses tested in this study were: Ho₁: µ[subscript 1968] bacteriological levels = µ[subscript 1979] bacteriological levels in the sewered area (i.e.: There is no significant difference between the mean bacteriological levels before and after municipal sewer connections.) Ho₂: u[subscript 1968] bacteriological levels = u[subscript 1979] bacteriological levels in the unsewered area (i.e.: There has been no significant change in bacteriological level in the unsewered area since 1968.) According to the 1968 investigating team, sampling sites were chosen within the guidelines of sampling procedures given in The Standards Methods for the Examination of Water and Wastewater. The current investigator closely followed the same guidelines, and since this was a comparative study, samples were obtained from the 22 sampling sites that were chosen in 1968 by previous investigators. From the findings of this study the following conclusions have been drawn. 1. The replacement of individual on-site sewage disposal systems with municipal sewage systems significantly reduced the level of surface water contamination. Subsequently, the sewage related health hazards has significantly diminished in the sewered areas. The null hypothesis was rejected at α = 0.01. 2. The levels of surface water contamination were still the same in the unsewered area. Thus, the sewage related health hazards still existed in the area. The null hypothesis was rejected at α = 0.01. The findings, therefore, strongly suggested that municipal sewer systems were more effective than the on-site sewage disposal systems in the area studied