Onsite Wastewater Risk Mapping

Non-point source septic system pollution is a known contributor to water quality degradation in Flathead County, but the spatial extent and distribution of the problem was not well understood. The Flathead Basin Commission retained RDG to conduct an initial risk assessment for Flathead County using publicly available GIS/remote sensing data sets to better understand the issue. This assessment analyzed physical characteristics that are known septic risk factors including soil type, groundwater depth, and slope.

Additionally, the assessment used the Flathead County septic permit geodatabase to analyze and map the existing septic systems and their age, which is primary risk factor for septic system failure or underperformance. Each of these individual parameters were combined into a cumulative risk model for all of Flathead County. The results were presented to the technical working group on the subcommittee along with a memo that summarized the methods used for the analysis.