SOIL SUITABILITY FOR ON-SITE SEWAGE TREATMENT IN THE FLATHEAD VALLEY, MONTANA: SOIL PERMEABILITY, VARIABILITY, AND GROUNDWATER CONTAMINATION

A field study of in situ soil hydraulic conductivity was conducted in the Flathead Valley of northwestern Montana. Of special interest was interpretation of soil properties in terms of suitability for on-site sewage treatment systems. Thirteen sites were selected, soil profiles described and sampled, and the gypsum crust method used to determine hydraulic conductivity at saturation and in the near-saturation range. Multivariate statistical techniques were employed for data analysis.; Results suggest that soil water movement is strongly influenced by the vertical variability (textural stratification) often noted in soil profiles in the study area. Complex glacial and proglacial depositional environments are responsible for this variability, which is also strongly expressed horizontally as lateral variation across the landscape. Soils formed from similar parent materials (and/or with similar textural properties) generally exhibit similar hydraulic characteristics in the saturated and near-saturated range. Substantial variability within these groups is not uncommon. This variability requires that determination of site/soil suitability for septic systems include on-site observations of soil profile characteristics. The implications of textural stratification within the soil profile need to be considered for proper design and long-term operation of individual on-site sewage treatment systems. Multivariate statistical techniques were employed in analysis of the physical and chemical properties of the soil horizons studied. Principle component analysis was shown to be an effective tool for graphical expression of soil profile variability. Cluster analysis demonstrated the ability of such methods to group horizons with similar properties.; Aquifer assessment should be included as an integral component of the site evaluation process for on-site sewage treatment systems. A simple model has been proposed that is designed to assist local regulatory officials in their efforts to minimize the environmental impacts of sewage treatment from suburban and rural housing developments. The model demonstrates that it is important to estimate the nitrogen load to the receiving aquifer (including the potential for denitrification), evaluate the diluting capacity of the aquifer, and also assess the relative importance of the particular ground-water system.