Preliminary Assessment of Hydrogeology and Water Quality in Ground Water in Teton County, Idaho

This report documents a preliminary investigation of the hydrology in Teton County, Idaho done by the Idaho Water Resources Research Institute (IWRRI) for the Idaho Department of Environmental Quality (IDEQ). IDEQ initiated this work in 2006 due to the high rate of housing development in Teton County, Idaho. IDEQ has mounting concerns over the potential introduction of water quality problems due to the installation of domestic onsite wastewater systems. An assessment of the area hydrogeology is necessary to evaluate Nutrient-Pathogen (N-P) Level 1 analyses submitted by developers (Howarth, et al, 2002). The N-P Level 1 evaluations include a spreadsheet analysis which requires hydrologic characteristics as input values. This project was intended to provide IDEQ with some guidelines regarding appropriate values for those inputs. The project work documented in this report includes an assessment of the hydrogeology in the Teton Valley, based on published reports and the ground-water model in the valley and an assessment of the sensitivity of the N-P Level 1 evaluation tool to various aquifer parameters. After the project was underway, an opportunity arose for further funding to conduct a synoptic measurement of water quality parameters in the Teton Valley, so the work on this project was expanded to support that effort. The water quality results are included in this report. Additionally, as work on this project was underway, it became apparent that one of the greatest water quality concerns in the Teton Valley is the potential impact to water quality if the valley were to fully build out using onsite wastewater systems. To address this need, we added a task to create a crude spreadsheet-based tool to analyze nitrate loading for user-defined build-out scenarios. The nitrate loading tool is documented in a separate report