GIS-BASED HYDROLOGIC AND ENVIRONMENTAL SYSTEMS ANALYSIS (HESA) FOR DETERMINING CRITICAL MULTISCALE GROUNDWATER SUPPLIES IN ALLUVIAL AND BEDROCK AQUIFER SYSTEMS: CASE HISTORIES IN PITKIN AND DELTA COUNTIES, COLORADO, USA

Hydrologic and Environmental System Analysis (HESA) is a GIS-based approach that integrates physical, chemical and biological information to derive a conceptual understanding or site model (CSM) of the various components of hydrological and environmental systems and their interactions. This approach has been formulated and applied in a series of projects to develop a basic understanding of the regional, subregional, and local groundwater resources, both alluvial and bedrock systems, of Pitkin and Delta Counties, CO. The purpose of utilizing HESA was to develop a critical understanding regarding the availability and sustainability of groundwater supplies; the vulnerability of the groundwater resource to contamination; the interaction between groundwater and wetlands and streams; and the effects of energy development and mining operations on groundwater. HESA relies on the extensive use of field observation, and is well suited for use in areas not subjected to previous groundwater investigations that lack quantitative hydrologic data.

The main result of the Pitkin and Delta County case histories are a set of reports, and consistent, continuous GIS maps with supporting hydrogeologic databases documenting the various multiscale groundwater subsystems. Groundwater systems were identified with respect to hydrogeological structure and characteristics, as well as their recharge/discharge relationships and direction of groundwater flow. A stepwise groundwater resources evaluation procedure was developed, to be used in conjunction with GIS maps and supporting hydrogeologic and hydrologic documentation, as a decision/land use management tool. The results are useful for developing drinking water source protection plans (groundwater and surface water), evaluating land use/water use changes, such as shifting from irrigated agriculture to residential developments, siting of wastewater treatment systems, and for county-wide watershed planning and management