Portneuf / Marsh Creek Aquifer

This boundary of the Portneuf/Marsh Creek aquifer was updated by IDWR to support ongoing work with the Portneuf River Basin hydrologic study. The boundary was manually delineated through an analysis of geological maps, well completion reports, and topographic data. The primary geological units that comprise the aquifer include basalt flows and alluvial deposits within the valley, while sedimentary and carbonate formations associated with the Basin and Range extension form the underlying bedrock and surrounding mountain ranges. Utilizing data from the 2012 Geologic Map of Idaho (IGS Map 9), the aquifer's extent was initially outlined by compiling identified water-bearing units. Well completion reports were examined across the study area to validate the consistency between subsurface hydrostratigraphy and surface geological units depicted in geological maps. Generally, water-producing strata described in well reports correlated with the mapped geological formations. In regions where surface geology was characterized by loess deposits, well reports provided supplementary data to refine aquifer boundary delineations. The interaction between primary water-bearing units and adjacent bedrock formations remains incompletely understood. Therefore, the delineated aquifer boundary encompasses areas where primary aquifer materials are confirmed, particularly where active wells exist. Notably, certain regions, such as those south of Lava Hot Springs and north of Inkom, feature numerous domestic wells completed in bedrock formations, suggesting they are outside the main valley aquifer system. In areas with limited well data, topographic features served as the primary criteria for boundary determination. In narrow valleys, an elevation contour approximately 200 feet above the valley floor delineated the aquifer's edge. Between Bancroft and Grace, the topographic divide defines the boundary, though additional groundwater level data is required to precisely locate the groundwater divide in this segment.