IMPLEMENTING GEOLOGIC FRAMEWORKS IN GROUNDWATER MODELS: THE DOS AND DON'TS

A geologic framework model (GFM) should not be equated to data when implementing into a groundwater model. Rather, GFMs are models of subsurface geologic settings that interpret data from well logs, surface outcrops, and geophysical surveys. GFMs infer the spatial extent and orientation of geologic units and structures. The use of interpreted and inferred geology commonly leads to structural errors in groundwater models because hydrologic data provide information about permeable pathways, groundwater barriers, or heterogeneities that often are not represented in GFMs. Groundwater modelers should consider revisions to GFMs as a necessary part of the calibration process. Revisions should be implemented in thoughtful ways to ensure consistency of hydrologic data with geologic evidence.

This presentation provides examples for how to implement GFMs in regional groundwater models and rules for revising these GFMs, based on geologic and hydrologic data. Examples of GFMs that should be revised include: (1) complex GFMs with more geologic units than hydrologic data can inform, resulting in insensitive hydraulic parameters; (2) GFMs that extend deeper than the active part of the flow system, so that vertical truncation is necessary; (3) simplified GFMs that require implementation of complex hydraulic-property heterogeneities; (4) GFMs that exclude hydrologically significant features; and (5) GFMs that do not honor all geologic data and require checking and modification. Computational methods will be described for implementing heterogeneity within and between geologic units, including pilot points and Tikhonov regularization.