Summary and Conclusions


A three-dimensional model of the major geologic units serving as aquifers within the Columbia Plateau regional aquifer system of Idaho, Oregon, and Washington, was developed. The model is at a scale and resolution appropriate for use as the foundation for a regional groundwater flow model. The model is completely consistent in that the computed bottom of any model-generated unit is coincident with the top of the uppermost model unit immediately underlying each unit, and the thickness of each unit is the difference between the unit top and unit bottom. Model-generated tops and bottoms of each unit are smooth where buried, reflecting the uncertainty in the estimated surface. These smooth surfaces were generated using loess trend modeling methods to decompose the data into well-supported trends and apparently random residuals. The variability of the residuals represents uncertainty and is the result of sampling an irregular buried paleotopography, measurement error, and interpretive error made when picking strata in well logs. Even if the data had been perfect, data density was insufficient to resolve the precise shape of the paleotopography. Because the residuals are representative of the uncertainty, the residuals may be used to estimate uncertainty in areas with no well picks or outcrop exposure of the unit. The final model was constructed using geologic principles to truncate trend surfaces against geologically bounding surfaces, thereby creating a complete representation of the geometry of all model-generated geologic units. 


First posted February 25, 2011