Translating Complex Regional Hydrostratigraphy in to MODFLOW: The Columbia Basin Groundwater Management Area (GWMA)

The Columbia Basin Ground Water Management Area (GWMA) is a grass-roots effort to develop a plan to manage depleted groundwater resources in Adams, Franklin, Lincoln and Grant Counties, WA. In support of this objective, efforts are underway to develop a formal structural framework for the Columbia River Basalt Group (CRBG) and interstitial/overlying sediments, and construct a groundwater model encompassing an area of about 8,300 square miles that comprises the four counties.

The CRBG comprises several hundred flood basalt flows that collectively reach over 10,000 feet thick and cover over 59,000 square miles in Washington, Oregon, and Idaho. The CRBG contains vertically discrete aquifers within basalt interflows, separated by low-permeability basalt flow interiors: aquifers are further compartmentalized laterally by faulting and folding. High-yielding wells can be over 2,000 feet deep, and drilling to depths exceeding 3,000 feet is becoming increasingly necessary. The complex system of buried structures is further complicated by the presence of open-borehole wells that connect multiple CRBG aquifers. Translating the maturing conceptual model, complete with these natural and man-made complexities, is challenging. Here, we highlight three particular considerations. First, the complex stratigraphy and distribution of surface-water features necessitated the development of methods and codes to generate native MODFLOW files from dozens of interpretive input sources, and visualize the developing model structure in three-dimensions using EVS (CTECH, 2010). Second, the development of lateral boundary conditions requires understanding of the CRBG-wide flow balance, including structural controls and recharge/discharge patterns far beyond the flow model domain. Third, since the resulting model comprises several million active cells, many of which experience drying throughout the simulation period, the MODFLOW simulation code has been modified to include various numerical techniques to achieve a rapid and stable solution for a reformulated (partially-saturated) groundwater flow equation (see Bedekar et al, this session).