Hydrogeologic properties of the Neogene Hillsboro Formation and Willamette Silt in the Tualatin basin are evaluated to provide a foundation for future groundwater use in the Portland, Oregon, metropolitan area, and are then compared to hydrogeological conditions in the neighboring Portland basin. Catastrophic flood deposits of the Willamette Silt in the Tualatin Valley have low hydraulic conductivities and well yields below 7 gpm (26.5 lpm). Fluvial sand aquifers within the Hillsboro Formation compose only 20 percent of the 1,400-ft (450-m) thick unit and most contain a significant quantity of silt and clay, with hydraulic conductivities on the order of 2–30 ft/day (0.6–6 m/day). Water quality is good, yet low well yields (6–100 gpm; 23–380 lpm), a low hydraulic gradient within the basin, and geochemical data point to residence times of tens to hundreds of years and slow recharge to the system. A numerical simulation verifies slow groundwater velocities and the presence of local and regional flow regimes. Aquifers of the Hillsboro Formation are limited to domestic use, and model simulations of hypothetical well fields show that these aquifers would not support large-scale withdrawals. High-yield aquifers of the coarse-grained Troutdale Formation in the Portland basin contrast sharply with those of the Hillsboro Formation and Willamette Silt in that they can be separated into multiple hydrogeologic units, have high hydraulic conductivities, and can be used for municipal purposes. Such drastic differences in hydrogeologic characteristics within a metropolitan area require different strategies involving groundwater use or remediation.

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