Introduction


The Mosier area lies in northwestern Wasco County between the cities of Hood River and The Dalles, Oregon (fig. 1). Water is needed for irrigation, municipal supply for the city of Mosier, and domestic use for rural residents. The primary source of water is groundwater within the Columbia River Basalt Group (CRBG) aquifers that underlie the area. Concerns regarding the sustainability of using the CRBG aquifers for long-term water supply have grown during the past 30–40 years as water levels in the aquifers have steadily declined. 


Groundwater levels began declining in the 1970s during a period of intense development of groundwater resources. Causes for the declines are pumping and leakage between aquifers through well boreholes open to multiple aquifers (commingling wells) (Lite and Grondin, 1988); however, the relative importance of these factors was unknown. Following a hydrogeologic assessment by the Oregon Water Resources Department (OWRD) (Lite and Grondin, 1988), a groundwater administrative area was delineated (fig. 1), and the Pomona and Priest Rapids aquifers in the area were withdrawn from further appropriations for any use other than domestic supply. Since that time, water levels in the area have continued to decline steadily. Among the adverse effects of the groundwater declines are (1) increased energy costs for pumping, (2) expense of deepening or replacing wells, and (3) reduced groundwater discharge to streams that can affect aquatic habitat (Lite and Grondin, 1988). Continued declines can further reduce flow in streams and make it infeasible for groundwater to support current water demand.


The Mosier Watershed Council and Wasco County Soil and Water Conservation District (SWCD) have established three goals for the watershed: (1) to reverse or stabilize water‑level declines in the principal aquifers of the Mosier area, (2) to increase summer base flows in Mosier Creek, and (3) to sustain productive, profitable agriculture in Mosier Valley (Jennifer Clark, Mosier Watershed Council, written commun., 2004). To meet these goals, the Mosier Watershed Council and SWCD are working with the OWRD to identify groundwater management strategies to ensure groundwater resources will sustain future water needs. 


In 2005, the Mosier Watershed Council and SWCD began a cooperative investigation of the groundwater system with the U.S. Geological Survey (USGS) to advance the scientific understanding of the hydrology of the basin and use that understanding to develop tools that can be used to evaluate management strategies. Another objective of the study was to advance the understanding of CRBG aquifers. These aquifers are some of the most productive aquifers in Oregon, Washington, and Idaho, and in some locations, these aquifers are heavily developed for agricultural, municipal, and domestic water supplies. Many other areas also have experienced significant groundwater-level declines, and water managers are seeking to achieve sustainable levels of groundwater development in the CRBG aquifers. 


Purpose and Scope


The purpose of this report is to identify the causes of long-term groundwater-level declines within basalt aquifers in the Mosier area. The first part of this report summarizes the purpose and scope of this study and provides a description of the study area and previous investigations. The second part describes the hydrogeology of the study area including the geologic and hydrogeologic frameworks, important components of the water budget, and groundwater flow. The final part summarizes the development and use of a three-dimensional numerical model of the groundwater-flow system to evaluate the causes of groundwater level declines and forecast the effects of management options. This report has six appendixes containing supplementary material. The technical details of construction of the groundwater-flow simulation model and supporting input data are summarized in appendixes A through E. The results of geophysical testing of well boreholes are summarized in appendix F. 


Description of Study Area


The Mosier area is in the eastern foothills of the Cascade Range in north central Oregon in a transitional zone between the High Cascades to the west and the Columbia Plateau to the east (fig. 1). The 78 mi² area is defined by the drainages of three streams—Mosier Creek (51.8 mi²), Rock Creek (13.9 mi²), and Rowena Creek (6.9 mi²)—all of which are tributary to the Columbia River. The area drains to the north with elevations ranging from more than 2,300 ft at Wasco Butte to about 70 ft at the Columbia River. The climate is semi-arid to dry sub-humid. The distribution of precipitation in the study area follows a strong gradient, decreasing from higher to lower elevations owing to orographic effects from west to east (associated with the Cascade Range) and south to north (associated with change in elevation along the watershed drainage), based on 1971–2000 average precipitation (PRISM Group, 2010). Average annual precipitation in the northwestern part of the study area is about 35 in., decreasing to 16 in. toward the northeast. Average annual precipitation in the southern part of the study area in the headwaters of Mosier Creek is 57 in., compared to 24 in. at the mouth of Mosier Creek. The distribution of ambient temperature in the study area also follows gradients from west to east and south to north. The Columbia River Gorge connects the moderate marine climate to the west with the interior climate to the east. Temperature increases due to orographic effects from the upland area in the south toward the lowland in the north.


Previous Investigations


In the earliest relevant work, a small part of the current study area was covered by Piper’s (1932) general description of the geology and hydrogeology of The Dalles area. This description was developed further in papers by Newcomb (1961, 1963, and 1969) describing the occurrence and flow of groundwater through important aquifers in the vicinity, especially the younger Dalles Formation (volcaniclastic deposits associated with Mt. Hood) and the older CRBG aquifers. Newcomb (1969) was the first to document the defining role of the Rocky Prairie thrust fault (then referred to as the Rocky Prairie anticline) as a significant hydraulic barrier to groundwater flow in the study area. Although the current study area is centrally located within the much larger Hood Basin groundwater resources area (Grady, 1983), efforts to understand the effect of the complex geologic on the hydrogeology near Mosier were limited. A detailed study (Lite and Grondin, 1988) of the area immediately to the south of the Rocky Prairie thrust fault (fig. 2) identifies the principal aquifers and their geometry over much of the current study area. This description of hydrogeologic units has been used for this and all subsequent studies (Keinle, 1995; Jervey, 1996).


The geologic map (fig. 2) is a compilation of work by Newcomb (1969), Swanson and others (1981), Bela (1982), Lite and Grondin (1988), Kienle (1995), and Jervey (1996). The primary sources for the refinement of the regional geologic maps were surficial and structural geologic interpretations by Lite and Grondin (1988), Kienle (1995), and Jervey (1996).


Objectives and Approach


To support the evaluation of causes for water level declines in the Mosier area, the study had three objectives:


1. Develop a better understanding of the hydrogeologic framework (the three-dimensional geometry and distribution of hydraulic properties of the aquifer system) (see section Hydrogeologic Framework and appendix A);

2. Estimate major groundwater system water fluxes for use in developing a groundwater system budget (see sections Conceptual Model of the Flow System, Recharge, and Discharge; and appendixes B, C, and D); and

3. Integrate the understanding of the hydrogeologic framework and the water budget into a quantitative tool that can be used to evaluate the causes of water level declines and forecast the effects of management options (see Groundwater-Flow Simulation and appendix E).


Because the geometry of the geologic units controls the storage and movement of groundwater, the first objective was achieved through the development of a three-dimensional geologic model and interpretation of hydrologic data in the context of this model. The second objective was achieved by using a watershed process model (precipitation-runoff model) to estimate the spatial and temporal distribution of recharge, and by conducting a 2-year intensive data collection period during which measurements were made of streamflow, pumping, and vertical borehole leakage in commingling wells. The final objective was achieved by combining the geologic model, a conceptual understanding of flow-controlling features, and the water budget to develop a groundwater-flow simulation model. Historical groundwater-level measurements were augmented with 2 years of intensive measurement to aid in development of the conceptual model of the groundwater‑flow system and to provide additional calibration data for the groundwater-flow simulation model.


First posted March 1, 2012