Mosier Valley Ground-Water Sustainability Study

In Cooperation with the Wasco County Soil and Water Conservation District
and the Mosier Watershed Council

APPROACH

The approach comprised several tasks designed to meet the study objectives:

Compile and Assess Existing Data

A wide variety of data are available for the area that has been collected as part of previous studies and as part of monitoring by various agencies. These include well data and ground-water levels, streamflow data, geologic mapping, climate and weather data, vegetation, soils, and many other types of natural resources mapping. All of these data were compiled and evaluated for use in this investigation. All pertinent data were documented, entered in project databases, and made available to the public through reports, the Internet, or the study GIS.

Collect Additional Data

Basic hydrologic data collection consisted of inventorying wells, ground-water level monitoring, and streamflow measurements. Additional types of data collection, such as borehole geophysical logs, are discussed in subsequent sections. Previous studies (Lite and Grondin, 1988; Grady, 1983; Kienle, 1995) have field-inventoried wells and springs in the study area. These data were reviewed and areas with data gaps were targeted for locating additional wells if new wells have been constructed in these areas. About 50 new wells were inventoried to provide geologic data and water levels.

The existing OWRD ground-water level monitoring network was supplemented with nearly 30 wells that were measured bi-monthly and 5-10 wells that had continuous monitors installed that collect levels every 2 hours. Two synoptic water-level measurements were made during the 2005 water year, one in the spring prior to the beginning of the irrigation season and one during the late summer immediately after the irrigation season. These data, from approximately 100 wells, were used to construct detailed hydraulic head maps. These data have enhanced our understanding of the horizontal and vertical directions of ground-water flow and how ground-water levels respond to seasonal pumping and recharge. The data were used to calibrate the ground-water model.

A gaging station was installed on Mosier Creek at the site of the former USGS station (14113200). The station measured discharge every 15 minutes and had telemetry to make data available in real time via the USGS NWIS site (http://waterdata.usgs.gov/or/nwis). Manual discharge measurements were made at several sites above and below the gaging station throughout the year to monitor gains and losses to and from ground water. These data allowed us to detect seasonal effects of climate and pumping on ground-water discharge to Mosier Creek, and they were used to calibrate the ground-water flow model.

Temperature sensors were deployed at several locations on Mosier Creek the summer and fall to aid in interpreting ground-water/surface-water interactions.

Define the Hydrogeologic Framework

Considerable work has been done to map the major hydrogeologic units of the basin by Lite and Grondin (1988) and Kienle (1995). The proposed study built upon this foundation to prepare a three-dimensional representation of the hydrogeologic units needed for the ground-water flow model. Any new well information available since 1985 was incorporated to produce maps of the thickness and extent of each major hydrogeologic unit.

The hydraulic characteristics of each aquifer and confining unit were also estimated. This study relied in large part on data and analyses from previous studies, well performance tests reported in driller’s reports, and values reported for similar hydrogeologic units in other locations in Oregon and Washington . The study also looked for opportunities to conduct aquifer tests in the Mosier Basin to fill data gaps. The best opportunities were where pumping and observation wells were completed in one unit, were located within distances that a measurable response could be expected within a reasonable time frame, and were available for pumping and monitoring outside the irrigation season.

Quantify the Hydrologic Budget

The hydrologic budget forms the basis of a quantitative understanding that is needed to model ground-water flow. Several of the budget components, such as recharge from precipitation and well pumpage are inputs that are typically specified in the model. Clearly, these components must be estimated by means other than the model. Other budget components, such as discharge to streams and springs, are simulated by the model; however, it is still important to have independent estimates of these budget components that can be used to compare with simulated values during the model calibration process.

Ground-Water Recharge Components

Recharge from precipitation was estimated by constructing a watershed model. The model used daily climate data and basic information on soils and vegetation to compute a mass balance that accounts for evapotranspiration and runoff. Moisture that infiltrates below the root zone was assumed to be available for recharge to the water table. The model was calibrated using the 1964-81 streamflow record from the USGS gauge on Mosier Creek. Recharge rates for the 1960-63 and 1982-2004 periods were simulated using the calibrated model with existing climate data for the periods.

A baseflow recession analysis was performed on the historic streamflow record from Mosier Creek to provide an independent estimate of ground-water contribution to streamflow for comparison with the watershed model results.

Recharge from irrigation return flow was estimated for the 1960-2004 period by reviewing information on irrigated acreage, crop type, and irrigation practices. This source of recharge was be minimal, particularly since sprinkler and drip irrigation are widely used.

Recharge from losing reaches of streams was evaluated using stream-gaging data and information from previous studies, seepage measurements that will be conducted for this study, and the gaging station installed for this study. See the section below on estimation of ground-water discharge to streams.

Recharge from subsurface inflow of groundwater into the Mosier Creek Basin was evaluated based on geologic and structural mapping, hydraulic head mapping, and as part of the ground-water flow model development process.

Ground-Water Discharge Components

Discharge to wells was estimated using data from previous studies (Lite and Grondin, 1988), the State Water Rights Information System database (WRIS), consumptive use data from the Oregon State Extension Service, and information obtained from local irrigators.  Selected irrigation wells were metered. Electric power consumption data was used to estimate pumping. Current (2005) irrigation withdrawals were estimated based on the best available information listed above; historical irrigation pumping was estimated for the 1960-2004 period by extrapolating from relations between current water rights and current actual water use.  Municipal pumping records wee obtained from the City of Mosier, and domestic well pumping was estimated based on average per capita household use typical of the area.

Discharge to streams was estimated using continuous streamflow data from the gaging station established for this study, data from previous studies, and detailed gain/loss measurements on Mosier Creek. Other techniques, including heat tracing, mini-piezometer surveys, and seepage meters, were employed to better quantify ground-water discharge to streams. Localized discharge to springs was inventoried, including springs that historically discharged but have stopped flowing due to pumping.

Discharge by evapotranspiration from the water table is typically difficult to estimate independently. This study will delineate areas where phreatophytes occur (from existing vegetation maps) and the water-table is within 10 feet of land surface (maximum root depth). The upper and lower limits of potential evapotranspiration were estimated for this area and used for comparison with model-simulated evapotranspiration during calibration.

Subsurface outflow from the Mosier Creek Basin was estimated in the same manner as subsurface inflow using geologic and hydraulic head mapping.

Estimate Interaquifer Flow

Ground-water flow between aquifers occurs by natural movement through undisturbed geologic materials and by vertical leakage through well bores that are open to multiple hydrogeologic units. Natural leakage is difficult to estimate accurately because of the difficulty in obtaining values for the vertical hydraulic conductivity of aquifers and intervening confining beds. Vertical hydraulic conductivity was estimated as part of the model calibration processes, and the vertical flow between aquifers was estimated using the model.

Leakage through well bores was estimated by first screening all wells to determine how many are potentially co-mingling ground water. The screening process looked at how many aquifers the well is open to, the elevation of the geologic contact between the aquifers, and the hydraulic heads in the well and the adjacent aquifers. A subset of about 10 wells was selected for analysis of well bore leakage. A suite of borehole geophysical logs, including electromagnetic flowmeter, video, acoustic televiewer and deviation, caliper, gamma, fluid resistivity, and temperature, was collected from each well. Flowmeter data provided actual measurements of flow, and the other logs were used to interpret the well construction and hydrogeologic conditions unique to each well.

Develop Ground-Water Model

The ground-water model integrated all of the data collected and concepts developed as part of the tasks described above. All of this information has value independent of the model; however, the model added value to the information by integrating it into a tool that can be used to evaluate the sustainability of the resource at various levels of development and management.

The general approach to development of the model was to (1) design and set up the model, (2) simulate historical conditions from 1960 through 2004, and (3) simulate alternative future scenarios.

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