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 comprises 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 will be compiled and evaluated for use in this investigation. All pertinent data will be 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 will consist of inventorying wells, ground-water level monitoring, and streamflow measurements. Additional types of data collection, such as borehole geophysical logs, will be 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 will be reviewed and areas with data gaps will be targeted for locating additional wells if new wells have been constructed in these areas. As many as 50 new wells may be inventoried to provide geologic data and water levels.

The existing OWRD ground-water level monitoring network will be supplemented with approximately 20-30 wells that will be measured bi-monthly and 5-10 wells that will have continuous monitors installed that collect levels every 2 hours. Two synoptic water-level measurements will be 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 50-100 wells, will be used to construct detailed hydraulic head maps. These data will enhance 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 will be used to calibrate the ground-water model.

A gaging station will be installed on Mosier Creek at the site of the former USGS station (14113200). The station will measure discharge every 15 minutes and have telemetry to make data available in real time via the USGS NWIS site (http://waterdata.usgs.gov/or/nwis). Manual discharge measurements will be 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 may allow us to detect seasonal effects of climate and pumping on ground-water discharge to Mosier Creek, and they will be used to calibrate the ground-water flow model.

Temperature sensors will be deployed at several locations on Mosier Creek for at least one summer and fall season 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 will build upon this foundation to prepare a three-dimensional representation of the hydrogeologic units that is needed for the ground-water flow model. Any new well information available since 1985 will be incorporated to produce maps of the thickness and extent of each major hydrogeologic unit.

The hydraulic characteristics of each aquifer and confining unit will also be estimated. This study will rely 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. This study will also look for opportunities to conduct aquifer tests in the Mosier Basin to fill data gaps. The best opportunities would be where pumping and observation wells can be found that are completed in one unit, are located within distances that a measurable response could be expected within a reasonable time frame, and are 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 will be estimated by constructing a watershed model. The model will use 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 will be assumed to be available for recharge to the water table. The model will be calibrated using the 1964-81 streamflow record from the USGS gauge on Mosier Creek. Recharge rates for the 1960-63 and 1982-2004 periods will be simulated using the calibrated model with existing climate data for the periods.

A baseflow recession analysis will be 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 will be estimated for the 1960-2004 period by reviewing information on irrigated acreage, crop type, and irrigation practices. This source of recharge may be minimal, particularly since sprinkler and drip irrigation have been widely used.

Recharge from losing reaches of streams will be 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 will be 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 will be 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 any information that can be obtained from local irrigators.  Selected irrigation wells will be metered if permission can be obtained from owners. Electric power consumption data may also be used to estimate pumping if a sufficient number of irrigators will agree to have power consumption monitored. Current (2005) irrigation withdrawals will be estimated based on the best available information listed above; historical irrigation pumping will be estimated for the 1960-2004 period by extrapolating from relations between current water rights and current actual water use.  Municipal pumping records will be obtained from the City of Mosier, and domestic well pumping will be estimated based on average per capita household use typical of the area.

Discharge to streams will be estimated using continuous streamflow data from the gaging station to be established for this study, data from previous studies, and detailed gain/loss measurements to be made on Mosier Creek. Depending on the results of the gain/loss measurements, other techniques including heat tracing, mini-piezometer surveys, and seepage meters might be employed to better quantify ground-water discharge to streams. Localized discharge to springs will be 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 will be estimated for this area and used for comparison with model-simulated evapotranspiration during calibration.

Subsurface outflow from the Mosier Creek Basin will be 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 very 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 can be estimated as part of the model calibration processes and the vertical flow between aquifers can be estimated using the model.

Leakage through well bores will be estimated by first screening all wells to determine how many are potentially co-mingling ground water. The screening process will look 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. Depending on the number of potentially co-mingling wells and the degree of cooperation from owners, a subset of 5-10 wells will be 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, will be collected from each well. Flowmeter data will provide actual measurements of flow and the other logs will be used to interpret the well construction and hydrogeologic conditions unique to each well. Analysis will determine if relations between geologic factors and water level can be used to accurately estimate well-bore leakage at other co-mingling wells without making direct measurements. If not, and the subset of wells tested suggest that the magnitude of well bore leakage is an important factor in water level declines, the study approach may have to be modified to test the remaining wells.

Develop Ground-Water Model

The ground-water model will integrate all of the data collected and concepts developed as part of the tasks described above. All of this information will have value independent of the model; however, the model will add 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 will be 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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