USGS Washington Water Science Center
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Objectives of Study
The overall objectives of the study are to fully describe the groundwater flow system and its interaction with and relation to surface water, and to provide baseline information for a management tool-a numerical model of the system. The conceptual model of the flow system and the results of the study can be used to guide and support actions taken by management agencies with respect to groundwater availability and to provide information to other stakeholders and interested parties. The numerical model was developed as an integrated tool to assess short-term to long-term management activities, including the testing of the potential effects of alternative management strategies for water development and use. The study includes three phases. The first phase included (1) project planning and coordination, (2) compiling, documenting, and assessing available data, and (3) initial data collection. The second phase consisted of data collection to support the following Phase 2 work elements: (1) mapping of hydrogeologic units, (2) estimating groundwater pumpage, (3) developing estimates of groundwater recharge, (4) assessing groundwater-surface water interchanges, and (5) constructing maps of groundwater levels. Together, these five elements provide the information needed to describe the groundwater flow system, develop the conceptual model, and provide the building blocks for the hydrogeologic framework. In the third phase, a regional-scale numerical model of the groundwater flow system was constructed in order to integrate the available information. The numerical model was used to enhance understanding of the flow system (including a water budget for the aquifer system) and its relation to surface water, and to test alternative management strategies.
The Yakima River basin aquifer system underlies about 6,200 square miles in south-central Washington. The aquifer system consists of basin-fill deposits occurring in six structural-sedimentary basins, the Columbia River Basalt Group (CRBG), and generally older bedrock. The basin-fill deposits were divided into 19 hydrogeologic units, the CRBG was divided into three units separated by two interbed units, and the bedrock was divided into four units (the Paleozoic, the Mesozoic, the Tertiary, and the Quaternary bedrock units). The thickness of the basin-fill units and the depth to the top of each unit and interbed of the CRBG were mapped. Only the surficial extent of the bedrock units was mapped due to insufficient data. Average mapped thickness of the different units ranged from 10 to 600 feet.
Lateral hydraulic conductivity (Kh) of the units varies widely indicating the heterogeneity of the aquifer system. Average or effective Kh values of the water-producing zones of the basin-fill units are on the order of 1 to 800 ft/d and are about 1 to 10 ft/d for the CRBG units as a whole. Effective or average Kh values for the different rock types of the Paleozoic, Mesozoic, and Tertiary units appear to be about 0.0001 to 3 ft/d. The more permeable Quaternary bedrock unit may have Kh values that range from 1 to 7,000 ft/d. Vertical hydraulic conductivity (Kv) of the units is largely unknown. Kv values have been estimated to range from about 0.009 to 2 ft/d for the basin-fill units and Kv values for the clay-to-shale parts of the units may be as small as 10-10 to 10-7 ft/d. Reported Kv values for the CRBG units ranged from 4×10-7 to 4 ft/d.
Variations in the concentrations of geochemical solutes and the concentrations and ratios of the isotopes of hydrogen, oxygen, and carbon in groundwater provided information on the hydrogeologic framework and groundwater movement. Stable isotope ratios of water (deuterium and oxygen-18) indicated dispersed sources of groundwater recharge to the CRBG and basin-fill units and that the source of surface and groundwater is derived from atmospheric precipitation. The concentrations of dissolved methane were larger than could be attributable to atmospheric sources in more than 80 percent of wells with measured methane concentrations. The concentrations of the stable isotope of carbon-13 of methane were indicative of a thermogenic source of methane. Most of the occurrences of methane were at locations several miles distant from mapped structural fault features, suggesting the upward vertical movement of thermogenic methane from the underlying bedrock may be more widespread than previously assumed or there may be a more general occurrence of unmapped (buried) fault structures. Carbon and tritium isotope data and the concentrations of dissolved constituents indicate a complex groundwater flow system with multiple contributing zones to groundwater wells and relative groundwater residence time on the order of a few tens to many thousands of years.
Potential mean annual recharge for water years 1950-2003 was estimated to be about 15.6 in. or 7,149 ft3/s (5.2 million acre-ft) and includes effects of human activities such as irrigation of croplands. If there had been no human activities (predevelopment conditions) during that time period, estimated recharge would have been about 11.9 in. or 5,450 ft3/s (3.9 million acre-ft). Estimated mean annual recharge ranges from virtually zero in the dry parts of the lower basin to more than 100 in. in the humid uplands, where annual precipitation is more than 120 in.
Groundwater in the different hydrogeologic units occurs under perched, unconfined, semiconfined, and confined conditions. Groundwater moves from topographic highs in the uplands to topographic low areas along the streams. The flow system in the basin-fill units is compartmentalized due to topography and geologic structure. The flow system also is compartmentalized for the CRBG units but not to as large an extent as in the basin-fill units. Regional groundwater flow discharges to surface-water drainage features in the lowlands in the structural basins. The gradient of the water table ranges from about 7 ft/mi to more than 400 ft/mi, with the smaller gradients in the topographically smooth parts of the structural basins. Typically, hydraulic gradients are similar to the topographic gradients. The lateral hydraulic gradient in the CRBG units also is highly variable and ranges from about 3 to 14 ft/mi in the flatter parts of the structural basins to as much as 800 ft/mi in areas of steep terrain. The hydraulic gradient of the CRBG units generally is within 5 degrees of the topographic gradient.
About 312,000 acre-ft (about 430 ft3/s) of groundwater was pumped in 2000 for multiple uses, about 60 percent of which was for irrigation. Allowable acreage of groundwater irrigation rights is about 130,000 acres. Mean annual surface-water diversions are about 5,800 ft3/s, of which about 66 percent was delivered for irrigation and 25 percent was for power production.
Excluding the initial water-level rises due to surface-water irrigation that started in the late 1800s, long-term measurements of the levels indicate they have been stable or have declined less than 20 ft in most of the aquifer system. Declines from 21 to 300 ft have occurred in some areas due to pumpage. The largest declines have been in the CRBG units. The pumpage from the basin-fill units, from which about 50 percent of the pumpage occurs, appears to have caused declines in only two small areas, but the withdrawals may be reducing groundwater discharge.
Five categories of data are analyzed to enhance understanding of river-aquifer exchanges-the processes by which water moves between stream channels and the adjacent groundwater system-in the Yakima River basin. The five datasets include (1) results of chemical analyses of water for tritium (3H, a radioactive isotope of hydrogen) and the ratios of the stable isotopes of hydrogen (2H/1H) and oxygen (18O/16O), (2) series of stream discharge measurements within specified reaches (seepage investigations or "runs"), (3) vertical hydraulic gradients (between stream stage and hydraulic heads the underlying aquifer) measured using mini-piezometers, (4) groundwater levels and water temperature in shallow wells near stream channels, and (5) thermal profiles (continuous records of water temperature along river reaches). Exchanges are described in terms of streamflow, vertical hydraulic gradients, groundwater temperature and levels, and streamflow temperature, and where appropriate, the exchanges are discussed in terms of their relevance to and influence on salmonid habitat.
A regional, three-dimensional, transient numerical model of groundwater flow was constructed for the Yakima River basin aquifer system to better understand the groundwater-flow system and its relation to surface-water resources. The model described in this report can be used as a tool by water-management agencies and other stakeholders to quantitatively evaluate proposed alternative management strategies that consider the interrelation between groundwater availability and surface-water resources.
Five applications (scenarios) of the model were completed to obtain a better understanding of the relation between pumpage and surface-water resources and groundwater levels. For the first three scenarios, the calibrated transient model was used to simulate conditions without: (1) pumpage from all hydrogeologic units, (2) pumpage from basalt hydrogeologic units, and (3) exempt-well pumpage. The simulation results indicated potential streamflow capture by the existing pumpage from 1960 through 2001. The quantity of streamflow capture generally was inversely related to the total quantity of pumpage eliminated in the model scenarios. For the fourth scenario, the model simulated 1994 through 2001 under existing conditions with additional pumpage estimated for pending groundwater applications. The differences between the calibrated model streamflow and this scenario indicated additional decreases in streamflow of 91 cubic feet per second in the model domain. Existing conditions representing 1994 through 2001 were projected through 2025 for the fifth scenario and indicated additional streamflow decreases of 38 cubic feet per second and groundwater-level declines.
Completed Major Study Components
Sediments and Basalts--Mapped Hydrogeologic Units
Hydraulic Characteristics of Units
Compilations and Analyses--Background Information and for Use in Regional Groundwater Flow Model
Compilation, Collection, and Analyses of Data--Provided Information on Flow System
Groundwater in the Yakima River Basin
Mapped Groundwater Levels and Described Flow System
Related Water Rights to Wells
Estimated Groundwater Pumpage from 1960 to 2001
Compiled Observed and (or) Estimated Surface-Water
Diversions--Monthly for 1960 to 2001
Compiled Historical Data and Input into National Database
Collected Water-Level Data
Mapped and Described Groundwater-Level Trends
Water Budget for the Yakima River Basin
Developed a Water Budget for the Yakima River Basin
River-Aquifer Exchanges in the Yakima River Basin
Described the exchanges and their controls for the shallow system
Construction of a Regional Groundwater Flow Model
Constructed, calibrated, and applied the regional model