Summary and Conclusions


The Columbia Plateau Regional Aquifer System (CPRAS) covers an area of about 44,000 square miles in a structural and topographic basin within the drainage of the Columbia River in Washington, Oregon, and Idaho. The primary aquifers occur in basalts of the Columbia River Basalt Group (CRBG) and in places, overlying sediment. The system consists of a series of productive basalt aquifers within permeable interflow zones separated by less permeable flow interiors, overlaid locally by aquifers within sedimentary deposits. Groundwater levels have declined throughout much of the CPRAS since the 1970s.


Information was compiled from about 60,000 wells and 450,000 water-level measurements from wells in the CPRAS. Data are from an inventory of published and unpublished well data from many agencies. A subset of these data for the CRBG aquifer wells were used to develop a simple linear groundwater-level trend map for 1968–2009, which illustrates a persistent pattern of widespread groundwater-level declines. Overall declines from the analysis of data from 761 wells in the CRBG aquifers were measured in 72 percent of the wells. The mean of the trends was a decline of 1.9 feet per year 
(ft/yr). Rates of declines greater than 1.0 ft/yr were measured in 50 percent of wells, declines greater than 2.0 ft/ yr in 38 percent of wells, declines greater than 4.0 ft/yr in 29 percent of wells, declines greater than 6.0 ft/yr in 9 percent of wells, and rates of decline greater than 8.0 ft/yr in 4 percent of wells.


The groundwater-level data also were used to identify groups of wells with similar hydraulic heads and temporal trends within limited subregions. Comparisons between adjacent well groups were used to define sets of well groups that delineate areas of overall similar groundwater-flow conditions. Discontinuities in hydraulic head between the sets of well groups were used to help infer the presence of barriers to groundwater flow. These barriers can include geologic features that influence the hydraulic properties of the aquifer, such as changes in lithology or the occurrence of folds and faults. In areas without flow barriers, dissimilarities in response of well groups over time result from the formation of irrigation-derived groundwater mounds or pumping induced regions of decline. The areas of focus for this analysis included the Umatilla area, Oregon, and the Palouse Slope/eastern Yakima Fold Belt (eYFB) in the Columbia Basin Ground Water Management Area (GWMA) consisting of Adams, Franklin, Grant, and Lincoln Counties, Washington.


In the Umatilla area, 286 wells were divided into groups with similar water levels and trends, and large hydraulic gradients between nearby groups defined barriers to groundwater flow. The barriers divide the groundwater-flow system in the Umatilla area into several leaky compartments that occur in vertical and horizontal directions. However, the degree of leakiness is highly variable. Significant findings for the Umatilla area include:


• Most CRBG aquifers are declining, except for a few shallow aquifers. Cumulative declines range from 100 to 300 feet since 1970.

• High horizontal hydraulic gradients and differences in temporal trends in water levels between adjacent well groups define horizontal flow barriers that generally correspond to mapped geologic structural features. Horizontal hydraulic gradients increase from north to south, which generally corresponds to an increase in structural complexity. This implies that recharge from the uplands into the heavily developed areas may be impeded.

• Significant vertical hydraulic gradients have been documented in a relatively small part of the Umatilla area, where the shallow aquifer system is distinct from the deep system. Since the 1970s, downward vertical gradients in these areas have been increasing as water-level declines have occurred in deeper wells due to pumping and commingling of these aquifers. Because the geology generally is conducive to creating vertical hydraulic gradients, the absence of vertical gradients over much of the area may be a consequence of flow through commingling wells resulting in the equilibration of the heads in the aquifers.


In the Palouse Slope of the central GWMA, the largest declining groundwater-level trends (1968–2009) follow a general north-south line through the middle of the GWMA. An analysis of 1,195 wells along major flow paths and through the area of persistent groundwater-level declines indicates that barriers to flow are not as evident in this area as near Umatilla, but well groups were still identifiable based on similar hydraulic heads and response to hydraulic stresses. This is consistent with the geologic interpretation of the Palouse Slope as being a gently folded structure created during subsidence when CRBG lavas were deposited in voluminous sheet flows. In this area, hydrographs of well groups were viewed along general flow paths that have developed as the result of irrigation stresses on the aquifer system. 


Significant findings for the Palouse Slope/eYFB include: 


• Groundwater levels in CRBG aquifers have risen since the 1950s in areas heavily irrigated with surface water and have declined since the 1970s in areas commonly irrigated with groundwater. For wells examined, typical rises in water level under surface-water irrigation areas are about 50 feet, with larger rises occurring locally (for example, in the fault bounded valleys to the south of the Frenchman Hills and the Saddle Mountains). Cumulative declines of 200 feet or greater are common in areas where pumping groundwater is the dominant source of irrigation water.

• Horizontal flow barriers are less apparent in this area than in the Umatilla area.

• Data indicate that significant vertical hydraulic gradients still exist, although much of the aquifer thickness is affected by commingling of wells.

• Prior to development, groundwater flow was from the uplands toward the Columbia and Snake Rivers. Late-time (2000–2010) flow patterns in the area are substantially altered by large-scale irrigation recharge and pumping patterns, and in some cases are reversed relative to pre-development conditions.

• The effect of commingling of water from multiple aquifers and its relative contribution to groundwater-level declines is not clear from these data; however, groundwater-flow modeling currently under development as part of the overall study may aid in quantifying these effects.

• The presence of a groundwater discharge boundary, where the upper aquifers are intersected at land surface by incised canyons and groundwater flows into the sediment-filled coulees, is an alternative to the hypothesis of a groundwater dam to explain local steepening of the hydraulic gradient along the Palouse Slope.


The use of stratigraphic coordinates to identify aquifers with similar water levels and trends worked well in the analysis of the groundwater conditions for the Umatilla area and Palouse Slope/eYFB. This suggests a way to advance the understanding of the geometry of local and regional aquifers by using the elevations of well bottoms for groups of wells with similar water levels and trends. This technique may be used to help identify the influence (or lack of influence) of structural features on groundwater flow or even to infer where previously unmapped structural features may exist. Trend surfaces representing aquifers and lava flow top elevations may be mapped in many areas across the Columbia Plateau, refining the current understanding of major geologic formations into smaller hydraulically important units.


First posted February 5, 2013