The policy stated in the Willamette Basin Report is to "minimize impairment of surface-water uses resulting from hydraulic connection between ground water and surface water." Except where wells are very close to streams or producing water from channel deposits directly connected to streams, there is currently no satisfactory way to evaluate the effects of ground-water pumping on streamflow. This is particularly true for evaluating the cumulative effects of many wells. A sound, quantitative understanding of the ground-water hydrology, the nature of the ground-water/surface-water connection, and streamflow characteristics is necessary for coordinated management of ground- and surface-water resources. This understanding has yet to be developed in the Willamette Basin.
Declining ground-water levels were documented locally in the Willamette Valley as early as the 1960s. Water-level declines cause serious water supply-problems and hardship for rural home owners as well as for municipalities and irrigators, and threaten the long-term sustainability of the resource. The Willamette Basin Report lists 8 areas with existing water-level decline problems and 4 with potential decline problems. Of these 12 areas, all but one involve aquifers within the Columbia River Basalt Group. The basalt aquifer is the principal or sole aquifer in uplands within and surrounding the Willamette Valley, and beneath basin-filling sediments in certain areas in the valley. Because of similar hydrogeologic conditions, the sustainability of ground-water supplies from all Columbia River Basalt Group aquifers in the basin is of concern.
The Willamette Basin Report proposes a policy to "prevent excessive water-level declines and restore aquifer stability while preserving limited storage for priority uses in the vicinity." Significant efforts have been made by OWRD staff over the years to collect water-level data and geologic information in some of the basalt areas in the basin and these efforts have led to a general, qualitative understanding of basalt hydrology has been developed. However, many aspects of basalt hydrology are still poorly understood, such as structural controls on ground-water flow, recharge mechanisms and rates, the nature of ground-water flow between water-bearing units, and details of the head distribution. Knowledge of all these hydrologic characteristics is necessary for effectively managing development and use of basalt aquifers, including well construction.
Low-yield aquifers are a problem primarily in the marine sedimentary rock of the Coast Range, and volcanic rock of the Cascade Range foothills. The marine sedimentary rock primarily consists of fine-grained sandstone, siltstone, and mudstone with very low permeability. Rock in the Cascade foothills consists of sediments of similar grain size but of volcanic origin, and weathered or altered lava flows. In general, these materials do not readily transmit water and well yields are small. In some cases, wells that intersect permeable fractures within these rocks will have yields that are larger then typically found in the area. These fractures, however, generally have limited storage, and because the water stored in the fracture ultimately comes from the fine-grained rock, the large yields are not sustained. Marginal aquifers are particularly sensitive to climate effects, such as drought, and to interference problems between wells. Low-yield aquifers can cause serious water-supply problems, particularly for rural landowners that have no alternative sources of water.
The Willamette Basin Report outlines a policy to "identify low-yield aquifers and inform local agencies of probable insufficient ground-water flow for some uses." Geologic maps showing the location of rock types with low-permeability are available for all of the Willamette Basin at various scales. In most of the basin, however, the water-bearing characteristics of these rocks have not been characterized or systematically described.
High salinity and high arsenic concentrations are the two major natural water-quality problems in parts of the Willamette Basin. Ground water with high salinity is commonly encountered in the marine sedimentary rocks in the Coast and Cascade Range foothills and the valley margins adjacent to these areas. The salt in these rocks most likely originates from sea water that was incorporated into the sedimentary rock when it was originally deposited millions of years ago. This saline water mixes with shallow ground water in certain areas where deep regional ground water is upwelling. Saline water is also encountered in the Columbia River Basalt Group aquifers where they overlie marine sedimentary rocks. Excessive pumping of basalt aquifers is known to induce the flow of saline water from the underlying marine sedimentary rock and increase the salinity of ground water.
Ground water with elevated arsenic concentrations is commonly encountered in the southern Willamette Basin. High arsenic concentrations are thought to coincide with the presence of volcanic rocks of one or two geologic formations. A systematic analysis of the distribution of arsenic in ground water has not been done, nor has the exact mineral source of the arsenic been identified.
In addition to high salinity and arsenic concentrations, elevated levels of radon and phosphorous are also encountered locally in the Willamette Basin. Ground water enriched with phosphorous in the Tualatin, Pudding, and Yamhill River basins is thought to contribute to eutrophic conditions as well as other water-quality problems.
The policy stated in the Willamette Basin Report related to ground-water quality is to "ensure safe municipal and domestic ground-water supplies by participating with DEQ [Oregon Department of Environmental Quality] and the Oregon State Health Division in a formal monitoring program to document changes in [water] quality and provide data for aquifer management." Information on the geology and hydrology of water with high salinity, arsenic, radon, and phosphorous concentrations would help in implementation of this policy.
Last modified: Fri Feb 9 08:49:09 1996