Hydrologic conditions and artificial recharge through a well in the Salem Heights area of Salem, Oregon

Abstract

In the Salem Heights area of Salem, Oreg., pumping from wells that tap a permeable zone of limited extent in basalt of the Columbia River Group caused serious year-to-year declines of ground-water levels. To determine the feasibility of reducing these declines by artificially augmenting the natural recharge and to develop techniques applicable to a municipal program of artificial recharge, a series of tests was made jointly by the Salem Heights Water District and the U.S. Geological Survey. A total of 24.5 million gallons of surplus water was purchased from the public-supply system of the city of Salem and injected under pressure into one of the municipal wells through the existing pump column during three periods, ranging in duration from 1 to 15 days, at an average injection rate of about 830 gallons per minute. The recharge water contained abundant dissolved air and, at times, excessive sediment; in other respects it was of excellent quality and was compatible with the native ground water. Before the experiments, water in the main aquifer contained unusually large amounts of dissolved oxygen, which apparently was introduced by water cascading from higher zones within unlined intervals of the wells. As a result of the injection, the specific capacity of the well (ratio of pumping yield to drawdown) was reduced temporarily because of clogging of the waterbearing material near the well by sediment and, probably, by bubbles of air which came out of solution in the recharge water. Following each of the last two periods of injection it was necessary to surge the well by intermittent pumping to restore the specific capacity. The artificial recharge had no apparent deleterious effects on the quality of the ground water. Sediment that was injected was virtually all removed from the recharge well during pumping and surging, and the chemical quality and bacteriological purity of the ground water did not deteriorate. Pressure rise from the injected water spread rapidly through the permeable aquifer, but the residual buildup of ground-water levels was soon masked by a seasonal rising trend of levels. The geologic and hydrologic conditions, however, preclude the escape of substantial volumes of the recharge water from the Salem Heights area. The conditions appear to be favorable for further artificial recharge of the main aquifer. Changes in the specific capacity of the recharge well provide valuable guidance for subsurface injection operations. Fl F2 ARTIFICIAL RECHARGE OP GROUND WATER INTRODUCTION PURPOSE AND SCOPE OF THE INVESTIGATION In 1960, the Salem Heights Water District and the residents then served by it faced a severe shortage of water. The water district, which supplied most of the water for a 3*4-square-mile suburban area adjacent to Salem, Oreg., owned 12 wells and pumped as much as 260 million gallons of ground water per year. Most of the water, however, was obtained from three wells that tap the same highly productive aquifer. Increasing pumpage associated with rapid suburban development of the area had resulted in progressive declines of water levels in the most productive wells to an extent that dewatering of the principal aquifer was inevitable unless withdrawal from it were decreased or recharge increased. After three deep wells drilled during 1958-60 failed to increase appreciably the capacity of the water district well system, the district arranged to purchase water from the city of Salem. Upon completion of a booster-pump station and pipeline in August 1961, additional water became available to help meet the large summertime demands within the district; consequently, pumping from the public-supply wells was decreased. The interconnection of the two water systems raised the possibility of artificially recharging the Salem Heights ground-water reservoir by using surplus water from the Salem system to build up the supplies of ground water for future long-term and emergency needs. The Salem Heights Water District, with the concurrence of the Oregon State Engineer, requested the U.S. Geological Survey to make a study of the technical feasibility of artificially recharging the principal aquifer and to develop practical techniques that could be used by personnel of a small water-supply agency in a continuing program of artificial recharge. The investigation consisted of a preliminary evaluation of the suitability of conditions for artificial-recharge tests, a study of the geologic and hydrologic conditions in the area, and the conduct and interpretation of artificial-recharge experiments in which water from the Salem water system was injected through one of the water district's wells. The investigation was financed cooperatively by the U.S. Geological Survey and the Salem Heights Water District, and personnel of both agencies participated in the collection of the field data. LOCATION AND EXTENT OF THE AREA Salem Heights is the name given to a rolling upland area of indefinite extent near the southern limit of the city of Salem. It is included in the northeastern part of the more extensive Salem Hills area (% I)SALEM HEIGHTS AREA OF SALEM, OREGON F3