Ground-water flow and water quality in the sand aquifer of Long Beach Peninsula, Washington

Abstract

This report describes an investigation of ground-water flow and water quality in the sand aquifer of the Long Beach Peninsula. The peninsula is located in the southwestern corner of the State of Washington, is about 27 miles long, and has an average width of about 1.5 miles. It is surrounded by seawater, by the Pacific Ocean on the west and Willapa Bay on the east. Water supplies on the peninsula are derived mostly from a local water-table aquifer composed largely of sand. The recent growth of population on the peninsula and the projected future growth have created concerns about the quantity and quality of the ground-water resource. Some issues include declining ground-water levels from increased pumpage, and ground-water contamination from seawater intrusion, pesticides or fertilizers from cranberrygrowing areas, and septic-system effluent. The ground-water system of the Long Beach Peninsula consists of a sand aquifer with some lenses of silt and clay that may act as confining beds in local areas. Data are lacking or inconsistent to define a confining bed that extends throughout the peninsula. Hydraulic conductivity calculated from slug tests in 58 shallow wells ranged from 10 to 37 feet per day with a median of 22 feet per day. Average annual ground-water recharge by infiltration and percolation of precipitation is estimated to be about 58 inches or 111,000 acre-feet, which is 72 percent of the average annual precipitation of 80 inches. Average annual ground-water discharge is estimated to be about 30,200 acre-feet to the Pacific Ocean, 56,000 acre-feet to Willapa Bay, and 24,800 acre-feet to surface-water drainage channels. Ground-water movement is generally perpendicular to the spine of the peninsula. A ground-water divide occurs along a north-south line and ground water flows west or east from the divide toward the Pacific Ocean or Willapa Bay. There does not appear to have been any long-term decline of the water table of the sand aquifer from 1974-92. Ground-water levels measured at three east-west cross sections in 1974-75 were at about the same altitude as water levels measured in 1992. Relatively accurate individual regression relations were developed at 45 wells with ground-water altitude as a response variable and cumulative precipitation for 4 months as an explanatory variable. The average coefficient of determination for all individual relations was 0.77, with a range of 0.11 to 0.89. Some empirical frequency or probability relations for precipitation and ground-water levels were used to estimate how often the maximum water levels measured in this study would be expected to occur in the future. These water levels reflected the lower-than-average precipitation that occurred during the study. Assuming that the annual maximum precipitation for 4 consecutive months is random and independent, the historical record of precipitation is representative of the future distribution of precipitation, and the relation between precipitation and water levels is accurate and stationary; a probability analysis of the historical record indicates that in any one year in the future there is a probability of 70 percent that the maximum water levels measured in wells during the winter of 1991-92 would be equaled or exceeded. The shallow ground water had generally low dissolved-solids concentrations in July 1992, with a median concentration of 92 milligrams per liter (mg/L) and a range of 56 to 218 mg/L. Sodium was the dominant cation and bicarbonate was the dominant anion. The distribution of hardness of the water samples was 84 percent with soft water and 16 percent with moderately hard water. The water quality of the shallow ground water was generally good, with a few small to moderate problems. A natural problem is locally high concentrations of dissolved iron. About 30 percent of the water samples had dissolved-iron concentrations of greater than 0.3 mg/L, which is the secondary maximum contaminant level established by the U.S. Environmental Protection Agency. No appreciable amount of seawater has intruded into the sand aquifer. The samples of shallow ground water collected in July 1992 had a median chloride concentration of 15 mg/L and a maximum concentration of 52 mg/L. The heavy average annual precipitation of about 80 inches, large average annual ground-water recharge of about 58 inches or 111,000 acre-feet, and small ground-water withdrawal rate (about 780 acre-feet per year in 1992) combine to maintain a thick freshwater lens of ground water that prevents seawater intrusion throughout the year. Agricultural activities do not appear to have appreciably affected the quality of shallow ground water on the Long Beach Peninsula. The concentration of nitrate in ground water was not significantly higher near cranberry-growing areas, and no sample of ground water or surface water had concentrations of selected pesticides or associated compounds that were above the analytical detection limits. Of the seven ground-water samples in which bacteria were detected, only one sample appeared to be related to agriculture; that sample was from a well located in an area where cattle graze for part of the year. Septic systems probably caused an increase in the concentration of nitrate in shallow ground water in areas of higher population density. Concentrations of nitrate were significantly related to population density. However, the concentrations were not generally high; median concentrations of nitrate increased from less than 0.05 mg/L in areas of low population density to 0.74 mg/L in areas of high density. Septic systems did not cause regional bacterial contamination of the ground water. Bacteria were detected in seven ground-water samples; however, only two of those samples were from wells that are close to septic systems. A limited amount of historical water-quality data is available for the peninsula; therefore, it is difficult to assess long-term changes. From 1968-92, chloride concentrations and values of specific conductance appear to have remained stable. Likewise, it appears that nitrate concentrations did not change from 1987-92.