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Tacoma, WA 98402

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Long Beach Ground Water

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WA371 - Water-Table Altitudes and Water Quality in the Shallow Aquifer of Long Beach Peninsula, Washington - Completed FY1995

Problem - Long Beach Peninsula, located in the southwestern corner of the State of Washington, extends about 20 miles north of the City of Long Beach, has an average width of about 1.5 miles, and separates Willapa Bay from the Pacific Ocean. Water supplies are derived mostly from a local, shallow, dune- and marine-sand water-table aquifer. In many locations during much of the year the water table is less than ten feet below land surface. Sewage treatment and disposal at most locations is by septic tanks with drainfields. Major industries on the peninsula are tourism, growing and processing of cranberries, processing of oysters that are harvested from Willapa Bay, and processing of other seafood. There is concern that leachate from septic systems and recharge water containing pesticides from cranberry growing areas may be degrading the water quality in the shallow aquifer, and that ground water containing pesticides, nutrients, and bacteria may be discharging to Willapa Bay and affecting oyster production.

Objectives - One objective of this study will be to determine the areal distribution of observed maximum water-table altitudes, minimum depths to the water table from land surface, and the range in water-table altitudes over one calendar year. To the extent that historical data allows, determine if present water-table altitudes are lower than they have been in the past and if present quality of water in the shallow aquifer is poorer than it has been in the past. Determine the directions and rates of ground-water flow and directions of flow between the shallow aquifer and the surface drainage system. Since there is concern that ground-water quality is being degraded, collections and analyzes of water samples to determine concentrations of nitrate, bacteria, chloride, and common ions in the shallow aquifer. Also analyze for the presence of pesticides in the shallow aquifer and surface drains, and if so, estimate for a few selected sites the rate at which pesticides are likely to be transported by ground-water to Willapa Bay.

Approach - As part of the proposed project, ground-water levels at about 90 locations and surface water levels at about 20 sites on streams and lakes that are in good hydraulic connection with the shallow aquifer will be monitored at least monthly for one year. Maps will be prepared showing maximum observed water-table altitudes and ranges and depth to the water table below land surface. Attempts will be made to relate the ground-water levels observed during the study to expected average and extreme highwater levels using relations between historical and ground-water levels and precipitation. Non-parametric statistical tests on individual and annual mean ground-water levels will be used to determine if present ground-water levels are lower than historical levels. Nitrates, bacteria, chloride, and other constituents will be determined at 40 ground-water and 15 surface-water sites. At one-half of these sites concentrations of all common ions will be determined. Pesticides will be determined at 20 ground-water and 8 surface-water sites.

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