The tidal efficiencies of wells tapping the principal artesian aquifer in Glynn County, Georgia decrease with distance from the influencing tidal body and also decrease with well depth. Although the magnitude of water‐level fluctuation of these wells is largely dependent upon the distance to a large tidal body, the time of a high or a low water level in the wells is mainly dependent upon the time of a high or a low tide in a small, nearby tidal body. A modified formula for computing tidal efficiency is developed:
From this formula, tidal efficiency can be calculated rapidly and easily
A gravity survey of the Toppenish Creek basin, tributary to the Yakima River basin, and surrounding area in the northwest part of the Columbia River Plateau shows a broad, 6-milligal gravity low over much of the basin. The low, which is somewhat smaller than expected, is probably due to about 300 m (1,000 ft) of the Ellensburg Formation and younger sediments that overlie the Yakima Basalt within the structural basin. Analysis of the gravity data suggests that: (1) the Toppenish Creek basin has a relatively flat bottom; (2) the basin is enclosed on the southeast by a broad, buried ridge between Toppenish Ridge and Snipes Mountain; (3) several nearby valley areas contain very little sediment; (4) the thickness of basalts within the study area varies considerably; and (5) as many as four small silicic volcanic cones of pumicite deposits may be buried within the younger Yakima Basalt along a possible north-south fracture zone. Differences in pattern between the main part of Toppenish Creek basin and the shallow valley areas on the aeromagnetic map lend support to the gravity interpretation.
Water‐level declines in the principal artesian aquifer have created a head imbalance between the aquifer and an underlying brackish‐water zone containing up to 4,550 mg/1 chloride. The brackish‐water zone leaks brackish water into the aquifer through several breaks in a confining unit.
A relief well tapping the brackish‐water zone was drilled near a suspected break and pumped at about 3,000 gpm to lower the potential in the zone and bring it into hydrostatic equilibrium with the aquifer. The pumping apparently succeeded in decreasing the rate of brackish‐water leakage into the aquifer. Successive samples of water from a well tapping the aquifer and downgradient from the relief well showed a decrease in the chloride content. Several more relief wells may be necessary to ultimately control chloride contamination of the aquifer.
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