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AAPG Bulletin

Abstract


Volume: 26 (1942)

Issue: 5. (May)

First Page: 838

Last Page: 851

Title: Base-Exchange and Sulphate Reduction in Salty Ground Waters Along Atlantic and Gulf Coasts

Author(s): Margaret D. Foster (2)

Abstract:

Salty waters encountered in water-bearing sands along the Atlantic and Gulf coasts do not appear to be simple mixtures of ground water with more or less sea water. When the salty waters are compared with theoretical mixtures of fresh ground waters and sea water--the ground waters being from comparable depths in the same formations and the amount of sea water being that indicated by the chloride contents of the salty waters--the natural salty waters are found to be characteristically lower in calcium, magnesium, and sulphate and higher in sodium content, suggesting that the waters have undergone base-exchange and reduction of sulphate.

The indicated replacement of magnesium and calcium in the salty waters by sodium from base-exchange minerals in the sands suggests that the base-exchange minerals are not in equilibrium with present-day sea water or with salty waters formed by its admixture with fresh ground waters in which sodium bicarbonate is the predominant constituent. These relations further suggest that the water-bearing sands have at some time been flushed of salt water, at least to a point farther down the dip than at present, by water having a lower Ca/Na and Mg/Na ratio and that the salt water now contaminating the fresh waters is a new advance inland of salt water.

The low sulphate content of the salty waters is attributed to reduction of sulphate, but whether the causative agent of the reduction was living organisms or inanimate organic matter or whether reduction took place at the time the sediments were deposited or subsequently can not be proved definitely at the present time. The parent salt waters of these salty waters may have been connate waters in which sulphate had been reduced--as reduction is known to take place in environmental conditions like those under which some of the sediments in the Coastal Plain were laid down. In the flushing implied by the low calcium and magnesium content of the waters the connate waters may have been forced seaward but may not have been completely flushed from the beds and subsequent lowering of the fres -water head relative to the head of sea water may have permitted the connate water to move farther inland.

The fact that unselected salty waters from widely different sources, both geographically and geologically, in the Atlantic and Gulf Coastal Plain have apparently undergone similar alterations in mineral composition seems to indicate that the conditions causing these alterations are rather general throughout the area.

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