ART. XXVII.—-Relation of Sea Water to Ground Water along Coasts;1 by JOHN S. BROWN.

Introduction.

Sea water in shallow wells.

Sea water in deeper, drilled wells.

CONTENTS.

Effect of special topographic and geologic conditions.
Law of equilibrium between fresh and salt water.
Nature of the contact between fresh and salt water.
Effects of pumping on salt content.

Seasonal variations in salt content.
Influence of tides on ground water.

Introduction.

Coastal ground-water problems:-The water from wells and springs near seacoasts is often salty. On most coasts saltiness due to sea-water is limited in shallow wells to a narrow zone, usually a few hundred feet in width, near the shore, but at some places it extends much farther inland. Many wells near the shore encounter fresh water near the surface and salt water at greater depths. Pumping plants often yield salty water, especially after long-continued operation at a high rate. The circumstances of contamination by sea water are modified greatly by the kind of rocks and their structure and by the topography adjacent to the shore, as well as by climatic factors.

Scope of field work. In 1919 the writer made a study of the groundwater resources of the New Haven area, Connecticut.2 This area includes a section of the coast of Connecticut, lying east and west of New Haven, which will be referred to hereafter as the New Haven coast. The length of this section is 30 miles in a straight line, but owing to small irregularities, the actual coast line is much longer than 30 miles. This work afforded a particularly favorable opportunity to observe some of the conditions under which ground water is contaminated by sea

Published by permission of the Director of the U. S. Geological Survey. The illustrations were prepared by the Survey.

2 Brown, John S.: Ground water in the New Haven area, Connecticut, U. S. Geol. Survey Water-Supply Paper [awaiting ¡ublication].

This work was done in co-operation with the Connecticut State Geological and Natural History Survey, under the direction of Herbert E. Gregory.

water.3 Some valuable information also was obtained during a few days spent on the coast of Florida in April, 1920.

Previous investigations.-In the United States many geologists and engineers have described contamination of ground-water by sea water, and a few writers have discussed at some length phases of the problem incidental to other investigations. However, there has been no systematic investigation of the problem or serious attempt to assemble and apply the data already published. In Europe, the denser settlement and more intensive development of water supplies caused the problem to be recognized earlier, and in Holland, Belgium, France, and Germany many data have been collected and numerous papers published on the subject. The present writer has used the published material to supplement the information obtained on the New Haven coast and confirm the conclusions reached.

Methods of investigation.-In the course of this investigation a geologic map of the region was made. Records of wells, springs, and pumping plants and details of their construction, use, and operation were collected. Topographic features were noted carefully. In order to determine the degree of contamination by sea water many samples of water were analyzed. As in many previous investigations, the chloride content of the samples has been used as the chief criterion for determining the presence or absence of sea water. Chloride is not always a reliable indicator of contamination by sea water, for it may come either from salt in the rocks or from sewage and factory wastes. The rocks of Connecticut, however, yield very little salt, and in areas free from polluted drainage the ground water contains but little chloride,— in fact, 10 or 12 parts per million is usually the maximum quantity present from both these sources.5 In this investigation, it was generally possible to tell from the surroundings whether chloride from sewage or waste was to

A complete report, of which this paper is a summary, entitled "A study of coastal ground water, with special reference to Connecticut," is to appear as a Water-Supply Paper of the U. S. Geological Survey.

The term chloride is used here to denote the chloride radicle (Cl).

5 See Jackson, D. D., The normal distribution of chlorine in the natural waters of New York and New England: U. S. Geol. Survey Water-Supply Paper 144, 1905.

be suspected, and samples collected where the circumstances seemed to indicate such pollution were rejected. A small assay kit was carried in the field for the purpose of making approximate determinations of chloride, but check samples were sent to the water laboratory of the United States Geological Survey for analysis. The laboratory analyses are given in this paper.

Sketch of the New Haven coast.-The New Haven coast is a "drowned coast" and is consequently very irregular in outline. There are large areas of flat tidal marshes and in some places many small islands near the shore. Portions of the coast have been cut away by waves, and other portions have been built up. Spits and bar beaches are common features.

The bed rocks of the New Haven coast consist mainly of gneiss and schist. At most places they are concealed by glacial drift, which is generally not more than 30 to 40 feet in thickness, but portions of the coast are bare and rocky, and small exposures of bedrock are common over much of the region. The drift consists of till and stratified drift. The till is generally variable in composition and texture and is in places somewhat clayey. The stratified drift has been re-worked by water and is usually clean, well-bedded sand or gravel. It is confined mainly to the valleys and certain low coastal plains of small extent. Besides these formations there is a recent deposit of black, slimy mud which covers the bottom of New Haven Harbor and the tributary tidal estuaries and whose thickness probably does not exceed 25 or 30 feet.

Occurrence of ground water.-In the bedrocks ground water is held mainly in open fractures, such as joints and cleavage planes, and the water table is irregular. In the stratified drift, which is very porous, the water table is in general fairly smooth and stands but little above the level of the sea or above adjacent streams. In the till, owing to its varying porosity, the water table is irregular, but usually water is found within a few feet of the surface.

Sea water in shallow wells.

Most of the shallow wells on the New Haven coast are either dug or driven and are less than 30 feet in depth. More than half of them penetrate stratified drift. The

table below shows the number of wells examined in several successive zones parallel to the shore and the number and percentage of wells contaminated by sea water in each zone. Three degrees of contamination, based on an arbitrary scale, are distinguished "trace" (from 25 to 100 parts per million of chloride), "moderate" (100 to 300 parts), and "high" (more than 300 parts). More than 300 parts per million of chloride is generally apparent to the taste. For a few wells where samples could not be obtained, positive statements of reliable persons have been accepted as evidence of contamination.

Contamination by sea water in shallow wells on the New Haven

The above table summarizes very briefly a large mass of data from which the conclusion is drawn that on the New Haven coast the zone in which the shallow ground water is contaminated by percolation or diffusion of salt water is very narrow. The greatest distance from the shore at which even slight contamination is suspected is 250 feet.

Similar conditions were observed at Cedar Keys, Fla., where the annual rainfall is very nearly the same as at New Haven and where the ground is sandy, comparable to the stratified drift of Connecticut. At Cedar Keys open pits are often constructed on the beaches of small islands to furnish water for stock (fig. 1). The water of the pit shown in fig. 1 contained 516 parts per million of chloride but was usable for stock. Water in a similar pit 120 feet from the reach of normal high tides contained only 92 parts per million of chloride. Of six wells in a zone between the limits of 125 and 350 feet from the hightide shore line, two were slightly contaminated and in another salt water apparently was induced by heavy pumping.

From the evidence set forth above and that given by other writers the conclusion seems warranted that sea water mingles with the shallow ground water over only a narrow area near the shore line. Wherever the natural conditions are disturbed, however, as through a heavy

drain on the ground water by pumping, conditions are immediately altered and sea water may be drawn great distances inland. Also, although data on the matter are lacking, it would be logical to suppose that on arid coasts, where the rainfall is insufficient to maintain a body of ground water, sea water would penetrate a much greater distance inland than in humid regions. Undoubtedly, too,