REPORT OF INVESTIGATIONS No. 40


water or surface water by industrial wastes; or structures that alter
drainage, infiltration, or runoff characteristics.
 LARGE DRAWDOWNS
 Industries usually require a continuous supply of water. Pumping of
ground water causes drawdowns of the water level in proportion to the
number and spacing of wells and the rate of pumping. Such drawdowns
increase the cost of pumping water, but more important, may cause the
cone of depression to extend outward farther than is desirable and may
cause local depletion and reduced well yield.
 An example of large drawdowns can be found at Cantonment. Figure
34 shows that the water level in well 036-719-1 declined more than 42
feet from 1941 to 1956. This decline was mainly the result of heavy
pumping although low rainfall was a contributing factor. During the 15-
year period, the water level declined at an average rate of 2.8 feet per
year. The reasons for the rise of the water level from 1957 to 1962 are
covered on page 68.
 SALT-WATER ENCROACHMENT
 Salt-water encroachment can be a serious "side effect" when water
 levels near bodies of salt water are lowered. If the sediments between
 the salt-water body and the ground-water aquifer are relatively imperme-
 able, the rate of salt-water encroachment is slow; if these sediments are
 relatively permeable, the rate of encroachment is much higher.
 An example of a slow rate of encroachment is shown by industrial
 wells near Bayou Chico where several years of heavy pumping lowered
 water levels below sea level (fig. 37). As a result of this lowering, water
 from wells nearest the bayou slowly became salty, and it became neces-
 sary to drill replacement wells farther away from the bayou. Usually
 several years were required after the chloride content of the water from
 a well started to increase before the water became too salty for use.
 Pumping of ground water at Newport Industries averaged 2.5 mgd
 from 1928 to 1933, The average ground-water level at Bayou Chico-
 prior to pumping was about 7 feet above sea level. Jacob and Cooper
 (1940, p. 60-64) calculated that the minimum time required for salt wa-
 ter from Bayou Chico to move 2,000 feet to the Newport Industries well
 field would be about 3.1 years, on the assumption that there was close
 interconnection of the aquifer and bayou and that permeability was
 constant, They further determined that salt-water encroachment from
 Bayou Chico would not begin until the ground-water level had been
 lowered 7 feet and that it would take a pumping rate of more than 3.4