Industry, commerce, and public utilities in 1954 withdrew about 1,500 mgd from surface- and groundwater sources in the New Orleans area. Most of the withdrawal was made from the Mississippi River. However, some withdrawal of surface water was made from Lake Pontchartrain. A large part of the withdrawal from both ground- and surface-water sources is available for reuse. Ground-water withdrawal amounts to about 100 mgd and is primarily for industrial and commercial uses. The average flow of the Mississippi River for the 23-year period, 1931--54, amounted to 309,000 mgd, and the approximate average flow of all the tributaries to Lake Pontchartrain is about 4,000 mgd. The flow of the Pearl River, which adjoins the tributary drainage area of Lake Pontchartrain, averages about 8,000 mgd. Total withdrawal of ground and surface waters amounts to less than 3 percent of the recorded minimum flow of the Mississippi River or less than 1 percent of the average flow. Although large quantities of water are always available in the Mississippi River the quality of the Water is not suitable for all uses.
Streams from the north that drain into Lakes Maurepas and Pontchartrain, and the aquifers in that area, offer one of the best sources of fresh water in the State. Industry, if located on the northern shores of Lake Maurepas or Lake Pontchartrain near the mouths of these tributaries, would be assured of an ample supply of either ground or surface water of excellent quality. All the tributaries north of Lake Pontchartrain have dry-weather flows which are dependable. The Pearl River above Bogalusa also is a good source of fresh water of excellent quality. At present it serves to dilute the tidal flow of salt water into Lake Pontchartrain through the Rigolets, the principal outlet of the lake.
In the area north of Lake Pontchartrain, wells 60 to 2,000 feet deep yield fresh water. There are no known wells tapping sands below 2,000 feet. However, electrical logs of. oil-test wells show that fresh water is available to a maximum depth of 3,000 feet. In the area south of Lake Pontchartrain, there is no withdrawal of ground water for public water supplies because of the saline content of the water. Three principal water-bearing sands, the '200-foot, ' '400-foot, ' and '700-foot'sands, are tapped in the New Orleans area south of Lake Pontchartrain for industrial and commercial use. In this area all deeper sands yield salt water.
In some areas the '200-foot' sand contains saline water of the sodium chloride type. Consequently, this sand is not developed extensively. Water from the 200-foot' sand is relatively fresh north of the Mississippi River and becomes increasingly saline to the south and west.
The 400-foot' sand is the second most highly developed aquifer in the New Orleans industrial district. The aquifer appears to be very prolific, but its full capabilities have not yet been determined. This aquifer yields a highly mineralized sodium chloride water in some areas; however, elsewhere it is a source of large quantities of fresh water.
The '700-foot' sand is the most continuous freshwater bearing sand in the area and is the principal source of fresh ground water in the New Orleans industrial district. Most of the wells tapping this aquifer yield soft water of the bicarbonate type. In the southern and western parts of the industrial district the water in the '700-foot' sand is too mineralized to be suitable for human consumption.