Ground-water exploration at Alexandria, Louisiana

Abstract

Serious overdevelopment of the ground-water supply in Alexandria and vicinity necessitated development of a well field in the Bayou Rapides area to supplement the supply from wells within the city. The development of this well field, located about six miles northwest of the City, to minimize the pumping interference of existing wells, was preceded by an exploration program designed to permit over-all planning before well construction. The geological and hydrological data obtained were recorded so that future development of ground-water in this area may be properly planned. The testing procedures are outlined in this article to serve as a possible guide for ground-water exploration in other areas.The exploration program in the Bayou Rapides well field consisted of drilling, electric logging, and testing of six test holes. Samples of the sands and water were collected and analyzed in the laboratory. The water level, temperature, yield, and drawdown were determined for each sand that appeared to have possibilities as an aquifer.The Bayou Rapides well field is underlain by water-bearing formations of the Miocene, Pleistocene, and Recent series. The Miocene sediments are composed chiefly of alternating sands, sandy shales, and tough clays of fluviatile or brackish water origin. The sands are generally fine-grained and very irregular in texture, thickness, and extent. Artesian conditions are present in the sands, recharge taking place in the hills to the north and west where the sands lie at or near the surface.The principal fresh-water sands of Miocene age range in depth from 150 to 970 feet below the land surface and may be grouped into three well-defined lithologic zones which dip south-southeast at a rate of 75 to 100 feet per mile. Brackish or salt water is found at a minimum depth of 1,100 feet. All of the water in the Miocene sands above this depth is soft and potable.Alluvial deposits of Pleistocene and Recent age blanket the Miocene sediments. The alluvium underlies Bayou Rapides well field to a depth of 120 feet, grading from clay downward into coarse sand and gravel. The water from the alluvium is hard and not generally considered satisfactory without treatment.After completion of the testing program in October 1942, six gravel-packed supply wells, 12 inches in diameter and 448 to 998 feet in depth, were constructed by reaming the test holes. The aggregate discharge of the six wells was 2,120 g.p.m. in a pumping test on August 25, 1943, and the individual yields ranged from 260 to 420 g.p.m. The specific capacities of the wells ranged from 1.5 to 8.1. The water is very soft and contains small amounts of fluoride.The benefits of an adequate exploration program prior to any large development of ground-water are apparent from the results obtained in this project. The definite knowledge of depth, character, and correlation of each water sand, along with the data on quality, temperature, and artesian head of the water therein, permits the careful consideration by the driller and well owner of the most suitable location, depth, construction, and pumping equipment for each well in relation to all other wells.