ARTESIAN WATER SUPPLY AT SAVANNAH
Savannah, the county seat of Chatham County, Ga., has a population of 80,000. The county is in the eastern section of the coastal plane, and has an area of 370 square miles. Savannah is 18 miles from the sea coast and is on the south bank of the Savannah River, and the elevation of the city above sea level ranges from 14 to 40 feet.
Previous to 1854, when the city had a population of 15,000, the domestic supply was obtained from wells ranging in depth from 15 to 30 feet. They were of the ordinary circular type lined with stone or brick, and ordinary hand pitcher pumps were placed at the top of them. The city maintained these wells and pumps, and the records show that the annual expenditures for obtaining a ground supply of water by maintaining some 150 wells, ranged from $3,000 to $7,000. The records state that the water furnished was soft and wholesome, but in one of the old city reports it was stated that mulberry trees growing anywhere near these wells were required to be removed, because they affected the purity of the water, in what way the author cannot imagine. The city also constructed and maintained apptoximately twenty large brick cisterns under ground, each with a capacity of from 5,000 to 10,000 gallons, which were kept filled for fire service only. These were abandoned many years ago. In 1851 the mayor and aldermen engaged the services of A. W. Craven, chief engineer of the Croton water works, New York, as consulting engineer to design a project for obtaining a surface water supply from the Savannah River. This supply was put in operation in 1854, and the author found in one of the old reports that the first Worthington type of pumping engine was installed at Savannah under this project. Water partially filtered was taken from the Savannah River at a point opposite the thickly populated portion of the city, and pumped to a standpipe, from which it was distributed by mains over a portion of the city. After twenty years operation of this plant a new pumping station, locally known as the river station, was constructed at a point one and three-quarters miles up the river, and this station was used for pumping direct from the river until 1887, when an artesian well supply was adopted.
Introduction of Artesian Wells.
In 1879 the mayor of the city brought up the question of the possibility of obtaining a water supply by boring artesian wells. As far as the author can learn, the earliest artesian or deep well in Georgia was bored in Savannah in 1882, and the boring of others soon followed, as they were self-flowing and furnished a most desirable quantity of water. The agitation for obtaining an artesian well supply for Savannah resulted in the boring of several wells, and at the end of 1887 fifteen wells had been bored near the river pumping station alluded to above, and a small reservoir was constructed into which the water from the wells entered by gravity, and the city’s supply was obtained from these self-flowing wells.
•Excerpts (rom a paper read at the St. Louis convention of the American Water W’orks Association.
The Atlantic coastal plane reaches from New York to the Florida straits. This plane is underlaid by a great sediment of superficial deposits consisting of gravel, sand, clay, marl and loam, and in certain localities lime rock full of cavities exists, which forms the aquifer into which the water pours from the catchment areas through a stratum of sand beds, which have a general seaward dip. These aquifers form valuable underderground water resources. Along the coast of Georgia there is a strip 25 to 40 miles wide bordering the coast overlying water-bearing strata or aquifers through which water comes from a catchment area located from 80 to 100 miles northwest of this coast line. The character of the water obtained from the aquifer along the Georgia coast is in almost every instance potable and suitable for a domestic supply, and at this time there are over 1,000 wells tapping the aquifer in this state alone. The direction of the flow from the catchment area to the aquifer at this locality is in a southeastern direction, and it is observed that the interference of one well over another is less when they are bored on an axis perpendicular to the line of flow than where they are promiscuously bored, as was the case with the first artesian well supply at the river pumping station above referred to. The strata penetrated in boring artesian wells at Savannah consist of 250 to 300 feet of clay with interbedded layers of marl and sands, and under this from 220 to 250 feet of porous limestone, or, more strictly speaking, limestone with voids or cavities. The principal water-bearing stratum is in the limestone. Below this, for 300 or 350 feet, a formation of marl intermixed with some shell is found until a shallow stratum of flint rock is struck at about 950 feet below the surface. Below this, for a depth of 50 feet, is another water-bearing stratum of limestone similar to the upper aquifer.
Gwinnett Street Pumping Station.
As noted above, the supply at the river station was insufficient to meet the needs of the city, and in 1891 a new project was adopted for supplying it with artesian water, which project at the time was one of the most carefully planned of any in the country. The site selected for the new pumping station is two miles southeast of the river station, well within the corporate limits of the city. The plan here consisted of boring 12 wells 300 feet apart in a continuous line nearly perpendicular to the line of the subterranean flow. The depth of the wells is from 500 to 600 feet. They are all 12 inches inside diameter, and the casing is driven to a depth of approximately 250 feet, where it is imbedded in rock so that there can be no contamination of the aquifer from surface water following the casing. A brick conduit 6 feet in diameter was constructed from 10 to 16 feet below the surface of the ground on a level grade, with elevation at mean low water, into which the wells flow, and the water is carried through this conduit to the pump well at the station. This pumping station was put in operation in 1893, and has been in continuous use without interruption since then. When the Gwinnett street station was put into operation the river station was shut down, and was not utilized, except for emergency use, for 11 years, and since then this station has only been used to make up any deficiency in supply that the Gwinnett street station could not furnish with one pumping unit, the supply from the river station being about 10 per cent of the total consumption.
Static Head of Wells.
When artesian wells were first bored in and near Savannah, the water rose to from 30 to 35 feet above mean low water. With a continuous draft upon the aquifer, the static head fell, and the rate and amount of lowering of the static head depends largely upon how concentrated the draft on the aquifer is at any locality. Take, for instance, the river station where 25 wells were bored promiscuously within a 10-acre tract. The diminution of the flow that occurred would indicate that within three years after they were put in operation the static head dropped at least 20 to 25 feet. Artesian wells were bored at the river station in 1887-1889, and between that year and 1893, the domestic supply, with the exception of a small percentage of pumpage at times from the river, came from these wells. In connection with the static head it is necessary to consider the draw-down, which is proportional to the draft placed upon the wells. The draw-down at the Gwinnett street station wells is from 12 to 15 feet, and at the river station approximately 6 feet, but if the air is cut off, the water rises rapidly to its ordinary static head elevation.
Interference of Wells.
As stated above, the wells at the river station were bored promiscuously in a small area, without any relation to the line of flow of the aquifer, and the wells interfere one with the other much more than at the Gwinnett street station. Nine months ago a new well was bored within 90 feet of one of the old wells at the Gwinnett street station, and there has been very little interference between the two. The static head has apparently only dropped two feet at this point, due to the additional draft on the aquifer. That this well has not interfered more with the old wells near it, can only be explained by the fact that the new well is a little deeper and evidently enters pockets that the old well does not draw from. We have one deep well 1,500 feet deep, within 1,000 feet of one of the old wells, which taps two aquifers. The static head is higher and the draw-down in this well is very much less than in the other two wells.
Capacity of Wells.
While the aquifer has been tapped for a period of twenty-nine years, there is just as much water in the aquifer at this time as there was originally, as shown by the capacity of the wells, but it is necessary to go a little lower for the supply. When the wells were first bored at the river station the capacity of the 22 wells was approximately 6,000,000 gallons and it is easto obtain this quantity now. Reference to the capacity of the wells at the Gwinnett street station for various periods gives better data as regards the strength of the aquifer. When the 12 wells at the Gwinnett street station were bored in 1892, the total flow from it was 10,300,000 gallons per day. In a year or two this flow diminished to 9,500,000 gallons and in 1897 the records show the flow to be further reduced to 5,500,000 gallons. In 1900 the flow was slightly less, recorded as 5,000,000 gallons, and in 1902 the flow was approximately the same, but when the air lift system was introduced in 1902, the capacity of the w’ells increased to 10,300,000 gallons per day. Tests made by the author of the capacity of the same w’ells in 1915 showed the supply to be at the daily rate of 15,000,000 gallons, and at this date the city is obtaining this quantity without any effort, in fact at this time it is pumping from eight wells and obtaining 10,000,000 gallons per day. This shows that there is a great subterranean flow, which is believed to be inexhaustible, and all that is needed is for a suitable distribution of wells to draw water without creating a too concern trated draft on the aquifer.
Consumption of Water.
Savannah at this date has a population of approximately 80,000. The average consumption is 10,300,000 gallons per day, or a per capita consumption of 129 gallons. The consumption has fluctuated in the past from 115 to 165 gallons. This has been due in part to abnormal pumping during extraordinary cold spells and the extension of house drainage system, but especially to the large waste due to faulty plumbing. A pitometer survey was made in Savannah in 1912 and considerable saving in pumpage was brought about by the work done following this survey. In the mains the pressure, which at one time was 30 pounds, has been increased to 50 pounds. The city has commenced the installation of meters and has passed an ordinance requiring stop and waste cocks on all risers. It is carrying out house to house inspection of all plumbing fixtures. The installation of only 700 meters last year brought about 4 per cent reduction in the per capita consumption.
Accurate records have been kept of the fuel consumption at the Gwinnett street station since it was first put in operation in 1893. When the new pumps were first installed the consumption was 2,000 pounds per million gallons pumped into the mains. Rather singularly the consumption sank regularly for a period of five years when 1,450 pounds of coal pumped the same amount of water. This was during the period when the wells were self-flowing. The, lift from the pump well is approximately 12 feet and the pressure in the force main was at that time from 20 to 30 pounds. The installation of the air compressor increased the amount of fuel required to 2,600 pounds of coal per million gallons pumped into the mains, but after efficiency was brought about in the operation of the air lift, the fuel required fell to 2,100 pounds. From 1904 to 1914 the annual increase in consumption of coal has been 90 pounds per million gallons pumped. This, of course, is accounted for by the slowly receding elevation of the static head, to the machinery becoming older and to the increase in pressure in the mains from 30 to 50 pounds, so that at this time the consumption is 3,240 pounds of coal per million gallons.
Deterioration of Well Casing.
Investigation made of one well casing removed from the river station showed it had suffered deterioration. On account of slight contamination of the water occurring at the river station two years ago, a thorough investigation was made to ascertain how this occurred, and this investigation has shown beyond a doubt that infiltration of surface water occurs through some of the well casings. In order to prove this, an attempt was made to withdraw one 10-inch casing and with the application of two 100-ton jacks the author succeeded in withdrawing 150 feet of the pipe, which then broke off at this depth. In the casing, at a point about 40 feet below the surface, a hole was found which it was thought at first might be due to electrolysis, but was finally concluded to be due to corrosion. At present, to avoid contamination of the water, in the morning before pumping occurs into the mains, it is customary to apply the air compressor and waste the water from the wells for a period of one hour. This eliminates the accumulation of infiltered water through the previous night and with this operation there has been no trouble, as continuous pumping creates an internal pressure in the casing greater than the outside pressure, thus keeping out infiltration. These casings have failed after 26 years life, and it has been planned to do away with this station within a few months.
The construction of the river station, which was met by a bond issue, cost $220,000; other outlays, such as the cost of the air lift system, for this station have been $57,000. The initial cost of the Gw’innett street station was $427,000, and since its construction to the present date $80,000 was expended for air lift plants and for new wells, making total construction cost $784,000.