How Geological Data Helps Locate Water Sources
Typical Case in Work of Illinois Geological Survey—Co-operation with Water Depts.—Determining Character and Structure of Strata
THE place that geological data takes in the choosing of the type of water supply or the location of a well system, reservoir, or in determining the advantages of certain sealersheds is often underestimated. In the following paper the author points out many uses for the geological survey in this respect:
Attention is being turned more and more to the development of water reservoirs, and to the treatment and use of surface waters. Indeed, it appears that we are entering upon an epoch of reservoir projects. In this case, it is exceedingly important that a drainage line be chosen which has adequate runoff and which affords a natural reservoir site. Data then are required as to the extent of the area drained, the amount of slope, the steepness of slope, and the relations of such an area to sources of pollution. Where topographic maps are not available expensive preliminary surveys are necessary.
Selection of Supply More than Surface Inspection
The execution of a water reservoir project is something more, however, than a consideration of adequate surface supplies and the economic construction of a satisfactory dam to impound the same. It is to be remembered that it involves the selection of a reservoir site after an examination that is something more than a surface inspection. The earth materials beneath the dam must be of a sort which will support the weight of the dam and will not develop weakness by virtue of an incipient condition which is prone to become hazardous under the weight of the dam and the impounded water as time goes by. The earth materials beneath the dam, at the ends of the dam, and in the natural walls of the reservoir must also be such as to prevent excess leakage. It is well appreciated by experienced engineers that the materials which enter into a dam may be first class in every respect, the construction of the dam may be in accord with modern standards of engineering, and that the structure itself may well invite the admiration of our engineering profession, but if it happens to be placed in a geologic environment, where, by reason of lack of strength in the materials beneath the dam or by reason of an incipient defect in the rock strata, the ideal structure is bound to give way, large sums of money will be lost, and another critical demand for water attend the catastrophe. And they further recognize that even though the geologic foundation of a dam may be strong, the primary purpose of the dam is defeated if excess leakage takes place beneath or at the ends of the dam, or through the natural walls of the reservoir.
Co-Operation in Illinois
In order to avoid such failures as have attended some water reservoir projects elsewhere, in order to prevent the possible loss of large sums of money, in order to maintain possession of an adequate reservoir supply so necessary to the health and industrial development of any municipality, the Illinois Geological Survey has prepared to co-operate to the full extent of its prescribed functions with the municipalities involved. and is ready to place at their disposal such information and facilities as it has available. It is prepared to give advice regarding the character of the preliminary tests to be made, in regard to the proper recording of the results of such tests, and in the interpretation of the data acquired. It is recognized that almost every new locality has a different set of geological conditions, and a special consideration of each is therefore imperative.
The Case of Central and Southern Illinois
In central and southern Illinois many of the county seats and larger towns have heretofore drawn their water supplies from the glacial drift. This is true of Urbana and Champaign. In the case of glacial drift, sand and gravel strata are likely to occur in old river valleys which were buried by the deposits of glacial drift. The existence of these old river valleys may not be revealed by the present topography, but may be ascertained by the asembling of well data. This is one reason why the Illinois Geological Survey stresses and appreciates the faithfulness of all drillers in sending in accurate logs of their wells. The old buried Rock River Valley of northern Illinois has been fairly well delineated and its buried gravel deposits furnish a part of the water supply of the city of Princeton. Less is known about the buried valleys of central and southern Illinois. Modern valley alluvium is a well-known source of supply, and locally is important, as at Peoria. At Springfield, however, it is proving inadequate, and a reservoir project is being planned. Normally, such wells are shallow, and while adequate for a while, it is only a question of time when many of them will have to look elsewhere for this great necessity. A critical situation having already developed for some of them, they are facing the question as to what other source they can draw upon. There are two possibilities: (1) deeper aquifers; and (2) surface water reservoirs. Let us examine these two sources.
Character and Structure of the Geologic Strata in Central and Southern Illinois
In the more northern part of Illinois, the Tertiary, Cretaceous, Pennsylvanian, Mississippian, and Devonian systems are absent, and the Silurian and Ordovician formations outcrop at the surface except where covered with glacial drift and recent deposits. Farther south, the Silurian, Ordovician, and Cambrian formations dip deeper and deeper as distance increases, and are buried by Devonian, Mississippian, and Pennsylvanian formations. The well driller can not help but be impressed with the notable changes which take place, especially south from the Upper Illinois River. When it is remembered that each of these systems comprises at least two formations, and in some cases, as in the Ordovician system, many different formations, and when it is remembered that most of these formations have thicknesses ranging from 100 to .300 or 400 feet and that their aggregate thickness is to be measured in terms of thousands of feet, we begin to conceive of the magnitude of the rock strata with which our study deals.
Geological Situation Differs with Localities
It is a common conception that the strata of Illinois lie flat. They appear to do so in many exposures whose horizontal extent is measurable, in some scores of feet. But it does not take more than a slight departure from horizontally to bring about a considerable vertical rise or fall when distance is measured in miles rather than feet. If the departure from horizontality is no more than half a degree, this would amount to about 46 feet in one mile, or about 460 feet in ten miles, and about 4,600 feet in 100 miles. Inasmuch as we are dealing with the dimensions of a state rather than a county or township, and inasmuch as our geologic formations are continuous over a great extent of territory, regardless of state boundaries, we should not be surprised to find that the geologic situation in one locality may be quite different from that in another.
In northeastern Stephenson County, the St. Peter sandstone reaches an altitude of approximately 800 feet above sea level, and again in Ogle and Lee counties, southwest from the center of Lee County, the formation dips to lower and tower levels, passing successively to the 600, 400 and 200-foot levels, and before reaching the southwestern corner of the counity, it descends to a depth of 400 feet below sea level. Southeast from here, our best evidence indicates that it continues to descend, reaching 800 feet below sea level in central Bureau County, 1,060 feet below sea level at Peoria, and more than 1,400 feet below sea level in southern McLean County. Still farther south, in Perry County, the well of the Illinois Central Coal and Salt Company at St. Johns, ended at a level of about 3,134 feet below sea level in the formation just above the St. Peter sandstone.
Difference Eastward and Westward in Illinois
Westward in Illinois from the line just traced, the strata rise gently but generally not reaching sea level, whereas eastward there is an abrupt rise to the crest of a fold known as the La Salle anticline which extends in a nortlnvest-southeast direction from Lee County to the oil fields in Clark, Crawford. Lawrence, and Wabash counties. From the crest of this fold, the strata dip much more gently eastward than to the west making the fold asymmetrical. It is to be noted that the crest of the fold declines gradually southeastward. In La Salle County, the St. Peter sandstone reaches an altitude in excess of 600 feet above sea level, and in places underlies immediately the surface soil, whereas farther and farther south it declines to more than 2,467 feet below sea level in Clark County, younger and younger formations coming in to bury it.
Now changing our direction to an east-west line we note the lay of the rock formations well shown in the illustration which is a cross-section by R. S. Blatchley from Valmeyer on the Mississippi to Lawrenceville, near the Wabash. This figure brings out the way the formations dip in the western part of the State and the marked change which takes place at the La Salle anticline.
It is clear now that the formations west of the La Salle anticline lie in a great basin and that those strata which are a continuation of shallow strata in northern Illinois are deeply buried. The uppermost bed-rock formations belong to the Pennsylvanian system and have a thickness of from a few feet near the edge to a thickness of 2,100 feet in Clay County. Beneath the Pennsylvanian system over a considerable part of the basin the drill must penetrate the formations of the Mississippian and Devonian systems before it reaches the well-known aquifers of the Silurian, Ordovician, and Cambrian systems. What higher horizons then may be regarded as potential aquifers? Within the whole Pennsylvanian system only the basal formation, the Pottsville, is a potential aquifer, but even this is doubtful, for while it is generally a sandstone formation, within short distance it may become a shale. This character, together with its small amount of voids due to the angularity of the sand grains and the presence of some clay bond, all tend to obstruct the free percolation of water and prevent it from being an aquifer of the type of the St. Peter sandstone.
Chemical Character of Bedrock Water in Central and Southern Illinois
The quantity of the bedrock water and the depth at which it may be secured are only two of the important factors in the consideration of ground water supplies, the third being quality or chemical character. It is generally appreciated that water from the bedrock formations is never pure or free from mineral matters. In fact there is probably no pure water in nature. But there is a considerable difference in the quantity of mineral matter which may be present in soltuion in the different formations and at different places, and a considerable difference in in the kind of mineral matter.
It is to be noted the greater prevalence of rock wells is in the northern part of the State. The dominant type of well in the central and southern parts is the drift well which is not shown on this map. Chemical analyses have been made of the waters of all of these rock wells, and their content of mineral residue determined in parts per million. It is possible that some addition to this number of wells might be made if the study were to become more intensive, but it is believed that the data at hand are adequate to show the general status of the situation.
Only 12 of the municipal wells of the State yield water with a mineral content less than 300 parts per million; 59 wells yield 300 to 500 parts per million; 60 wells, 500 to 1.000 parts; 37 wells, 1,000 to 2,000 parts; and 14 wells yield in excess of 2,000 parts. All of those which yield less than 300 parts per million are located in the northern part of the State, excepting two, namely Mounds and Mound City which are located in the extreme southern part of the State, south of the Illinois Ozarks. None of the wells in the central and southern parts of the State yields less than 400 parts per million, and the great majority of them yields more than 1,000 parts, at Decatur, there is a well which, according to the Illinois State Water Survey, yields 76,377 parts per million. We do not know the chemical character of the water from all of the water-bearing formations in every part of the state, and it is to be hoped that such a study can soon be made. Nevertheless it seems that the high mineral content of water from central and southern Illinois prevails without regard to any specific bedrock formation. It is, therefore, quite evident that drilling to the deeper horizons would be a discouraging venture, not only from the standpoint of cost but also from the standpoint of industrial use.
(Excerpts from a paper read before the annual convention of the Illinois Section, American Water Works Association.)