Importance of Proper Locations of Reservoirs
Most Watertight Varieties of Rock Formation Should Be Chosen—Limestone Does Not Make Impervious Bed— Fxperiences of Bloomington, Ill., and Lexington, Ky.
BLOOMINGTON is located on the summit of the divide between the east and west forks of White River in the driftless area of southern Indiana. A ground water supply is not available. The topography is rugged and good reservoir sites for an impounded supply are apparently plentiful on the contour map. Unfortunately the impounded supply that has actually been developed by the city has been unsatisfactory and inefficient. To appreciate one of the serious defects in the existing impounded supply, and the possibility of a satisfactory reservoir supply, it is necessary to consider the geological structure in this vicinity.
The rock formations at Bloomington are stratified and dip gently to the west southwest, so that the successive outcrops form north and south strips from one quarter mile to four miles wide, their lines of contact made very irregular by surface erosion. The lowest formation exposed in the vicinity of Bloomington is the Knobstone, consisting of dense sandstones and shales several hundred feet in thickness which appear in the bottoms and sides of the valleys to the north, east and southeast of the city. Above this lies the Harrodsburg limestone which outcrops on the uplands north, south and east of Bloomington, and in the lower portions of the city. Immediately above this formation occurs a stratum about 60 feet thick of Bedford limestone which underlies the surface in the higher portions of Bloomington, and outcrops along the sides of ravines to the north and south of the city. Next above the Bedford limestone lies the Mitchell limestone which outcrops over an area about four miles in width, extending west from Bloomington. It is in this latter formation that the present city water supply has been developed. The rocks of this whole region are overlaid on the uplands by a thin, rather impervious soil cover and the alluvial deposits in the valleys are shallow.
The sandstones and shales of the Knobstone are unusually impervious and tough, but resist weathering very poorly, and are deeply eroded where exposed. The limestones are in general soluble and are honeycombed with solution channels. This is particularly true of the upper strata of the Mitchell formation.
The geological conditions are in general distinctly adverse to the development of an underground water supply for a city as large as Bloomington which has a present population of 12,000. In fact good wells for domestic water supply are infrequent.
The influence of the rocks is evident in the varying topography around Bloomington. To the west in the central portion of the Mitchell limestone outcrop, the uplands are rolling, and pitted with sink holes. The water courses are shallow and often disappear into the cavernous limestone to issue later as springs at the contact plane with less soluble rocks below. Eastward toward the edge of the Mitchell limestone outcrop and beyond, the topography is rough and the streams deeply eroded in the lower limestones, and in the sandstone and shales of the Knobstone formation. The topography in this area offers numerous reservoir sites, but on account of the varying character of the rock formations, they are not all equally desirable from the standpoint of watertightness. This has been demonstrated by local experience.
Present Impounded Supply
The two older city reservoirs are located entirely in the cavernous Mitchell limestone formation. The dam of the newer Leonard Springs reservoir rests on Bedford limestone, while the upper part of the reservoir is in the Mitchell limestone.
The experience with these three city reservoirs has demonstrated that a water-tight reservoir should not be expected in these limestone formations, particularly in the Mitchell limestone. At the old reservoirs leakage occurs in solution channels through which the water finds its way around the dams, and under the spillways in natural rock to such an extent that several years ago the entire city water supply was taken from a spring or leak below the lower dam at the main pumping station. When the reservoirs are not full, water not pumped to the city is pumped back into the reservoirs to help conserve the supply. At the newer Leonard Springs reservoir, there is a large marsh immediately in front of the dam with indications of a considerable flow of water from the reservoir above. The leakage from these reservoirs is so great that even though all unused visible leakage is pumped back into the upper reservoirs, they become rapidly depleted in dry weather. Efforts to correct the leakage have been unsuccessful.
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Proper Location of Reservoirs
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Advantages of Knobstone Area
Satisfactory reservoir sites are available in the creek valleys east and north of Bloomington at no greater distance than the existing city reservoirs built in the Mitchell limestone area. In these creeks the Knobstone formation provides an impervious basin which is ideal for retaining an impounded supply, and the narrow deeply trenched valleys afford numerous good dam sites.
In striking contrast to the city reservoirs in the limestone is the very successful reservoir built in the Knobstone formation by the Indiana University for its own water supply. This reservoir was located on the recommendation of Professor E. R. Cumings, geologist at the university. The reservoir is formed by a concrete arched dam 40 ft. in height at the mouth of a small ravine entering Griffy’s Creek, and has a watershed of about 200 acres. This reservoir is absolutely tight and clearly shows that the city of Bloomington could obtain equally satisfactory reservoirs by utilizing the available reservoir sites in this formation instead of those in the limestones to the west.
The relation of the geology to the water supply of this region has been very fully described in a paper by Prof. Cumings. The conclusions of Prof. Cumings are reiterated by the report on the city water supply made by Morris Knowles in 1914 which included a report on the local geology by Prof. W. O. Crosby, the Geologist. Professor Crosby pointed out the defects of the Mitchell limestone for an impounded supply, and recommended that the city supply be developed in the impervious Knobstone formation.
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Proper Location of Reservoirs
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The new Leonard Springs reservoir was built in the limestone formation subsequent to these recommendations. As above pointed out this reservoir has repeated the experience with the old reservoirs in the limestone, and Bloomington is again confronted with serious water shortage. The firm of Alvord & Burdick reported on the water supply in 1920 and recommended that the city abandon the reservoirs in the limestone area in favor of an adequate impounded supply in the Knobstone formation.
Deficiency in Present Supply
Even if the impounding reservoirs of the Bloomington municipal supply were free from leakage, there is good reason to believe that the present development would be insufficient to fully supply the city during years of greatest drought. There are no records of the local watershed yield, but for the purpose of judging the adequacy of the existing development at Bloomington, comparison may be made with the long record of watershed yield at Lexington, Ky.
This latter record covers a period of 26 consecutive years. It is interesting to observe in this record the importance of a long record in determining the safe yield of a watershed, that is, the yield which may be depended upon in the year of greatest drought. If we consider the yield of the driest year in the 26 years’ record at Lexington as 100 per cent., the safe yield, as indicated by shorter records which did not include the driest years, would be as follows in percentage of the safe yield for the entire 26 year period:
Driest year in 26 years ……………………… 100%
Second driest year, once in 18 years ……… 120%
Third driest year, once in 15 years ……….. 125%
Fourth driest year, once in 11 years ……….. 160%
It is apparent from these figures that the application of a short record of yield in developing an impounded supply may lead to a very serious overestimate of the safe yield.
The diagram on page 744 summarizes the experience at Lexington and shows the safe yield that may be expected from given amounts of storage per square mile of watershed.
At Bloomington the estimated safe yield of the city’s impounded supply, based on the Lexington experience, is approximately one million gallons per day as compared to a present average pumpage rate of about 1.6 million gallons per day. It is evident from this that the existing impounded supply at Bloomington is insufficient to provide the city with water throughout the driest year that may be expected. The deficiency might be as much as 60 per cent.
The frequent water shortages experience at Bloomington are due in part to the fact above pointed out that the development is deficient in safe yield. This deficiency is made more serious by the excessive leakage from improperly located reservoirs.
Note—Excerpts from paper read before annual meeting of the Iowa-Illinois Sections, American Water Works Association, at Chicago.