Variables affecting well success in a Kentucky limestone aquifer

.roumal of'Hydrology, 20 (1973) 3 2 7 - 3 3 3 © North-Holland Publishing Company, Amsterdam - Printed in the Netherlands

VARIABLES AFFECTING WELL SUCCESS IN A KENTUCKY LIMESTONE AQUIFER J.T. JOHNSON* and JOHN THRAILKILL

Department of Geology, Universityof Kentucky, Lexington, Ky. (U.S.A.) (Accepted for publication April 3, 1973)

ABSTRACT Johnson, J.T. and ThrailkiU, J., 1973. Variables affecting well success in a Kentucky limestone aquifer. J. HydroL, 20: 327-333. Ten variables were examined using non parametric statistical tests (Mann-Whitney U and x 2) to evaluate the influence of these variables on ground water supply in the Centerville Quadrangle, Kentucky. Variables found to be significant were t h o ~ directly related to lithology (wellhead stratigraphy and well-bottom stratigraphy) and those interpreted as being strongly influenced by lithology (well de pth, wellhead elevation, and well-bottom elevation). In addition, wells judged adequate for water supply were closer to surface streams and bottomed closer to stream level than dry holes. Adequate wells were also closer to synclinal axes than dry boles. There was no significant difference between dry holes and adequate wells in their distance from sinkholes or regional structural position. Wells producing H2Scontaining water differed from adequate wells only with regard to variable: related to lithology but saline-water wells were more abundant in areas distant from surface stream~ as well as in unfavorable Uthology. INTRODUCTION

It is widely recognized that limestone aquifers in which the voids are mainly of solutional origin are qualitatively different from granular aquifers. One of these differences is the wide variations in well yields over short distances, and the difficulty of predicting whether a well drilled at a given location will yield water sufficient for the purpose for which it was drilled. A number of geologic factors have been suggested as influencing the yield of wells in limestone. For example, Brown and Lambert (1963) state that in the Mississippian Plateau region of Kentucky in relatively pure limestones, " . . . the largest yields are obtained from drilled wells distant from streams but in which the water lew.q is not far above the local perennial stream levels." They also state t h a t " . . , in general higher yields can be expected from wells drilled close to s i n k h o l e s . . , if the water level in the well is close to perennial *Present address: St. Joe Miner01s Corp.; Box 172; Balrnat, N.Y. 13609, U.S.A.

328

LT. JOHNSON AND J. THRAILKILL

stream levels." Burchett and Moore (1971) state that wells (in Tennessee) in narrow valleys and draws have the best change of yielding over 50 gallons per minute. Lattman and Parizek (1964) found that in Pennsylvanian wells drilled on fracture traces had higher specific capacities than those not so located. This brief and far from exhaustive list is not intended to ascribe priorities to the ideas, and most of the authors cited discussed factors other than those mentioned. It is appare.nt, however, that there is little agreement as to the relative importance of various factors in controlling well success. Much of this lack of agreement is undoubtedly due to real differences in the controlling factors in different regions. It seems likely, however, that in many previous studies alternative explanations were not thoroughly investigated and that the apparent correlations observed may have limited statistical validity. The present study, therefore, was undertaken to investigate statistical correlations between well yield and a number of possibly influencing variables. AREA STUDIED

The area covered by the 7 !/2 minute Centerville Quadrangle, Kentucky (1:24,000, contour interval 10 ft. ~_ 3 m), whose center lies about 10 km north of the city of Lexington, was selected for this study because of its accessibility and the relatively large amounts of geological and hydrological data available. The following description of the geology is taken from Kanizay and Cressman (1967).

MATSON(1909)

illlU,llORAPHIC GROUND-WATER AVAILABILITY tCTION ~IF'IrER THIS STUDY P&LMQUIST WELl. HAMILTON(1950) AND HALL (1940) WELLHF.ADS BOTTOMS m~t mills i~l|l~Iggll

small quantities nearly oli

moderate quantities of water

more

wells dry most wells adequate for ( 5 0 0 gol Idoy (< 2 m31doy)

5 0 % o f wells adequate

adequate wel Is ;

: o o t

more adequate ~,mme ~ e wells

___3

abundant quantities of water

neody all wells

adequate nearly all

most wells adequate for ) 500

fewer adequate

gollday

wells

() 2 m31doy)

wells dry

F:g. 1. Relationship of lithoiogy to groundwater availability in the Centerville Quadrangle

VARIABLES AFFECTING WELL SUCCESS

329

The Centerville Quadrangle is underlain by nearly flat-lying Middle and Upper Ordovician rocks. The Lexington Limestone, which crops out over most of the area, is largely limestone with lesser amounts of interbedded shale. Three tongues of the Tanglewood Member of the Lexington limestone (unpatterned units on the stratigraphic section of Fig. 1) are bioclastic, medium-bedded limestone with little or no shale. The Clays Ferry Formation (uppermost patterned unit in Fig. ! ) which consists of interbedded limestone and shale, overlies the Lexington Limestone and occurs in a few small patches on ridge tops. The structural relief of the area is about 30 m with the regional dip to the north. Several broad anticlines and synclines and a few faults with displacements up to about 10 m have been mapped. Maximum topographic relief is about 75 m. The quadrangle is drained by several perennial streams, of which North Elkhorn Creek is the largest. Sinkholes are numerous and widely scattered. Groundwater studies by Matson (1909), Hamilton (1950), Palmquist and Hall (1960, 1961 ), Hendrickson and Krieger (1964), and Mull (1968) have included all or part of the Centerville Quadrangle. METHOD OF INVESTIGATION

The data gathered for this study/lad, in common with the data in many other groundwater investigations, three characteristics which influenced the selection of statistical evaluation techniques. Firstly, much of it consisted of nominal or ordinal measurements. Nominal measurements are those in which the numbers assigned to measurements have no magnitude relationship, and ordinal measurements simply rank the measurements with no connotation of equal intervals between numbers (Siegel, 1956). Secondly, the assumption of the normal distribution of observatior, s, which is necessary for many statistical tests, seemed to be doubtful for many of the variables. Finally, much of the data was based on interviews with landusers and is of uncertain accuracy. Because of the lack of information on well yields, it was decided to categorize each welJ as either "adequate", "dry", "salt" or "sulfur". It was realized that there are many factors influenc~mg the use of a well. These include availability of other water supplies, whetlaer or not proper completion practices were employed, and the amount of yield considered satisfactory. Data was obtained from Hamilton (1950), open-file well records of the United States Geological Survey, and field investigations by one of the authors (J.T.J.). 52 adequate and 32 inactequate wells distributed fairly evenly over the quadrangle were used in l:he study. Of the inadequate wells, 9 were dry, 5 were "salt" (wells yielding tmacceptably saline water) and 18 were "sulfur" (wells yielding water with an unacceptable amount of dissolved H2S).

330

J.T. JOHNSON AND J. THRAILKILL

TABLE 1 Results of statistical *.ests

Variable (test used)

Apparent relationship of adequate wells to inadequate wells Probability of error in accepting apparent relationship Conclusion (apparent relationship accepted at 0.05 level) Dry holes

Salt wells

Sulfur wells

Inadequate wells (all three combined)

1 Well depth (Mann-Whitney)

shallower 0.0007 accept

shallower 0.03 accept

no difference (0.50) -

shallower 0.0055 accept

2 Wellhead elevation (Mann-Whitney)

higher 0.047 ac~pt

higher 0.10 reject

higher 0.008 ac,~ept

higher 0.0013 accept

3 Structural elevation (Mann-Witney)

lower 0.20 reject

higher 0.13 reject

lower 0.44 reject

lower 0.17 reject

4 Structural ratio (Mann-Whitney)

closer to syncline

no difference (0.50)

closer to syncline 0.19 reject

closer to syncline 0.09 reject

0.035 accept 5 Topographic ratio (Mann-Whitney)

closer to valley 0.024 accept

closer to valley 0.0087 accept

closer to valley closer to valley 0.12 0.0066 reject accept

6 Distance to sinkhole (Mann-Whitney)

closer 0.30 reject

closer 0.496 reject

farther 0.053 reject

farther 0.19 reject

7 Well-bottom elevation (Mann-Whitney)

higher 0.0007 accept

higher 0.0047 accept

higher 0.36 reject

higher 0.00023 accept

8 Angle from well bottom to stream (Mann-Whitney)

smaller 0.0075 accept

smaller 0.24 reject

smaller 0.36 reject

smaller 0.0027 accept

9 proportion of wellheads in upper rocks (x 2)

more 0.025 - 0.05 accept

more 0.35 - 0.40 reject

more 0.10 - 0.05 reject

more 0.025 - 0.01 accept

10 Proportion of well bottoms in upper rocks (x 2)

more

more

more

more

0.0005 - 0.005

0 . 0 0 0 5 - 0.005

0.025 - 0.05

0.0005

accept

accept

accept

accept

VARIABLES AFFECTING WELL SUCCESS

331

Information on about a dozen "independent" variables was gathered for the wells used in the study. Most of these were variables that have been suggested as being influential in determining well yields by earlier workers. S,ome variables were not used in the statistical analysis, either because it was felt that they were too inaccurately determined or because there was an insufficient population of either adequate or inadequate wells assignable to the interval: used to measure the variable. Included in this category was the fracture trace variable. The ten variables retained and analyzed statistically are listed in Table I. Three of the variables: well depth, wellhead elevation and well-bottom elevation, are self-explanatory. Structural elevation is the elevation of the base of the lower Millersburg Member of the Lexington Limestone at the well, as determined from structure contours of this horizon (Kanizay and Cressman, 1967). "Lower" wells are thus regionally downdip from "higher" wells. The structure ratio is the position of the well relative to adjacent anticlinal and synclinal axes drawn to conform with the structure contours. Topographic ratio is the well position relative to adjacent ridge lines and valley bottoms determined from the topographic contours. Distance to sinkhole is the distance between the well and the nearest closed depression contour on the topographic map. Angle from well bottom to stream is the vertical angle between the well bottom and the nearest perennial stream shown on the topographic map. "Smaller" angle indicates a well bottom slightly below the stream level (all well bottoms were below the stream levels). An attempt was made to evaluate the influence of stratigraphy in the final two variables: proportion o f wellheads in upper rocks and proportion of well bottoms in lower rocks. For the former the stratigraphic column was divided into "upper" and "lower" parts by a horizon which followed the base of the "fossiliferous shale" member where it was present and the base of the lower Millersburg Member elsewhere (Fig. 1). A horizon 50 ft. below the top of the Grier Member was used to divide tlae section for the well bottom variable (Fig. 1). For analysis of variables 1-8 a scale was set up which would yield a numerical value for each well. As an example, the structural ratio was defined as the ratio of the distance from the well to the synclinal axis and the distance between the synclinal and anticlinal axis measured along a line passing through the well. This yielded a value for each well between 0 (well on synclinal axis) and l (well on anticlinal axis). It is obvious that such values will not, in general, be normally distributed and that little meaning should be attached to the absolute magnitude of each value. The appropriate statistical test was judged to be the Mann-Whitney U Test (Siegel, 1956) which tests whether two groups belong the same population. This test was used by Siddiqui and Parizek (1972)

332

J.I. JOHNSON AND J. THRAILKILL

who describe it. Only the relative rank of the values is used and no assumptions of the distribution of the population are required. Its power approaches that of the (parametric) t test (Siegel, 1956). Variables 9 and 10 could only be grouped (into "upper" and "lower" rocks) with no numerical scale. These variables were therefore tested in a 2 X 2 contingency table with the ×2 test (Siegel, 1956). RESULTS AND CONCLUSIONS

Insofar as possible, variables were chosen which would allow comparison with previous studies of the Centerville Quadrangle. All earlier workers (Matson, 1909; Hamilton, 1950; Palmquist and Hall, 1961; Hendrickson and Krieger, 1964; Mull, 1968) considered lithology to be an important variable controlling yield. The lithologic characterization of the upper Lexington Limestone and lower Clays Ferry Formation which underlie the quadrangle is far from simple. Repetition of lithologies and lateral facies changes are common, and there have been numerous revisions of the stratigraphic nomenclature. There is, therefore, some uncertainty in the assignments (shown in Fig. 1) o f groundwater availability judgments by earlier workers. It can be seen, however, that the results of this study are reasonably consistent with the evaluation of earlier workers. The best discrimination was with the "well bottom" variable (Table I) as might be expected. The authors agree with the conclusion of Matson (1909), Hamilton (1950) and Palmquist and ttall (1961 ) that the medium-bedded, relatively pure Tanglewood Member of the Lexington Limestone (blank areas on Fig. 1) is probably the most favorable stratigraphic target for groundwater. It is unfortunate that the geologic situation and distribution of wells did not allow comparisons to be made among the three tongues of the Tanglewood. Well depth was considered an important variable by both Hamilton (1950) and Palmquist and Hall (1960) who stated that there was a strong possibility of encountering sulfurous or saline water at depths greater than 100 ft. (30 m). The data for the well variable (Table I) show, however, no differences in depth between "sulfur wells" and adequate wells. F~:rther, the correlation with depth of "salt wells" was rather weak (P = 0.03) although accepted at the P = 0.05 level. Mull (1968)judged proximity to streams to be an important variable in an area which included a portion of the Centerville Quadrangle. This would seem to be born out by the topographic ratio variable (Table I) which suggests adequate wells are closer to valleys than dry holes or salt wells (but not necessarily sulfur wells). Mull (1968) also suggested that wells with the largest yields tended to be located in synclines, and analysis of the structural ratio shows such a relationship between dry holes and adequate wells.

VARIABLES AFFECTING WELL SUCCESS

333

It was thought that wells drilled near sinkholes might have a better chance of being adequate. As mentioned earlier, this was suggested as an important variable by Brown and Lambert (1963) in rocks of Mississippian age in Kentucky. The distance to sinkhole variable designed to test this showed no relationship, however (Table I). There was no relationship between the adequacy of the well and its regional structural setting (structural elevation variable in Table I). Dry holes tend to have their bottoms farther below stream level (angle from well bottom to stream) than adequate wells (Table I). The remaining variables (well depth, wellhead e'.evation, and well-bottom elevation) can have their significance explained most easily by their dependence on stratigraphy. ACKNOWLEDGEMENT

The authors thank the staff of the Louisville office of the United Statcs Geological Survey for making available well records, and the University of Kentucky Research Committee for providing funds for the study. REFERENCES Burchett, C.R. and Moore G.K., 1971. Water resources in the upper Stones River basin, central Tennessee. Tenn. Div. WaterResour. Water Resour. Set., No.8, 62 pp. Brown, R.F. and Lambert, T.W., 1963. Reconnaissance of groundwater resources in the Mississippian Plateau region, Kentucky. U.S. Geol. Surv. Water-Supply Pap., No. 1603, 58 pp. Hamilton, D.K., 1950. Areas and principles of groundwater occurrence in the inner Blue Grass region, Kentucky. Ky. Geol. Surv., Set. 9, Bull. 5, 68 pp. Hendrickson, G.E. and Krieger, R.A., 1964. Geochemistry of natural waters of the Blue Grass region, Kentucky. U.S. Geol. Surv. Water-Supply Pap., No. 1700, 135 pp. Kanizay, S.P. and Cressman. E.R., 1967. Geologic map of the Centerville Quadrangle, central Kentucky. U.S. Geol. Surv. Map, GQ-653. Lattman, L.H. and Parizek, R.R., 1964. Relationship between fracture traces and the occurence of groundwater in carbonate rocks. J. liydrol., 2: 73-91. Matson, G.C., 1909. Water resources of the Blue Grass region, Kentucky. U.S. Geol. Surv. Water-Supply Pap., No 233, 223 pp. Mull, D.S., 1968. The Hydrology of the Lexington and Fayette County. Kentucl:y Area. Lexington and Fayette County Planning Commission, Lexington, Ky., 24 pp. Palmquist, W.N. and Hall, F.R., 1960. Availability of groundwater in Bourbon, Fayette, Jessamine and Scott counties, Kentucky. U.S. Geol. Surv. ttydrol. Invest. Atlas, HA-25. Palmquist, W.N. and Hall, F.R., 1961. Reconnaissance of grc.undwater in the Blue Grass region, Kentucky. U.S. Geol. Surv. Water-Supply Pap., No. 1533, 39 pp. Sicldiqui, S.H. and Parizek, R.R., 1972. Application of nonparametric statistical tests in hydrogeology. Ground Water, 10(2): 26-31. Siegel, S., 1956. Nonparametric Statistics for the Behavioral Sciences. McGraw-Hill, New York, N.Y., 312 pp.