Correlating Hydraulic Conductivity Values to Geologic Well Log Descriptions of Licking County, Ohio

Aq

uif

er T

ype

Gra

vel

San

d-&

-Gra

vel

Cla

y-&

-Gra

vel

Cla

y

Cla

y w

ash

San

d

Cla

y, S

an

d &

Gra

vel

Lim

est

on

e

Be

dro

ck

San

dst

on

e

Sha

le-&

-Sa

nd

sto

ne

Sha

le

Silt

sto

ne

David H. Scott, William A. Avalon and Jonathan LevyDepartment of Geology and Environmental Earth Science, Miami University

Well Construction & Design

Hydraulic Conductivities Map

Aquifer Type MapIntroduction Results

Conclusion

Bibliography

Data & Results

Figure 1: Sample Driller Report & Well DiagramWell design is determined by geological site selection. A typical design consists of a casing to position the well and screen to capture water and prevent sediment from entering the well.

Figure 3: Geology-Aquifer Type MapTwo major aquifer types found in Licking County are sandstone and sand-&-gravel. Individual well sites are indicated and kriging tools were used to represent generalized geologies.

Figure 4: Hydraulic Conductivity Map Estimate hydraulic conductivities in ft/d throughout Licking County based on aquifer draw-down pumping tests. Highly permeable zones correspond to the occurrence of sand-&-gravel (Figure 3).

Table 1: Summary TableHydraulic conductivity data from driller reports in Licking Co., Ohio, organized by aquifer type and order from largest-to-smallest geometric mean values.

Table 2: Aquifer Type Significance TableAquifer types with bars indicating no significant difference between their geometric means at a p-value = 0.05.

Figure 5: Major Aquifer Types in OhioThe major aquifer types within Licking Co. published by the Ohio EPA closely resemble the distribution of sandstone and sand & gravel aquifers predicted in Figure 3.

Figure 2. Hydraulic Conductivity Histograms for Sand-&-Gravel and Sandstone. Histograms indicate the data are log-normally distributed and demonstrate the higher average K values associated with the sand-&-gravel.

When considering the use and management of groundwater systems, the most important properties of geologic materials are the porosity and the hydraulic conductivity. Hydraulic conductivity is a measure of the ease with which fluids can pass through a geologic medium. It is the most important parameter to estimate when predicting aquifer yields, sustainable pumping rates or rates of contaminant transport. Estimations of hydraulic conductivity are also difficult because for any given type of geologic medium, there can be variations through four or five orders of magnitude.

In many states, including Ohio, when a borehole is drilled for a water-well installation, the driller is required to file a well driller’s report (shown in Figure 1 with an example well log and well diagram). In Ohio, these reports are filed with the Ohio Department of Natural Resources (ODNR), and the reports include an aquifer pumping test that gives the well owner an idea of the pumping capacity of the well. This pumping test also allows estimation of the hydraulic conductivity of the geologic formation surrounding the well. The ODNR has an electronic data base of all the filed drilling reports and these reports allow estimations of hydraulic conductivity at hundreds of thousands of well sites across Ohio. The well logs also give a description of the geologic formation where the pump is set.

The purpose of our research was to estimate the hydraulic conductivity at as many well sites as possible within Licking County, Ohio and relate those estimations to the geologic setting. Our objective was to provide a quantitative description of the mean and range of hydraulic conductivities for each major aquifer type. Such descriptions could then be used in future work to estimate hydraulic conductivities based on the underlying geology.

Records provided by the ODNR, to the Department of Geology

and Environmental Earth Science at Miami University, include

over ten-thousand recorded pumping tests performed in Licking

Co. These pumping test reports include, the well radius (rw), the

pumping rate (Q), the length of the test (t), and the drawdown in

the well (Sm). To calculate hydraulic conductivity, we first

calculate the transmissivity of the formation according to:

Equation 1

where S is the storage coefficient estimated to be 0.0005 for all formations. Note that T falls on both sides of Eq. 1. We therefore had to apply an iterative solution technique to converge on the proper T values. Hydraulic conductivity (K) is the transmissivity per unit length that the well is exposed to the aquifer (L):

Equation 2

The length used in Eq. 2 corresponded to the screen length for wells in unconsolidated formations or the open interval of wells set in rock.

Initial calculations of K revealed many obvious errors in the information transcribed to the ODNR database, necessitating our review and correction of thousands of records.

Well logs typically contain a recorded geologic aquifer type description, usually corresponding to the deepest formation encountered. Drillers’ descriptions comprised about 160 different aquifer types. We consolidated these into 13 major aquifer types to allow better county-wide characterizations and comparisons.

Methods

Each major aquifer type comprised a wide range of K values (Table 1). K distributions for each aquifer type were log-normal (as exemplified for sandstone and sand-&-gravel, Figure 2). All statistics were therefore performed on the log-transformed values. Geometric means were used to characterize the central values and the 95% confidence intervals for the means reflect the log-normal distributions. T-tests were performed to indicate significant differences between aquifer types. Table 2 shows each aquifer type with bars drawn under the types whose means are not significantly different at the p = 0.05 confidence level.

As expected, the formations with the highest K values were gravel followed by sand-&-gravel; sand and combinations of clay, sand-&-gravel had lower average K values. Rock formations had yet lower values with limestones and sandstones having higher average K values than shales and siltstones. Formations that were described as clay or clay wash, however, had unexpectedly high K values. The discrepancies might be due to the low numbers of wells tested and the fact that other formations may fall within the well-screen intervals, and these were not represented in the aquifer-type descriptions.

Spatial distributions of the aquifer type (Figure 3) and K values (Figure 4) were compiled for all of Licking County using ArcMap®. For Figure 3, we focused on only the two aquifer types reported as the most prevalent according the Ohio EPA (Figure 5). The occurrence of sand-&-gravel and the resulting higher K values are clearly correlated with the major river valleys.

Correlating hydraulic conductivity values to geologic well log descriptions can provide valuable regional aquifer characteristics that support sustainable use and development of aquifer systems. The ODNR has access to an abundance of well-log information for the entire state. It is possible to conduct similar studies anywhere with access to driller reports. However, we found many errors in the drilling-report information transcribed to the electronic database. The database needs to be carefully checked for all future analyses.

Bradbury, K. R. and E. R. Rothschild. "A Computerized Technique for Estimating the Hydraulic Conductivity of Aquifer from Specific Capacity Data." Ground Water, v.23, no.2 (1985): 240-246.

Cobb, Michael. "Worksheet for Estimating Transmissivity and Hydraulic Conductivity from Specific Capacity Test Data." Madison: UW-Madison Department of Geology and Geophysics, 16 December 2001.

Fetter, C.W. Applied Hydrology, Fourth Edition. Upper Saddle River: Pentice Hall, 2001. Text Book.

Ohio Department of Natural Resources (ODNR). "Well drillers information from Licking County, Ohio." Columbus, 3 February 2015.

Ohio Environmental Protection Agency. "Major Aquifers and Ground Water Quality in Ohio." February 2014. OhioEPA. PDF. April 2015.

Summary Table

Correlating Hydraulic Conductivity Values to Geologic Well Log Descriptions of Licking County, Ohio

GravelSand-&-

GravelClay-&-Gravel Clay Clay wash Sand

Clay, Sand &

GravelLimestone Bedrock Sandstone

Shale-&-

SandstoneShale Siltstone

Wells Tested 781 2006 153 9 9 768 434 78 430 1703 1866 1496 55

Minimum (ft/d) 1.17E-02 2.62E-02 6.81E-03 8.47E-02 1.45E+00 1.58E-03 2.20E-03 5.87E-03 9.37E-03 1.04E-03 1.04E-03 1.17E-03 2.69E-03

Maximum (ft/d) 4690 4095 2157 440 479 5614 1728 2014 532 1772 1772 1049 488

Median (ft/d) 70.1 66.8 35.9 33.8 12.8 13.5 19.1 6.55 4.72 4.85 4.77 3.70 1.94

Geometric mean (ft/d) 58.0 56.9 34.8 22.0 18.9 14.2 13.8 7.51 4.51 4.44 4.29 3.23 1.98

95% Confidence Interval

of the Mean (ft/d) 51.5-65.5 52.9-61.1 24.8-48.8 2.45-197 4.22-85.1 12.3-16.4 11.2-17.2 4.06-13.9 3.78-5.38 3.97-4.98 3.84-4.78 2.88-3.62 1.11-3.53

Std.Dev. (ft/d) 389 302 340 184 170 327 165 324 44.2 132 127 70.8 66.0