USE OF AQUIFER TESTING AND GROUNDWATER MODELING TO EVALUATE AQUIFER/RIVER HYDRAULICS AT LOUISVILLE WATER COMPANY, LOUISVILLE, KENTUCKY, USA

By

David C. Schafer David Schafer & Associates 1 White Pine Road North Oaks, Minnesota 55127, USA

In 1999, the Louisville Water Company completed construction of a radial collector well adjacent to the Ohio River in Louisville, Kentucky at their B. E. Payne Water Treatment Plant. The well was completed in a sand and gravel aquifer to a depth of 105 feet as part of a pilot study to evaluate the feasibility of converting their surface water supply to riverbank infiltration. One of the objectives of the study was to estimate the total yield capacity available along the shoreline on the Payne Plant property. It was hoped that the supply developed at this location could supply 25 percent or more of the water company’s requirement of 240 million gallons per day.

Beginning in August 1999, a 70-day constant-rate pumping test was conducted on the well to evaluate aquifer properties. The parameters of interest included aquifer transmissivity, leakance between the Ohio River and the aquifer, and vertical anisotropy ratio of the aquifer sediments. The aquifer coefficients determined from the pumping test were applied in a groundwater flow model to predict yields of various extraction facilities designs for the site.

Three design options were considered for the Payne Plant site. One design incorporated two or more new collector wells in addition to the pilot well, connected by a subterranean tunnel drilled in the shale and limestone bedrock beneath the sand and gravel aquifer. The second option was to install a large diameter tunnel within the sand and gravel aquifer and extend well screen laterals from the tunnel to produce water. The third option was to drill conventional vertical wells, but connect them to a subterranean tunnel drilled in the bedrock.

Modeling showed that all three of the design options could produce the desired yield. This meant that the design and construction decision could be driven by the economics of the project. Modeling was used further to track the decline in yield of the pilot collector well over time caused by clogging and compaction of the riverbed sediments. Modeling showed average riverbed leakance reductions of approximately an order of magnitude.

INTRODUCTION

Louisville Water Company in Louisville, Kentucky is investigating converting their surface water supply of 240 million gallons per day (mgd) [more than 10,000 liters per second] to riverbank filtration to improve the quality, consistency and reliability of the supply. To that end, a 20 mgd radial collector well, shown on Figures 1 and 2, was constructed at their B. E. Payne Water Treatment Plant where they hope to develop a capacity of 60 million gallons per day (mgd), or about 25 percent of their supply needs. The collector well was used to test the aquifer and also will serve as one component of the extraction facilities installed at the Payne location. Results of testing this pilot well provided the basis for making yield projections for the site.

The B. E. Payne Water Treatment Plant is located along the Ohio River between the river and River Road in northeastern Louisville as shown on Figure 3. Louisville Water Company’s new collector well is located in the northern-most corner of the property, about 120 feet from the river’s edge.

The collector well is completed in a glacial sand and gravel aquifer approximately 70 feet thick, extending from slightly above elevation 400 feet above mean sea level (amsl) to just below 330 feet amsl. The aquifer is confined by a clay unit from 400 feet amsl to land surface which is about 435 feet amsl across much of the site, including the well location. Because of the high permeability of the aquifer, its piezometric surface matches the pool level of the adjacent Ohio River, averaging about 420 feet amsl under most conditions. The aquifer is underlain by relatively tight shale and limestone bedrock. About 2300 feet from the river, the bedrock rises, essentially truncating the sand and gravel aquifer at that location. Thus, for all practical purposes, the aquifer is a finite-width strip, paralleling the Ohio River.

The collector well was installed by excavating a caisson to a depth of about 105 feet below land surface to the top of the bedrock at the base of the sand and gravel aquifer. After pouring a 12-foot high concrete plug in the bottom of the caisson, seven 12-inch diameter screen laterals were installed at an elevation of 346 feet amsl in the radial pattern shown on Figure 4. Laterals 1, 2 and 7 extended to a length of 200 feet while 3, 4, 5 and 6 were 240 feet in length. 2 During test drilling, prior to constructing the collector well, several piezometers were installed at the locations shown on Figure 4 to be used in a subsequent pumping test. In addition, Louisville Water Company had divers install three shallow piezometers under the river near Lateral 4. These piezometers were installed in about 20 feet of water to depths of two feet, five feet and ten feet below the riverbed.

PUMPING TEST

A long-term, constant-rate pumping test was conducted to provide data from which to compute aquifer properties. Pumping began at noon on August 9, 1999 and continued for 70 days until mid-afternoon on October 18, 1999. Water level measurements were made in the collector well and piezometers during the pumping period and for a week following shut down. In addition, periodic measurements were made of the discharge rate from the collector well and the surface water elevation of the Ohio River.

Figure 5 shows water levels recorded in the collector well during the test. The pumping rate was maintained near 19.4 mgd throughout most of the test. One exception to this can be seen by the increase in drawdown on the hydrograph when the rate was increased to 21.4 mgd for several days during early September. Subsequently, the rate was returned to 19.4 mgd and was maintained at that rate for the remainder of the test. Similar hydrographs (not included here) were developed for each of the piezometers monitored during the test.

Examination of the hydrograph shows that water levels stabilized during the first half of September (except for the temporary pumping rate increase) and then drifted downward somewhat for the remainder of the pumping test. During this time period, the temperature of the water in the river and aquifer was dropping because of the onset of colder weather. The reduction in water temperature had the effect of reducing the hydraulic conductivity of the aquifer, thereby causing an increase in drawdown. A detailed analysis of the changes in water temperature and drawdown showed that all of the observed increase in drawdown was temperature induced. Thus, for all practical purposes, the pumping had reached steady-state conditions during the aquifer test. This conclusion allowed the well and aquifer system to be simulated with a steady-state model rather than a transient one.

3 GROUNDWATER FLOW MODEL

Computer modeling was used to “interpret” the pumping test. Pumping test analysis was accomplished by building a computer model of the site and adjusting key aquifer properties so that a match was achieved between observed and simulated water levels in the piezometers at the site. Adjustable parameters included aquifer conductivity, vertical anisotropy ratio, leakance between the aquifer and the Ohio River and the conductance value of the drain cells used to represent the collector well laterals.

The computer code selected for modeling the pumping test was the US Geological Survey Groundwater Flow Code MODFLOW (McDonald and Harbaugh, 1988). MODFLOW was implemented using Waterloo Hydrogeologic Software’s Visual MODFLOW. Visual MODFLOW is a sophisticated pre-processing/post-processing graphical user interface, which automatically creates the input files required by MODFLOW and downloads and interprets output. This fully integrated modeling environment enhances model integrity by virtually eliminating errors associated with handling cumbersome data input files and output files.

The model constructed to represent the site covered an area of 4000 feet by 14,000 feet and consisted of 119 rows and 53 columns. As shown on Figure 3, the rectangular model domain was oriented parallel to the Ohio River. Model grid cells ranged in size from a minimum of 20 feet by 20 feet to a maximum of 200 feet by 200 feet, as diagrammed on the model grid shown on Figure 6. The model domain was divided into eight vertically stacked layers as shown on Figure 7.

Constant-head cells were used to represent the Ohio River and the conductivity of these cells was varied to adjust the leakance parameter in the model. Drain cells were used to simulate the pumping well’s screen laterals as shown on Figure 8. In MODFLOW, a drain removes water from the aquifer based on a specified head value and an assigned conductance term, which represents the ease with which water passes from outside the drain cell to inside the drain.

Figure 9 shows an artistic rendering of what the cone of depression around the collector well looked like at the end of the pumping test. Drawdown occurred not only in the aquifer onshore, but also in the sediments beneath the Ohio River.

4 The model was calibrated to the pumping test data by adjusting the variable parameters mentioned above. By varying these parameters, it was possible to achieve an accurate calibration as shown on Figure 10. This figure shows a bivariate plot of observed versus simulated drawdown. In a perfect match, all of the plotted data points would fall directly on the straight line shown on the graph. As indicated, the simulated drawdown values matched the actual values nearly perfectly, with an error of only 1 percent of the range in water levels spanned by the data points. Calibration parameters for October 1999 water temperatures included a hydraulic conductivity of 390 feet per day, a leakance of 2.35 inverse days and a vertical anisotropy ratio of 3:1. The calibrated model was used to project yields of various proposed extraction facilities for the site.

YIELD PROJECTIONS

The calibrated model was run for various extraction facility designs and water temperatures to predict yields that could be obtained from the Payne Plant site via riverbank filtration. Figure 11 shows one such design in which the existing well would be retrofitted with additional laterals and two additional wells would be constructed on 800-foot centers adjacent to the existing well. Simulations also were run for a four-well system spanning a greater length of shoreline. It was possible to simulate various combinations of river water and groundwater temperatures by applying temperature correction factors to the model inputs, such as hydraulic conductivity, leakance, and drain conductance.

Figure 12 shows another extraction design that was under consideration at the time in which a large diameter tunnel would be excavated through the base of the sand and gravel aquifer and screen laterals would be installed beneath the river from within the tunnel.

Figure 13 shows the extraction system design that was ultimately chosen for the project. It incorporated 31 conventional vertical wells spanning about 6000 feet of shoreline. To minimize aesthetic impacts to the beautiful landscape along river, the design called for capping the wells flush with the ground surface and connecting them to a pumping plant via a subterranean tunnel constructed in the shale/limestone bedrock beneath the aquifer. Figure 14 shows a section view, looking toward the river, of the bedrock tunnel and vertical well system. Figure 15 also shows a section view, but looking upriver, in the direction of the tunnel alignment.

5 CHANGING HYDRAULIC CONDITIONS IN THE RIVERBED

Following the conclusion of the pumping test, the collector well was operated steadily, providing a portion of the water demand for the Payne Plant. After several months, a decline in well production was detected based on a reduction in specific capacity beyond what could be explained based on water temperature reduction alone.

Preliminary analysis suggested that the cause of the reduction was probably related to reduced leakance between the Ohio River and the aquifer rather than clogging of the screen laterals. To confirm this, another pumping test was conducted for three days in late March 2000. Data from this test were used to re-calibrate the computer model to determine the cause of the reduced yield. Calibration confirmed that there had been a significant reduction in leakance from the river to the aquifer. It was assumed that infiltrating river water had caused clogging of the riverbed sediments, as well as compaction of the aquifer, resulting in a reduction in hydraulic conductivity of the uppermost portion of the aquifer beneath the river.

Figure 16 shows how operation of the well induces flow of river water into the aquifer, carrying with it the solids normally suspended in the water. Buildup of these solids on the riverbed could reduce the hydraulic conductivity of the upper layer of riverbed materials, thus reducing the leakance.

It is expected that the greatest reduction in riverbed conductivity would be near the well initially, as shown on Figure 17. Clogging of the sediment near the well would then force more water to enter the aquifer at a greater distance from the well, exacerbating clogging at greater and greater distances. It is expected that the clogged zone would grow over time.

The effect of reduced river leakance is increased drawdown. Figure 18 compares the original cone of depression around the collector well to a new, deeper cone of depression that has resulted from reduced hydraulic conductivity of the riverbed sediments. Note that as the cone of depression expands, the area of dewatered sediments beneath the river increases, further restricting water entry into the aquifer because of the degradation in permeability of partially saturated sediments compared to fully saturated sediments. The ultimate effect of the increased drawdown is increased interference among multiple wells, eventually limiting the quantity of groundwater that can be pumped.

6 As the Louisville Water Company pilot collector well has continued operation, there has been regular monitoring of the pumping rate, drawdown and water temperature to track the pumping performance and riverbed conditions. The river leakance has been evaluated by computer modeling of the pumping performance at regular intervals. Figure 19 shows the results of the leakance evaluations. As shown on the graph, the greatest leakance reductions occurred during the first year of operation. Over the next two years of operation, the rate of leakance reduction was substantially less.

Finally, the last couple of measurements showed an increase in leakance, presumably in response to episodes of river scour that may have repaired some of the damage caused by clogging. Note that the last data point on the graph, determined from December 2003 pumping data, corresponds to a time when the Ohio River was four feet above the normal pool level. It is possible that riverbed scour associated with this higher than normal flow in the river may have allowed a transient improvement in performance, greater than what is generally observed at normal pool level.

As time goes on, operation of the pilot well will continue with regular monitoring of the pumping rate, drawdown and water temperature to see if pumping performance stabilizes or continues to change over time. The data collected will help identify the range of river leakance values that my be expected during long term operation of the extraction facilities.

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