Stormwater Injection Well Performance
Water Quality and Quantity, and
Maintenance
Warren Campbell
Civil Engineering
Western Kentucky University
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Introduction
Central and southern Kentucky comprise one of the world’s best known karst (cave and
limestone) areas and contains the longest cave in the world. Surface streams are rare and the common
practice of routing runoff from developments to the nearest surface stream is not economically feasible
in Bowling Green, Warren County and surrounding areas. This is also one of the most rapidly developing
areas in the Commonwealth.
For decades the only alternative to surface stormwater drainage has been to drill stormwater
injection wells and deliver runoff directly to groundwater. Bowling Green alone has more than 1200
stormwater injection wells. The common practice is to drill in the lowest part of the nearest sinkhole,
and there are many of these, until a bedrock cavity at least 1 ft high is encountered. In Bowling Green, it
is almost impossible to drill through 100 ft of bedrock without encountering one of these. Normally, a
casing is grouted into the bedrock for a depth of at least 3 ft and the casing or a pipe is left standing
above the level of the box, presumably to prevent debris from clogging the cavity below. This is
normally the only treatment the runoff receives on the surface. This approach to stormwater
management will not be environmentally sustainable as population densities increase.
There are many different types and configurations of injection wells. Figure 1 shows one of the
more elaborate designs of a well placed in a well-landscaped sinkhole in Bowling Green. Figure 2
illustrates another approach. A pipe is oriented vertically and rests on bedrock. This pipe prevents soil
from collapsing in and clogging the cavity. The galvanized pipe in the figure is a crest cage constructed
by Western Kentucky University freshman engineering students and it is used to record maximum water
depths.
Figure three shows five different injection wells in the Batsel Avenue sinkhole. Four are the
elaborate gray pipes that are obvious in the figure. Notice the elaborate concrete pad at the base of the
foreground well. This pad and grout below the surface prevents soil leaking into the cavity through gaps
in the casing. This happens when injection wells are not properly constructed. The result is the
formation of a sinkhole and often the collapse of the casing and stand pipe.
The foreground well has another interesting feature that can not be shown here. In drilling the
well a small cavity less than the required 1 ft height was encountered. The crew kept drilling until a
larger cavity was encountered. During wet weather or during the wet season, the smaller cavity carries
flow and creates a waterfall inside the well.
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Figure 1. Stormwater injection well showing debris grate and standpipe within
Figure 2. Injection well using a bedrock cavity in the Nahm sinkhole in Bowling Green
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Figure 3. There are five injection wells in this figure. Can you identify them?
The candy cane piping on the injection well at the right in the figure is intended to prevent
clogging of the cavity. The fifth injection well is more difficult to identify but it consists of a grate
covering a bedrock cavity and can be seen in the background in the figure and just below and to the left
of the car.
Much of the work on Batsel sinkhole was done following the April 16, 1998 storm in Bowling
Green that flooded several houses in the area. In response, the City with support from FEMA acquired
two flood prone houses and removed them.
Figure 4 illustrates another strategy for stormwater management in karst. The water in the
figure is flowing over a concrete weir and directly into Bypass Cave on the right. The entrance to the
cave is about 5 ft high and it receives runoff from approximately 90 acres of mixed residential and
commercial land. The flow into the cave was significantly polluted and contained parking lot debris
from commercial areas. The City spent a significant amount of money to install a water quality unit that
would remove some of the debris and hydrocarbons from the runoff. Figure 5 shows this unit as it was
installed. From the figure it is obvious that installing a water quality unit in each of the 1200 Bowling
Green injection wells is not economically feasible.
Some of the injection wells are not as elaborate as some of the ones shown in the first five
figures. Figures 6 and 7 show more typical injection well standpipes. Under the greatest head that most
wells experience, the maximum flow is typically only 10 cfs. The inflow to the injection wells may be
several hundred cfs. In an attempt to increase the capacity of the wells, holes have been cut in the sides
of the standpipes using welding torches. This is a very common practice. Notice that the pipe was also
cut to length using a welding torch.
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Figure 4. Runoff flowing into Bypass Cave in Bowling Green
Figure 5. Installation of the Bypass Cave water quality unit
Injection wells come in many sizes and shapes. They also come with many grate styles. Figure 1
shows a circular grate. Figures 8 through 11 show different grate styles used in Bowling Green. Each
grate has its own advantages and disadvantages. Experience has shown that triangular grates will often
clog with leaves and flow will go by leaving only a trickle to enter the injection well. All grates will clog,
especially in an area with many trees. Figures 12 – 14 illustrate problems with grate clogging. While the
presence of leaves offers some pollution control benefits, it hampers flood control and creates
maintenance problems.
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Figure 6. Corrugated metal standpipe cut with a welding torch
Figure 7. Injection well in operation with flow through the side orifices
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Figure 8. Trapezoidal grate covering an injection well
Figure 9. Triangular injection well grate
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Figure 10. Flat circular injection well grate
Figure 11. Rectangular injection well grate
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Figure 12. Trapezoidal grate clogging
Figure 13. Flat grate clogging
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Figure 14. Clogged round grate at Batsel Avenue sinkhole
The preceding discussion has shown that injection wells come in many forms. All of these wells
hold one thing in common and that is the potential to pollute groundwater. One of the great unsolved
mysteries in karst stormwater management is the economical and effective treatment of runoff going to
injection wells without significantly increasing either flood risk or maintenance costs. The purpose of
this proposal is to suggest an approach to find the solution to this puzzle.
Approach
We are proposing the development of a stormwater injection well performance test bed to
include both a lab facility and a prototype test bed. It would be used to measure hydraulic performance,
pollution control, and clogging potential (maintenance performance) for a general injection well
configuration. Figure 15 provides a conceptual diagram of the laboratory facility. The pump and sump
would be fixed, but the test section would be removable and could be replaced with different
configurations. The capabilities required for the test bed would include the following:
1.
2.
3.
4.
5.
6.
Ability to provide a wide range of flow rates
Enough flow to move the debris normally encountered by stormwater systems
Pump system that can handle the smaller debris that would come through a strainer
Accurate flow rate measurement
Instruments and facilities for measuring water quality parameters
Flexibility to accommodate most possible injection well configurations
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Figure 15. Stormwater injection well test bed schematic
The advantage of a lab test facility is that testing could be performed continuously without the
necessity of cooperation by Mother Nature. However, there are advantages of a special field prototype
facility. The hydraulic performance of an injection well may depend not only on the performance of the
inlet works, but also on the performance of the solution cavity below. It is well known that under some
conditions, injection wells can discharge. In these situations, the well becomes a spring which is exactly
opposite of the desired performance. This can only be measured for realistic situations in the field.
Measurements of inflow are necessary for any field facility. The lab test facility could be used to
calibrate the inflow for the field facility by measuring discharge vs. depth of the inlet works. Then in the
field, a measure of inflow depth would provide discharge.
Figure 16 provides a schematic of the
prototype test facility. The inlet works would be designed so that a general standpipe and grate system
could be attached to the well. Instruments would be able to measure water depth around the inlet
works as well as in the well. Depths in the well are necessary to assure that the inlet works are
discharging freely into the well.
Measurements would be recorded with a data logger. These measurements would include
rainfall, depth around the inlet works and within the well, water quality parameters such as pH,
conductivity, concentrations of various contaminants, and temperature. The measurements should be
recorded at a high sampling rate and should be triggered by rainfall and/or runoff.
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Figure 16. Schematic of field prototype facility
It is also desirable to instrument existing injection wells in the field. Ideally, we would like a
mobile instrument package. We would use the lab test facility to calibrate flow into the injection wells
so that inflow could be determined with a depth measurement in the field. One problem with field
measurements is the vagaries of rainfall. However, for smaller injection wells the fire department could
assist. For larger storm events, we would want to have an instrumentation package with data logger
that would trigger when rain begins.
The University owns land that would be suitable for both the prototype well and the lab facility.
The Western Kentucky University for Research and Development sits on a 27 acre site that was formerly
a shopping mall and parking lot. Most of the runoff from the site drains to a detention pond with
approximately 1.5 acre-ft of storage. Rough calculations indicate that runoff from the site would deliver
up to 40 cubic feet per second (cfs) to the pond on average about once a year. This would correspond
to a total runoff of about 1.26 acre-ft. By comparison, the 100-yr, 24-hr Bowling Green storm would
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deliver a peak flow of about 220 cfs to the pond with a total runoff of about 15 acre-ft from the site.
The pond is a simple trapezoidal pond so that the actual runoff could be estimated from changes in
pond depth with time.
The injection well at the site is shown in Figures 17 and 18. The injection well takes advantage
of a bedrock cavity as can be seen in Figure 18. The bedrock in the cavity is scalloped indicating that the
injection well has been a natural drain for many years. It is a very suitable facility for both instrumenting
an existing well and creating a prototype well for study.
Figure 17. WKU Center for Research and Development detention pond
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Figure 18. Injection well from Figure 17
If some lab and office space could be made available in the Center, this would be a perfect
location for both.
Benefits
A dedicated test bed offers many benefits in addition to providing a means for improving
injection well performance. The system could be configured to test different designs for culverts and
storm drain inlets. This is useful for any area and not just for the karst of central Kentucky.
The test bed could be configured for testing the outlet works of detention and retention ponds
used to reduce flood risk from development. Many communities require that development does not
increase runoff from a design storm above that for pre-existing conditions. Many communities use the
10-yr storm. Increasingly, some communities are requiring multi-storm design. For example, Franklin
Tennessee requires that the post-development runoff from a site not be increased for the 1-yr, 2-yr, 5yr, 10-yr, and 25-yr storms. For certain watersheds, Franklin may add the same requirement for the 50yr and 100-yr storms. This requires multi-stage detention.
Anacortes, Washington requires that the post-development peak runoff be less than or equal to
one-half the pre-development peak for the 2-yr storm.
Anacortes also requires that the postdevelopment runoff for the 10-yr and 100-yr storms not be increased.
Another benefit involves training both of students and stormwater professionals. The WKU
Mission is:
Western Kentucky University prepares students to be productive, engaged leaders in a global
society. It provides service and lifelong learning opportunities for its constituents. WKU is responsible
for stewarding a high quality of life throughout its region.
These facilities would offer outstanding opportunities for student training that will serve them
well in their professional careers. In addition, stormwater professionals from the region will be
interested in short courses that could be developed around these facilities. Aside from injection well
design, The Kentucky Transportation Cabinet has expressed an interest in having Western teach short
courses in areas such as culvert design, highway drainage, and detention pond design. This will provide
lifelong learning opportunities for constituents. Additionally, if the facility is located at Western’s Center
for Research and Development, the site could be instrumented to create a hydrologic teaching and
research laboratory. Few of these exist for commercial property.
By providing the means to improve groundwater quality, these facilities will contribute to
stewarding a high quality of life throughout the region. The facilities would also offer opportunities for
public outreach which is a cost effective way of improving the quality of life throughout the region. By
teaching developers and construction companies economic means of protecting groundwater, and by
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Injection Well Test Facilities | Western Kentucky University
teaching citizens how to avoid detrimental practices such as washing cars on driveways rather than on
grass, or to maintain cars to avoid oil and gas leakage, we would enhance the regional quality of life.
The information on inlet works hydraulic performance would allow better flood maps for
sinkholes in the region. The principal flood problems in Bowling Green arise out of sinkhole flooding. By
improving flood maps, we improve floodplain management and improve the quality of life throughout
the region.
Finally, it is well known that karst aquifers are extremely vulnerable to contamination. In nonkarst areas typical groundwater flow velocities may be inches per day. In karst areas, groundwater flow
velocities of a mile per day are common. Contamination in karst spreads very quickly.
Drainage engineers receive very little formal training in stormwater system design in karst areas
despite the major differences in karst and non-karst hydrology. Currently, it is impossible to find a
standard engineering hydrology text that even mentions the word karst. One of the primary goals of
this facility would be to improve karst stormwater system design.
Partnerships
We anticipate interest in this work from several potential partners. These would include, but
not be limited to the following.
Western Kentucky University
Bowling Green/Warren County Planning Commission
City of Bowling Green Public Works Department
Other communities in the region
Private Sector companies (construction, environmental, engineering, etc.)
State of Kentucky
Environmental and Public Protection Cabinet
Kentucky Transportation Cabinet
Division of Water
U.S. Environmental Protection Agency
We have had discussions with Tim Slattery, Bowling Green City Hydrologist who has said that
Public Works would be very interested in being involved during the planning stages of this project. We
would offer the opportunity to each of these stakeholders to be involved in planning and supporting this
work.
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Summary
We are proposing to develop a facility to test the hydraulic, water quality, and maintenance
performance of injection wells. Bowling Green alone has more than 1200 of these currently and more
are being drilled all the time. The information from this facility will be used to improve injection well
performance and improve groundwater quality for the rapidly developing south-central Kentucky region
and for other karst areas.
The proposed facility would provide many additional benefits that include student and
stormwater professional training, public outreach, and improved stormwater system design in karst and
non-karst areas. The facilities could be configured to test both detention pond outlet works design, and
improved culvert design.
We anticipate that many organizations will be interested in using and supporting this work.
These include the University, local governments, and state and Federal agencies.
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