Along
with thousands of other small municipalities, Bowling Green, KY, became
responsible for Phase II implementation of stormwater regulations under the
National Pollutant Discharge Elimination System (NPDES) in 2003. As a part of
its Phase II program, the city installed a structural water-quality device
(SWQD) at the entrance of Bypass Cave. The cave entrance is located within a
sinkhole and contributes recharge to the Lost River karst groundwater basin. The
sinkhole is actively used for stormwater drainage and contains several
drainpipes that empty into the cave entrance. The SWQD was installed to filter
out surface debris and trash, and to improve the quality of the water entering
the local karst groundwater system. City and county stormwater coordinators have
been working with Western Kentucky University (WKU) to ensure that the NPDES
Phase II stormwater program is rigorous and produces scientifically usable
monitoring data.
For
many years, the WKU, in coordination with the city of Bowling Green, has been
studying Bypass Cave and its impact on local groundwater. Before installation of
the SWQD, a set of water samples was collected to establish a water-quality
baseline (Figure 1). An assessment of the cave passage was conducted to document
the impact of stormwater flow and surface debris. A second set of samples was
collected a year after the device was installed. Water-quality analyses
subsequent to the SWQD installation indicated that the device had a negative
effect on water quality and actually increased the amount of debris entering the
cave system. A follow-up cave passage assessment documented a substantial
increase in surface trash and debris in the cave, and also revealed that this
material was being carried deeper into the cave system. This is likely a result
of the overload of stormwater on the SWQD and an inadequate detention basin.
Installing the unit caused a decrease in detention basin capacity, and nearly
all infiltration within the basin was blocked so that the entire volume of
stormwater must now pass through the cave system. The unit is unable to handle
the increased volume of water that results from storm runoff that initially
fills the detention basin behind the device. As the volume of water increases,
it develops enough hydraulic head to flow over the device, and is injected into
the far reaches of the cave passage. Implementation of a SWQD must include a
corresponding increase in detention basin storage capacity so that the slower
movement of water through the system and decreased infiltration can be
accommodated. A sizable amount of infiltration generally occurs in sinkhole
detention basins, and SWQD installation must actively minimize the negative
effects on this process for remediation to be successful.
Karst
Terrain
Karst
is a type of landscape formed from the dissolution of soluble rocks including
limestone, dolomite, and gypsum. Karst regions are characterized by sinkholes,
sinking streams, caves, and springs. Nearly all surface karst features are
formed by internal drainage, subsidence, and collapse caused by the development
of underlying caves (Ford and Williams 2007). Karst terrain is present in 18% of
the lower 48 states of the US and 25% of the world (Figure
2).
Rather
than overland flow through streams, karst water flows belowground through
systems of conduits and fractures until it emerges as a spring. Surface drainage
through stream networks disappears, and sinkholes replace these features as the
subsurface flow increases due to ever-enlarging conduits. Subsurface water in
these systems moves very quickly to a spring, similar in speed to pipeflow
(Brosig et al. 2008). A myriad of local planning problems are specific to karst
landscapes, including sinkhole collapse, sinkhole flooding, and an easily
pollutable groundwater supply (Crawford 2001). In non-karst areas, groundwater
moves far more slowly, and this laminar flow and contact with soil and soil
organisms allows for greater removal of contaminants from groundwater than in
karst regions (White 1988).
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Figure 3. Bypass Cave before installation of the structural BMP. The weir to measure water volumes blocks most of the entrance from view. |
Bypass
Cave is an insurgence cave in Bowling Green, KY, that has been dye-traced to the
Lost River Cave system (a major tourist attraction). It most likely intersects
the main Lost River Cave stream downstream from the terminal breakdown in
Alexander Cave (Schafstall 1984) and eventually flows into the Barren River,
which is the drinking water source for the city. Bypass cave has a surveyed
length of 378 meters (1,247 feet) with a vertical extent of approximately 11
meters (35 feet). The cave entrance is situated within a small sinkhole located
approximately 100 meters from US Route 31. A weir was installed by Crawford
& Associates in the 1980s to monitor water quality and storm flow volumes
(Figure 3). Before the SWQD installation, the entrance sink periodically
contained small amounts of trash consisting of old clothes, aluminum cans,
plastics, tire segments, and hubcaps. Many commercial businesses are located on
US Route 31 near the entrance to the cave, including restaurants, clothing
stores, a car repair shop, banks, and other small businesses. The Bypass Cave
entrance sinkhole drains a large paved area of streets and parking lots. The
cave also extends under residential areas located just off of the
road.
A
general understanding of stormwater quality during wet weather can be
ascertained by monitoring water within the cave to evaluate how effectively
specific best management practices (BMPs) improve water quality. Water sampling
within the cave was conducted for baseline water-quality and post-SWQD
water-quality comparisons. The SWQD was installed with the intention of
filtering the flood-water runoff, which would ultimately improve water quality
entering the Barren River system where Bowling Green’s drinking water intake is
located (Figure 4). The SWQD has six baffles—three high, three low—and a
collection grid in the back (Figure 5). The three top baffles filter floating
debris, and the lower three baffles collect all of the contaminants that are
heavier than water.
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| Figure 4. The sinkhole entrance after structural BMP installation. A student is standing at the cave entrance. |
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| Figure 5. The structural BMP installation and internal construction |
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| Figure 6. Water quality in the cave directly below an auto repair facility |
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| Figure 7. Trash and debris deep within the cave in an area that had been clean for decades prior to the installation of the structural BMP |
Cave
Passages
Most
of the passages in Bypass Cave are narrow stream canyons with dimensions of 3
meters high by 1 meter wide. Several small infeeders bring water into the stream
canyon during wet weather. The floor of the stream canyon contains a series of
discrete, shallow pools of water, which form a stream during very wet weather.
The canyon is decorated with flowstone and small stalactites, which indicate
input from epikarstic water.
The
foundation of a building intersects the cave at approximately 60 meters into the
cave. The foundation extends one-half meter below the natural roof of the cave,
but does not block water flow through it. At this point in the cave, the water
quality changes from a stormwater gray color with oily residue (which occupied
all interior pools) to include an orange organic growth, possibly associated
with the deposition of metals and other automobile wastes (Figure 6). This
change begins approximately below an automobile repair facility located on US
Route 31.
A
significant amount of surface debris (both natural and anthropogenic) was
observed to litter the floor, walls, and ceiling of the passage during the
initial assessment. Natural surface debris consists of leaves, sticks, and
branches. The manmade debris includes glass bottles, aluminum cans, plastic
bags, hubcaps, tire pieces, old pipe segments, bricks, and pieces of
rebar—likely from the building foundation. The debris reaches almost to the
ceiling in places, indicating that the first 200 meters of passage can flood to
the ceiling.
At
this point, the cave passage changes morphology from a simple canyon to a
T-shaped canyon. The upper segment of the canyon is much wider in cross-section
than the lower segment. A series of two flowstone ramps have formed short
segments above the canyon floor. In between the ramps, the stream canyon can be
observed below. After the second ramp, the passage remains above the stream
channel and is very dry. After 40 meters, this dry (i.e., never flooded) section
becomes nearly blocked with breakdown from past roof collapses. Beyond this
point, the passage opens into the eastern edge of a large terminal chamber, 40
meters long by 20 meters wide. One has to climb down the breakdown to enter the
room. A second passage also enters the chamber from the east. This passage
contains an actively flowing stream, and the passage is a low crawl space that
has been mapped for 90 meters, at which point it becomes too low to
continue.
Secondary
deposits of flowstone, rimstone dams, stalactites, stalagmites, and small soda
straws are present throughout the cave, but mostly in the first 200 meters of
the cave. The speleothem deposits appear to be active, which means that surface
water is entering the cave through the cave’s bedding planes. Water is also
entering through the various small joints and fissures that occur throughout the
cave. Additionally, there are two minor and one major side passage water
infeeders in the cave that do not flow past the SWQD location. The constant
inflow of water from a variety of locations, even during dry periods, would
limit the overall effectiveness of a cave-mouth SWQD for downstream water
quality.
Because
the biology of stream passages within a cave is an important indicator of water
quality, a preliminary inventory of cave life was done at Bypass Cave. Organisms
observed included tubifex worms, pouch snails, small flies, cave crickets,
spiders, washed-in earthworms, beetles, and on the upper walls and ceiling small
patches of actinomycetes (a filamentous bacteria). Tubifex worms are indicative
of low oxygen concentrations, which characterize sewage-type pollution. Pouch
snails generally indicate nutrient-enriched conditions and poor water quality.
The fauna of the entire Lost River Cave system consists of troglobitic
amphipods, isopods, crayfish, and blind fish. Yet none of these was observed in
the stream canyon of Bypass Cave, which indicates that the water quality of the
cave is seriously compromised.
Stormwater
Impact Assessment
A
preliminary investigation of Bypass Cave was conducted on July 23, 2004, and it
was determined that water-quality sampling prior to SWQD installation would be
appropriate. A second visit was made on September 30, 2004 to continue
stormwater assessment of the cave and conduct baseline water sampling at five
locations within the cave (Figure 1) to evaluate the changes in water quality as
it moves through the cave system. A cave cleanup by Western Kentucky University
students was organized in conjunction with the city stormwater program on
October 23, 2004 to ensure that all anthropogenic debris was removed from the
cave before SWQD implementation. The SWQD, which was designed to handle 25 cubic
feet per second, was installed on February 21, 2006. A second set of samples was
collected on November 29, 2006.
The
last significant rainfall occurred on August 12, 2004, prior to collection of
the pre-SWQD installation samples, and, thus, they represented “low-flow”
conditions. This was the longest dry period for several months prior to
sampling, and only intermittent pools of water covered passage floor. Normally,
these pools are much deeper. Because the sampling was done at low flow, water
quality is indicative of the contaminants that remain in the cave after flood
flows and is a better representation of baseline water quality. The second set
of samples was collected in 2006, shortly after a storm event with several prior
weeks of dryness so that conditions within the cave were similar to those on the
initial sample date.
Methods.
Two sets of samples were collected from the cave stream in 40-ml and 1,000-ml
bottles. A visual assessment of the cave passage was conducted and
photo-documented. Analysis was conducted at Western Kentucky University’s WATERS
Laboratory both before (2004) and after (2006) the installation of the SWQD.
Analysis was performed for cations, anions, metals, atrazine, fecal coliform,
and oil and grease (Table 1). In addition to water sampling, other properties
were also measured (pH, conductivity, turbidity, temperature, and dissolved
oxygen) using the Horiba U-10 Multiparameter Water Quality Meter. This sampling
regime was developed by the Bowling Green and Warren County stormwater
coordinators and Western Kentucky University scientists in a cooperative
endeavor to ensure that Phase II stormwater monitoring leads to scientifically
usable data. This sampling framework is used at a variety of surface karst
features through Bowling Green to evaluate water quality and stormwater program
effectiveness quarterly. This project extended the research belowground to
evaluate water quality under more difficult circumstances.
Sampling
Locations.
Sample 1 was collected 3 meters after an in-feeding stream shortly past the cave
entrance at the end of an overflow passage. Sample 2 was collected shortly just
beyond the 90-degree turn in the main passage before the first climb. Sample 3
was collected between the second and third up-climbs.
Sample
4 was collected from an in-feeding stream shortly before its confluence with the
main stream in the terminal big room. The water quality at this site should have
minimal improvement from the SQWD because it is primarily fed by a second cave
stream.
Sample
5 was collected in a stream after the confluence of the main stream and
infeeding stream, located several meters upstream of a monitoring well. Because
the water in this area is downstream of the confluence of the main stream and
the last infeeding passage and it is impossible to ascertain contaminant
sources, no sample was collected in 2006.
See
Figure 1 for a map of sampling locations.
Results.
The results of the analysis for 2004 and 2006 (Table 1) indicate that there is
no significant improvement in water quality and that there is a decline in water
quality in some areas. The post-installation cave passage assessment showed a
major increase in the amount of surface debris and garbage entering the cave.
The debris traveled far deeper into the cave than in previous years (Figure 7).
Sediment coatings continued to cover all flooded surfaces, and trash was
distributed much farther downstream in the cave passage than had previously been
observed. There was an increase in lead (Figure 8) from 2004 to 2006. Chromium
and other metals were pushed farther into the cave, from sample site 2 to sample
site 3 (Figure 8). Copper and other metals increased close to the surface of the
cave and were being carried in from parking lots and other paved areas. Calcium
and chloride both increased dramatically as well. Fecal coliform increased at
all sample sites except site 4 (Figure 8).
The
most notable result for both sampling dates was the fecal coliform levels from
site 1 (10,000 c/mL) and the lesser, but still high, levels at the other
stations. Site 1 is located adjacent to the first infeeder encountered in the
cave directly below a Chinese food restaurant. During a cave cleanup by Western
Kentucky University students (October 23), a very septic odor filled the cave
starting at the infeeder (where it was strongest) and continuing to the main
room. Fecal coliform levels were also high at sites 4 and 5. The high fecal
counts at site 1, 4, and 5 are strongly indicative of multiple septic system
failures. This situation will not be resolved by using a structural
water-quality device at the entrance of the cave.
Levels
of several metals and of oil and gasoline at sites 2 and 3 were noticeably
higher than at the rest of the sites for both sampling dates. These levels are
indicative of possible wastewater leakage or chemical dumping from industrial or
commercial sources. A transmission repair shop is located directly over the cave
near this location and is almost certainly the source of these
contaminants.
Dissolved
oxygen levels at sites 1 through 3 are too low to support aquatic life (and lack
of aquatic organisms was observed in the cave). Dissolved oxygen levels
increased toward the back of the cave and reached nearly 6 mg/L, which is enough
to support aquatic life, although no organisms were observed
there.
The
observed sediment deposits within the cave far exceed natural levels observed in
other area caves. Sedimentation can negatively affect aquatic communities.
Nutrients and toxic chemicals can attach to sediment particles, ride the
particles in the water, and either settle with the sediment or become soluble in
the water column. SWQD installation should have most directly improved this
situation but appears to have made the situation worse.
Though
nitrate levels at each site are below the EPA’s drinking water standards (10
mg/L), sites 4 and 5 had levels that were higher than natural background levels
(>2 mg/L). Because this part of the cave does trend toward a residential
area, it is possible that the above-background nitrate levels result from lawn
fertilizer application.
Installation
of the SWQD does not appear to have reduced the overall amount of trash and
large debris entering the cave. Observations on the physical impact of
floodwater in the cave passage have been ongoing for many years. Prior to the
SWQD installation, all reports noted thick sediment coatings on flooded
surfaces. However, the trash was typically reported to reach the area before the
first up-climb in the cave—just before sampling site 3. The terminal chamber
contained some plastic bag segments and a few bottles and cans, which is the
typical flood debris that one would expect in a cave of this type. However,
unlike the stream passage, the main walls and ceiling of the main room were not
coated with flood debris. Past research teams have reported that this area
remained dry and safe even during heavy rain and flooding of the entrance
passage. The low stream passage that enters the main room from the east
contained a gasoline odor, so it could not be safely
assessed.
Since
the SWQD installation, trash is now found in all sections of the cave, including
as far as the terminal breakdown room at sampling station 5. Our assessment also
shows that the trash has not simply been washed in, but has entered under
hydraulic head. Trash has been observed crammed in small cracks and crevices in
the walls and ceiling, and between and under breakdown blocks. Much of this
material is broken, torn, and tattered into much smaller pieces during transport
into the cave passage. The location and state of the trash indicates that the
SWQD has a limit on how much stormwater it can filter. Surface runoff from a
typical storm pulse begins to accumulate and then back up at the entrance of the
cave, and infiltration into the sinkhole epikarst that had previously occurred
has been blocked by the SWQD and its concrete apron. As more water accumulates,
the flow develops enough hydraulic head to flow over the device and into the
entrance of the cave under pressure. Trash that has washed off the surrounding
streets and parking lots is literally injected into the cave and transported
more than 300 meters to the terminal room of the cave.
Conclusions
Based
on two sets of water samples collected before and after installation of the
SWQD, the device does not appear to have improved the quality of the water
within the cave, nor did it effectively filter any trash to keep it from
entering the cave. The baseline water-quality data are from only one set of
samples and provide only a snapshot of the pre-SWQD water quality. A series of
samples, ideally for an entire year, would have given a more accurate assessment
of existing water quality. The post-SWQD sampling set is also only a snapshot of
the water quality at Bypass Cave. Our water-quality monitoring program will
provide more accurate information on the state of the water in the cave in the
future.
Visual
inspection of the trash within the cave passage does indicate a significant
increase in trash within the cave and also shows that the trash is being
transported farther into the cave passage than it was previous to the
installation of the SWQD. Treating the water before it enters the cave would be
very effective in improving the overall groundwater quality if the entrance was
the only source of water. However, two infeeders that enter the cave beyond the
entrance have significant water-quality issues that will not be addressed with
the entrance SWQD. In addition, water also enters the cave by infiltration from
the surface and into fractures, fissures, and bedding planes that occur at the
soil-rock interface. In order to improve the overall groundwater quality in
Bypass Cave, the pollution for two infeeders and from nonpoint sources at the
surface need to be identified and remedied. Commercial and residential land use
above the cave most certainly impacts the groundwater not only during storm
events, but also during normal flow conditions.
These
initial findings are crucial for evaluating stormwater mitigation activities in
Bowling Green. It appears that much of the pollution (i.e., fecal coliform)
entering the cave cannot be directly tied to stormwater flows. If subsequent
research demonstrates this to be true, then total maximum daily load (TMDL)
levels in karst surface water bodies cannot be used to measure the success or
failure of the stormwater program. Instead, this type of targeted in-cave
monitoring will best demonstrate the contribution to water quality made by
Bowling Green stormwater mitigation activities.
Overall,
structural water-quality devices in karst areas are not going to provide a magic
bullet to address stormwater quality. Land use controls and planning, combined
with required sewering of homes and business, are critical. It is also critical
to determine the locations of caves, which are the major conduit of subsurface
water flow, so that polluting activities are not located directly above cave
streams, or so they are effectively regulated if they are currently located in
the area. Most importantly, while structural water-quality devices have a role
to play as part of an integrated stormwater management program, it is critical
that their installation provide for an increased volume in the attached
detention basin and that infiltration into the epikarst is maximized so that the
devices are not regularly overwhelmed and, thus, create a worse water-quality
situation.