Stormwater Infiltration Practices in Karst
Michael J. Byle, P.E., F. ASCE
Abstract
The soluble nature of carbonate geologies makes them sensitive aquifers. Solutions
features create an open structure that produces a groundwater regime that provides
little in the way of filtration and little resistance to groundwater flow. Cavities in the
rock, formed over geologic, time lie in wait beneath the surface to open as sinkholes
as soil is eroded into the voids. This process can be greatly accelerated by changes to
natural drainage and increased or concentrated infiltration. The key to successful use
of infiltration in carbonate geologies is to consider this sensitivity in mitigating the
affects of development. Detailed subsurface investigation is required to define the
soil, rock, and groundwater conditions. The design of infiltration systems must be
customized to individual site conditions. Where sinkholes are a consideration,
selected infiltration sites should have a well defined rock surface, a groundwater level
above the top of rock, and soils of adequate permeability. Due to the aquifer
sensitivity, Adequate cleaning measures should be provided prior to infiltration and
infiltration sites should be located distant from structures. There are a great many
benefits of infiltration as a management practice for stormwater runoff. Infiltration is
the primary natural source of recharge to groundwater, and infiltration practices can
be used to mitigate the impacts of impervious surfaces created by land development.
Infiltration may also serve as a means of disposal for excess runoff to reduce surface
water discharges. Infiltration may also mitigate temperature increases in stormwater
basins, as the water is cooled as it percolates through the ground. Filtration of the
infiltrating water can remove some contaminants. One significant benefit of
infiltration to site development is that it can often be effected invisibly, with no
visible surface structures.
Introduction
The effectiveness of infiltration practices for stormwater is limited by the soil, rock
and groundwater conditions within the site. Clogging is a significant issue for many
soils and for runoff containing heavy sediment loadings. Since infiltration systems
are largely below-ground, they may be difficult to maintain, particularly if installed
beneath a pavement or other structure. Contaminants may be introduced into the
groundwater when soils either provide insufficient filtration or when the capacity of
the soils to adsorb additional contaminants is exceeded.
In designing infiltration systems one must consider factors related to the particular
1
application. The expected concentrations of sediment, and other contaminants must
be assessed and appropriate design criteria established for the system to operate
effectively. Both the rate and quantity of water must be considered. A detailed site
evaluation is required to assure performance.
The detailed site evaluation should include subsurface investigation to determine the
soil, rock and groundwater characteristics of the infiltration site. Soil properties
should be determined including the following:
+
Soil Gradation and Permeability to assess the ability of the soil to pass
water
+
Soil Cation Exchange Capacity to measure the ability of the soil to
remove contaminants
+
Soil Thickness to establish its filtration capacity
+
Groundwater level for use in computing infiltration rate and capacity
+
Soil Anisotropy to assess the potential for differential flow paths
+
Topography to assess surface flows, and potential seeps from the
groundwater mound created during infiltration
+
Soil Susceptibility to Erosion and/or Moisture Sensitivity
+
Other appropriate site specific parameters affecting infiltration
Design Challenge
Generally stormwater management practices are regulated and applied in a
prescriptive manner. Regulating authorities generally mandate one or more
approaches. Many regulatory bodies allow for site specific designs, though most
designers rely on published “standards” rather than a more involved individualized
design. This is largely a product of the competitive design market and the lack of
perceived value on the part of developers. The accepted standard designs are usually
based on local experience with little in the way of supporting studies. The
performance of these systems in the published literature is largely based on averaged
experience and many subjective criteria. Published design requirements are often
qualitative with little to form a basis for design.
Most typical infiltration standards call for “free draining soil” This is often mistakenly
taken by some as “well drained,” soil description employed by the NRCS soil surveys.
Infiltration can occur in any soil regardless of permeability. However, the rate and
capacity of some soils may be unacceptable.
Karst
The ground surface features resulting from solution activity in carbonate bedrock are
referred to as karst. These features include surface depressions, sinkholes, rock
2
pinnacles and caverns. Carbonate Rocks include limestone, dolomite, evaporite, and
some marbles. The karst features are produced by the action of water dissolving the
rock and transporting the overlying soils into solution openings in the rock. Sudden
subsidence or collapse occurs when the overburden soil or rock is undermined by
dissolution or soil erosion to the point where it can no longer span over the opening
created.
One of the main difficulties in investigating karst sites is that there are frequently a
number of openings present beneath the ground surface that cannot be detected.
Some of these openings are stable and some are not. The natural process of
infiltration and subsurface flow of groundwater is constantly at work rearranging
things. In most rocks, with the exception of evaporites, the rate of dissolution of rock
is quite slow, taking millennia to erode a few millimeters. In these environments, the
rate of soil movement into preexisting openings is the critical mode of failure. The
introduction of surface water infiltration can accelerate the process and precipitate
collapse in area where natural processes may have posed no significant risk. The
introduction of water at a lower pH than the natural condition will also accelerate the
rate of dissolution of the carbonate rock. Acid rain and runoff from certain industrial
or agricultural lands can be a factor. In some cases the replacement of native
deciduous vegetation with coniferous plantings can cause a significant change in Ph
of runoff.
Changes in the groundwater level pose a significant risk for sinkhole formation. In
most cases, a decline in groundwater levels will produce an increased risk for
sinkhole formation. This has frequently been observed in the vicinity of limestone
quarries that have lowered the groundwater level to enable mining to deeper levels.
Fluctuating groundwater levels that oscillate above and below the rock surface are
probably the highest risk environment for sinkholes.
Infiltration Difficulties Particular to Karst
Best Management Practices for infiltration in karst prone areas are limited. For
example, The Pennsylvania Handbook of Best Management Practices (CH2MHILL,
1998) states specific limitations for infiltration in karst bedrock. In fact, the manual
states that channels and ponds should be lined to prevent concentrated infiltration that
could induce sinkholes.
In the temperate climate of the northeastern United States the carbonate rocks often
weather to form low permeability residual soil. This results in overburden soils
consisting of clays and silts that limit the practical amount of infiltration. The relative
low permeability of these soils forces seepage to follow fractures or discontinuities in
3
the soil which in effect concentrates the erosive force of water in these areas. Where
these seepage paths encounter opening in the rock, there will be erosion of soil form
the overburden into fractures and voids in the rock.
Where surface grades promote runoff, this process is slow. However, where local
ponding occurs, or in relatively flat areas that do not shed stormwater quickly, the
process can be more rapid. One significant problem in site investigation is
determining at what state the subsurface erosional process is to predict future
behavior. Changes in drainage patterns due to construction can rapidly accelerate this
erosional process and cause sinkholes to appear almost over night.
Likewise, introduction of new and more concentrated infiltration into the subsurface
from a stormwater management system will similarly accelerate the subsurface
erosional process. This can result in an increased potential for collapse and sinkhole
subsidence.
Through this process an erosional channel can open through the overburden soils, to
form a direct path to groundwater. This circumvents the filtration that is normally
associated with infiltration Best Management Practices (BMP’s). Contaminants
present in roadway, parking lot, and landscaping runoff flow directly into the
groundwater. This is especially problematic due to the sensitivity of carbonate
aquifers to contamination. Carbonate aquifers generally have large free flowing
reservoirs within interconnected solution voids and as a result provide very little to no
restriction on the migration of contaminants once they enter the system.
Where the limestone bedding is upturned and eroded, pinnacles of rock may be
present. The presence of pinnacles, cavities in the rock, collapsed zones, and
differential weathering result in a variable and complex stratigraphy. It is not feasible
in many cases to adequately define the true condition of the subsurface with
conventional borings or test pits. This can result in surprises in construction such as
having a pinnacle in the middle of an infiltration structure, or opening up a sinkhole
that happened to fall between borings.
The subsurface conditions in karst areas seldom meet the prescriptive requirements of
regulations or design manuals. For example, the Pennsylvania Handbook of Best
Management Practices notes that infiltration systems require free draining soils such
as sand, which is seldom the case in karst sites. Horner et al (1994) indicates that
systems with artificial media (peat and sand) may be considered and have been
successful in where native soils are insufficient.
Practicality of Infiltration in Karst
4
One might ask based on the above discussion if infiltration is indeed possible in karst.
The answer is of course yes, but the real question is whether it is practical. The
answer to that question is “sometimes.” The feasibility of infiltration practices in
karst sites can only be determined on a site by site basis. The topography, depth to
rock, groundwater conditions, subsidence history and site area are all factors.
In any case, there are a number of design features that are essential for infiltration
systems in carbonate geologies with karst potential. Where there is potential for
contaminants in the runoff, provisions must be made to provide adequate cleaning
prior to infiltration. Sediment is lethal to infiltration systems and can contain metals
and organic contaminants. The facility design should include measures to reduce the
generation of sediment and measures to remove it before it can get into the infiltration
system. With the special sensitivity of carbonate aquifers and the potential for
channels to erode through the overburden, it is critical to remove as much of the
contamination as possible prior to infiltration. The overburden in karst areas should
not be relied upon for filtration and adsorption of contaminants. Filter strips, grass
lined swales, basins and other filtration systems should be considered.
In siting an infiltration system, consider the site characteristics that will increase the
likelihood of success. The optimal site will be underlain by intact rock, have a
groundwater level above the top of rock, have a reasonably permeable overburden soil
and have no critical structures within the potential zone of infiltration induced
subsidence. However, to obtain a reasonable measure of success, a site should have at
least one of these two. Where rock is intact, the likelihood of sinkhole subsidence is
reduced. Likewise where the groundwater is level is above the top of bedrock, there
is less potential for subsurface erosion of the soils.
5
One common misunderstanding arises from the presumption that where there is a
deep overburden, there is less potential for sinkholes to form. While there is a certain
logic to this assertion, the reverse is often true. A thick soil overburden will provide
greater opportunity for filtration and dispersion of infiltration. However, in low
permeability soils, infiltrating water tends to concentrate along discontinuities in the
soil. The greater depth of soil will require a longer period of time for a sinkhole
migrating from the bedrock to reach the ground surface. But the size of the sinkhole
that can open in deep soil overburden is much larger than would open where there is
shallower soil cover.
Infiltration Balance
Two common reasons for using infiltration BMP’s are to maintain the predevelopment groundwater recharge and to reduce surface discharges to streams and
waterways. In general, the former is often achievable in karst. The surface discharge
reductions to pre-development levels may not be practical. When development
occurs, typically vegetation is removed and the site is covered with impervious
surfaces. The removal of vegetation reduces the amount of evapotranspiration and
decreases the initial abstraction ( the amount of rainfall it takes to wet the surface
before any runoff occurs). Because of evapotranspiration due to vegetation, not all of
the pre-development infiltration goes to recharge groundwater. Depending on the
climate and site conditions, the evapotranspiration losses can be greater than the
groundwater recharge. The introduction of impervious surfaces does three things, 1)
it reduces the infiltration in the impervious areas, 2) due to a reduced initial
abstraction, increases the volume of runoff, and 3) due to a higher runoff coefficient,
Figure 1 Deeper soil over bedrock permits larger sinkholes to form.
6
increases the rate of surface discharge.
Balancing the groundwater recharge in karst areas can often be achieved with
relatively modest and practical infiltration systems. This is because the volume of
recharge is only a fraction of the pre-development infiltration. Detailed modeling is
required to assess the pre-development groundwater recharge. And statistical
projections of rainfall and stormwater generating events can be used to compute the
infiltration capacity and size infiltration systems appropriately.
Attempting to balance the entire site discharge through groundwater infiltration is not
usually a viable option for karst sites. This is primarily due to the low permeability
overburden soils and the increased potential for subsurface erosion that occurs when
infiltration increases above pre-development rates. If appropriate conditions are
present, it may be possible to completely balance the stormwater flows through
infiltration. This will likely only be practical with large sites having significant open
space for infiltration structures. If this is considered, one must assess the consequence
of inducing a local rise in the groundwater level during storm events.
Infiltration Systems for Karst
Many of the normal infiltration BMP’s may be considered for Karst areas, but require
close scrutiny to assure that they can function and do not adversely affect the safety of
structures, people and the environment. Infiltration trenches, basins and beds
generally should be sited away from buildings, roadways or other structures where
subsidence could damage the structure and create an unsafe condition. Only where,
detailed geotechnical studies and investigations indicate that there are no karst
features present, should one consider locating the infiltration structures close to or
beneath structures. Due to the potential for concentrated flow in the subsurface,
filtration by the native soils should not be relied upon for cleaning and groundwater
protection. This requires the use of additional BMP’s such as grassed swales, filter
strips, sedimentation basins and others to remove contaminants before they reach the
infiltration structure.
Carbon and peat filters may be considered where space is at a premium. The
concentrations anticipated in the runoff should be assessed and the filtration media
designed to provide adequate life. Maintenance of stormwater management structures
is often neglected due to lack of awareness, ignorance, poor or lost maintenance plan
or a change of property ownership. This factor should be considered in any
stormwater management design, but is especially important for filtration systems
where failure of the filter may go completely undetected causing damage to aquifer.
7
Treatment wetlands are a good choice for promoting infiltration, if proper conditions
can be established. These should be designed to retain and infiltrate up to about six to
eight inches (150 to 200 mm) of water for each storm event. The addition of this
small hydraulic head effectively enhances infiltration even in relatively low
permeability soils. The actual achievable infiltration will depend on the nature of the
soils, since truly low permeability clays will not permit significant infiltration.
Another alternative system would be to use infiltration beds. These are especially
practical in areas where rock is relatively shallow and consistent. In such situations
the overburden clay can be removed down to the top of bedrock and it can be replaced
with sand over a geotextile filter. A peat filter layer can be placed over the sand and
the surface planted in grass. This type of structure promotes surface filtration by the
grass, followed by secondary filtration through the peat and tertiary filtration through
the sand. The geotextile is used primarily to prevent migration of sand particles into
rock fractures and should not be relied upon for filtration. A structure so designed
will resist clogging while providing a high capacity infiltration and protecting
groundwater quality. This type of system is represented in Figure 2.
Installation of such a system is illustrated in Figures 3 and 4. In this particular site,
the rock was found to be relatively uniform and within 5 to 10 feet (1.5 to 3 m) of the
ground surface. This relatively simple system can be installed where conditions are
appropriate. This system was installed at the base of a grassed slope so that the grass
can act as a filter strip to remove a larger suspended solids and extending the life and
effectiveness of the infiltration bed. The surface of the rock was broken into gravel
sized fragments in the upper 20 to 30 feet (6 to 9 m) with groundwater located more
than 40 feet (12 m) deep. The overburden clay soil and upper weathered and broken
rock was easily excavated for construction of the sand and peat bed.
8
Figure 2 Infiltration Bed for shallow carbonate rock
Figure 3 Construction of Infiltration Bed in
Limestone
Investigations
A proper investigation of sites is especially important in carbonate geology where
there is a karst potential. The entire site should be observed by a geologist or
geotechnical engineer familiar with karst processes and features. Locations of dolines
9
Figure 4 Close up of broken
rock excavation and
geotextile
or historic sinkholes should be identified and mapped. Geologic features such as
contacts between formations, fracture traces and fault lines should be identified as
these areas will have a higher potential for sinkholes. Subsurface investigations
should include rock coring to evaluate the presence of voids, solution features and
collapse features. Ground water levels must be accurately measured and groundwater
gradients identified. Soil permeability should be measured either through laboratory
or field tests.
Siting
The site topography and proposed development must be considered together with the
geologic/geotechnical information to identify likely locations where infiltration may
be safely considered. Some of the things to consider in siting infiltration facilities in
karst include:
a.
b.
c.
d.
e.
Keep infiltration distant from critical structures
Locate where groundwater is above the top of rock
Best where 5 to 10 feet of soil over rock
Downstream from cleaning and sediment removal
Locate in areas of more pervious soils
10
These may be considered as guidelines. The actual viability of infiltration at a given
site must determined on a site specific basis. There may be sites and conditions that
preclude the use of infiltration for stormwater management. Developments in highly
pinnacled rock or highly sensitive structures in densely developed areas will not likely
be good candidates for infiltration of stormwater. However, infiltration may be a
practical solution for many other sites. A properly designed system can provide safe
and effective stormwater infiltration to protect and maintain the groundwater
resources.
Summary
While not a solution for every site, with sufficient investigation and proper design,
infiltration may be effectively used for stormwater management and groundwater
recharge. A thorough investigation of site specific geologic, groundwater and
geotechnical conditions is essential. Selection of infiltration locations and the overall
stormwater management system should identify and minimize risks associated with
potential sinkholes and direct discharges to groundwater. The stormwater
management for the site should be designed as complete system of components to
include discharge rate controls and facilities to clean the water before infiltration.
Site selection is critical and should be considered early in the site planning. Creativity
in application of Best Management Practices is required to fit the solution to the
unique site conditions. A properly designed system can provide safe and effective
stormwater infiltration as a tool to protect and maintain groundwater resources.
References
CH2MHILL Pennsylvania Handbook of Best Management Practices for Developing
Areas. Pennsylvania Association of Conservation districts, Keystone Chapter, Soil
and Water Conservation Society, Pennsylvania Department of Environmental
Protection, Natural Resources Conservation Service, Spring 1998
Horner, R.R., Skupien, J. J., Livingston, E.H. Shaver, H.E., Fundamentals of Urban
Runoff Management: Technical and institutional Issues. Terrene Institute ans Us.
Environmental Protection Agency, Washington, D.C. August 1994.
About the Author
For further information contact: Michael J. Byle, P.E., Fellow ASCE, Geotechnical
11
Manager, Gannett Fleming, Inc. Valley Forge Corporate Center, 1010 Adams
Avenue, Audubon, PA 19335. Phone: (610) 650-8101. Fax: (610) 650-8190. E-mail:
[email protected]
12