Stormwater Infiltration Practices in Karst Michael J. Byle ... · Groundwater level for use in...

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1 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

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Stormwater Infiltration Practices in KarstMichael J. Byle, P.E., F. ASCE

Abstract

The soluble nature of carbonate geologies makes them sensitive aquifers. Solutionsfeatures create an open structure that produces a groundwater regime that provideslittle in the way of filtration and little resistance to groundwater flow. Cavities in therock, formed over geologic, time lie in wait beneath the surface to open as sinkholesas soil is eroded into the voids. This process can be greatly accelerated by changes tonatural drainage and increased or concentrated infiltration. The key to successful useof infiltration in carbonate geologies is to consider this sensitivity in mitigating theaffects of development. Detailed subsurface investigation is required to define thesoil, rock, and groundwater conditions. The design of infiltration systems must becustomized to individual site conditions. Where sinkholes are a consideration,selected infiltration sites should have a well defined rock surface, a groundwater levelabove the top of rock, and soils of adequate permeability. Due to the aquifersensitivity, Adequate cleaning measures should be provided prior to infiltration andinfiltration sites should be located distant from structures. There are a great manybenefits of infiltration as a management practice for stormwater runoff. Infiltration isthe primary natural source of recharge to groundwater, and infiltration practices canbe 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 surfacewater discharges. Infiltration may also mitigate temperature increases in stormwaterbasins, as the water is cooled as it percolates through the ground. Filtration of theinfiltrating water can remove some contaminants. One significant benefit ofinfiltration to site development is that it can often be effected invisibly, with novisible surface structures.

Introduction

The effectiveness of infiltration practices for stormwater is limited by the soil, rockand groundwater conditions within the site. Clogging is a significant issue for manysoils and for runoff containing heavy sediment loadings. Since infiltration systemsare largely below-ground, they may be difficult to maintain, particularly if installedbeneath a pavement or other structure. Contaminants may be introduced into thegroundwater when soils either provide insufficient filtration or when the capacity ofthe soils to adsorb additional contaminants is exceeded.

In designing infiltration systems one must consider factors related to the particular

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application. The expected concentrations of sediment, and other contaminants mustbe assessed and appropriate design criteria established for the system to operateeffectively. Both the rate and quantity of water must be considered. A detailed siteevaluation is required to assure performance.

The detailed site evaluation should include subsurface investigation to determine thesoil, rock and groundwater characteristics of the infiltration site. Soil propertiesshould be determined including the following:

� Soil Gradation and Permeability to assess the ability of the soil to passwater

� Soil Cation Exchange Capacity to measure the ability of the soil toremove 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 aprescriptive manner. Regulating authorities generally mandate one or moreapproaches. Many regulatory bodies allow for site specific designs, though mostdesigners rely on published “standards” rather than a more involved individualizeddesign. This is largely a product of the competitive design market and the lack ofperceived value on the part of developers. The accepted standard designs are usuallybased on local experience with little in the way of supporting studies. Theperformance of these systems in the published literature is largely based on averagedexperience and many subjective criteria. Published design requirements are oftenqualitative with little to form a basis for design.

Most typical infiltration standards call for “free draining soil” This is often mistakenlytaken 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 andcapacity of some soils may be unacceptable.

Karst The ground surface features resulting from solution activity in carbonate bedrock arereferred to as karst. These features include surface depressions, sinkholes, rock

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pinnacles and caverns. Carbonate Rocks include limestone, dolomite, evaporite, andsome marbles. The karst features are produced by the action of water dissolving therock and transporting the overlying soils into solution openings in the rock. Suddensubsidence or collapse occurs when the overburden soil or rock is undermined bydissolution or soil erosion to the point where it can no longer span over the openingcreated.

One of the main difficulties in investigating karst sites is that there are frequently anumber of openings present beneath the ground surface that cannot be detected. Some of these openings are stable and some are not. The natural process ofinfiltration and subsurface flow of groundwater is constantly at work rearrangingthings. In most rocks, with the exception of evaporites, the rate of dissolution of rockis quite slow, taking millennia to erode a few millimeters. In these environments, therate of soil movement into preexisting openings is the critical mode of failure. Theintroduction of surface water infiltration can accelerate the process and precipitatecollapse in area where natural processes may have posed no significant risk. Theintroduction of water at a lower pH than the natural condition will also accelerate therate of dissolution of the carbonate rock. Acid rain and runoff from certain industrialor agricultural lands can be a factor. In some cases the replacement of nativedeciduous vegetation with coniferous plantings can cause a significant change in Phof runoff.

Changes in the groundwater level pose a significant risk for sinkhole formation. Inmost cases, a decline in groundwater levels will produce an increased risk forsinkhole formation. This has frequently been observed in the vicinity of limestonequarries that have lowered the groundwater level to enable mining to deeper levels. Fluctuating groundwater levels that oscillate above and below the rock surface areprobably the highest risk environment for sinkholes.

Infiltration Difficulties Particular to Karst

Best Management Practices for infiltration in karst prone areas are limited. Forexample, The Pennsylvania Handbook of Best Management Practices (CH2MHILL,1998) states specific limitations for infiltration in karst bedrock. In fact, the manualstates that channels and ponds should be lined to prevent concentrated infiltration thatcould induce sinkholes.

In the temperate climate of the northeastern United States the carbonate rocks oftenweather to form low permeability residual soil. This results in overburden soilsconsisting of clays and silts that limit the practical amount of infiltration. The relativelow permeability of these soils forces seepage to follow fractures or discontinuities in

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the soil which in effect concentrates the erosive force of water in these areas. Wherethese seepage paths encounter opening in the rock, there will be erosion of soil formthe overburden into fractures and voids in the rock.

Where surface grades promote runoff, this process is slow. However, where localponding occurs, or in relatively flat areas that do not shed stormwater quickly, theprocess can be more rapid. One significant problem in site investigation isdetermining at what state the subsurface erosional process is to predict futurebehavior. Changes in drainage patterns due to construction can rapidly accelerate thiserosional process and cause sinkholes to appear almost over night.

Likewise, introduction of new and more concentrated infiltration into the subsurfacefrom a stormwater management system will similarly accelerate the subsurfaceerosional process. This can result in an increased potential for collapse and sinkholesubsidence.

Through this process an erosional channel can open through the overburden soils, toform a direct path to groundwater. This circumvents the filtration that is normallyassociated with infiltration Best Management Practices (BMP’s). Contaminantspresent in roadway, parking lot, and landscaping runoff flow directly into thegroundwater. This is especially problematic due to the sensitivity of carbonateaquifers to contamination. Carbonate aquifers generally have large free flowingreservoirs within interconnected solution voids and as a result provide very little to norestriction on the migration of contaminants once they enter the system.

Where the limestone bedding is upturned and eroded, pinnacles of rock may bepresent. The presence of pinnacles, cavities in the rock, collapsed zones, anddifferential weathering result in a variable and complex stratigraphy. It is not feasiblein many cases to adequately define the true condition of the subsurface withconventional borings or test pits. This can result in surprises in construction such ashaving a pinnacle in the middle of an infiltration structure, or opening up a sinkholethat happened to fall between borings.

The subsurface conditions in karst areas seldom meet the prescriptive requirements ofregulations or design manuals. For example, the Pennsylvania Handbook of BestManagement Practices notes that infiltration systems require free draining soils suchas sand, which is seldom the case in karst sites. Horner et al (1994) indicates thatsystems with artificial media (peat and sand) may be considered and have beensuccessful in where native soils are insufficient.

Practicality of Infiltration in Karst

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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. Theanswer to that question is “sometimes.” The feasibility of infiltration practices inkarst sites can only be determined on a site by site basis. The topography, depth torock, groundwater conditions, subsidence history and site area are all factors.

In any case, there are a number of design features that are essential for infiltrationsystems in carbonate geologies with karst potential. Where there is potential forcontaminants in the runoff, provisions must be made to provide adequate cleaningprior to infiltration. Sediment is lethal to infiltration systems and can contain metalsand organic contaminants. The facility design should include measures to reduce thegeneration of sediment and measures to remove it before it can get into the infiltrationsystem. With the special sensitivity of carbonate aquifers and the potential forchannels to erode through the overburden, it is critical to remove as much of thecontamination as possible prior to infiltration. The overburden in karst areas shouldnot be relied upon for filtration and adsorption of contaminants. Filter strips, grasslined swales, basins and other filtration systems should be considered.

In siting an infiltration system, consider the site characteristics that will increase thelikelihood of success. The optimal site will be underlain by intact rock, have agroundwater level above the top of rock, have a reasonably permeable overburden soiland have no critical structures within the potential zone of infiltration inducedsubsidence. However, to obtain a reasonable measure of success, a site should have atleast one of these two. Where rock is intact, the likelihood of sinkhole subsidence isreduced. Likewise where the groundwater is level is above the top of bedrock, thereis less potential for subsurface erosion of the soils.

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Figure 1 Deeper soil over bedrock permits larger sinkholes to form.

One common misunderstanding arises from the presumption that where there is adeep overburden, there is less potential for sinkholes to form. While there is a certainlogic to this assertion, the reverse is often true. A thick soil overburden will providegreater opportunity for filtration and dispersion of infiltration. However, in lowpermeability soils, infiltrating water tends to concentrate along discontinuities in thesoil. The greater depth of soil will require a longer period of time for a sinkholemigrating from the bedrock to reach the ground surface. But the size of the sinkholethat can open in deep soil overburden is much larger than would open where there isshallower soil cover.

Infiltration Balance

Two common reasons for using infiltration BMP’s are to maintain the pre-development groundwater recharge and to reduce surface discharges to streams andwaterways. In general, the former is often achievable in karst. The surface dischargereductions to pre-development levels may not be practical. When developmentoccurs, typically vegetation is removed and the site is covered with impervioussurfaces. The removal of vegetation reduces the amount of evapotranspiration anddecreases the initial abstraction ( the amount of rainfall it takes to wet the surfacebefore any runoff occurs). Because of evapotranspiration due to vegetation, not all ofthe pre-development infiltration goes to recharge groundwater. Depending on theclimate and site conditions, the evapotranspiration losses can be greater than thegroundwater recharge. The introduction of impervious surfaces does three things, 1)it reduces the infiltration in the impervious areas, 2) due to a reduced initialabstraction, increases the volume of runoff, and 3) due to a higher runoff coefficient,

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increases the rate of surface discharge.

Balancing the groundwater recharge in karst areas can often be achieved withrelatively modest and practical infiltration systems. This is because the volume ofrecharge is only a fraction of the pre-development infiltration. Detailed modeling isrequired to assess the pre-development groundwater recharge. And statisticalprojections of rainfall and stormwater generating events can be used to compute theinfiltration capacity and size infiltration systems appropriately.

Attempting to balance the entire site discharge through groundwater infiltration is notusually a viable option for karst sites. This is primarily due to the low permeabilityoverburden soils and the increased potential for subsurface erosion that occurs wheninfiltration increases above pre-development rates. If appropriate conditions arepresent, it may be possible to completely balance the stormwater flows throughinfiltration. This will likely only be practical with large sites having significant openspace for infiltration structures. If this is considered, one must assess the consequenceof 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 requireclose scrutiny to assure that they can function and do not adversely affect the safety ofstructures, people and the environment. Infiltration trenches, basins and bedsgenerally should be sited away from buildings, roadways or other structures wheresubsidence could damage the structure and create an unsafe condition. Only where,detailed geotechnical studies and investigations indicate that there are no karstfeatures present, should one consider locating the infiltration structures close to orbeneath structures. Due to the potential for concentrated flow in the subsurface,filtration by the native soils should not be relied upon for cleaning and groundwaterprotection. This requires the use of additional BMP’s such as grassed swales, filterstrips, sedimentation basins and others to remove contaminants before they reach theinfiltration structure.

Carbon and peat filters may be considered where space is at a premium. Theconcentrations anticipated in the runoff should be assessed and the filtration mediadesigned to provide adequate life. Maintenance of stormwater management structuresis often neglected due to lack of awareness, ignorance, poor or lost maintenance planor a change of property ownership. This factor should be considered in anystormwater management design, but is especially important for filtration systemswhere failure of the filter may go completely undetected causing damage to aquifer.

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Treatment wetlands are a good choice for promoting infiltration, if proper conditionscan be established. These should be designed to retain and infiltrate up to about six toeight inches (150 to 200 mm) of water for each storm event. The addition of thissmall hydraulic head effectively enhances infiltration even in relatively lowpermeability soils. The actual achievable infiltration will depend on the nature of thesoils, since truly low permeability clays will not permit significant infiltration.

Another alternative system would be to use infiltration beds. These are especiallypractical in areas where rock is relatively shallow and consistent. In such situationsthe overburden clay can be removed down to the top of bedrock and it can be replacedwith sand over a geotextile filter. A peat filter layer can be placed over the sand andthe surface planted in grass. This type of structure promotes surface filtration by thegrass, followed by secondary filtration through the peat and tertiary filtration throughthe sand. The geotextile is used primarily to prevent migration of sand particles intorock fractures and should not be relied upon for filtration. A structure so designedwill resist clogging while providing a high capacity infiltration and protectinggroundwater 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 theground surface. This relatively simple system can be installed where conditions areappropriate. This system was installed at the base of a grassed slope so that the grasscan act as a filter strip to remove a larger suspended solids and extending the life andeffectiveness of the infiltration bed. The surface of the rock was broken into gravelsized fragments in the upper 20 to 30 feet (6 to 9 m) with groundwater located morethan 40 feet (12 m) deep. The overburden clay soil and upper weathered and brokenrock was easily excavated for construction of the sand and peat bed.

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Figure 2 Infiltration Bed for shallow carbonate rock

Figure 3 Construction of Infiltration Bed inLimestone

Investigations

A proper investigation of sites is especially important in carbonate geology wherethere is a karst potential. The entire site should be observed by a geologist orgeotechnical engineer familiar with karst processes and features. Locations of dolines

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Figure 4 Close up of brokenrock excavation and

geotextileor historic sinkholes should be identified and mapped. Geologic features such ascontacts between formations, fracture traces and fault lines should be identified asthese areas will have a higher potential for sinkholes. Subsurface investigationsshould include rock coring to evaluate the presence of voids, solution features andcollapse features. Ground water levels must be accurately measured and groundwatergradients identified. Soil permeability should be measured either through laboratoryor field tests.

Siting

The site topography and proposed development must be considered together with thegeologic/geotechnical information to identify likely locations where infiltration maybe safely considered. Some of the things to consider in siting infiltration facilities inkarst include:

a. Keep infiltration distant from critical structuresb. Locate where groundwater is above the top of rockc. Best where 5 to 10 feet of soil over rockd. Downstream from cleaning and sediment removal e. Locate in areas of more pervious soils

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These may be considered as guidelines. The actual viability of infiltration at a givensite must determined on a site specific basis. There may be sites and conditions thatpreclude the use of infiltration for stormwater management. Developments in highlypinnacled rock or highly sensitive structures in densely developed areas will not likelybe good candidates for infiltration of stormwater. However, infiltration may be apractical solution for many other sites. A properly designed system can provide safeand effective stormwater infiltration to protect and maintain the groundwaterresources.

Summary

While not a solution for every site, with sufficient investigation and proper design,infiltration may be effectively used for stormwater management and groundwaterrecharge. A thorough investigation of site specific geologic, groundwater andgeotechnical conditions is essential. Selection of infiltration locations and the overallstormwater management system should identify and minimize risks associated withpotential sinkholes and direct discharges to groundwater. The stormwatermanagement for the site should be designed as complete system of components toinclude discharge rate controls and facilities to clean the water before infiltration. Site selection is critical and should be considered early in the site planning. Creativityin application of Best Management Practices is required to fit the solution to theunique site conditions. A properly designed system can provide safe and effectivestormwater infiltration as a tool to protect and maintain groundwater resources.

References

CH2MHILL Pennsylvania Handbook of Best Management Practices for DevelopingAreas. Pennsylvania Association of Conservation districts, Keystone Chapter, Soiland Water Conservation Society, Pennsylvania Department of EnvironmentalProtection, Natural Resources Conservation Service, Spring 1998

Horner, R.R., Skupien, J. J., Livingston, E.H. Shaver, H.E., Fundamentals of UrbanRunoff 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

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Manager, Gannett Fleming, Inc. Valley Forge Corporate Center, 1010 AdamsAvenue, Audubon, PA 19335. Phone: (610) 650-8101. Fax: (610) 650-8190. E-mail:[email protected]