United States
           Environmental Protection
           Agency
Office of Water
(WH-550)
EPA/810-K-93-001
September 1993
EPA    Wellhead Protection  in  Confined,
            Semi-Confined,  Fractured,  and  Karst
            Aquifer Settings                             	
                Protection areas around wells producing from confined, fractured, and karst aquifers are,
           because of their complex hydrogeology, more difficult to define than protection areas for wells in
           porous media settings. This factsheet provides background information explaining the need to
           define protection areas for wells that draw public drinking water from several complex
           hydrogeologic settings: confined, semi-confined, fractured, and karst aquifers. These settings v
           include aquifers in which the ground water is not open to the atmosphere, or the aquifer does not
           consist of unconsolidated porous media.  Several figures illustrate these settings in a general way.

                A wellhead protection area (WHPA) is the surface and subsurface area surrounding a
           public water supply well or wellfield that contributes recharge to, and through which contaminants
           are likely to reach, that well. Because contaminants from sources within a WHPA are likely to
           reach the well, EPA has developed the Wellhead Protection Program to prevent ground water
           contamination in those areas.

                Figure 1 shows a geological cross section through an unconfined (that is, water-table)
           aquifer. These are aquifers in which ground water at the top of the saturated zone is at
           atmospheric pressure (open to the atmosphere). In Figure 1, the aquifer is a porous medium
           comprised of sand and gravel. Well pumpage has caused the cone of depression that is shown
                                             Pumping Well
                                           \\
             ,-v
                    EXPLANATION

                           Sand and Gravel Aquifer

                           Relatively Impermeable Material Underlying Base of Aquifer

                           Direction of Row in Aquifer to Well

                           Screen
                           Potentiometric Surface
            Figure 1. Well Pumping From an Unconfined Aquifer
                            Recycled/Recyclable
                            Printed with Soy/Canola Ink on paper that
                            contains at least 50% recycled fiber

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Confined
Aquifers
in the potentiometric surface (also referred to as "water table" in a water table aquifer) around the
well.  Because ground water in such aquifers is open to the atmosphere, wells in these aquifers
are particularly susceptible to contamination from sources in close proximity.

      In confined-aquifer settings, it is important to protect, not only the WHPA surrounding the
well, but also that portion of the aquifer recharge area that supplies recharge, and potentially
contaminants, to the well. Because this recharge area can be at a great distance from the well, it
is important to understand the nature of the ground water flow paths in order to determine which
part of the aquifer recharge area is to be protected.

      In a confined aquifer, ground water is not open to the atmosphere and is generally above
atmospheric pressure.  A confining layer (aquitard) of lower-permeability material restricts the
upward and downward movement of the ground water into or out of the confined aquifer. In many
cases, a confined aquifer is found between two aquitards in a geologic sequence.

      Water levels in cased wells that tap confined aquifers are usually above the top of the
aquifer. Water may even flow to the land surface.  In the context of wellhead protection, a
confined aquifer is still considered confined  even if water levels have dropped below the base of
the confining bed as a  result of, for example, well pumpage. The imaginary surface defined by
the level to which ground water would rise in wells that are open to the atmosphere is referred to
as the potentiometric surface.  In the figures that follow, the cone of depression in the
potentiometric surface around the well is caused by well pumpage.

      The degree to which an aquifer is confined varies.  Water flow in a truly confined aquifer
(Figure 2) -- one in which the overlying aquitard is very highly  impermeable - lacks a downward
component. Therefore, the aquifer receives no recharge of water (or contaminants) from directly
above.  Recharge is limited to areas beyond the extent of the overlying aquitard, where the
                                                          Pumping Well
                             EXPLANATION

                                    j Confining Layer of Highly Impermeable Material

                                    | Sand and Gravel Aquifer
                             IV\V\\| Relatively Impermeable Material Underlying Base of Aquifer
                                ^  Direction of Row in Aquifer to Well

                                     Screen
                                	 Potentiometric Surface
                   Figure 2.  Well Pumping From a Truly Confined Aquifer

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Fractured
and Karst
Aquifers
                  aquifer is not confined. These recharge areas are frequently at great distances from the wells that
                  tap into the aquifer. Therefore, except for a small protection area immediately surrounding the well
                  casing to ensure no contaminant movement along any imperfections in the casing, grouting or
                  backfill around the well, a WHPA surrounding the well itself may serve no protective function
                  in the case of a truly confined aquifer.

                        Most aquitards, however,  contain "breaches." Some of these breaches are natural, caused
                  by local variations in the materials that make up the aquitard, by local thinning or "pinching out" of
                  the aquitard, or by conduits such as sinkholes, faults, or fractures.  Other breaches, such as open
                  boreholes and cracked well seals,  may be caused by humans.  Figure 3 depicts a confined aquifer
                  in which a sandy zone of much higher permeability and a borehole breach the overlying aquitard.
                       Abandoned
                       Bore Hole
                                                          Pumping Well
                             EXPLANATION

                                      Confining Layer with Localized Area of Sand of Relatively Higher Permeability

                                      Sand and Gravel Aquifer

                                      Relatively Impermeable Material Underlying Base of Aquifer

                                      Direction of Row in Aquifer to Well

                                      Direction of Row to Aquifer


                                      Screen
                                      Potentiometric Surface
Figure 3.  Well Pumping From a Confined Aquifer with Breaches

      When potentiometric levels in a confined aquifer are low enough (perhaps as a result of well
pumpage) to permit ground water to flow downward through the overlying aquitard, breaches serve
as conduits for the movement of water and contaminants into the aquifer.  These breaches may
provide a relatively short path between a contaminant source and a well screen.

      In leaky, or semi-confined aquifers, a layer of moderately low permeability (such as the
sandy clay shown in Figure 4) overlies the aquifer.  Even if a leaky aquitard is continuous
(unbreached), the aquifer will slowly receive recharge water from above if pumping or other factors
cause ground water flow to have a downward component.  From the perspective of wellhead
protection, a leaky-confined setting is similar to the breached-confined setting: In both
cases, contaminants may enter the aquifer not only in remote recharge areas but also near
the well.

      Many aquifers are composed of rocks that transmit water primarily (or exclusively) through
cracks, fractures, cavities, and caverns. These aquifers consist of coarsely or finely  fractured rock
and rock with karst and mature karst features.  These aquifers may be confined, partly confined, or
unconfined.

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                                         Pumping Well
           EXPLANATION
                    Moderately Confining Layer of Sandy Clay

                    Sand and Gravel Aquifer

                    Relatively Impermeable Material Underlying Base of Aquifer

                    Direction of Row in Aquifer to Well

                    Direction of Row to Aquifer

                    Screen
                    Potentiometric Surface
Figure 4. Well Pumping From a Semi-Confined Aquifer

      Water movement in a rock with pervasive, fine, intersecting fractures (Figure 5) may be
very similar to water movement in a porous medium such as sand.  For this kind of fractured-rock
setting, WHPA delineation techniques used for unconsolidated porous settings may be
appropriate, although some methods have been found to be more applicable to finely fractured
settings than other methods (USEPA, 1991 a).  The larger and more widely spaced the fractures,
the less is the similarity to unconsolidated porous media, and the less appropriate  is the use of
delineation methods designed for wells in porous media.

      Karst features develop in rocks where ground water has widened fractures and porous
zones into solution cavities by dissolving soluble minerals.  Generally, this widening is limited to
carbonate rocks such as limestone and dolomite.  Karst aquifers (Figure 6) typically contain
solution cavities along fractures and along contacts between  rock layers. Karst aquifers are
generally within several hundred feet of the land surface.

      Sometimes a karst aquifer with fairly uniform porosity and without cavernous flow may be
similar enough, at the scale of WHPA delineation, to a porous medium that porous-media WHPA
delineation techniques  may be used.  However, in mature karst  (Figure 7), solution openings are
large, well-developed, and often partially cavernous. Sinkholes, closed depressions, and pipes
(vertical solution cavities often filled with weathered  rock and soil) are common features of mature
karst aquifers.  In this setting, contaminant movement may be measured in feet per minute rather
than feet per year, as is common for unconsolidated porous media.  Because large volumes of
water move very rapidly through the large solutional openings, use  of delineation techniques
developed for  porous media is not appropriate.

      Protection of ground water quality of wells and springs tapping karst aquifers is particularly
difficult because:  (1) ground water flow is complex,  and different fractures or cavities may contain
waters from totally different sources that mix where openings intersect; (2) wide fractures and
large, well-developed solution cavities in mature karst provide little if any contaminant attenuation
by the aquifer; and (3)  sinkholes or open fractures and  cavities may provide a direct connection

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                                             Pumping Well
            EXPLANATION

                    \ Finely Fractured Aquifer

                     Relatively Impermeable Base of Aquifer

                     Direction of Flow in Aquifer to Well
                     Screen
                     Potentiometric Surface
Figure 5. Well Pumping From an Unconfined Fractured Aquifer
                                            Pumping Well
           EXPLANATION
            I""/- l~f\ Karst Aquifer
            KV\\\ Relatively Impermeable Material Underlying Base of Aquifer
                     Direction of Flow in Aquifer to Well
                     Screen
                     Potentiometric Surface
            Well Diameter is Exaggerated
Figure 6.  well Pumping From an Unconfined Karst Aquifer

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                                    Pipe/Fracture
                                                          Pumping Well
                                                             Pipe/Fracture
                                                                                     ^^>
                             EXPLANATION

                                     Karst Aquifer with Conduits
                             \X\\\ Relatively Impermeable Material Underlying Base of Aquifer
                                     Direction of Row in Aquifer to Well

                                     Screen
                                     Potentiometric Surface
                             Well Diameter is Exaggerated
                  Figure 7. Well Pumping From an Unconfined Mature Karst Aquifer
WHPA
Delineation
Methods for
Confined,
Fractured,
and Karst
Aquifers
from the land surface to the aquifer. Although WHPA delineation is difficult in these settings,
wellhead protection is particularly important because contaminants can move rapidly and
extensively throughout karst aquifers.

      Because of potentially high flow velocities and complex flow routes in karst and coarsely
fractured settings, and remote locations of recharge areas in confined settings, methods such as
arbitrary and calculated fixed radii, may be of limited or no value when applied to fractured, karst,
or confined aquifers. Additionally, application of some porous media  methods to finely fractured
settings has been found to more poorly approximate a well's recharge area than application of
other porous media methods.

      Hydrogeologic mapping, particularly when combined with dye tracing and lineament
analysis can be used to delineate WHPAs in karst settings.  USEPA, 1988, provides helpful
information for using dye tracing to determine flow paths in karst settings.  The publication
describes both qualitative and quantitative dye tracing.  Qualitative dye tracing involves "tagging" a
sample of water with a tracer and then monitoring several ground water locations for the
reappearance of the dye-laden water.  The reappearance of the dye may be observed visually or
through passive detectors and then identified via chemical or instrumental analyses.  After
qualitative dye tracing is used to identify ground water sites that are in hydraulic connection with
the injection site, quantitative dye tracing may be performed  to give estimates of peak
concentration, dispersion, and persistence.  Quantitative dye tracing is considerably more labor
intensive than qualitative dye tracing and is performed less frequently.

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      USEPA, 1991 a, describes several methods for delineating WHPAs in fractured aquifers that
behave as porous media and discusses method effectiveness.  The methods presented are:
Vulnerability Mapping, Flow-System Mapping, Flow-System Mapping With Time-of-Travel
Calculations and With the Uniform Flow Equation, Residence Time Approach and Numerical
Flow/Transport Models. Figure 8 shows a comparison of WHPAs delineated with three different
methods. The document also assesses which of the methods presented are applicable to
fractured aquifers that do not behave as porous media.
      -     Ground Water Divide
         ^    Test Well (MW-1)
       Row-System Mapping
      ........ Uniform Flow Equation

      ^ Numerical Modeling

              SCALE 1:24 000
         FEET 500  0 500 1000   2000

     (From USEPA, 1991 a, p. 62)    	
Figure 8.   Comparison of wellhead protection areas delineated with three different
            delineation methods. The hydrogeologic setting is a fractured aquifer that
            behaves as a porous medium.
      USEPA, 1991b, describes Cone of Depression methods and Time of Travel methods that
can be used for delineating WHPAs in confined aquifers whose regional potentiometric surface
has a negligible slope.  The document also presents a Zone of Contribution With Identification of
Flow Boundaries method and several Zone of Transport With Time of Travel Contours methods
that are applicable where the regional potentiometric surface has a non-negligible slope.  Figure 9
depicts the difference in capture zones calculated with the WHPA Model for two hydrogeologic
settings, identical except for the slope of the regional potentiometric surface.  The publication also
describes approaches for determining the presence and degree of confinement provided by an
overlying aquiclude.

      The reader is referred to these publications for further information on delineating WHPAs in
karst, fractured, and confined settings.

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                                 20,000
                              8  10,000 -
                                                  40-yrTOT.
                                              -regional gradient = 0.002
                                                                       40-yrTOT,
                                                                     no regional gradient
                                          gradient
                                                                        (Irom USEPA, 1991D, p. 128)
                                                              10,000

                                                              Feet
                                                                           15,000
                                                                                        20.000
                 Figure 9.   Wellhead protection areas of 5-, 10-, 20-, 30-, and 40-year time of travel (TOT)
                            assuming a potentiometric surface with a regional slope of 0.002, and a
                            wellhead protection area of 40-year time of travel assuming a flat regional
                            potentiometric surface.  The method used was the semianalytic WHPA
                            computer program.
References
U.S. Environmental Protection Agency. October 1988.  Application of Dye-Tracing Techniques for
      Determining Solute-Transport Characteristics of Ground Water in Karst Terrains, U.S. EPA
      904/6-88-001.  103pp.

U.S. Environmental Protection Agency. June 1991 a.  Delineation of Wellhead Protection Areas in
      Fractured Rocks, U.S. EPA 570/9-91-009. 144 pp.

U.S. Environmental Protection Agency. June 1991b.  Wellhead Protection Strategies for
      Confined-Aquifer Settings, U.S. EPA 570/9-91-008. 168 pp.
8

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