Leetown Karst Studies Revised Concepts and Regional Implications

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Leetown Karst Studies Revised Concepts and
Regional Implications

• Mark Kozar, U.S. Geological Survey, Charleston WV
• Topics to be discussed
• Epikarst in the Leetown, WV area.
• Geologic, lithologic, and topographic controls
  on
• ground-water flow in the Leetown area.
• 3) Ground-water flow velocities in conduit and
  diffuse
• fracture dominated portions of the aquifer
  in the
• Leetown area.

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What is Epikarst

• Epikarst is a relatively new concept in the field
  of Hydrology and there is much debate over what
  the term epikarst means.
• Epikarst is simply the upper surface of karst,
  comprised of the network of intersecting
  fractures, residuum, and solution conduits, that
  collect and transport water and other
  constituents to the deeper karst aquifer.

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Why is Epikarst Important

• Since the epikarst is a zone of very permeable
  solutionally enlarged fractures and residuum, the
  epikarst constitutes a major avenue of shallow
  ground-water flow.
• Also, contaminants from septic effluent, urban,
  or agricultural runoff, or other contaminant
  sources can rapidly infiltrate epikarst and
  easily flow through the epikarst or percolate
  deeper into ground water.

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Epikarst in the Leetown Area

• Thickness of the upper clay layer ranged from 1
  to 24 feet and averaged 13 feet..
• Typical depth of epikarst for 17 wells drilled in
  Leetown ranged from 22.5 to 61 feet below land
  surface and averaged 40 feet.
• Pinnacles and large high angle solution voids gt1
  ft wide are common in the epikarst.
• Solution voids were found at depths gt200 feet but
  are not common at such depths.

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Avoiding Epikarst Problems Preventative Measures

• Due to the permeable nature of epikarst, shallow
  wells or wells with short casings are more
  susceptible to contamination than are deeper
  wells or wells with longer casings (Mathes, 2000,
  Kozar and Mathes, 2001).
• Casing water wells through the epikarst can
  dramatically improve water-quality.
• Grouting of wells can seal off the epikarst and
  mitigate contamination of wells.

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Dye Tracer Studies

• Past tracer tests at Leetown (Jones,1997)
  documented ground-water velocities from 70 to 150
  feet per day (ft/d).
• Recent tracer tests at Leetown documented flow
  velocities in the range of 20 to 100 ft/d.
• Tests show that dye movement is controlled by
  hydraulic gradients and precipitation (storms)
  with faster ground-water velocities typical in
  the Winter and Spring and slower velocities in
  the Summer and Fall.

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Dye Tracer Studies

• Dye tracer tests document mostly flow in the
  epikarst and therefore only quantify flow in one
  portion of the aquifer (primarily conduit flow).
• Additional major components of ground- water flow
  occur within a broadly distributed network of
  interconnected diffuse fractures and in smaller
  solution conduits at depths greater than that of
  the epikarst.

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Aquifer Tests

• Short and long-term aquifer tests conducted in
  Jefferson and Berkeley Counties indicate three
  ground-water flow regimes with unique properties.
• These regimes are commonly 1) within the shallow
  epikarst (conduit flow), 2) within fractures with
  little or no carbonate dissolution (diffuse
  flow), 3) or within bedrock with components of
  both flow regimes (mixed flow).

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Aquifer Tests - Continued

• Typical transmissivity of the conduit portion of
  the system ranges from 3,900 to 20,000 ft2/d,
  from 0.00001 to 10.0 ft2/d for the diffuse
  portion of the aquifer, and from 10.0 x to 3,900
  ft2/d for the mixed zone.
• Turbidity is a major problem for wells completed
  in solution conduits but wells in the mixed zone
  are typically less vulnerable to turbidity
  problems.

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Ground-Water Modeling

• A ground-water flow model prepared for the
  Leetown area helped test and revise the
  conceptual model of ground-water flow in the
  region and develop a water budget for the
  Hopewell Run watershed.
• Ground-water modeling confirmed the conceptual
  model and also revealed the importance of
  geologic (structural and lithologic) and
  topographic controls on ground-water flow.

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Ground-Water Modeling- Continued

• Particle tracking produced velocities in solution
  conduits of approximately 20 to 50 ft/day similar
  to that documented in recent tracer tests
  conducted at Leetown.
• Calculated flow velocity for the diffuse flow
  fractured limestone dominated portion of the
  aquifer system was 1.0 to 4.0 ft/day.
• Calculated flow velocity for the Martinsburg
  (0.35 to 0.7 ft/day) and Conococheague Formations
  (0.9 to 1.0 ft/day) were slower.

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Geologic Mapping/Aerial Imagery

• Geologic mapping is critical for accurate
  simulation of ground-water flow in the Jefferson
  and Berkeley County area.
• Aerial imagery including aerial and satellite
  photography, digital elevation models, and Lidar
  imagery can significantly improve fracture
  trace/lineament analysis.
• Location of significant fracture zones is
  possible with recent developments in aerial
  imagery and can aid development of models.

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Summary

• The aquifer is comprised of both conduit and
  diffuse flow components and a continuum between
  the two end members.
• Dont forget to account for the slower compo-nent
  of flow in the diffuse flow portion of the
  aquifer which comprises the major surface area of
  the aquifer.
• Consider grouting and casing off the epikarst to
  avoid potential contamination.
• Ground-water flow modeling of Great Valley karst
  aquifers is possible but requires detailed
  mapping of potential conduits to be realistic.