Evaluating Karst Geology Using The Hazard Ranking System

Under karst conditions, contamination from a hazardous waste source canbe expected to travel in ground water rapidly and
erratically and with less dilution than in most other aquifer conditions. Because of this, sites overlying karst may pose a
greater threat to human health and the environment. The Hazard Ranking System1 (HRS) contains special considerations
to account for the increased threat posed at sites where karst underlies any part of a source. This fact sheet will discuss the
definition and identification of karst, the ways in which karst conditions at the location of a source affect the HRS scoring
process, and commonly-asked questions and answers.
United States Office of Publication 9320.8-02FS
Environmental Protection Solid Waste and PB98-963327
Agency Emergency Response EPA 540-F-98-052
September 1998
SEPA Evaluating Karst Geology Using
the Hazard Ranking System
Office of Emergency and Remedial Response
State, Tribal, and Site Identification Center (5204G) Quick Reference Fact Sheet
INTRODUCTION
What is karst? Consistent with the definition of karst in the
HRS, The Hazard Ranking System Guidance Manual2
defines karst as:
A kind of terrain with characteristics of relief and
drainage arising from a high degree of rock
solubility. The majority of karst conditions occur in
limestone areas, but karst may also occur in areas of
dolomite, gypsum, or salt deposits. Features
associated with karst terrain may include irregular
topography, abrupt ridges, sinkholes, caverns,
abundant springs, disappearing streams, and the
lack of a well-developed surface drainage system of
tributaries and streams. Karstaquifers generally are
associated with karst terrain on the surface. Karst
aquifers at depth may not be associated with karst
terrain.
Karst aquifers and karst terrain are formed by dissolution
of certain types of rocks by ground water and rain Where
extensive dissolution has occurred (mature karst), ground
water flow is dominated by conduits that act as tributaries
to cave streams. Subterranean openings in karst range in
size from minute voids to large caverns. Ground water
flow velocities are potentially very high, and contaminants
in karst can travel long distances with little dilution in
comparison to contaminants in granular porous media
aquifers.
In the United States, karst is most commonly found in the
midwest, eastern, southern, and mountain states, but small
pockets of karst can be found in almost every state in the
Nation.3 The presence of a karst aquifer underneath a site
must be based on site-specific information.
In the scoring of karst aquifers in the ground water pathway
and the ground water to surface water component of the
surface water pathway, the HRS accounts for differences in
the fate and transport of hazardous substances by assigning
higher factor values if karst aquifers are present under the
site. Karst is evaluated differently for several HRS factors
in these pathways.
IDENTIFYING KARST FOR SCORING PURPOSES
To score the ground water pathway or ground water to
surface water component of the surface water pathway, the
presence of karst conditions underlying any portion of the
sources should generally be documented using site-specific
information The Hazard Ranking System Guidance
Manual suggests that the site evaluator:
1. Use geologic maps and other readily available
information to determine if karst features are expected
within 4 miles of the site. If a karst formation is identified

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within the target distance limit (TDL), continue with the
following steps.
2.	Compile the available site-specific evidence that
indicates the presence ofkarst. Such information can be
obtained fromtopographic maps, aerial photographs, maps
of caves, and visual observations.
3.	Estimate the lateral extent of karst. Based on the
distribution of the karst features within the formation, use
professional judgement to delineate laterally the areas
containing karst features. Documentation of karst
underlying a source may include, but is not limited to:
•	A drilling or boring log from on-site wells that
indicates voids beneath the source, illustrated by a
lithologic log, loss of drill mud, or intermittent plunges
of the drill bit into solution cavities.
•	Surficial features of karst terrain, such as a sink hole,
are evident on the site.
•	Features of karst terrain are extensive surrounding the
site, within the target distance limit, and indicate the
karst formation extends beneath the site.
4.	Estimate the thickness of karst. As an initial
determination, the depth and thickness of the formations)
containing the karst features should be evaluated.
Indications of depth and thickness may be available from
well log data, scientific literature, or other information
compiled during the evaluation of aquifer boundaries.
5.	Define the aquifer boundaries for karst aquifers. To
identify karst aquifer boundaries, start with geologic maps
and information compiled during the identification and
definition of aquifers. Based on this information, compile
a list of geologic materials and/or formations that are
known to contain karst features.
6.	Identify wells that draw drinking water from a karst
aquifer that underlies sources at the site. These drinking
water wells qualify for special consideration when scoring
potential contamination.
The steps provided in The Hazard Ranking System
Guidance Manual allow the use of professional judgement
in identifying and evaluating karst aquifers. The rationale
for evaluating a karst aquifer should be supported by site-
specific and regional geologic references.
SPECIFIC CONSIDERATIONS OF KARST IN THE
HRS
The factors that are potentially impacted when karst is
present are listed in Exhibit 1. The following discussion
summarizes how the affected factor values are adjusted
when karst is present.
EXHIBIT 1
HRS GROUND WATER PATHWAY
FACTORS EVALUATED
DIFFERENTLY
FOR KARST
Ground Water
Pathway
Evaluate
d
Differentl
y
Likelihood of Release
Observed Release
No
Potential to Release
YES
Containment
Net Precipitation
Depth to Aquifer
Travel Time
No
No
YES
YES
Waste Characteristics
Toxicity
Mobility
Hazardous Waste Quantity
No
YES
No
Targets
Nearest Well
Population
Resources
Wellhead Protection Area
YES
YES
No
No
Likelihood of Release
The adjustments in the likelihood of release to the potential
to release factor value show that contaminants move rapidly
through a karst aquifer.
Depth to Aquifer
In evaluating the depth to aquifer factor value for a site
located in karst terrain, assign a thickness of 0 feet to a karst
aquifer that underlies any portion of the sources at the site.
Travel Time
The HRS gives special consideration in the travel time
factor value by stating that "[if], for the interval being
evaluated, all layers that underlie a portion of the sources at
the site are karst assign a value of 3 5."1 If the entire interval
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is not karst, continue the evaluation for the "Other Than
Karst" layers. Assign a thickness of 0 feet to a karst layer
that underlies any portion of the sources at the site.
Waste Characteristics
The adjustment to the waste characteristics mobility factor
value shows that contaminants may move more rapidly in
solution channels, or other karst features, than through a
non-karst aquifer.
Mobility
The Superfund Chemical Data Matrix4 (SCDM) gives
mobility values for chemicals in karst and non-karst
settings. Use the value given in the "karst" column if the
entire interval from a source at the site to the aquifer being
evaluated is karst. If karst is present in the interval, but the
entire interval is not karst, use "non-karst" values given in
SCDM.
When using HRS Table 3-8 to assign a mobility factor, use
the distribution coefficient category "karst" if the entire
interval from a source at the site to the aquifer being
evaluated is karst. If karst is present in the interval, but the
entire interval is not karst, use "non-karst" values given in
the table.
Targets
Adjustments in the evaluation of targets show that the
individuals drinking water from a karst aquifer can be
exposed to higher concentrations of contaminants than they
would be if they were drinking from other aquifer types.
Nearest Well
If none of the target drinking water wells is subject to level
I or level II concentrations for the aquifer and if one of the
target aquifer is a karst aquiferthat underlies any portion of
the sources at the site and if any well draws drinking water
from this karst aquifer within the TDL, assigna value of 20
for the nearest well factor for the aquifer.
Population
For potentially contaminated drinking water populations,
use the "Karst" portion ofHRS Table 3-12 to assign values
only for that portion of the target population served by
points of withdrawal of drinking water from a karst aquifer
that underlies any portion of the sources at the site.
Continue the evaluation with use of "Other Than Karst"
values from HRS Table 3-12, applied to the remainder of
the target drinking water population.
QUESTIONS AND ANSWERS
Q: Are surficial features of karst, such as sinkholes, springs,
and disappearing streams, necessary to establish the
presence of an underlying karst aquifer?
A: No. A karst aquifer may exist at such a depth that
surface features do not exist. Consideration of an
aquifer as karst does not require surface features.
Q: Are surficial features of karst sufficient to document the
existence of a karst aquifer if found at or very near a
source?
A: Usually. Although these features may be absent in the
case of karst existing at depth, the presence of sinkholes
and other surficial features is indicative of karst.
However, in the western United States, lava tubes,
fissures, open sinkholes, and caves have been formed by
extrusion of the still-liquid portion of cooling lava.
These surface features may bear a resemblance to karst.
"Sinkholes" in lavagenerally lack the symmetry of those
developed in solution terrain.
Q: Can a non-karst area riddled with mining shafts or lava
tubes be evaluated as karst?
A: No. These features may, however, be adequate to
document aquifer interconnection, which may lead to a
higher site score.
Q: Does the presence of a limestone aquifer necessarily
mean that the aquifer is karst?
A: No. The area must either show surficial karst
expression or the aquifer must have karst features.
Q: Can the existence of akarstformationlying between two
non-karst formations be used to document
interconnection between the aquifers above and below
the karst layer?
A: Not necessarily. It still should be shown that the
hydraulic conductivities are less than two orders of
magnitude between each formation. For example, karst
features can actually channel water horizontally and
stop vertical migration.
Q: When some of the individuals within the TDL are
obtaining water from a surficial karst portion of an
aquifer, but others are obtaining water from a non-karst
portion, how are the targets evaluated?
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A: The two sets of targets are evaluated separately and
then added together. Use the karst portion of HRS
Table 3-12 to assign values for the population that
obtains water from an eligible karst aquifer. The
remaining non-karst water-drawing population is
assigned a valuefrom the "other than karst" portion of
HRS Table 3-12. These values are then assigned to the
potential contamination formula in HRS section
3.3.2.2.
Q: A karst aquifer found 2.1 miles away from a source is
proven to be interconnected with an aquifer underlying
the source. Can the aquifer beneath the site be
evaluated as karst? What if the interconnected karst
aquifer is found 1.5 miles from a source?
A: At greater than 2 miles, interconnection is not used in
scoring a site, so distant karst formations are not
relevant to site evaluation. At less than 2 miles, an
interconnected karst aquifer that is used for drinking
water is evaluated as karst only if that karst aquifer
underlies a source on the site.
REFERENCES
1.	40 C.F.R., Appendix A (1997). Hazard Ranking
System; Final Rule. 55 FR 51582.
2.	U.S. Environmental Protection Agency, November,
1992. The Hazard Ranking System Guidance Manual.
Interim Final. Office of Solid Waste and Emergency
Response. Directive 9345.1-07, pp. 137 to 146.
3.	U.S. Department of the Interior, U.S. Geological
Survey, 1986. National Atlas of the United States of
America, "Engineering Aspects of Karst," Map with
Narrative.
4.	U.S. Environmental Protection Agency, June, 1994. The
Superfund Chemical Data Matrix (SCDM). Office of
Solid Waste and Emergency Response. Directive
9360.4-18.
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