New Mexico Geological Society Guidebook, 5th Field Conference
Caves and Karst of Southeastern New Mexico, 2006, p. 233-242.
INTRASTRATAL KARST AT THE WIPP SITE, SOUTHEASTERN NEW MEXICO*
CAROL A. HILL
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, [email in linked article]
ABSTRACT. — The possibility of intrastratal karst at the WIPP site has been evaluated with respect to karst principles and processes and to dissolution/hydrological studies. While little direct evidence exists for intrastratal karst at the WIPP site, many indirect lines of evidence suggest the possible presence of karst: surface topographic depressions, negative gravity anomalies, lack of surface runoff, collapse breccias, insoluble residue horizons, WIPP-33 sinkholes and caves, anomalous drawdowns, and salinity variations within boreholes. Intrastratal karst, if it exists, would bypass slow matrix flow in the Culebra Dolomite aquifer and be subject to fast flow in cave conduits.
SUMMARY
Karst aquifers pose more problems to the hydrologist than any
other because their characteristics are poorly defined and water
flow within them is of a very particular type. With direct observations
limited to caves, boreholes, and recharge and discharge, the
rest of the aquifer characteristics must necessarily be deduced from
indirect evidence. One cannot treat the system as a “black box”
– such treatment can be far removed from physical reality and tell
one little about the structure of the aquifer and how it really operates.
This is especially true for intrastratal (covered) karst.
In the case of the WIPP site, documenting intrastratal karst by
direct evidence is very difficult. The method of recharge is diffuse,
so that dye cannot be easily put into the system in order to
trace water from input to output. The discharge point is unknown,
and possible discharge springs (e.g., Malaga Bend) have not yet
been monitored hydrographically to correlate with rainfall on and
around the WIPP site. And, without the parameters of recharge
and discharge, water balance studies are hypothetical at best.
With deep, phreatic, intrastratal karst such as may possibly exist
at the WIPP site, caves cannot be explored and geophysical techniques
cannot detect caves that deep. Many boreholes have been
drilled at the WIPP site, but even their lack of intersection with
caves is not a guarantee that instrastratal karst is not present.
It should be kept in mind that while well and drawdown tests
have characterized fracture and matrix flow in the Culebra dolomite
aquifer, they have not characterized possible channel network
flow that may be occurring between wells. To quote Palmer
(1995, p. 61): “The laminar flow so commonly detected by well
tests is tributary to conduits that convey water and contaminants
at high velocities to spring outlets. Unless monitor wells happen
to intercept these conduits, contaminants can pass right between
them without the slightest chance of detection.”
ACKNOWLEDGMENTS
This project was funded by DOE/Sandia Labs under Contract
#BF-4551. Derek Ford and Ken Johnson reviewed the manuscript.
