springs in the area of a TSDF sited in a karst terrane. This is a necessary step in the process

of determining whether conduit or diffuse flow predominates.

Springs, seeps, and directly accessible cave springs may need to be tested by tracing

not only during moderate flow, but also during flood flow and base flow, to prove their

usefulness for monitoring. Testing must be conducted during the extremes of expected flow

conditions because flow routing in conduit flow systems commonly varies as stage changes.

During flood conditions, water levels in streams, rivers, and lakes may rise, and some of this

water may move temporarily through conduits that are dry during low flow conditions and

discharge into adjacent ground water basins.

There are certain characteristics of water movement in a karst aquifer dominated by

conduit flow that should be recognized if a monitoring strategy is to be effective. One

characteristic is distributary flow. During periods of high stage, water (and contaminants if

they are present) from the headwaters or mid reaches of a ground water basin may flow to all

springs in its distributary system. Distributary flow is most pronounced in areas of

aggradation of river valleys (e.g., Mammoth Cave, Kentucky) where many alternate conduits

at and below base level are available (deep sediment fill covering the bedrock floor of the

valley). Such a system is shown in Figure 11. Radial flow also may be identified during

dye tracing and monitoring of springs (Aley, 1988). Radial flow is most common in highly

fractured mountain areas. Another unusual characteristic encountered is subsurface drainage

paths that cut across surface drainage divides without any surface indication. Ground water

flow can parallel surface drainages, but usually not over long distances. This subterranean

piracy of water from one surface watershed to another is not uncommon in many karst

terranes. An example of watershed piracy that was discovered by tracer studies is described

by Jones (1973). Given the possibility of variable ground water movement in aquifers

dominated by conduit flow, owners and operators may need to perform tracer studies to

delineate flow paths for ground water monitoring accurately.

5.2.3

Using Wells as Monitoring Sites in Aquifers Dominated by Conduit

Flow

The placement of wells in karst terranes with subsurface conduits is rarely effective.

Installing a suite of wells to intercept cave streams that have been shown by tracing to flow

from the facility is a good strategy, but cave streams can be practically impossible to locate.

A second alternative for selecting monitoring well locations in aquifers with conduit flow is

to place wells along fractures or fracture trace intersections. Tracer studies should be used to

verify a hydraulic connection with the ground water beneath a facility under base flow and

flood flow conditions. Although some cave passages coincide with various types of fracture

traces and lineaments, not all fractures and fracture related features are directly connected

with cave passages. Many cave streams are developed along bedding planes, and are thus

unaffected by vertical fractures. This fact lessens the probability that a well drilled on a

November 1992

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