to 264.98(d) and 264.99(f), or must be an alternate sampling procedure specified under
264.97(g)(2). This minimum monitoring frequency is inadequate for karst terranes
dominated by conduit flow, because the storage time of the water is low and the
concentrations of contaminants in conduit systems can vary over a short period of time.
Therefore, hourly sampling of aquifers dominated by conduit flow is recommended before,
during, and after storm or other runoff (e.g., snowmelt) events, although this is a site specific
determination. As described by Quinlan (1990), in aquifers dominated by conduit flow,
"sampling should start at base flow, before the beginning of a storm or meltwater event, and
continue until 4 to 30 times the time to the hydrograph peak has elapsed, depending upon the
extent to which an aquifer is characterized by conduit flow as opposed to diffuse flow.
Sampling may have to be done as often as at 1 to 6 hour intervals in the early part of a
precipitation event and at 4 to 24 hour intervals in the waning part of its hydrograph."
Data from the samples collected during the peak runoff should be compared with
samples collected during base flow under fair weather conditions at other times of the year.
This should enable a reliable assessment of the ground water quality to be made. Quinlan
and Alexander (1987) discuss the rationale for sampling frequencies of ground water in karst
aquifers dominated by conduit flow in much greater detail. The work performed by Quinlan
and Alexander (1987) is site specific, and while it illustrates the considerable extent to which
spring discharge should be evaluated, the results obtained by Quinlan and Alexander should
not be taken as representative of all sites located above karst aquifers dominated by conduit
The previous discussion highlights the various difficulties associated with ground
water monitoring in karst aquifers. Site characterization of these hydrogeologic settings is
complex, time consuming, and potentially costly. The Agency again emphasizes its belief
that most facilities sited in karst settings will not be able to meet the ground water monitoring
requirements of Subpart F; therefore, alternative locations for land disposal of hazardous
wastes are preferred.
Fracture Trace Analysis
The detection of ground water contamination in fracture controlled aquifers can be
problematic due to the difficulties in locating the fracture systems that often dominate the
ground water flow system. Fracture traces have been mapped for the purpose of locating
zones of increased weathering, porosity, and permeability that act as preferential pathways of
contaminant migration. Strong correlations between well yields and fracture traces in
carbonates, igneous rock, metamorphic rock, fractured siltstones, and fractured sandstones
have been documented by numerous authors (Jansen and Taylor, 1988). Fracture trace
analysis should be performed at sites where hydrogeologic data indicate that contaminant
migration may occur along fractures.