Determining Sampling Frequency
The frequency at which ground water samples will be collected should be described in
the QAPjP. Sampling frequency, in nearly all cases, should be based on the hydrogeology of
the site. There is no maximum sampling frequency set by the Agency. The minimum
frequency for sampling is at least semiannual (  264.98(d) and 264.99(f)). As stated in
  264.97(g) and 264.99(f), the Regional Administrator will specify the frequency for
sampling in the permit. Proposed sampling frequencies should be submitted by the owner or
operator as part of the permit application. Furthermore, regulations at  264.97(h) require the
owner/operator to use one of the several specified statistical procedures, or to use an
alternative method that meets specified performance standards. The method chosen should be
approved by the Regional Administrator, and specified in the operating permit. EPA's
guidance document "Statistical Analysis of Ground Water Monitoring Data at RCRA
Facilities," Interim Final Guidance (EPA/530 SW 89 026, NTIS PB 89 151 047; USEPA
1989a) suggests a method for choosing a sampling frequency that will reflect site specific
hydrogeologic conditions. The method uses the Darcy equation to determine the horizontal
component of the average linear velocity of ground water for confined, semiconfined, and
unconfined aquifers. This value is used to determine a sampling frequency that will yield an
independent sample of ground water in diffuse flow regimes.
Recent research performed in the area of ground water sampling frequency (Barcelona
et al., 1989) indicates that ground water monitoring data should be carefully collected over
long periods of time (i.e., greater than two years) to determine optimal sampling frequency
and to delineate seasonal trends in ground water monitoring results. In Barcelona et al.'s
study, ground water was collected biweekly for 18 months and analyzed for 26 water quality
and geochemical constituents. The researchers determined that for the study site, ground 
water sampling performed four to six times per year would result in an estimated information
loss below 20% and would minimize redundancy. The researchers concluded that by using
careful sampling and analytical procedures, sampling and analytical errors could be controlled
to approximately  20% of the annual mean inorganic chemical constituent concentrations in
ground water.
Alternative methods should be employed to determine a sampling frequency in
hydrogeologic settings where conduit flow predominates and where Darcy's law is invalid
(e.g., karst terrane). Section 4.5.5 discusses how to determine monitoring frequencies in these
environments. More detailed information may be found in Quinlan and Alexander (1987).
In addition to the routine analyses to be performed as specified in the facility's permit,
all land disposal facilities applying for a RCRA operating permit that have contaminated
ground water must identify the concentration of each Appendix IX constituent throughout the
plume or identify the maximum concentrations of each Appendix IX constituent in the plume
( 270.14(c)(4)(ii)). This analysis is conducted for the purpose of characterizing the
chemistry of the background and downgradient ground water.
November 1992
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