7.2.1

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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