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Amy Eschberger Reference No 20144265-REV1
<br /> Colorado Division of Reclamation,Mining and Safety September 4,2020
<br /> larger minimum data set(i.e., more sampling events)utilized in other methods, in turn allowing for determination of
<br /> differences on an expedited schedule. Since variability in concentrations is observed between sampling events, this
<br /> comparison will be conducted for three sampling rounds once the water levels and data are stable. If at least one
<br /> sampling round demonstrates the results are comparable, a demonstration can be made that the concentrations
<br /> observed in MW-4, MW-6, and MW-7 are reflective of background (i.e., similar to MW-8).
<br /> RPD values will be calculated when both the MW-8 results and the mean of the recent results from MW-4, MW-6,
<br /> and MW-7 were greater than 5 times the practical quantitation limit(PQL; USEPA 2017). RPDs are calculated
<br /> according to the following formula:
<br /> %RPD = I A +B I x 200
<br /> Where: A is the concentration of the applicable result at MW-8; and
<br /> B is the corresponding concentration mean of recent data at MW-4, MW-6, and MW-7.
<br /> RPD values can range from 0%, indicating perfect correlation between results, to 200%, indicating a significant
<br /> divergence between results. Results are considered comparable when the RPD is less than 20%, per the National
<br /> Functional Guidelines for Inorganic Data Review (USEPA 2017).
<br /> The RPD is not used when results are less than 5 times the PQL for a given analyte. In that circumstance, the
<br /> absolute value of the difference between the two results is calculated and the results are considered comparable
<br /> when the absolute difference is less than the PQL (USEPA 2017). When one of the two results for comparison is
<br /> below the PQL for a give analyte, the difference is calculated using the PQL as the value of the result that was
<br /> below the PQL. No comparison is performed when both results are below the PQL.
<br /> Box and whisker plots of recent results (2017-2019, following the shift to a semi-annual sampling schedule)
<br /> showing the variability of barium, iron, manganese, boron, selenium, uranium, fluoride, chloride, sulfate, nitrate,
<br /> nitrate+nitrite, gross alpha, and TDS are presented in Attachment B and time series graphs are presented in
<br /> Attachment C. Large differences interpreted to be due to natural variability, are observed between MW-4, MW-6,
<br /> and MW-7 for barium, iron, gross alpha, and to a lesser extent manganese. For these parameters, comparisons
<br /> will be made on an individual well basis, rather than pooling data from MW-4, MW-6, and MW-7. If any of the
<br /> comparisons show that the results are comparable, a demonstration can be made that the concentrations
<br /> observed in MW-4, MW-6, and MW-7 are reflective of background (i.e., similar to MW-8).
<br /> 3.3 Evaluation of MW-9
<br /> If installed, the data from well MW-9 will be compared to water quality standards and site background water
<br /> quality. First, the data will be compared to the BSGW. If all concentrations are below the standards, a
<br /> demonstration can be made that there is no off-site migration of constituents of interest. However, concentrations
<br /> from well MW-9 are anticipated to be similar to MW-4, MW-6, MW-7, and MW-8, but there is a potential for higher
<br /> concentrations at MW-9 due to the increased residence time for groundwater in the aquifer further downgradient
<br /> to this location. Therefore, if concentrations are above the BSGW they will be compared to the background well
<br /> MW-8 using the approach outline for MW-8 above, assuming its concentrations are also above the BSGW. If
<br /> concentrations in MW-9 are comparable to MW-8, a demonstration can be made that the concentrations in
<br /> groundwater in samples collected from MW-9 reflect natural conditions.
<br /> If neither of the above demonstrations can be made, discussions with the DRMS will be initiated about making
<br /> demonstrations related to risk to potential downgradient receptors.
<br /> V GOLDER 5
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