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Amy Eschberger Project No 20144265
<br /> Colorado Division of Reclamation, Mining and Safety July 24,2020
<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 showing the variability of barium, iron, manganese, TDS, and chloride are presented in
<br /> Attachment A and time series graphs are presented in Attachment B. Large differences in the concentrations of
<br /> ' these parameters are observed between MW-4, MW-6, and MW-7 for barium and iron, and to a lesser extent
<br /> manganese. Consequently, the comparison for these parameters will be made on an individual well basis, rather
<br /> than pooling data from MW-4, MW-6, and MW-7.
<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, TDS, and chloride are presented in Attachment B and time
<br /> ' series graphs are presented in Attachment C. Large differences interpreted to be due to natural variability, are
<br /> observed between MW-4, MW-6, and MW-7 for barium and iron, and to a lesser extent manganese. For these
<br /> parameters, comparisons will be made on an individual well basis, rather than pooling data from MW-4, MW-6,
<br /> ' and MW-7. If any of the comparisons show that the results are comparable, a demonstration can be made that the
<br /> concentrations 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 /> tIf 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 /> 4.0 REPORTING
<br /> ' 4.1 Field Investigation Summary
<br /> Following the field program, a technical memorandum summarizing the field investigation will be prepared. The
<br /> report will include discussion of the field activities performed and subsurface conditions encountered or inferred.
<br /> ' The borehole geologic logs, well installation logs, and water level measurements will be presented as tables or
<br /> attachments. The geologic logs will include depths, elevations, descriptions of subsurface materials (soil and rock)
<br /> encountered, and depths to groundwater. Finally, monitoring well permit applications will be filed with the
<br /> ' Colorado Division of Water Resources.
<br /> 4 GOLDER 5
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