2021-07-21_PERMIT FILE - C1981019 ‎(5)‎

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 7/21/2021
• Doc Name: Point of Compliance Well Investigation Report
• Section_Exhibit Name: Exhibit 07 Item19 Point of Compliance Well Investigation Report
• Media Type: D
• Archive: No

Point of Compliance Well Investigation

AECOM
6

3. Data Analysis
3.1 Establishing Site Background Values
To evaluate the temporary well constituent concentration data, it was necessary to establish
groundwater threshold values for comparison. Numeric standards for unclassified groundwater
in Colorado are contained in Regulation No. 41 – The Basic Standards for Ground Water (5
CCR 1002-41). Per 41.5.C.6. of Regulation No. 41, the “Interim Narrative Standard” is
applicable to all groundwater, to which standards have not already been assigned in the state,
with the exception of those areas where the TDS is equal to or exceeds 10,000 mg/L. The
Interim Narrative Standard applies to alluvial groundwater at Colowyo Mine because the
groundwater TDS is typically less than 10,000 mg/L, and site-specific standards have not been
established. Section 41.5.C.6. of the regulation goes on to state that:
“Until such time as use classifications and numerical standards are adopted for the
ground water on a site-specific basis throughout the state…ground-water quality shall be
maintained for each parameter at whichever of the following levels is less restrictive:
(A) existing ambient quality as of January 31, 1994, or
(B) that quality which meets the most stringent criteria set forth in Tables 1 through 4 of
“The Basic Standards for Ground Water.”
The simplest interpretation of the Interim Narrative Standard is that groundwater at the Mine
could be required to meet the “most stringent criteria set forth in Tables 1 through 4 of The Basic
Standards for Ground Water” (BSGW). Table 2 lists the potentially applicable human health,
drinking water, and agricultural standards for constituents monitored by Colowyo, and identifies
the most stringent value for comparison (the table only includes constituents with defined
groundwater criteria). However, the approach of selecting the most stringent criteria may not be
appropriate for Colowyo Mine because past studies have shown that upgradient, ambient
groundwater unaffected by mining does not always meet the BSGW for certain constituents
(AECOM 2018; Peterson Hydrologic, LLC 2015). In such cases, compliance with the most
stringent groundwater standard is not achievable, and an alternate method is needed to
establish comparison standards for point of compliance wells.
Ambient groundwater quality for the site was evaluated using statistics to develop background
comparison values that were based on site-specific data. Two different approaches were used
for the background statistics. The first approach relied on concentration data from the Mine’s
upgradient monitoring wells to calculate site-specific comparison values. Analytical data
collected from upgradient wells A-6 and A-8 between 1984 and 2017 were pooled to form a
background dataset, and were used to develop statistical upper tolerance limits (UTLs) with 95
percent confidence. The second approach used was to estimate existing ambient water quality
in the downgradient site area prior to January 31, 1994, as prescribed in Section 41.5.C.6.b.i.A
of Regulation No. 41. Under this method, AECOM compiled pre-1994 concentration data from
the NGSW and Gossard Well locations, and again developed UTLs for the combined dataset
with 95 percent confidence.
ProUCL software (Version 5.1) published by the EPA was used for the statistical analyses (EPA
2015). For both the upgradient and pre-1994 datasets, background concentrations were
developed by calculating parametric or nonparametric UTLs for each constituent based on the
frequency of non-detect values and whether the background dataset for that constituent