2020-12-15_HYDROLOGY - M1983194 ‎(2)‎

• Permit No: M1983194
• IBM Index Class Name: Hydrology
• Doc Date: 12/15/2020
• Doc Name: Water Monitoring - Groundwater
• From: Daub & Associates,Inc.
• To: DRMS
• Email Name: ACY
• Media Type: D
• Archive: No

DAUB &ASSOCIATES, INC. 1985 Y2 SOUTH BROADWAY
GRAND JUNCTION, CO 81507-9649
[phone]
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email: [e-mail]
www.daubandassociates.com
December 9,2020
Re: Natural Soda LLC Nahcolite Project Permit No. M-1983-194 Summary Letter of the BG-9 Groundwater
Departure from Baseline Conditions
The BG-9 groundwater monitoring well is exhibiting elevated chemistry values for conductivity, total
dissolved solids, and alkalinity. The BG-9 is a B-Groove (BG) Aquifer and subsurface subsidence monitoring
(SSMW)well, recompleted from the original DS-5,which was initially drilled as a core hole and completed as
a Dissolution Surface (DS) Aquifer and subsurface subsidence monitoring well. The DS-5 began showing
elevated chemistry and temperatures, indicating possible communication with the proximal solution mining
operations. A video log inspection indicated that the casing was intact, so the decision was made to
recomplete the well to utilize it both as a BG Aquifer monitor and a subsurface subsidence monitoring well.
This recompletion was completed in August 2018 by setting a bridge plug near the end of the casing, placing
cement on top of the bridge plug,and then perforated the casing across the BG Aquifer interval.
A chemistry departure in the BG-9 began in late 2019, but has accelerated since April of 2020. Conductivity
values have subsequently elevated to the 15-16K micromhos range. An investigation was initiated to
determine the cause. A video log was performed in the BG-9 on October 12, 2020. The video log showed
very little fluid movement in the well until the cement plug was reached near 1,823 feet below ground level
(BGL), where there was evidence of minor upflow. Subsequent scrutiny revealed four slight casing partings
just above casing couplings at depths of 1676.9, 1760.9, 1782.2, and 1804.2 feet BGL. While haloclines due
to saline stratification were observed at various depths, no apparent inflow or outflow was observed from
these partings.
Due to the casing partings but lack of apparent flow, it was decided to be proactive and remediate the well
by plugging it with a second bridge plug and cementing it back to just below the BG Aquifer perforations in
order to continue utilizing it as a BG monitoring well. This work was completed in November 2020 (refer to
BG-9 Well Recompletion Diagram). Subsequent to the plug back,the well was airlifted to clean out the well
and ensure good flow through the perforations. Field samples indicated conductivity in the range of 45-50K
micro-Siemens (field measurements), appreciably above the 16K micromhos (lab analysis) seen prior to
plugging back the well. The well was airlifted at intervals for three days to remove high conductivity fluids;
this activity concluded on 11/6/2020. Conductivity fell to 30k micro-Siemens before the effort was halted.
It is believed that DS Aquifer water was migrating up inside the casing and mixing with the BG up through
the first cement plug, and possibly through the casing partings. During subsequent operations,the DS water
was forced out of the perfs,creating a localized concentration of higher conductivity water.
NS has engaged in a dual-use strategy by converting exploratory wells to GMWs. This strategy was done in
an attempt to reduce NS's environmental impact by reducing the number of drilled wells, well pads, and
subsequent surface and subsurface disturbances. There may be some evidence however, that NS wells
which were originally drilled as exploratory core holes and then completed as GMWs may be more
susceptible to being a conduit for high conductivity fluids migrating up from depth, either from casing
damage at depth and the subsequent infiltration of mining or DS Aquifer fluids up through the inside of the
casing or via plug-back cement to a point where the GMW sampling may be detecting these high
conductivity fluids (coming up through the inside of the deeper casing) rather than the actual aquifer fluids.
Video logging in the BG-8 (plugged and abandoned)and BG-9 seem to support this position.
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