DERR-2024-005215

EPA-Superfund and UDWQ-GWDP, May 2022

Principal Study Questions and DQO Study Area for
Supplemental Geo-Hydrology Information Required for CERCLA Site-Characterization
and Utah Groundwater Discharge Permit Requirements

Principal Study Questions
The RI to date indicates that groundwaters moving through the PRI-15 area (the CERCLA Zone) are in contact with the
western side of the uncontained Facility Operating Area (FAO) and the Retrofitted Waste Pond (RWP). Because sparse
RI data exists for this area compared to the FOA-RWP area proper, further characterization of the geohydrologic
conditions (along the 1.6+ mile long N-S axis) along the PRI-15 CERCLA-Zone is needed in order to support the
development of an appropriate long-term groundwater monitoring plan (as required in the UDWQ GWDP), to assure
RWP-containment and sitewide groundwater protectiveness, The RWP containment plan-design assumes the
groundwater flow velocity and direction are consistent and uniform across the cross-sectional area to the west of the
FOA-RWP. Additional data is necessary to more fully characterize groundwater flow and support an eventual long-term
monitoring plan.
Accordingly, additional data within PRI-15 is needed to answer the following Principal Study Questions:
1. Is the lateral and vertical variability in hydraulic conductivity sufficiently low to produce the uniform flow
required to maintain containment of the Retrofit Waste Pond for PRI-15 groundwater protection?
a. Geostatistics may be used to determine if existing data is sufficient, but this is unlikely given the sparse
data available in the area of interest and the heterogeneity observed throughout the site.
b. EPA/UDWQ recommend collecting additional data along western boundary to characterize K at high
vertical and lateral resolution to allow targeted monitoring wells to be constructed in key zones (such as
high K zones throughout the upper aquifer).

2. Are the groundwater flow directions and magnitudes sufficiently uniform and consistent to assure containment
at the western boundary of the RWP?
a. Existing data suggests that flow direction is not uniform. Little is known about magnitudes. Can vertical
and lateral variability in direction and magnitude of flow be differentiated and estimated using existing
data? How does this flux compare to model and water budget? Is this flux uniform?
b. EPA/UDWQ recommends installing appropriate monitoring wells along the eastern side of PRI-15 and
the western boundary of the FOA-RWP to identify heads, gradients, flow directions, and calculate flux.
Include multiple vertical completions to allow for the upper aquifer to be monitored throughout the
water column and to determine if flow is uniform laterally and vertically along the cross-sectional plane
of the North/South hydraulic barrier zone.

3. After demonstrating 1 and 2 above, how will the upgradient groundwater flow system be affected by the Facility
Operating Area (star pond, gypsum pile, plant, etc.) and the RWP? Where (and in what manner) should
monitoring occur to account for variability in flow?
a. The agencies are most concerned about northwestern and southwestern corners of RWP and the
influence of the FOA-RWP on those zones.
b. Compare to predicted or potential FOA-RWP effects.
c. UEPA/UDWQ recommend utilizing short screen monitoring wells in transmissive/high K zones to
monitor the most likely transport pathways.

4. What are the Total Dissolved Solids (TDS) of groundwater outside of the Retrofit Waste Pond? For example,
how does TDS vary spatially throughout the study area, and how can the TDS be utilized in a monitoring system?
a. Map existing data
b. Collect TDS samples at new MWs and update maps

EPA notes that the NMR SAP included the following DQOs which remain directly related and remain relevant:
1. Determine the localized variability of horizontal hydraulic conductivity and porosity within the Upper Aquifer
Zone (UAZ) along the western end of the RWP and the adjoining PRI-15 area.
a. The NMR/CBS data report did not address this DQO (directly related to 2022 PSQ/DQOs).

2. Define the characteristics of groundwater flow direction and relative flux in the UAZ along the western side of
the RWP.
a. Groundwater flow direction is not uniform. The RWP design assumes uniformity (directly related to #2
above).
b. Flux has not been presented.
3. Determine the depth and screen interval of UAZ groundwater wells that will adequately monitor potential
migration of wastewater-impacted groundwater across the open western end of the RWP.
a. This objective has not been discussed. The Agencies review of existing data suggests that additional
investigations are required.

DQO Study Area