HomeMy WebLinkAboutWQ0004972_Hydrogeologic Review_20230711DWR
Division of Water Resources
State of North Carolina
Department of Environmental Quality
Division of Water Resources
Non -Discharge Branch
Hvdrogeological Investigation Report Review Form
WQ0004972
MHC TT, L.P.
Forest Lake Preserve WWTP
192 Thousand Trails Drive
Advance, NC 27006
Primary Permit Reviewer: Zachary Mega
Date submitted for Review: April 28, 2023
Hydrogeological Investigation Report Reviewer: Leah Parente
Date of review completion: July 11, 2023.
1. Are the following elements included in the hydrogeological investigation report?
a. Report signed and sealed by a PE, LG, or LSS yes
b. Cursory examination of nearby properties and wells within 500 feet no
c. Field observations no*
d. Maps (topographic, overview, and site detail) no*
e. Published reports no
f. Sufficient number of bore holes with drilling/sampling logs in the area
of system operation no*
g. Sufficient number of wells with drilling/sampling logs in the area of
system operation no*
h. Shallow aquifer tests with detailed supporting data and appropriate
analyses yes
i. Groundwater modeling results with supporting test data and reasonable
assumptions yes
WQ0004972
Forest Lake Preserve WWTP
Page 2 of 6
*List any missing elements, including others not mentioned above:
• In the "Approach" section of the Hydro Report (page 5), it is stated that 3
borings were installed in areas identified for new spray fields. However, there
is no mention of these borings later in the report.
• Well construction data for the existing monitoring wells was insufficient to
assess the site. On page 10 of the report, it was stated that data collected from
MW-1 was not used to determine the Geomean value for hydraulic
conductivity because it was not known if the well was at least partially
screened into the fractured bedrock. The media in which these wells are
located should be known and verified to determine if the calculated hydraulic
conductivity value is appropriate for this site.
• The topographic map of the site that was provided is an overview of the site
and the surrounding area. Because of the scale of the map, it is not possible to
determine the topography of the site.
• No physical description of the subject site was provided. Review of records for
the site shows that there is a large lake on the property with a stream that
appears to flow from Forest Lake into the Yadkin River. This stream appears
to go through or near one of the proposed spray fields.
2. Was a field visit conducted? A site visit was conducted by the Regional Office Staff
a. Was the physical information contained in the hydrogeological investigation report
verified by the field observations? See the "Site Visit Summary" conducted by
Caitlin Caudle.
3. Aquifer parameters were determined by falling head slug tests.
4. Was a mounding analysis submitted? Yes
5. Was the methodology used for analysis adequate? Yes
6. Approximate depth to groundwater mound at steady state: Not provided
7. Site conditions to be maintained as assumed in the analysis (e.g. drainage features): This site
will be undergoing construction of new spray irrigation fields and therefore will be
changing.
WQ0004972
Forest Lake Preserve WWTP
Page 3 of 6
8. Was a contaminate transport analysis submitted? No
Please provide commentary on the necessity of a contaminate transport analysis:
A contaminant transport analysis was not provided as a part of the report. Due to
the historic use of the site as a spray irrigation fields, a contaminant transport
analysis should be provided.
9. Effluent quality used in analysis demonstrating protection of 2L standards: Not provided
10. Are there concerns with protection of 2L standards at the Compliance Boundary? N/a
List the areas of concern:
11. Are monitoring wells needed at this facility? Yes
a. The number and locations of the monitoring wells should consider such factors as the
size of the application area, the locations of the Compliance and Review Boundaries,
and the existence of nearby water supply wells. Do the number and locations of the
monitoring wells proposed in the hydrogeological report concur with the
recommendations of the APS? No
MW-4 is currently located in the middle of the proposed new spray field. This well
must be permanently abandoned prior to the operation of the new spray field.
There was no mention in the Hydro Report of plans to abandon this well. Please
comment on the future of this well.
There is no discussion in the hydro report of where the new downgradient
monitoring well will be located that will be replacing MW-4. Additionally, it is
recommended that a second new monitoring well be located on the western edge of
the proposed spray irrigation area. Please provide a map showing the proposed
location of all new monitoring wells.
b. The recommended substances to be monitored are as follows: same as previous
permit issuance.
WQ0004972
Forest Lake Preserve WWTP
Page 4 of 6
12. The hydrogeologic report should meet the following standards described in the Aquifer
Protection Section's Hydrogeologic and Reporting Policy and Groundwater Modeling
Policy of May 31, 2007. Does the hydrogeologic report:
a. Focus on the waste application area?
Yes
b. Include borings advanced to a depth of 20 feet or more? Yes
c. Include enough borings in appropriate locations to create a reasonable
hydrogeologic conceptualization of the waste application area? No
d. Include a sufficient number of slug tests or pumping tests that were
properly performed and analyzed for basic hydrogeologic parameters? No
e. Utilize appropriate calculations or computer software to assess the
potential for mounding beneath the application area and/or
contaminant transport beyond the Compliance Boundary? Yes
f. Use recognized assessment methods that are consistent with standard
scientific practices and interpretations? Yes
g. Have analyses and/or conclusions which include "safety factors" such as
conservative assumptions to compensate for gaps in the field data or
questionable test results? No
13. List in detail any additional information or items that are needed to evaluate the site:
• The topographic map of the site that was provided is an overview of the site and the
surrounding area. Because of the scale of the map, it is not possible to determine the
topography of the site. Please provide a legible topographic map of the subject site.
• There is no discussion of the 100-year floodplain in the report. A review of publicly
available data from the Federal Emergency Management Agency shows that a
portion of the proposed spray irrigation area may be located in the 100-year
floodplain. Please provide comment on this and how it will affect site conditions.
• Page 10 of the report states that boring logs were used to determine subsurface
conditions. These boring logs are not included anywhere in the report. Please
provide a copy of all boring logs from the subject site.
WQ0004972
Forest Lake Preserve WWTP
Page 5 of 6
• Well construction data for the existing monitoring wells was insufficient to assess
the site. On page 10 of the report, it was stated that data collected from MW-1 was
not used to determine the Geomean value for hydraulic conductivity because it was
not known if the well was at least partially screened into the fractured bedrock. The
media in which these wells are located should be known and verified to determine if
the calculated hydraulic conductivity value is appropriate for this site. Please
provide well construction data for all existing monitoring wells.
In the "Approach" section of the Hydro Report (page 5), it is stated that 3 borings
were installed in areas identified for new spray fields. However, there is no mention
of these borings later in the report. Were these borings completed? If so, please
provide a map showing the locations of these borings, construction records, and all
other relevant data regarding these borings.
Page 10 of the report states that the calculated geomean value for hydraulic
conductivity of the surficial unit is 2.63 ft/day. However, on page 12 of the report, it
is stated that a value of 3 ft/day was used for hydraulic conductivity in the model.
Please provide justification for the use of 3 ft/day instead of the calculated value of
2.63 ft/day.
In several places in the report, it is stated that the site is located within the Carolina
Slate Belt. This is not corrected. The subject site is located in the Charlotte and
Milton Belts. The hydraulic conductivity of Unit 2 was assigned an initial value of
0.01 ft/day in the model based on literature values for fractured granite and rocks of
the Carolina Slate Belt. Please comment on how the model will be affected when
correct values for the Charlotte and Milton Belts are used.
• Please provide a justification for the use of a variable drainage coefficient for the
existing spray fields in the mounding analysis on page 20 of the report and explain
why a variable drainage coefficient was not used for the proposed spray fields.
In Table 5 on page 20, the mounding analysis shows that there is no irrigation
capacity for existing spray fields during the month of April. This is not explained
anywhere in the report. Please provide an explanation for the lack of irrigation
capacity during that time of year.
On page 20 of the report, it states that the areas shown in red in Figures 11 through
22 indicate where modeled depth to groundwater is one foot for less. However, the
captions for those figures state that the figures show areas where modeled
groundwater is less than two feet in depth. Please clarify the depth to groundwater
being depicted in the model results for Figures 11 through 22.
WQ0004972
Forest Lake Preserve WWTP
Page 6 of 6
Several errors were noted in the water balance shown on page 33 (Table 6). The
area listed for the new spray fields is not correct. Additionally, the water balance
was calculated assuming there is no water in the storage pond. This is an incorrect
assumption as the storage pond will contain effluent when the new package plant is
installed. The storage pond cannot be completely drained because a minimum liquid
level will always be maintained to protect the irrigation pumps. Please resubmit the
water balance calculations taking these two factors into consideration.
• In the Conclusion on page 33 of the report, it is stated that groundwater flow
patterns are consistent with the conceptual model. Groundwater flow paths are not
discussed in the report. Please provide clarification on this matter.
• A contaminant transport analysis was not provided as a part of the report. Due to
the historic use of the site as a spray irrigation fields, a contaminant transport
analysis is necessary to assess conditions at this site. Please conduct this analysis and
provide a report summarizing the results.
• MW-4 is currently located in the middle of the proposed new spray field. This well
must be permanently abandoned prior to the operation of the new spray field. There
was no mention in the Hydro Report of plans to abandon this well. Please comment
on the future of this well.
• There is no discussion in the hydro report of where the new downgradient
monitoring well will be located that will be replacing MW-4. Additionally, it is
recommended that a second new monitoring well be located on the western edge of
the proposed spray irrigation area. Please provide a map showing the proposed
location of all new monitoring wells.
14. List in detail any special conditions related to groundwater monitoring or hydrogeological
issues that should be included in the permit: not applicable at this time
15. Other areas of concern or importance: mounding is a potential limiting factor in the water
balance
16. Recommendation on permit issuance based on hydrogeological investigation report:
Request additional information based on above comments.