HomeMy WebLinkAboutCorrespondence - Engineering - 08/14/2025 Miller Groundwater Engineering, LLC
Consulting, Contracting, Numerical Modeling
324 Remington Street,Suite 110 Fort Collins,CO 80524 970.492.5710
August 14, 2025
Jim Birdsall
Fort Collins Interstate Corner, LLC
P.O. Box 7388
Colorado Springs, CO 80933
RE: Groundwater report for the Gateway at Prospect development
Dear Mr. Birdsall:
This letter report, provided for the Gateway at Prospect development ("the Site"), presents
information about groundwater conditions at the Site that is required by the City of Fort Collins
per Section 5.6 (Subsurface Water Investigation) of the Larimer County Urban Area Street
Standards (LCUASS).
(1) Site Location
Much of the Site is currently an irrigated cornfield encompassing roughly 150 acres located
northwest of the intersection of Prospect Road and Interstate 25 (Figure 1). An additional 30
acres (approximate) lies between the Timnath Reservoir Inlet Canal and the Lake Canal. The site
also includes an area between 1-25 and the frontage road. The full site occupies most of the
southeast quarter, and part of the northeast quarter, of Section 16 of Township 7N, Range
68W. The Site is generally bounded on the south by Prospect Road, on the north by the Lake
Canal, and on part of its east side by the 1-25 frontage road, 1-25, and Boxelder Creek. The creek
also flows through the southeastern portion of the site (Figure 1).
(2) Information Referenced or Relied Upon
Our evaluation has used information from the following sources:
• Ayres Associates (2017). Final Report - Groundwater Study and Underdrain
Recommendations for the Gateway Residential Development, Prospect & 1-25, Fort
Collins, Colorado. Report dated August 8, 2017.
• CTL Thompson (2023). Supplementary Groundwater Monitoring, Gateway at Prospect,
Fort Collins, Colorado. CTLT Project Number: FC10412.001-112 L2. Report dated August
1, 2023.
• EEC (2000). Preliminary Geotechnical Exploration Report, Interstate Land Parcel,
Prospect Road and 1-25, Fort Collins, Colorado. EEC project No. 1002156
JIM BIRDSALL
AUGUST 14,2025
• EEC (2015). Subsurface Exploration Report, Proposed Development— Prospect Road and
Interstate 25, Evaluation of Groundwater/Subsurface Conditions and Installation of 4
Piezometers, Fort Collins, Colorado. EEC Project No. 1152086. Report dated Sept. 23,
2015.
• EEC (2021). Subsurface Exploration / Pavement Design Report. Proposed Bridge—
Gateway at Prospect development, Northwest of Prospect Road and 1-25 frontage road.
Fort Collins, Colorado. EEC Project No. 1212028. Report dated May 14, 2021.
• EEC (2025). Supplement Subsurface Exploration /Groundwater Study, Gateway
Development— N/W/C of Interstate 25 and E. Prospect Road, Fort Collins, Colorado. EEC
Project No. 1252033. Prepared for Fort Collins/1-25 Interchange Corner, LLC, P.O. Box
7388, Colorado Springs, Colorado 80919. Report dated May 15, 2025.
• EPS Group (2025). Review Set of site development plans titled, "Public Improvement
Plans for Gateway at Prospect" dated June 25, 2025. EPS Project No. 892-002.
• SCI Engineering, Inc. (2023). Geotechnical report, Quiktrip 4248, Fort Collins Colorado,
Prepared for QuikTrip Corporation, SCI No. 2023-0686.10. Report dated June 2023.
In addition to the site-specific reports listed above, we have also used common public reference
information such as aquifer maps developed by the Colorado Division of Water Resources
(DWR) in their South Platte Decision Support System (SPDSS) project, well permit records
available online through DWR, common published geologic maps, USGS topographic maps, and
other relevant and available public information.
(3) Hydrogeologic Setting and Conditions
The site is underlain by the Boxelder Creek alluvial aquifer. The creek alluvium merges with the
alluvium of the Cache la Poudre (CLP) River immediately southwest of the site. Counting both
the river and creek alluvial deposits, the combined aquifer is roughly 3 miles wide at this
location with the Site approximately in the middle.
The CLP River is approximately 0.6 miles southwest of the site, and the confluence of the
Boxelder Creek channel and the river is about 1.2 miles south of the site.
(3.1) Lithology. In general, the shallow subsurface at the site consists of a layer of lean clay that
contains varying amounts of sand per geotechnical reports, with this clay layer estimated to
have an average thickness around 10 ft and observed to range from 4 ft to 16 ft thick. Below
this surficial clay layer is a sand and gravel layer (the aquifer) which appears to be relatively low
in silt. The sand layer is around 25 to 30 ft thick along the northeast portion of the site and
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thinner (e.g., 10 ft) in the southern portion of the site (Figures 2, 3, and 4). Underlying the
aquifer is a regional siltstone/claystone bedrock (Pierre Shale) which is thick across the region
and with permeability much lower than the alluvium.
Depth to bedrock from existing grade ranges generally from 20 to 30 ft, but with some areas in
the northeast over 35 ft and some areas in the southern portion near 15 ft.
(3.2) Groundwater. The water table depth varies in part with variable surface topography. It
has been observed to range from 7 feet below existing grade along the north part of the
cornfield to 5 feet below grade between the canals, 5 ft in the south-central part of the site,
and to as shallow as 2 ft below grade in the southeast portion of the site. Direct measurements
of depth-to-water are shown as labels on Figure 1.
Water table elevation contours estimated from the depth observations are shown in Figure 5.
The water table is typically above the top of the sand layer. As shown by the contours,
groundwater flow is generally from north to south, which is expected for this aquifer regionally.
The depth-to-water (DTW) measurements shown in Figure 1 and used in Figure 5 represent a
mix of measurements made in different years and at different times of year. Variations over
time have been seen in certain past measurements. For visual assessment of variability, Figure
6 plots measurements for locations where we have multiple measurements over time. This plot
suggests reasonable consistency to date despite a rise on the scale of 1.5 feet which was
recorded in late May 2025.
Automated continuous monitoring is currently ongoing at MW-3, MW-6, P-2, and D-2. P-1 is
periodically measured manually.
Estimated depth-to-water contours for observed conditions and the current site grade are
shown in Figure 7. To construct this map, we subtracted the interpolated water table elevation
surface shown in Figure 5 from the ground surface topography. (Note: This process means the
detail implied in Figure 7's contours comes from subtracting a smooth and approximate water
table map from a detailed ground surface topography map. Therefore, the spatial detail for
depth-to-water should not be interpreted as being as precise as the contour map implies.)
(3.3) Hydraulic Conductivity. One of the past site investigations (listed Section 2) included a set
of slug tests performed using monitoring wells (MWs) that are no longer present. Those tests
were interpreted to show hydraulic conductivity("K") in the range of K = 15 to 35 ft/day.
However, our current opinion is that those test results were likely too low. The test results were
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potentially influenced by the process of drilling through the shallow fine-grained layers, or by
the MW's sand pack, or by limitations of MW development, etc. Based on: (1) DWR's SPDSS
maps, (2) the clean and coarse sand and small gravel we observed when MW-3 and MW-6 were
recently redrilled by EEC, and (3) our professional experience with this aquifer in the wider
aquifer, we have set horizontal K=400 ft/day for the groundwater flow modeling presented
herein. Vertical K is variable in the model (described later).
(4) Potential Sources of Water
LCUASS Section 5.6.2.A.1 requires that information be provided about potential sources of
groundwater. This discussion is provided below.
The primary source of water is the underlying Boxelder Creek alluvial aquifer. It is a relatively
long and narrow aquifer that begins north of the Town of Wellington. The aquifer first picks up
some water north of Wellington through the creek's very large surface watershed, and then
picks up irrigation return flows from farms, reservoirs, and numerous irrigation canals that cross
the aquifer far north of the subject Site. The aquifer's regional footprint generally parallels
Boxelder Creek, its groundwater flows from north to south, it discharges some of its
groundwater water laterally to Boxelder Creek, discharges significant water via numerous
irrigation wells across the region, and then ultimately discharges to the CLP River just
downgradient of the Site. In our opinion, this water in the aquifer, from the regional hydrologic
balance of the overall aquifer, is the primary source of groundwater at the Gateway site.
Additionally, there are likely additional contributions to groundwater locally from the two
canals on site and from the Cooper Slough drainage. The Cooper Slough drainage appears to
currently run down the west side of the site (at least in part) and it is our understanding that in
the future it may be redirected, all or in part, between the two canals on the north side of the
Site.
We have seen an anecdotal report (comments in a 1983 dewatering well permit for the
basement at 3435 E. Locust Street) that the water table in the area may rise to some extent at
times when the Timnath Reservoir Inlet Canal is running.
Even though the cornfield is flood irrigated, it is our current opinion that this has a minor
contribution to the groundwater conditions at the site relative to the canals and relative to the
regional groundwater flow under the site through the regional aquifer. Therefore, if all else is
the same in the future (e.g., not considering subsurface drains or changes in canal conditions),
we reasonably expect that the Site's groundwater conditions would be roughly similar even if
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irrigated farming ends at the property. (Potential exceptions to this statement are a potential
change toward drier conditions in low-ground surface areas due to reduced farm irrigation
runoff to the lower part of the Cooper Slough or to some of the areas along the creek.)
Standard Limitations on Hydrogeologic Predictions: These opinions are based on currently
observed conditions. We have not assessed potential impacts (including potential
improvements) of what we understand to be the City's plan to reroute the Copper Slough
through the area. Our comments herein are also about average or typical conditions and do
not necessarily include potential transient fluctuations such as infrequent but potentially large
flood events on Boxelder Creek that may temporarily raise groundwater levels near the creek.
(5) Water Rights and Permitted Wells
LCUASS Section 5.6.2.A.4 requires information be provided about water rights that may be
relevant for the site. For this information we reviewed DWR's Hydrobase tool, which is an
online database and GIS tool for water rights and water data in Colorado. Hydrobase shows
there are many small domestic wells and a few irrigation wells in the vicinity. Their locations,
taken mostly from the DWR well permit database, are plotted on Figure 8.1
Shallow subsurface drains (akin to French drains) are proposed to be constructed under the
development's future excavated detention ponds. Their purpose is to keep the ponds from
exposing shallow groundwater (discussed later in Section 6). The proposed drain locations are
shown on Figure 8 for reference. In Colorado, a subsurface drain is expected to be permitted as
a dewatering well, and the Colorado DWR requires that the owners of existing permitted wells
located within 600 ft of a proposed new well be given notice of the applicant's plans. DWR
requires the new-permit applicant to obtain a waiver from those owners, if the owners consent
to the construction, or to pursue a technical hearing in front of DWR if the owners decline to
grant the waiver. Figure 8 shows circles with a 600-ft radius placed at key locations on the
proposed drains.
Based on documents available from the DWR well permit database that are associated with the
permit the City of Fort Collins obtained in 2018 for the existing drain that passes under Prospect
Road at the south end of the site (Permit No. 82269-F), the locations of two nearby domestic
wells (Permit Nos. 9580 and 9964) are apparently incorrect in the DWR database. We corrected
1 The DWR well database locations are often approximate such as when based on centers of quarter sections or
quarter-quarter sections rather than individual well coordinates.
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the location, to what is shown in Figure 8, based on that documentation. The City's drain
permit indicates the locations for those two wells were confirmed by the City's consultants
during the permit process and that those two well owners signed waivers agreeing to the
construction of the existing drain.
When the existing drain was permitted, it was sized for, and also reported to DWR to eventually
include, receiving flows from future drains on the Gateway development. It was anticipated
that additional subsurface drains on the Gateway site would be connected to that drain.
However, the City's report in the drain's permit documents states that the additional drains
from the Gateway site would require going through the DWR well permitting process again.
Domestic Well Permit No. 19308, with a database location seen along the road in the center of
the Site's cornfield (Figure 8), has a permit date of 1964. We are not aware of any well
currently at that location.
Domestic Well Permit No. 285804 (Figure 8) appears to be very close to 600 ft southwest of the
west end of the southwestern Gateway drain (the drain for the proposed Gateway Pond E). A
professional survey of that existing well's location, along with confirmation of exactly what
endpoint location is actually needed for the Pond E drain, may be required to determine if that
well would be included in the permit notice and waiver requirements.
No other domestic wells appear to be within 600 ft of the proposed new drains, per DWR
database locations.
Well Mapping Note: We double checked the locations of only the closest wells in the database.
As indicated in Figure 8,there are eight domestic wells in the DWR database plotted simply at
the center of the quarter-quarter section to the southwest of the site (SE quarter of the SW
quarter of Section 16). Those wells are likely at other locations, but the database's quarter-
quarter section spot is beyond the 600-ft distance and, more importantly, we presume their
actual locations are within the home lots along South Summit View Drive, which puts them
even further away.
One irrigation well (19201-R-R) is about 550 ft southeast of the Site's proposed southeast
detention pond drain (Pond H). Permit records show it to be owned by the CSU Research
Foundation. Given that the proposed detention pond drains will be shallow, at the top of the
aquifer, they are reasonably expected to have little to no influence on any area wells.
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Two historical water rights, apparently ditch diversions, show up on DWR's Hydrobase for the
area covered by Figure 8 (not shown). Four more are shown in the region just outside the area
covered by Figure 8. All of these are listed by DWR as either no longer existing (four of them) or
as inactive with no diversion records maintained (two). Since none of these appear to be
relevant, they are not plotted on Figure 8.
(5.1) Opinion: No Reason to Expect Material Impact on Area Wells. The blue contour lines in
Figure 9 show model-estimated water table drawdown expected to be caused by the proposed
detention pond drains. This drawdown map was created using a groundwater flow modeling
tool we constructed for the site (discussed in next section).
The drawdown created by the drains will be naturally limited since the drains will be relatively
shallow. The drains will be shallow since their sole purpose is to keep the bottom of the
stormwater detention ponds free of exposed groundwater. Based on the limited drawdown
expected at the irrigation wells and the domestic wells, and also based on the low production
rates of the domestic wells and the high permeability of this aquifer, we do not anticipate
material impacts from the proposed drains on the area wells.
(6) Groundwater Model Evaluation for Proposed Drains
Our estimation and mapping of groundwater elevations between observation locations was
done first via simple interpolation between the direct observations (presented previously in
Figure 5) and then also, secondarily, by constructing a groundwater flow model of the site. We
also used the model to estimate inflow rates to the proposed drains (Table 1) and the water
table drawdown expected to be created by the drains (Figure 9).
We constructed the model through mapping site structural features and surfaces (existing
grade, proposed grade, water table, top of sand, and top of bedrock) and by including water
level "influencers" (boundary conditions) such as the two canals, nominal net recharge rates,
and the creek. As a final step we compared the model to observations and made moderate
adjustments in setup assumptions until the model reasonably approximated observations on
the south end and had desired higher-than-observed conditions on the north end.
The completed model closely matches observations at the south end of the site (Figure 10) and
is higher than observed across the north portion of the site. The model is higher than observed
along the north portion of the site since, as a conservative step, we assumed the canals could
be completely full at some time in the future (conservative for a steady-state model such as
this) and that the canals seep some water into the ground (likely).
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For brevity, the model construction will be summarized with only the more notable features
and observations highlighted. Additional model details and the model files can be provided
upon request.
Model Code. Groundwater flow simulation was performed using the USGS modeling code
MODFLOW-2005 within the common commercial graphical user interface called Groundwater
Vistas. A MODFLOW model is useful in this case because it helps organize and compare the
various site features and data, such as to account for the spatially variable and potentially
intersecting slopes of the ground surface, clay and sand layers, water table surface, bedrock
surface, and drain paths.
Model Layering and Grid. The model was constructed with two model layers that have spatially
variable top and bottom elevations. The top of Model Layer 1 represents the ground surface
(per USGS topo and topo provided by EPS Group). The bottom of Layer 1 represents the bottom
of the shallow sandy-clay layer, which is also the top of Model Layer 2 and is also the top of the
sand layer. The bottom of Layer 2 is the top of the shale bedrock. In other words, Model Layer
1 corresponds to the sandy-clay layer and Model Layer 2 corresponds to the sand and gravel
aquifer. The modeled layers therefore correspond to the lithologic descriptions provided in
Section 3.1. The model grid has a 10 x 10 ft resolution over the Site and over an area extending
south to the south end of the existing drain. This resolution increases smoothly to 50 ft over a
slightly wider peripheral area.
Note on a Standard Technical Limitation: All lithologic transitions and/or contact surfaces (clay
to sand and sand to bedrock) are approximate, inferred, interpolated, and extrapolated from
various site boring logs (Section 2). It is also a standard simplifying assumption, and used here,
that these layers are assumed to be homogenous and/or can be represented as an "average"
condition, even while actual conditions may be more heterogenous and more variable at the
local scale. This limitation, though standard and common, is potentially consequential here
since it means, for example, it is difficult to predict precisely whether and how much the
bottom of each excavated detention pond will intercept the deeper sand layer (not expected
but possible) or intersect localized shallow sand layers (possible), and whether and to what
extent the installed underdrains below the ponds will intersect sand layers (currently estimated
as likely under Pond B and Pond H, and less likely to uncertain elsewhere). For these reasons,
we recommend that conditions encountered during pond excavation and drain installation be
observed and well documented and reviewed for any significant deviations from the current
assumptions and current information.
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Hydraulic Conductivity. We set horizontal hydraulic conductivity (Kh) of Layer 2 (sand/gravel
aquifer layer) to be 400 ft/day based on SPDSS mapping and our general experience nearby in
this aquifer. We set Layer 2 vertical K (K,) to be 40 ft/day based on a common rule of thumb for
setting K„at 1/10th of Kh in permeable layers. We added a few small zones along the south side
of the site with slightly lower Kh (50 ft/day) to better calibrate water levels in that area.
Based on the distinct contrast in materials observed in the field, it is clear that most
groundwater flow will be through Layer 2, so we expect the general model results (with
exceptions noted later) to be less sensitive to K in Layer 1 (the sandy-clay layer). We set most
of Layer 1 at Kh = Kv= 2 ft/day. Our rationale was based on our regional experience that even a
sandy clay layer will often have effective K in this range at the larger scale, even when local-
scale testing may find samples or areas with lower K.
We also added local zones in Layer 1, across the south side of the site, with Kh = Kv= 0.5 ft/day.
This was created to fine-tune the water level calibration. There were numerous water level
targets in this area.
(6.1) Notable Model Assumptions Regarding Stormwater Detention Pond Subdrains in Model
A few unique considerations for the stormwater pond subdrains are presented here to help
understand how the drains were represented in the groundwater flow model.
Model Detail Limitation: A primary purpose of this groundwater model was to model
groundwater flow behavior at the overall site scale. We did not simulate drains at the scale of
individual drain trenches. We have limited the local-scale scope of our modeling at this time
because, once the drains are installed, the pond bottoms can be modified if the pond bottoms
remain wetter than desired. For example, the pond bottoms could be underlain by coarse sand
layer and/or additional drain stubs could added, if the pond bottoms are not dry enough. For
important context, note that certain types of underdrains would be very difficult to modify
after installation (e.g., deep drains, drains in sewer trenches, or drains under roads) but here
the detention pond subdrains will be accessible and shallow through the ponds so that they can
be adjusted, if needed, based on future observations of their as-built effectiveness.
The subdrains' purpose is to keep the water table below the stormwater detention pond
bottoms, so that shallow groundwater is not exposed via the excavated ponds. More commonly
when draining entire developments there is incentive for subdrains to take in as much water as
possible (for maximum impact), but here the goal is to lower the water table only locally at the
empty ponds since, to our knowledge, no other significant below-grade construction is
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currently planned for this development that would require the drains to create drainage
beyond the pond footprint. And there is incentive to minimize produced water in this case in
order to remain under the capacity limitation of the existing drain that goes under Prospect
Road.
As is typical for subdrains, the subdrains will be constructed with permeable bedding (e.g., sand
or fine gravel) in the trench around them. The pond bottoms themselves may also be modified,
if needed, such as adding a layer of sand under the topsoil of the pond bottom to ensure the
subdrains are in good contact with the water table along the edges of the ponds. As noted
above, there is also a somewhat unique incentive here for the drains to collect no more
groundwater than necessary, and we believe this may be accomplished by limiting the depth of
the subdrain trench bedding so that the drain trench is fully within the upper sandy-clay layer
(see profiles views in Figures 2, 3 and 4) and not in direct contact with the sand and gravel
aquifer unless unavoidable based on local variations in top-of-aquifer depth.
In other words, since the pond bottoms are likely to be solely in the upper sandy-clay layer, it
may help limit how much groundwater is produced by the drains by limiting if and how much
the subdrain bedding penetrates through the sandy-clay layer into the aquifer. In our opinion,
these design adjustments are highly localized in scale and are localized details that are best
adjusted during construction and/or after observing as-built behavior of the drains.
The different scenarios described above—about drain and pond details that are at a scale finer
than the model grid, and finer than feasible for mapping detail—create a need to make
professional judgment decisions about how to most accurately or most reasonably
(conservatively) represent the drains in the sitewide groundwater model. A groundwater
model of this scale necessarily has limited resolution at the scale of individual drain trenches.
Additionally, here, as is typical, despite numerous borings on the south side of the site, we also
necessarily have some practical limits on lithologic detail at the scale of pond drain trenches
until excavation begins.
Based on the factors outlined above, we used the standard MODFLOW "drain package" for the
model cells containing the drain. We set the drain conductance to be high so that groundwater
freely flows into the drains,2 and we modified the K of a few cells in Layer 1 over a fairly wide
lateral distance (e.g., 20 to 40 ft) along the subdrain route to represent the potentially
engineered modifications to the ponds. For the modified zone, we set Kh = 400 ft/day (to be no
Z Conductance is a MODFLOW parameter that governs ease of groundwater inflow into the drain.
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higher than the modeled native aquifer Kh) and set Kv to be 2 ft/day to represent that, in the
vertical direction, the effective Kv is a combination of the modified sand bedding and the native
sandy-clay between the drain and the upper surface of the aquifer. This modified zone will be
wide but likely not deep so as to not contact the sandy aquifer, if that can be avoided.'
These K-zonation changes apply only to Model Layer 1. The current mapping suggests that the
subdrains for Pond B and Pond H may penetrate into Layer 2 (the sandy aquifer) where these
engineered modifications will not apply because a portion of those drains is likely to be in the
sand and gravel layer. The other pond subdrains are modeled to remain in Layer 1 (sandy-clay
layer) which will have the native conditions modified as needed along the drains.
(6.2) Model Simulation Results, including "Cone of Influence"
The modeled water table was presented previously in Figure 10. As noted, the model closely
matches observations in the south part of the site, and by design it is higher than observed
conditions on the north part of the site to represent (approximately) more conservative
conditions such as full and seeping canal channels.
Figure 9 previously presented the drawdown that is estimated by the model to be created by
the detention pond subdrains. These drawdown contours also represent the "cone of
influence" required to be presented per LCUASS Section 5.6.2.A.8.
Table 1 lists the estimated flow rates for the detention pond subdrains. It also notes which
drains will discharge to the creek and which will discharge to the existing drain.This discharge
routing is important since the design capacity of the existing drain is reported to be around 450
gpm. Figure 11 shows the pond outlines and pond names that correspond to the list in Table 1.
For illustration of why the pond drains are proposed, Figure 12 presents depth to water for the
observed conditions and the proposed grade and no drains, i.e., with the excavated ponds in
place but no drains. This shows the pond bottoms might otherwise expose groundwater if no
subsurface drainage were provided. Figure 12 can be compared to Figure 7 (previously
presented), which plots depth to water based on observations and current grade, to see the
changes created by excavation.
s The current mapping suggests that the drains for Pond B and Pond H may necessarily penetrate the top of the
sandy aquifer, but that the other pond drains might remain fully within the sandy-clay layer.This can be confirmed
once the ponds are excavated.
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Figure 13 zooms into the detention pond area and changes the depth-to-water contouring
scheme to better show the small local variations. Figure 13 also uses the modeled water table
(not the observe conditions used in Figure 12), so it presents water levels that may be slightly
different than observed to date. Figure 13 is from a simulation with no drains.
Figure 13 uses the modeled water table to provide an apples-to-apples comparison to Figure 14
which is the same model simulation but with the pond drains represented in the model. Based
on comparing these two figures, the model simulations indicate that the pond subdrains should
be able, with proper installation (and possible pond bottom modifications), to keep the pond
bottoms free of exposed groundwater.The pond bottom soils may still be very moist to wet
(shallow water table below the bottom bottom) but the drains should be able to prevent
exposed water.
Cautions and Caveats: The results presented in Figure 14 are reasonable and are based on
sound site observations and estimation methods. The results provide a reasonable guide to
general expectations. However, these results are at a very fine scale (e.g., water depth
thresholds that pass/fail with only a few inches of variation), therefore these results are not and
cannot be guaranteed. We present these results with you, the site owner, acknowledging that
if the currently proposed design of the subdrains does not keep the pond bottoms adequately
free of water, then further engineered modifications may later be necessary. The later
modifications would be based on observations made during pond excavation and drain
installation and/or and based on observations of as-built effectiveness. Such modifications may
include altering the pond bottom materials (e.g., underlaying the ponds with a sand layer),
adding more perforated pipe under the ponds, or other modifications as necessary to achieve
desired conditions. The plan and ability to respond to as-built field conditions, and to respond to
additional information that is often revealed during excavation, is an important assumption and
basis in this work.
(7) Simulation and Evaluation Limitations
The scope of this evaluation considered drain function with respect to the perforated drain pipe
depths and locations, and it assumes groundwater can freely flow into the perforated drain
pipe and into the trench holding the perforated pipe. This scope has not included reviewing
drain pipe specs (such as pipe flow capacities), detailed specs for bedding materials, or other
trench details. Our simulations assume the drains are sized and installed properly and that
their function and ability to take in and remove groundwater to the drain invert level is well-
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maintained over time. Periodic inspection and maintenance of the drains may be necessary
and is not within the scope of Miller Groundwater.
Standard Technical Limitations in Subsurface Evaluations
Subsurface data is always limited in its spatial coverage and subsurface hydraulic testing
produces only approximate results. Additionally, numerical models are greatly simplified
approximations of a typically complex subsurface. Therefore, estimates and projections about
groundwater and subsurface drain behavior have inherent and unavoidable degrees of
uncertainty. Providing certainty is not an attainable or expected goal in this work. This is
particularly true for potential local-scale(i.e., lot-scale or pond-scale) variations in subsurface
conditions. By using good,common, and accepted methods,we believe this work provides
useful and reasonable guidance for expected site behavior, but actual field results may be
different from model-projected results, expected results, or desired results.
This evaluation is based on data available to Miller Groundwater Engineering at this time. The
estimates and opinions contained herein may be revised if significant additional information
becomes available. Nevertheless,the opinions are well-founded and consistent with observed
conditions at the site.
Please contact me if you would like to discuss this evaluation further.
Sincerely, _
Calvin Miller, PE, PhD � F
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Miller Groundwater Engineering,uC Page 113
Table 1. Model-estimated flow rates to detention pond drains.
Drain ID Flow(gpm) discharge location
Pond B 161 existing City drain
Pond C 118 existing City drain
Pond D 91 creek channel(east side)
Pond E - existing City drain
Pond F 28 existing City drain
Pond G 12 creek channel(east side)
Pond H 61 creek channel(east side)
existing City drain 25 now,9 after existing City drain
Total through City drain: 315
Total to Creek: 165
Note: Rates are based on reasonble but approximate aquifer parameters.Actual
rates will likely vary from estimated and may vary seasonally as well.
Miller Groundwater Engineering,LLC
1
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11• TH-136 7 -4898
6 4902 4902 4901 6 B-la f
TH-5• TH-6• TH-7•
11�� 6 6 6 4902
P-2•
4899 4896 5 p TH-8• TH-99 /
7 ,
4896 '
MW-1• 4897 / 4900
10 TH-12• I=Z.� BA 54 C-1
' 8 4897 - coB ` v 4
r — TH-10• =o 5 B-2bv Y 8 = 0 4 o T_1 7
.°► 489 = C-2 2 i
N\ P-q9,4 'r y �1898 D-24 4
11 . 111 . 11 111 11 111 11
Figure 2. Profile A-A'.
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3,135,500 3,136,000 3,136,500 3,137,000 3,137,500 3,138,000 3,138,500
easting(feet)
A At
4920 Profile A-A' (15:1 vertical scale) Observed water table-light blue line
Modeled water table- dotted blue line
4910
c
4900
m
w 4890 sandy alluvium
06
bedrock
4880 —
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400
Distance(feet)
Project:Gateway Miller Groundwater Engineering,LLC
Figure 3. Profile B-B'.
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easting(feet)
4920 B B'
4915 Profile B-B' (15:1 vertical scale) Observed water table-light blue line
,-. 4910 Modeled water table- dotted blue
-- _ - - - - -
4905 ' -
- - - - - - - - - - - - - - - - - -
- --� -
0 4900 - - - - - - - - - - - - - - - - - - -
4895 clayey
w 4890 sandy alluvium
4885
i bedrock
4880
1
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400
Distance(feet)
Project:Gateway Miller Groundwater Engineering,LLC
Figure 4. Profile C-C'.
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easting(feet)
C C'
4920 Profile C-C' (15:1 vertical scale) Observed water table-light blue line
_ - Modeled water table- dotted blue line
4910-1
c
2 4900 clayey` ' ' -
>
4890 sandy alluvium -
w
4880 bedrock
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800
Distance(feet)
Project:Gateway Miller Groundwater Engineering,LLC
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API
TH-8•
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TH 12• _c BA,5• � C 1 r j
10 , 8 4897 = m 4 i -4^ • r '
TH 10• `o` �4
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11 . 111 . 11 111 11 111 11
Figure 6. Gateway at Prospect depth-to-water data: Spring/Summer 2025 plus misc. older data.
D2(2025)
0 • D2(2025 manual)
X D2(2023)
1
MW-6(2025)
♦ MW-6(2025 manual)
MW-3(2025)
2
■ MW-3(2025 manual)
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3 P-2(2025)
O P-2(2025 man.)
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4/1 5/1 6/1 7/1 8/1 8/31 10/1
Date(Day of Year)
8/14/2025 Miller Groundwater Engineering, LLC
Figure 7. Depth to water from current grade based on recent observations.
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easting(feet)
Project:Gateway at Prospect Miller Groundwater Engineering,LLC
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Figure 11. Stormwater detention pond outlines and subdrains.
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easting(feet)
Note: Location ID in yellow text, observed groundwater elevation in blue (ft NAVD88).
Project:Gateway at Prospect Miller Groundwater Engineering,LLc
Figure 12. Depth to water (observed conditions)from proposed grade and no drains.
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easting(feet)
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Project:Gateway at Prospect Miller Groundwater Engineering,LLC
Figure 13. Depth to water (modeled) from proposed grade and no detention pond subdrains.
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Detention
Rim Outlines
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3,136,000 3,136,200 3,136,400 3,136,600 3,136,800 3,137,000 3,137,200 3,137,400 3,137,600 3,137,800 3,138,000
easting(feet)
Depth to Water(FEET)
<0 0 1 2 3
Project:Gateway at Prospect Miller Groundwater Engineering,LLc
Figure 14. Depth to water (modeled) from proposed grade and with detention pond subdrains.
0
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0
Detention Pond
OutlinesRim
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3,136,000 3,136,200 3,136,400 3,136,600 3,136,800 3,137,000 3,137,200 3,137,400 3,137,600 3,137,800 3,138,000
easting(feet)
Depth to Water(FEET)
<0 0 1 2 3
Project:Gateway at Prospect Miller Groundwater Engineering,LLc