HomeMy WebLinkAboutSP201900006 Study 2019-05-21Boyd Tavern Market
TM P 94-39
Tier 3 Groundwater Assessment
Groundwater Management Plan
Prepared for:
Shimp Engineering, P.C.
Charlottesville, VA
Draft submitted March 31, 2017
Modified in response to comments May 4, 2018
Nick H. Evans PhD CPG
Virginia Groundwater LLC
PO Box 1424
Charlottesville VA 22902
nick(a-)-virginiagroundwater.com
434-466-1280
Key Findings
Hydrogeologic setting: Located in gently rolling topography in eastern
Albemarle County, the property is underlain by about 45 feet of soils and
saprolite, and fractured mica schist and metagraywacke bedrock. The parcel is
at the crest of a low divide between the Middle Rivanna and Mechunk drainages,
about 1000 feet east of the Mountain Run fault zone.
Groundwater availability: Favorable.
Are hydrogeologic conditions favorable to proposed use? Yes.
Contamination threats on record within 2000 feet of parcel? None.
Additional contaminant threats observed in field reconnaissance? None.
Anticipated impacts of proposed use on existing users of groundwater:
None.
Groundwater management plan:
Implement runoff -neutral development to the extent possible.
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Project Overview
The proposed development is a convenience store to be located on Black Cat
Road (Rt. 616) immediately southwest of Interstate 64 interchange 129, in
eastern Albemarle County (Figure 1).
Figure 1
The parcel includes 12.486 acres located on a drainage divide between the
Middle Rivanna River and Mechunk Creek drainages (Figure 2). The parcel
slopes gently westward, with elevations ranging between about 420 and 460 feet
above sea level. Existing land cover on the parcel is mixed forest in the eastern
portion and open field in the remainder. Surrounding land use is a mix of small
lot residential, farmland and forest. A site plan -showing the proposed
development layout and approximate land disturbance is attached to this report.
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Projected average daily water consumption for the facility, based on historical
records of water usage in a similar facility operated by the applicant, is 800
gallons. Wastewater will be disposed of through an on -site drainfield. There is
no existing well on the parcel.
Figure 2: Watershed boundaries and topography, TMP 94-39
Hydrogeologic Assessment
Bedrock -geology, fracture density and water well productivity
The parcel is located a few hundred feet east of a regional geologic structure
known as the Mountain Run fault zone. This zone of fractured bedrock rock
including limestone juxtaposes mica phyllite to the west against schist and
metagraywacke to the east. Bedrock on the parcel includes schist and
metagraywacke which are likely to be heavily fractured due to proximity to the
fault (Figure 3). No bedrock exposures were observed on the parcel during field
reconnaissance.
E
Figure 3 : Bedrock geology in the vicinity of TMP 94-39
(adapted from the 1993 Virginia Geologic Map)
In the absence of good bedrock exposures with which to directly observe
bedrock fractures, the yields of randomly -sited water wells can be used as a
proxy for fracture density. Table 1 (below) summarizes data from 290 wells in
the County database that were constructed in the same bedrock formation as
underlies this property. The average yield of this group of wells is 7.7 GPM
(gallons per minute), which is low to moderate relative to average yields from
other geologic formations in the county. (Note that dry holes or "zero" GPM wells
are under reported in the database).
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Geologic map unit
yield
total well depth
casing length
count
(gallons per minute)
(feet)
(feet)
CZpm
(mica schist and
average:7.7
average:179
average:50.4
290
metagraywacke)
maximum: 75
maximum: 630
maximum: 322
There are 11 domestic wells in the database and 4 public water supply wells
within 2000 feet of the property (Figure 4). The average yield of the 11 nearby
domestic wells is 21.5 GPM. This is almost 3 times the overall county -wide
average for wells drilled in this geologic formation. The greater yields are likely
due to increased bedrock fracture density in proximity to the Mountain Run fault.
Figure 4: Existing water wells and known yields
in the vicinity of TMP 94-39
scale: 1000 feet
N
Key. -
public water
supply well
(VDH oversight)
private well
in County
database
well yield
6 (gallons per
minute)
drainage
divide
� C-1 zoning on
\\\ TMP 94-39
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Groundwater recharge and flow paths
Groundwater recharge and flow in the Virginia Piedmont begins at the surface
where rainwater soaks into soils, saprolite and weathered rock. The groundwater
percolates downward to enter fractures in the underlying bedrock. At shallow
levels, recharge and flow direction are influenced by both topography and the
thickness and character of soils and saprolite.
The proposed site is positioned on the western flank of a local drainage divide,
with the divide almost coinciding with the eastern margin of the parcel (Figure 5).
Because there is no adjoining land topographically above the parcel, the parcel
does not receive hydraulically -driven shallow recharge from off -site.
Figure 5: Drainage divide and predicted shallow groundwater
flow paths in the vicinity of TMP 94-39
Middle Rivanna -
Mechunk Creek
drainage divide
predicted
groundwater
flow direction
On the basis of surface topography, shallow -level groundwater flow on the parcel
is predicted to be westward, toward discharge into Limestone Creek.
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USDA soils mapping shows the parcel contains Nason and Tatum soils (Figure
6). These are classified as well -drained and moderately permeable, meaning
they serve as a favorable sponge to soak up rainwater and feed it downward to
recharge bedrock fractures.
Figure 6: USDA soils mapping (Mason 6213, 62C; Tatum, 8013)
Casing lengths are a good indicator of the thickness of the soil and saprolite
"sponge". The average length of casing reported for the 11 nearby domestic
wells in the County database is 43 feet. This represents a significant volume for
potential groundwater storage on the parcel.
At deeper levels, below the soil-saprolite "sponge", groundwater in bedrock
fracture networks on the parcel would potentially receive recharge from off -site,
including possible connection with the highly productive fractured rock aquifer
associated with the Mountain Run fault zone. Overall natural flow at deeper
levels is likely southwestward, driven by a regional gradient toward discharge into
the Rivanna River (2.5 miles southwest of the site).
Water budget estimate
It is instructive to review the proposed use of water relative to the amount of
water available to the C-1 zoned portion of TMP 94-39 (3.28 acres; Figure 4)
from natural recharge.
Annual regional precipitation: 44 inches
Conservative estimate for the percentage of precipitation contributing to
groundwater recharge, subtracting runoff and evapotranspiration:
15%
Annual regional groundwater recharge: 6.6 inches
Average regional daily groundwater recharge:.0181 inches = .0015 feet
Daily recharge per acre: .0015 feet X 43560 square feet per acre = 65.6
cubic feet recharge per acre
Gallons recharge per day per acre: 65.6 cubic feet X 7.48 gallons per
cubic foot = 491 gallons per day per acre
Gallons per day recharge on C-1 zoned portion of parcel: 491 gallons per
acre X 3.28 acres = 1610 gallons per day
Gallons per day recharge to the parcel from off -site up -drainage lands: 0
Predicted maximum daily groundwater withdrawal on site: 800 gallons.
These numbers indicate that the proposed net use of groundwater is
substantially less than naturally occurring recharge on the parcel. A significant
portion of the groundwater withdrawn will be returned to the ground on site via
the drainfield.
Potential for proposed use to affect existing users of groundwater
The proposed use of groundwater is unlikely to affect existing nearby domestic
wells because 1) the proposed usage is substantially non -consumptive (most
water pumped from the ground will be returned to the ground through an on -site
drainfield); 2) the usage is very modest relative to available on -site recharge; and
3) the site is separated from nearby existing users of groundwater by a drainage
divide.
Significant quantities of groundwater are being withdrawn at the Keswick Hall
community well field, about 1500 feet northwest of the site. It is very unlikely
there will be interference between these wells and a future well on TMP 94-39
because 1) the proposed usage is very modest and substantially non -
A
consumptive; and 2) dominant recharge feeding the Keswick wells is predicted to
be from the northeast, away from the parcel under study.
Contaminant threats
There are four documented petroleum releases on the current Virginia DEQ
database within 2000 feet of the parcel (Figure 7). Each of these is listed as a
"closed case", meaning remediation no further action is required. None poses a
threat to TMP 94-39.
Figure 7: Contamination threats in the vicinity of TMP 94-39
r.�
j
616
scale: 1000 feet
NI
petroleum release
listed on Virginia DEQ
database as of 03-28-2017
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Reserve wellfield
The C-1 zoned portion of the parcel is sufficiently large that there should be room
to site a replacement well in the event of failure of the primary well.
Groundwater management plan
Implement runoff -neutral development to the extent possible.
Attachment: Site Plan
95-39 SP (2.6.17).PDF
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Submitted by Nicholas H. Evans, CPG # 2801 001041
March 31, 2017
Updated in response to comments May 4, 2018
DEPARTMENT OF PROFESSIONAL AND OCCUPATIONAL REGULATION
COMMONWEALTH OF VIRGINIA
06-31-2017, 9%.6 hisyland Dr. Swtv 400, Rithmand, VA 2323) 9801001041
roll .phone 1604) 3674SOO
BOARE)FoR PROFESSIONAL SOIL SULNmTt; WETLAND PROFESSIONALS & GEOLOGIS'Is
. CERTIFIED PROFESSIONAL GF0LO<iIST
NICHOLAS i I EVANS
4609 BURNLEY STATION HOAC)
BARBOUR "141-iY VA 22923
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