28b_NCS000250_TurbidityReductionOptionSheet

NCDOT TURBIDITY REDUCTION OPTIONS FOR BORROW PITS*
Tier I Methods
January 1, 2009
Method
Description
Positive Aspects
Negative Aspects
Comments
Pit Dewatering Basin
These are detention basins that include a forebay
Basins are easy to build and maintain.
Basins are only 69-72 % effective at removing total
Outlet water should be drawn from the surface.
(Compensatory BMP)
and baffle where pit water is pumped and
They provide further treatment before
suspended solids under normal conditions. If
Basin sized according to pump capacity and
allowed time for suspended material to settle.
pit water is discharged to the
turbidity (>50 NTUs) is encountered then additional
detention time.
The baffle is constructed with coir fiber material.
environment on a continuous basis
BMPs may be needed. These types of basins alone
Flashboard riser can be used to control the
are ineffective at removing fine or colloidal particles.
water level. Outlet comprised of rigid riser/barrel.
Non Compensatory
BMPs
1) Land
Water from pit is pumped to irrigate
Rain and pit water are used by agricultural
Irrigation activity can have no discharge into surface
No chemicals can be added to water being
Application
agricultural crops
crops and there is no non -discharge permit
waters and there is no violations of the Capacity Use
applied.
(Irrigation)
required.
permit. Limits exist as to the distance that irrigation
pipe can be extended.
2) Silt Bag
Pit water is pumped through a water
Easy to install and remove bag. Effective
Silt bag is limited to a certain flow rate and bag does
Addition of PAM may cause floc to seal bag.
permeable fabric bag resting on a bed of washed
at removing large size particles. Only a
not remove fine or colloidal particles unless a PAM
aggregate to increase bag discharge area.
small footprint is required.
treatment is also used.
PAM can also be introduced in the pump
system for enhanced sediment removal
3) Aluminum
A granular coagulant material added by
Inexpensive and relatively easy to apply.
A toxicity (tox.) test is required because of a potential
pH needs to be above 5.5 to avoid toxic level of
Sulfate
spreader to pit water and settle out
Works well on clay particles. PAM can
pH shift. Also, a background test of iron and
aluminum. May need lime for pH adjustment.
(Alum)
suspended material. Maximum rate is
also be used when a re -suspension occurs
aluminum present in the pit should be conducted.
25 Ib./1,000 cu. ft. of water and keep
May take 1-2 days to clear water.
below 250 PPM sulfate.
4) Gypsum
A powdered coagulant material added by
Relatively easy to spread and takes
Requires much larger quantities of material (100 times)
Also can produce pH shifts.
spreader to pit water and settles out suspend-
around two -days to clear the water
that of aluminum sulfate and a toxicity (tox) test. Can
ed material. Maximum rate is 30 lb per 1,000
column before pumping from water
resuspend in large pits on windy days.
cu ft of water to keep below 250 ppm sulfate
surface.
5) Polyacrylamides
A broad range of flocculants in liquid,
Works well under a variety of conditions.
Needs technical oversight for setup and water test
Keep sulfate levels below 250 ppm. Use only
(PAMs)
powder, & solid forms to chemically bind
Does not affect pH and is not toxic to
for best product and equipment match. May not
apionintypes. Should not be applied directly to
sediment particles together and settle out
aquatic organisms at recommended levels.
work on some clay materials.
surface waters of the state. Instead, application
should be through a pit dewatering basin or
other structure. Use PAMs approved by DWQ.

NCDOT TURBIDITY REDUCTION OPTIONS FOR BORROW PITS"
Tier II Methods —
January 1, 2009
Method
Description
Positive Aspects
Negative Aspects
Comments
Non Compensatory
BMPa
6) Well Point
Pit is dewatered by a series of
Water can be directly discharged to
Runoff is a problem and can create turbid waters
If iron levels are high in pit water, discharge
Pumping
shallow wells surrounding the pit at
the environment without tox. testing
in the pit. Must be treated before being discharged to
must pass through a stilling basin.
(Soil Filtration)
approximately 20' intervals
the environment usually with flocculant because
drawdown of pit exceeds filtering capacity of soil.
7) Impoundment
Large detention basin used for storage,
There would be a slow release from this
In some areas, land for impoundment may be hard
Storm events often resuspend settled particles.
(Detention)
evaporation, and sedimentation of pumped
basin after material has settled out and
difficult to find due to the size and location of the basin.
water from the pit
discharged through an outflow pipe
Very fine material will not settle in some cases.
S) Cell Mining
The borrow pit is divided into cells and water
There is no immediate discharge from the
Extra movement of discharged water from one cell to
Limiting factor may be volume of water to be
is pumped out of one cell into another so a
pit
another within the pit. Wastewater from the pit will
moved.
specific cell can be mined dry
have to be discharged some time during the active
life of the pit.
9) Sand - Media
Water from the pit is passed through a floc
Treated water can be discharged directly
The rental rate for this equipment is very costly. May
Proper pump rate and prefiltration must take
Filtration
sock if needed and into multi -chamber sand
to the environment.
want to consider buying equipment and moving
place and monitored closely. Can be used
media filter for treatment
system around to different locations.
after flocculants.
10) Wet Mining
Material from pit is removed wet and placed
There is no discharge from the pit
Material from pit is handled twice, land needed for
No water quality impact (self contained)
on higher ground to drain before being
stockpiling material, and time needed for pile to dry.
moved to job site
NOTES:
* 1) An evaluation of the pit soil's cation exchange capacity should be considered as the contractor develops his bid.
** 2) Tier II Methods will be considered when 401 WO Certification requires protection for rare or unique resources
3) Many of these turbidity reduction techniques can be combined to provide further treatment