Assessing Pollutant Loads and Stormwater Treatment System Design

The Importance of Assessing
Pollutant Loads from Land
Development Projects and the
Design of Effective Stormwater
Treatment Systems

2010 Watershed Management
ASCE/EWRI

8/24/2010 Trinkaus Engineering, LLC

Steven Trinkaus
Licensed Professional Engineer (CT)
Certified Professional in Erosion and
Sediment Control (CPESC)
Certified Professional in Storm Water
Quality (CPSWQ)
Over 28 years in the Land Development Field
Expertise in the field of Stormwater, Water
Quality Issues & Low Impact Development


Current Local Stormwater Focus
Peak rate control for large storm events
No consideration of small frequent
storms
No evaluation of pollutants found in
stormwater
No evaluation of the effectiveness of
stormwater systems to remove
pollutants


Storm Water Runoff


Storm Water Quality Issues


Water Quality Issues


Water Quality Impacts

Lawns to edge of water –
Increased nutrient loads

Connected Impervious
Areas

Results of Non-Point Source Pollutants


How we design today
Stone berm across level No provisions to ensure long
bottom of basin does NOT flow paths in basin
create a proper forebay
This cell may trap some
coarse sediment but will
NOT prevent resuspension of
sediments
Berms of modified riprap do
not function well at trapping
of fine sediments (void
spaces are too large)
Flows may “short circuit”
parts of basin due to location
of inlet & outlet structures Forebay will not
trap sediments
over long term


What we see today

Outlet set too close
to inlet – flows will
short circuit –
inadequate
treatment for water
quality

Riprap Berm
Does Not
create forebay

Inadequate Forebay
– No Depressed
Sediment Storage -
Lack of
Maintenance


Why should we Assess Pollutant
Loads?
Are permit performance standards being
met?

Will there be short or long term adverse
impacts to receiving waterways?

Are the treatment systems designed
correctly?


How do you Assess Pollutant Loads?

SLAMM
Source Loading and Management Model
P8
Program for Predicting Pollutant Particle
Passage through Pits, Puddles and Ponds
The Simple Method
Estimates pollutant loads for urban areas

The Simple Method

Equation developed by Tom Schueler in
1987 to estimate pollutant loads on an
annual basis
Requires easily obtainable information
to use:
Annual Precipitation
Pollutant Concentrations
Percent Impervious Cover per land use type
Watershed Area


The Simple Method

L = 0.226(P)(Pj)(Rv)(C)(A) where:
L = Pollutant Load in Pounds
P = Rainfall Depth (inches)
Pj = Factor that corrects P for storms that produce no
runoff, use Pj = 0.9
Rv = Runoff Coefficient, fraction of rainfall that turns to
runoff, Rv = 0.05 + 0.009(I)
I = Site Impervious Coverage (percent)
C = Flow weighted mean concentration of pollutant (mg/l)
A = Area of Site (acres)
0.226 = Unit Conversion Factor


Best Source of Pollutant
Concentrations
National Stormwater Quality Database
(NSQD), Version 1.1 – (Maestre & Pitt,
2005)
Land Use Category TSS TP TN
2-8 units/ac 60 0.38 2.1
8+ units/ac 60 0.38 2.1
Commercial 58 0.25 2.6
Industrial 80 0.23 2.1
Transportation 99 0.25 2.3
Forest Deciduous 90 0.1 1.5


Pollutant Concentrations for Metals &
Total Petroleum Hydrocarbons (TPH)
Land Use Category Zn Cu TPH
Medium Density Res. 0.176 0.047 0.344
High Density Res. 0.218 0.033 0.344
Commercial 0.156 0.037 0.324
Transportation 0.156 0.037 0.375
Sources: Metals (NURP 1983, Horner 1994,
Cave 1994); TPH’s – UNHSC & NY Stormwater
Manual 2003


How do the Systems Work?

LID Systems:
Filtration thru soil columns,
Uptake by vegetated biota,
Settlement due to slow flow velocities, and
Infiltration into underlying soils,
Biological and chemical reactions within the
soil media and plants assimilation.


Pollutant Removal Efficiencies

I. Event Mean Concentration (EMC)
II. Mass Efficiency
EMC gives equal weight to large & small
storms and averages incoming & outgoing
concentrations for all storms
Mass efficiency is affected by the
volume of water in the system & losses
that occur within the practice, such as
evapotranspiration & infiltration


Pollutant Removal Efficiencies

2. Mass Efficiency
Mass efficiency is affected by the volume of
water in the system & losses that occur within the
practice, such as evapotranspiration & infiltration

Method is based upon a summation of incoming &
outgoing loads and is considered the more accurate
method

Not easily applied for a proposed project due to
lack of monitoring data


Caveats of “Removal Efficiencies”
Removal efficiency is closely related to
influent quality. I.E. the “dirtier” the influent
water is higher the pollutant removal rate will
likely be as a percent.

Removal efficiency rates may encourage
designs which do not address ‘source’ control.

A system which has a ‘high’ removal efficiency
rate may still be discharging high pollutant
concentrations in the effluent.



Many Best Management Practices have not
been monitored long enough to establish valid
data to determine a supportable removal rate.

Removal efficiency does not always account
for how much water is treated. If a system
is bypassed due to clogging, a stated removal
rate is not likely to be valid.



When using ‘median’ removal rates, it is
imperative to design the treatment system
fully in accord with the specifications as
provided for in the State’s Water Quality or
Storm Water Manual.

I.E. A forebay for a Wet Extended Detention
Pond needs to be 4’ deep and contain 10% of
the WQV within the forebay. A 1’ deep
forebay does not work.


IRREDUCIBLE CONCENTRATIONS
“If pollutant concentrations in the influent approach
the “Irreducible” concentrations noted below, then it
is not possible to change the effluent concentrations
very much (Schueler)”

Irreducible Pollutant Concentrations (CWP)
Water Quality Parameter Irreducible Concentration
Total Suspended Solids 20 to 40 mg/l
Total Phosphorous 0.15 to 0.2 mg/l
Total Nitrogen 1.9 mg/l
Nitrate-Nitrogen 0.7 mg/l
TKN 1.2 mg/l


Pollutant Removal Efficiency Rates –
Conventional Systems


Pollutant Removal Efficiency Rates LID
Systems


Pollutant Removal

LID systems are most effective when
used as part of a “Treatment Train”.
This is a system when more than one
system is used in series to treat runoff.

As you can observe on the prior tables,
by using multiple treatment systems,
significant pollutant removal rates can
easily be achieved.


Pollutant Removal

After const., the goal is met by the
utilization of LID systems to trap
sediment as many other pollutants are
attached to soil particles.

Proper design and construction is very
important, especially for infiltration
systems


Bioretention
-Designed to provide
groundwater recharge &
water quality
- Infiltrate runoff into
underlying soils
- Set ponding depth per
soil type to fully drain
within 24 hrs.
- Max. drainage area = 5
acres
- Gravel layer with raised
underdrain can provide
addl. storage in system


“Rain Garden/Bioretention”
Rain Garden at Sibley Residence - Newtown, CT

Designed & Constructed by
Homeowner


Residential Rain Garden (dry)


Residential Rain Garden (wet)


Infiltration Systems
-3’ separation from bottom of
system to SHGW
- Native soils must have < 20%
& 20-40% silt/clay
- Native soils must have in-
situ infiltration rate of 0.5”/hr
- 25% of WQv to be provided
by pretreatment
- Must be installed “off-line)
- Install on slopes < 15%
- Basin to fully infiltrate WQv
through bottom of basin only


Infiltration Basin
Mulvaney Subdivision – Ridgefield, CT

Very sandy
soils – has
never
discharged
via overflow
pipe


Extended Detention Shallow
Wetlands
- Min. drainage area = 10 ac.
- Maximize flow paths by use of
high & low marsh area, islands
- Required forebay with 10% WQv
- Surface area of system = 1.5% of
drainage area
- 65% of area shall have a depth <
18”
- 35% of area shall have a depth <
6”
- Deep water areas (>4’) shall
contain 25% of WQv
- Min. L:W ratio = 3:1


“Subsurface Horizontal Flow
Gravel Wetlands”
UNHSC

Deep Forebay & Two Treatment Cells


Subsurface Gravel Wetlands

Required Design Elements
Forebay – 10% WQv, 4-6’ depth
Two treatment cells, each holding 45% of
WQv
Minimum length of treatment cell – 15’
(longer is better)
Water quality outlet pipe to maintain
saturated condition just below soil surface



Required Design Elements
Surface layer – 8” wetland soil
Filter layer – 3” pea gravel
Treatment layer – 24” of 1” clean crushed
stone (washed, no fines)
Appropriate wetland plants to be used to
survive inundation depth to provide WQv.
(larger cells, less depth of ponding better)


Pond / Wetland System
Min. drainage area = 25 ac.
Forebay required – 10% WQv
Create long flow paths within system
by using high & low marsh areas
Surface area of system must be a min.
of 1.5% of drainage area
Outlet pool must contain 10% WQv
35% of surface area must be shallow
marsh (<6”)
50% of surface area must be less than
18” in depth


Wet Swales
Max. slope = 4%
Max. drainage area = 5 ac.
Linear applications are best
Pretreatment is required and
must contain 10% WQv


Wet Swale

Swale side slopes shall be 3:1, Bottom width
shall be min. of 8’, with a maximum ponded
depth of 12”

Non-erosive velocities must be provided for 1-
yr, 24 hr storm event

Swale shall handle flow rate from 10-yr, 24-
hr storm event on contributing drainage area


Water Quality Swales
Wet Swale – G&F Dry Bioswales – High
Rentals – Oxford, CT Point – Seattle, WA


Grass Filter Strip
Ledgebrook Lane – Southbury, CT


Filter Strips
Maximum slope = 6%

Stone
trench or
raised
concrete
lip – very
Generally – important
berms are not to achieve
needed or overland
desired as flow
concentration
flow can
develop


Sediment Forebays
Forebay must
hold as a fixed
volume – 10%
WQv
Ideal depth is WQv
4-6’ to promote
settlement and
sediment
storage

Forebay needs to have 3:1, L to W ratio to
promote residence time and settlement


Wet Extended Detention Pond
Most important features:
- Forebay
- 6-8’ permanent pool
- Aquatic shelf around pond
- Appropriate plants for
hydrologic conditions

Pond system must be
designed in accord with
state manual to be
effective


How not to apply LID
Failed Bioretention: No sizing calculations, out of date
soil mixture, too few plants, soil compaction

8/24/2010 Copyright Trinkaus Engineering, LLC
Trinkaus Engineering, LLC

Failed Bioretention: water can enter from one side
only, 2’ of soil mix on top of compacted structural fill
with no underdrain, overflow grate set flush to
bottom of facility, no sizing calculations


Failed
Bioretention:
Overflow
grate set
flush to
bottom of
facility, no
storage
volume, no
plants


Commercial Site

Goodhouse Flooring – Newtown, CT

1+ acre – Open Meadow in Industrial Park

Soils consist of Hinckley, excessively well drained
sand (Class A)

Slight slope (average 3%)


Existing Conditions
Ex.
Conventional
Slight slope – drainage
south to north

Hinckley Soils


Commercial Site

Revised Building Program (LID)

Building: 8,000+ with parking/loading for commercial
flooring company

Stormwater: Grade paved surface direct all runoff to one of
eight bioretention facilities

Stormwater storage: Bioretention will fully infiltrate all
storm events up to 100-yr (7.2”/24hr)

Stormwater treatment: 85%+ removal of TSS, TPH &
metals, roughly 50% removal of TP

Proposed Conditions
8 Bioretention systems will treat & infiltrate WQv for entire site Approx. cost
saving vs.
conventional
drainage &
galleries = $ 95,000

Site is graded to
direct runoff to one of
the bioretention
systems

NO STRUCTURAL
DRAINAGE


THE END


Assessing Pollutant Loads and Stormwater Treatment System Design

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