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Constructed Wetlands for Non-point Source Runoff. Reference: ENVIRONMENTAL QUALITY TECHNICAL NOTE NO. N4 NUTRIENT AND SEDIMENT CONTROL SYSTEM. Carl A. DuPoldt, Jr., Water Quality Specialist, Chester, PA Robert W. Franzen, Wildlife Biologist, Chester, PA

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Constructed Wetlands


Non-point Source Runoff





Carl A. DuPoldt, Jr., Water Quality Specialist, Chester, PA

Robert W. Franzen, Wildlife Biologist, Chester, PA

Charles R. Terrell, National Water Quality Specialist,Washington, DC

Robert J. Wengrzynek, Jr., Biologist, Orono, ME


Constructed Wetlands (CW) for Non-point Source Runoff

-- follow Practice StandardConstructed Wetland (656) for non-point source

- applies to non-point source runoff from

agriculture (cropland, pastureland)

urban, commercial runoff

- does not apply to animal waste treatment from CAFOs and AFOs which are more appropriately handled as a point source (follow 656 Constructed Wetland for waste treatment)


Constructed Wetlands for Non-point Source Runoff

Function -

a biological filter and physiochemical treatment system.

Purpose -

The goal of a constructed wetland is to maximize reduction of total and soluble phosphorus, and reduction of nitrogen,

organic matter, bacteria and fine sediments reaching lakes and streams.



Avoid placement in valuable natural wetlands.

The use of natural wetlands needs to be carefully

assessed to prevent damage to valuable wetlands.

The components of the CW are installed as needed to intercept surface and ground water flow.

Runoff from the contributing area will be passed through the CW. No bypass is needed or desirable because the largest storm events carry the most sediment and other pollutants.

The CW can help reduce shock loadings of

pesticides to the receiving water.


CW Components

- sediment basin- vegetative barrier

- a vegetated shallow marsh

- a deep pond, and

-a vegetated \'polishing\' area.

The particular hydrologic sequence should be based on

practical experience, typical site topography, soils, hydrology, economics, and the resulting effectiveness in reducing pollutants.

Not all components may be required, and design of individual components will vary .

There is no cookie cutter CW design!!


CW Components

Sediment basin:

The sediment basin is designed to collect large sediment particles and organic matter from runoff water prior to routing it through the treatment system. It provides pretreatment which protects the functions of the other components. It also serves to regulate flow, which minimizes excessive flushing of the ponds.

The sediment basin consists of a trapezoidal trench located across the slope. There is no pipe outlet. The bottom width should be a minimum of ten feet to facilitate cleaning with a front-end loader. Side slopes should not be steeper than two (horizontal) to one (vertical).

Sediment Basin


CW Components

Vegetative barrier :

The vegetative barrier serves to protect the CW\'s aquatic plants and animals in the event of excessive pesticides, sediments and nutrients

m surface runoff.

The grassed buffer area needs to be level from side-to-side (cross-slope) to provide a consistent sheet flow over the grassed buffer. The minimum and maximum slope from the influent end to effluent end needs to be five-hundredths feet per foot or five percent.

Vegetative Barrier

Sediment Basin


CW Components

Vegetated Shallow Marsh (includes the vegetated shallow marsh, the training dike and the stable outlet).

- The vegetated shallow marsh is planted to emergent wetland plants and receives sheet flow and subsurface flow from the grassed buffer.

- The vegetated shallow marsh maintains conditions suitable for a dense growth of emergent aquatic vegetation.

- The depth of the vegetated shallow marsh should range from saturated soil at the grassed buffer to one and one-half feet near the interface with the deep pond.

Vegetative Barrier

Vegetated Shallow Marsh


CW Components

Deep Pond (includes a number of biological and physical elements)

- The deep pond is created by impounding or by impounding and

excavating to obtain the required depth and size.

- The pond should have a maximum depth ranging from seven to twelve feet.- Establishment of plants and animals within this component is required.- Many pollutants are transformed into biomass through the food

chain and are eventually converted into organic materials or removed from the system.

Deep Pond

Vegetated Shallow Marsh


CW Components

Deep Pond -Minnows

-Golden shiners (Notemigonus crysoleucas), fathead minnows (Pimephales promelas) and lake chubsuckers (Erimyzon sucetta), are recommended species that can survive and reproduce in these ponds.- Cyprinids (minnows) are typically chosen because their food requirements result in removal of nutrients during all seasons of the year

- the fish will also feed on insect larvae helping keep the mosquito population to a minimum.

Deep Pond

Vegetated Shallow Marsh


CW Components

Deep Pond - Fish

-Bottom feeders, such as carp (Cyprinus carpio) and catfish or bullheads (Ictaluridae family) are not suitable.- These fish cause excessive water turbidity, due to their feeding behavior.- This turbidity affects biological activity within the pond, reduces pollutant removal efficiency and can be harmful to mussel or clam populations.

Deep Pond

Vegetated Shallow Marsh


CW Components

Deep Pond - Fresh Water Mussels or Clams.

- Fresh water mussels or clams are common in most lakes and rivers in North America.- They live on the bottoms of rivers and ponds.

- Mussels and clams are natural "filter" feeders that are able to tolerate considerable amounts of nutrient pollution, turbidity, temperature variations and low dissolved oxygen.

Deep Pond

Vegetated Shallow Marsh


CW Components

Deep Pond - Fresh Water Mussels or Clams.

-Each mussel or clam "filters" one and one-half to two tablespoons of bottom water per minute which results in 10 to 12 gallons of water per day.- Mussels and clams only stop feeding when disturbed or during periods of high sediment concentration.- Therefore, 500 adult mussels possess the ability to "process" about 5,000 gallons of water per day.

Deep Pond

Vegetated Shallow Pond


CW Components

Deep Pond - Fresh Water Mussels or Clams.- Stocking should be at a rate of approximately 500 adult (two and one half inches or larger) mussels or clams per 10,000 square feet of pond surface area.- Mussels should be placed into the permanently flooded areas of the pond.

- Stocking should be accomplished one year after normal water level is established.

- Mussels or clams should be collected by trained personnel or purchased through biological supply sources.

Deep Pond

Vegetated Shallow Pond


CW Components

Deep Pond -Plants in the Deep Pond.

- The majority of the area of the deep pond will be free of aquatic vegetation.- Aquatic plants that form submerged, net-like growths are planted in the deep pond.

- Sago pondweed (Potamogeton pectinatus) has been used successfully.

- These plants should be planted one growing season after the completion of construction. Sago pondweed and other plants will be limited by the depth and transparency of the water.

Deep Pond

Vegetated Shallow Marsh


CW Components

Deep Pond -Plants in the Deep Pond.

- The perimeter of the pond will develop a fringe of cattails and other emergent plant growth.- However, if the side slopes of the pond are two (horizontal) to one (vertical), the width of this fringe will be narrow.

- ‘Halifax’ maidencane would make a good shoreline planting.

Deep Pond

Vegetated Shallow Marsh


CW Components

Vegetative "Polishing" Area (optional)

(includes a distribution spreader, a vegetated "polishing" filter and a stable outlet)- This area, which is down gradient of the deep pond, consists of a

stable, relatively level, vegetated site.

- Sheet flow will be required to preclude gullying and short- circuiting of flow. - The area may be grassland, wetland, riparian area, or forested area, either natural or constructed between the distribution spreader and a stable outlet at the receiving water body.

Vegetated Polishing Area Outlet

Deep Pond





(AC) (FT2) ( FT2) (FT) (FT2) (FT2) (FT2) (FT2)


<25 1,000 10,000 100 10,000 10,000 3,750 7,500

50 1,250 12,500 125 12,500 16,000 5,000 10,000

75 1,500 15,000 150 15,000 22,000 6,250 12,500

10 2,000 17,500 175 17,500 28,000 7,500 15,000

150 3,000 22,500 225 22,500 40,000 10,000 20,000

> 150 - Increase size of components proportional to contributing area

/1 - Barrier width (and corresponding area) may be reduced in accordance with Vegetative Barrier Practice Standard 601.

‘Alamo’ switchgrass would be a viable planting in both the vegetative barrier and vegetative polishing area.


TABLE 1a. Pesticide Detention added to Table 1.


Cont. Apparent Pesticide Detention/1

Drain. Detention on runoff of

Area Table 1 1.01 in 0.46 in/2

(ac) (yds3) (yds3) (yds3)---------------------------------------------------------------------------------------

<25 2,600 3,400 1,550

50 4,000 6,800 3,100

75 5,400 10,200 4,650

100 6,800 13,600 6,200

150 9,700 20,400 9,300

>150 - increase size of components proportional to contributing area.

/1 Pesticide detention based upon: Moore, M. T. 1999. Fate of Chlorpyrifos, Atrazine, and Metolachlor from Non-point Sources in Wetland Mesocosms. A Dissertation presented for the Doctor of Philosophy Degree, University of Mississippi, Oxford, MS./2 86% of all precipitation in MS occurred in events of less than 2.5” with 0.46” of runoff. Therefore, treatment design based upon lower runoff events appears



General Guidance

1) Use vegetated swales rather than pipes to convey flow between major components and at outlet.

2) Prevent “short-circuiting” of flows, use training dikes where needed.

3) Keep flows as shallow as possible, and in contact with as much vegetation as possible (the larger the vegetated shallow marsh , the better)

4) Temporarily detain as much water as possible (contact time).

5) Retain as much runoff as possible in CW components (processing time).

6) If suitable outlet exists, a trickle pipe can be used to lower deep pond (w/o removing water from shallow vegetated marsh) to increase detention/retention before next runoff event.



Operation and Maintenance

- costs include mowing, trash removal, brush control, fish harvesting, filamentous algae mats and other vegetation removal, and sediment clean out.

- average about $50/year


Example- Red River Exp. Sta., LA, CW

Proposed site of CW in LA. Note drainage way, which CW will follow in shape (meander) , although deeper and wider. Trees retained.


Example- Red River Exp. Sta., LA, CW

400 ac. CDA- row crops, pasture.

Blue- Deep Pond5 ac, 6-8’deep

Green- Veg. Shallow Marsh 3 ac, 12-18” deep

Red - fencing, vegetative barrier enclosed


Example- Lake Beasley, MS, CW

Site was naturally linear, creating “block” design”. Should follow natural contours to provide natural look. Site was designed with flow-over dikes (TRM reinforced), contractor was more comforable with pipes.


Example- Lake Beasley, MS, CW

Lake Beasley, MS, CW shortly after construction.



Paul B. Rodrigue, P.E., CPESC


NRCS Wetland Science Institute

598 McElroy Dr.

PO Box 1157

Oxford MS 38655-2117

662-232-2973 (ph)

662-232-2915 (fax)

[email protected] (e-mail)