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        Wet Stormwater Ponds

The United States Environmental Protection Agency (US EPA) defines Wet Stormwater Ponds as: “Wet ponds (a.k.a. stormwater ponds, wet retention ponds, wet extended detention ponds) are constructed basins that have a permanent pool of water throughout the year (or at least throughout the wet season). Ponds treat incoming stormwater runoff by allowing particles to settle and algae to take up nutrients. The primary removal mechanism is settling as stormwater runoff resides in this pool, and pollutant uptake, particularly of nutrients, also occurs through biological activity in the pond. Traditionally, wet ponds have been widely used as stormwater best management practices [BMP].”

This type of BMP is widely used around the Country.  The effectiveness of a wet pond can often be measured in the wet season when the pond is fully functionally and discharging.  This allows water quality testing to take place as the treated stormwater is discharging as designed.  This information can, in part, be utilized to help develop your future maintenance activities to keep your wet pond optimally functioning.  While maintenance can take place at different times of the year, it is often recommended to perform the needed maintenance activities during the dry season or in drought conditions.  This will limit or eliminate the need to pump down the wet pond.  As an example, in many areas of Florida, the water levels of wet ponds are very low.  This allows easier access to structural elements and for the removal of nuisance vegetation, such as cattails. The removing of cattails and other non-beneficial vegetation during low water conditions may help to prevent the need to chemically control them later.

Below is a table from the US EPA that provides an overview of suggested wet pond maintenance activities and schedules.

Activity

Schedule

  • If wetland components are included, inspect for invasive vegetation.

Semi-annual inspection

  • Inspect for damage.
  • Note signs of hydrocarbon build-up, and deal with appropriately.
  • Monitor for sediment accumulation in the facility and forebay.
  • Examine to ensure that inlet and outlet devices are free of debris and operational.

Annual inspection

  • Repair undercut or eroded areas.

As needed maintenance

  • Clean and remove debris from inlet and outlet structures.
  • Mow side slopes.

Monthly maintenance

  • Manage and harvest wetland plants.

Annual maintenance
(if needed)

  • Remove sediment from the forebay.

5- to 7-year maintenance

  • Monitor sediment accumulations, and remove sediment when the pool volume has become reduced significantly or the pond becomes eutrophic.

20-to 50-year maintenance

Typical maintenance activities for wet ponds (Source: WMI, 1997)

The following Table, also produced by the US EPA, provides an overview of the treatment efficiencies of wet ponds from studies conducted around the country.

 

Wet Pond Removal Efficiencies

Study

TSS

TP

TN

NO3

Metals

Bacteria

Practice Type

City of Austin, TX 1991. Woodhollow, TX

54

46

39

45

69.76

46

wet pond

Driscoll 1983. Westleigh, MD

81

54

37

-

26.82

-

wet pond

Dorman et al., 1989. West Pond, MN

65

25

-

61

44.66

-

wet pond

Driscoll, 1983. Waverly Hills, MI

91

79

62

66

57.95

-

wet pond

Driscoll, 1983. Unqua, NY

60

45

-

-

80

86

wet pond

Cullum, 1985. Timber Creek, FL

64

60

15

80

-

-

wet pond

City of Austin, TX 1996. St. Elmo, TX.

92

80

19

-17

2.58

89-91

wet pond

Horner, Guedry, and Kortenhoff, 1990. SR 204, WA

99

91

-

-

88.90

-

wet pond

Horner, Guedry, and Kortenhoff, 1990. Seattle, WA

86.7

78.4

-

-

65.67

-

wet pond

Kantrowitz and Woodham, 1995. Saint Joe's Creek, FL

45

45

-

36

38.82

-

wet pond

Wu, 1989. Runaway Bay, NC

62

36

-

-

32.52

-

wet pond

Driscoll 1983. Pitt-AA, MI

32

18

-

7

13.62

-

wet pond

Bannerman and Dodds, 1992. Monroe Street, WI

90

65

-

-

65.75

70

wet pond

Horner, Guedry, and Kortenhoff, 1990. Mercer, WA

75

67

-

-

23.51

-

wet pond

Oberts, Wotzka, and Hartsoe 1989. McKnight, MN

85

48

30

24

67

-

wet pond

Yousef, Wanielista, and Harper 1986. Maitland, FL

-

-

-

87

77.96

-

wet pond

Wu, 1989. Lakeside Pond, NC

93

45

-

-

80.87

-

wet pond

Oberts, Wotzka, and Hartsoe, 1989. Lake Ridge, MN

90

61

41

10

73

-

wet pond

Driscoll, 1983. Lake Ellyn, IL

84

34

-

-

71-78

-

wet pond

Dorman et al., 1989. I-4, FL

54

69

-

97

47.74

-

wet pond

Martin, 1988. Highway Site, FL

83

37

30

28

50.77

-

wet pond

Driscoll, 1983. Grace Street, MI

32

12

6

-1

26

-

wet pond

Occoquan Watershed Monitoring Laboratory, 1983. Farm Pond, VA

85

86

34

-

-

-

wet pond

Occoquan Watershed Monitoring Laboratory, 1983. Burke, VA

-33.3

39

32

-

38.84

-

wet pond

Dorman et al., 1989. Buckland, CT

61

45

-

22

-25 to
-51

-

wet pond

Holler, 1989. Boynton Beach Mall, FL

91

76

-

87

-

-

wet pond

Urbonas, Carlson, and Vang 1994. Shop Creek, CO

78

49

-12

-85

51.57

-

wet pond

Oberts and Wotzka, 1988. McCarrons, MN

91

78

85

-

90

-

wet pond

Gain, 1996. FL

54

30

16

24

42.73

-

wet pond

Ontario Ministry of the Environment, 1991. Uplands, Ontario

82

69

-

-

-

97

wet extended detention pond

Borden et al., 1996. Piedmont, NC

19.6

36.5

35.1

65.9

-4 to-97

-6

wet extended detention pond

Holler, 1990. Lake Tohopekaliga District, FL

-

85

-

-

-

-

wet extended detention pond

Ontario Ministry of the Environment 1991. Kennedy-Burnett, Ontario

98

79

54

-

21.39

99

wet extended detention pond

Ontario Ministry of the Environment 1991. East Barrhaven, Ontario

52

47

-

-

-

56

wet extended detention pond

Borden et al., 1996. Davis, NC

60.4

46.2

16

18.2

15.51

48

wet extended detention pond

References available at US EPA Reference Link

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