Agricultural Runoff

Nonpoint runoff from cultivated fields adds pollution to receiving water in the form of sediments and nutrients, particularly phosphorus. The Natural Resources Conservation Service (NRCS) has developed a process for treatment and management of these runoff waters. A schematic diagram of the system is shown in Figure 6.5; components include an underdrained wet meadow, a marsh, and a pond in series. An optional final component is a vegetated polishing area. The combined concept is referred to as a Nutrient/Sediment Control System (NSCS) by the NRCS. Several of these systems have been used successfully in northern Maine for treatment of runoff from cultivated fields. The NSCS should not be installed as the sole control system. It should only be used in conjunction with best conservation practices applied for erosion control on the agricultural fields of concern.

Equations 6.3 through 6.7 are used to size the components in the NSCS concept. These are based on an assumed modular width of 100 ft (30.5 m) for the general case. Dimensional modifications are possible to fit the system to specific site constraints as long as the surface area of each NSCS component remains about the same. The design procedure is considered valid for agricultural land including row crops, hay, and pasture with average slopes up to 8%.

Typically, the agricultural runoff will be conveyed to the NSCS in an appropriately sized ditch. The first NSCS component is a trapezoidal sedimentation trench that runs the full width of the system. The bottom width of the trench should be 10 ft (3 m) to facilitate cleaning with a front-end loader. The vegetated side slopes should not be greater than 2:1, and the depth should be at least 4 ft (1.2 m). A ramp is constructed at one end of the trench to allow access for cleaning. The top, downstream edge of the trench includes a level-lip spreader constructed of crushed stone to distribute the water uniformly over the full width of the system. This spreader consists of an 8-ft (2-m)-wide zone of stone, extending the full width of the system and very carefully constructed to ensure a level surface. Within that zone is a trench that is 1 ft (0.3 m deep and 4 ft (1.2 m) wide, also filled with the same stone. The stone size may range from 1 to 3 in. (25 to 76 mm). The necessary surface area of this trench can be calculated with Equation 6.3:

Metric units: AST = [78 + 1.074WA + 0.04WA] U.S. units: AST = [843 + 4.54WA + 0.07WA ]

where AST is the surface area of sedimentation trench (ft2; m2), and WA is the area of contributing watershed (ac; ha).

The wet meadow is composed of underdrained, permeable soils planted with cool season grasses (other than Reed Canary grass). This unit must be absolutely level from side to side to promote sheet flow and should slope from 0.5 to 5% in the direction of flow. Underdrain pipe (4 in.; 100 mm) is placed on about 20ft (6-m) centers perpendicular to the flow direction. These drains are backfilled with a gravel pack, which is covered with an appropriate filter fabric. These drains discharge, below the water surface, in the marsh component. The first drain line should be about 3 m (10 ft) downslope from the level lip spreader. At least 3 in. (76 mm) of topsoil should be spread over the entire wet meadow area prior to grass planting. The surface area of this wet meadow can be calculated with Equation 6.4 and the required slope length in the flow direction with Equation 6.5:

Metric units: AWM = [783 + 10.4WA + 0.37WA ] (6.4a)

where AWM is the surface area of wet meadow (ft2; m2), and WA is the area of contributing watershed (ac; ha).

The wetland or marsh component is the same area as the wet meadow and also extends the full width of the system. Equation 6.4 can be used to determine the surface area of this component. The marsh should be level from side to side of the system and range from zero depth at the interface with the wet meadow to 1.5 ft (0.46 m) deep at the interface with the deep pond. Typha is the recommended plant species. The habitat values of the system will be enhanced by planting sago pond weed where the water depth in the marsh will exceed 1.2 ft (0.4 m).

The deep pond (DP) provides a limnetic biological filter for nutrient and fine sediment removal. The area of the pond can be determined with Equation 6.6:

The pond should be stocked with indigenous fish that feed on plankton and other microorganisms. Common or golden shiners are often used. The stocking rate should be 250 to 500 fish per 5000 ft2 (465 m2) of pond area. The fish may be periodically harvested and sold as bait fish. Freshwater mussels are also stocked at a rate of 100 per 3000 ft2 (900 m2). The pond should be between 8 ft (2.4 m) and 12 ft (3.7 m) deep. The principal discharge structure from the pond should be designed to maintained the desired water level and accommodate the expected

TABLE 6.12

Performance of Agricultural Runoff Constructed Wetland

TSS VSS TP

TABLE 6.12

Performance of Agricultural Runoff Constructed Wetland

TSS VSS TP

Inflow

Outflow

In

Out

In

Out

In

Out

Season

(m3)

(m3)

(kg)

(kg)

(kg)

(kg)

(kg)

(kg)

1990

Spring

648

1768

7

8

3

7

0.06

0.13

Summer

292

0

1144

0

113

0

3.06

0

Fall

7296

12,295

3884

144

546

35

4.63

1.26

Total

8236

14,062

5036

152

663

42

7.76

1.38

1991

Spring

1387

7685

54

107

7

26

0.30

0.76

Summer

2023

743

3505

11

393

4

12.4

0.11

Fall

1526

3102

644

34

84

10

3.9

0.70

Total

4936

11,530

4203

152

484

40

16.6

1.57

Note: TSS, total suspended solids; VSS; volatile suspended solids; TP, total phosphorus.

Note: TSS, total suspended solids; VSS; volatile suspended solids; TP, total phosphorus.

Source: Higgens, M.J. et al., in Constructed Wetlands for Water Quality Improvement, Moshiri, G. et al., Eds., Lewis Publishers, Chelsea, MI, 1993, 359-367. With permission.

flow from up to a 5-year storm. A grass-covered emergency spillway is sized and located to accommodate flows in excess of the 5-year storm.

The final optional component is a grassed polishing area that receives the discharge from the deep pond. If practical, another ditch and level lip spreader are desirable to ensure uniform flow in this polishing area. This area can be determined using Equation 6.7:

The performance of a NSCS system in northern Maine, over two operational seasons, is summarized in Table 6.12. This system collected the runoff from a 17.3-ac (7-ha) cultivated watershed growing potatoes (Higgens et al., 1993). This system, over the 2 years, achieved an average sediment removal of 96% and total phosphorus removal of 87%.

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