Nursery Case Study

In the following paragraphs, we describe an instance of successful constructed wetland usage for removal of nutrients from nursery runoff. There are three main sections:


Implementation and Description

map of constructed wetland

In 1997, Wight Nursery of Monrovia Growers, Cairo, GA installed a constructed wetland to remove nutrient and pesticide residues from their runoff water. This wetland system was designed to process water from a 120 acre production area.

The wetland's watershed was used for growing large container trees and shrubs (primarily 13 ga containers) that were microirrigated three to five times daily for 3 to 5 min each cycle as needed. Plants were fertilized with a combination of controlled-release fertilizers and irrigation injected liquid fertilizers. Production beds drained into a 1500-ft collection channel that flowed into a 1.0-acre retention pond.

nursery watershed and plants

The first ½ inch of a rainfall (typically most of the nutrients are present in the first ½ inch of runoff) event was captured in the retention pond. Excess rainfall was routed over a diversion dam into a storm water retention basin.

The wetland was a surface flow design that was heavily vegetated. There were two stages (delineated based on depth) in the wetland - 30 in and 8 in. Water from the retention pond was pumped into two first-stage wetlands (through three 2.5 in PVC pipes at a rate of 66 -92 gal/min per section) that were each divided into three sections by dikes that extended to within 30 to 40 ft of the end of the wetland (to allow mixing prior to discharge). The two first-stage wetlands gravity fed two second-stage wetlands, which were also divided into three sections. Discharge from second-stage wetlands flowed by gravity through two stilling ponds (for suspended sediment finishing) prior to discharge into a nearby stream. Flow through the wetlands averaged 555000 gal/day but exceeded 819000 gal/day during periods of excess rainfall.

Plant cover in the first-stage wetlands was dominated by giant bulrush (Schoenolectus californicus), maidencane grass (Panicum hemitomon), pickerelweed (Pontederia cordata), common cattail (Typha latifolia), floating pennywort (Hydrocotyle ranunculoides), duckweed (Lemna valdiviana), water meal (Wolffi a brasiliensis), and alligator weed (Alternanthera philoxeroides).

In the second-stage wetlands, dominant plant species included common cattail, pickerelweed, water pennywort (Hydrocotyle umbellata), maidencane grass, floating pennywort, common duckweed, and broadleaf arrowhead (Sagittaria latifolia).

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Monitoring

The wetland was monitored carefully for research purposes.

The typical nursery wetland requires minimal effort with regard to monitoring. Three sites should be sampled at the end of the wetlands where water exits and flows off of the property. Sampling should be conducted at least once per month. If there is a single place where water empties into a stream, this would be an excellent sampling location. Water samples should then be sent to a lab where they can be analyzed for nutrient concentration. Other tests to determine pesticide concentrations may also be conducted.

If a large rain event occurs immediately following fertilizer application, it may be a good idea to conduct an immediate sampling. This will give you a measure of worst case scenarios for nutrients in water runoff.

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Maintenance

There are only a few major rules with regard to maintenance:

  • Remove willow trees from the wetland before they grow to any appreciable size. Willow trees can crack the clay liner of the wetlands and allow the water to seep out into the surrounding soil before it is treated properly.
  • Ensure that flow is high enough to promote proper treatment. The wetland should not become stagnant. Careful control of outflow pipe levels and inflow amounts can be used to insure adequate flow is achieved.
  • Insure that flow is not excessively high. When flow is too fast, water does not have time to be treated properly. If flow is too high, a retention pond may need to be constructed to serve as a reservoir for water as it waits to be treated.
  • Do not allow the wetland to dry out. This will destroy plant life and kill off microbial communities responsible for treatment.

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