Evaluation of A Small In-Stream Constructed Wetland in North Carolina's Coastal Plain

Abstract

The use of in-stream wetlands is a growing practice being used to mitigatethe impacts of non-point source (NPS) pollution. Wetlands promote physical, chemical, and biological processes that attenuate and convert nutrientswhich can lead to improved water quality. Wetland performance is sensitiveto site conditions, making it difficult to precisely quantify their possibleimpact. Factors such as site specific soil, hydrologic, and vegetativecharacteristics influence wetland effectiveness. Typical design criteriainclude surface area/depth, retention time, plant coverage, and other hydrodynamic recommendations. One recommendation made by Scheuler (1992) for stormwater wetlands is that the design surface area be at least 1% of the contributing watershed size. Most urban areas are severely limited with regard to availableland areas for wetland creation, thus, to be practical, smaller wetlandswill be necessary in many locations.

This research involved a two year study to quantify impacts of an in-streamconstructed wetland on water quality. The one hectare (2.4 ac) in-streamwetland was built to intercept drainage waters from approximately 240 hectares(600 ac) of agricultural and urban watershed, which resulted in a wetland:watershedarea ratio of 0.004:1. The wetland was instrumented to monitor hydrologyand water chemistry.

Water level recorders were used to measure stage at the wetland inletsand at the outlet. Weir equations and discharge curves combined with statistical modeling and calibration techniques were used to determine the flows throughthe wetland. A water balance was computed using inflow, outflow, precipitation,and potential evapotranspiration. A watershed scale balance showed that the total volume of flow leaving the wetland was comparable to the estimatedvolume of drainage and runoff from various land uses.

Water quality samples were analyzed for total Kjeldahl nitrogen (TKN),ammonium-nitrogen (NH-N),total phosphorus (TP), and ortho-phosphorus (OP). Measurements of watertemperature and dissolved oxygen levels were also made within the wetland.Background data acquisition began in early 1996. The evaluation period began in August of 1997 and continued through December of 1999.

Over the evaluation period, NO-N concentrations were reducedthrough the wetland by 60%, NH-N concentrations by 30%, andTKN levels by 9.5%. This resulted in a 20% drop in total nitrogen concentration.Phosphorus levels increased 55% between the wetland inlets and outlet.Actual reduction of NH-N and TKN concentrations may be slightlyunderestimated due to unaccounted for inputs. This may also contributeto the increases in phosphorus concentrations observed between the inletsand the outlet. In the first full year, NO-Nlevels were reduced 70% and 33%, respectively. A significant decrease inNO-N concentrations through the wetland was detected duringthe first winter, and in other nitrogen forms in the first full growingseason. NO-N levels were 60% lower at the wetland outlet throughoutthe year. Ammonium nitrogen concentrations dropped 30% through the wetlandduring the growing season and 20% during the dormant season. TKN levelswere reduced (15%) in winter months, but not during the growing season.Phosphorus concentrations were higher at the wetland outlet than at theinlet throughout the year, but showed larger increases during the growingseason.

Monthly nutrient reductions were generally associated with temperaturechanges. Higher temperatures resulted in greater reduction of NO-Nand NH-N concentrations. Larger increases in TKN and phosphorus concentrations were also associated with higher temperatures.