CREP Site 3

Site 3 is located near Enfield, NC in Halifax County. This area is part of the upper coastal plain of North Carolina and is located in the Tar-Pamlico River watershed. The buffer is located on an unnamed stream that feeds into Fishing Creek, which is a tributary of the Tar-Pamlico River. The North Carolina Division of Water Quality (DWQ) identified the Fishing Creek subbasin of the Tar-Pamlico as a targeted watershed for best management practices and restoration efforts. The buffer was established in 1999 and has been monitored since 2005.

The portion of the buffer being monitored at the site is about 850 m (2800ft) long and has an average width of 46m (150ft). At the pasture edge switchgrass (Panicum virgatum) was planted, followed by a zone of loblolly pine (Pinus taeda), and finally a zone of hardwoods directly adjacent to the stream. The stream is 0.3 -0.6m (1–2ft) in width and is incised between 0.6-1.5m (2-5ft) throughout the buffer.

The upland area at the site is currently being utilized as pasture for cattle. Poultry litter is applied annually as a nitrogen amendment and is believed to be the primary source of nitrate (NO₃-) contamination in the shallow groundwater that moves through the Site 3 buffer.

Site Instrumentation

• 24 Water Quality Wells (3.0-3.4m deep or 9-12ft)
• 27 Water Quality Wells (1.5-2.1m deep or 5-7ft)
• 7 Water Quality Wells (4.2-9.8m deep or 14-32ft)
• 6 Water Table Elevation Wells (records elevations hourly)
• 9 Soil Reduction-Oxidation Potential Probe Nests
• 3 In-Stream Automated Samplers
• 1 Tipping Bucket Rain Gage
• 1 Manual Rain Gage

Site Monitoring

• Groundwater Sampling (monthly)
• NO₃^--N, NH₄-N, DOC, Cl^-, Na²⁺ and Ca²⁺
• Surface Water Sampling (biweekly)
• NO₃-N, PO₄-P, NH₄-N, Cl^-, TP, TKN, TSS and turbidity
• Redox Potential (monthly)
• Rainfall Timing and Intensity (constantly)
• Water Tables (hourly)
• Water Tables manually (monthly - measured in all water quality wells)

Figure 1: Monitoring blocks and equipment layout for Site 3

Currently NO₃-N concentrations have been shown to decrease in the surficial aquifer across the Site 3 buffer.

Figure 2: Mean well NO3-N concentrations for all monitoring blocks

Overall NO₃-N concentrations were reduced by 84% for shallow wells and 81% for deep wells in the surficial aquifer. These reductions can be attributed to either denitrification or dilution of surficial aquifers by deep, nitrate free groundwater.

• Hydrology within the buffer drives denitrification. Wetter conditions promote reduced soil conditions needed for the process to occur. The hydrology of each monitoring block was variable. The water table in Block 1 reached within 60 cm of the ground surface for about 35% of the monitoring period, Block 2 only reached within 60 cm for 12% of the monitoring period, and Block 3 never climbed above 60 cm. Block 1 is the most downstream monitoring block and also receives higher NO3-N concentrations inputs than the other monitoring blocks; however, the stream edge NO3-N concentrations were comparable to the other blocks. The frequency the water table was near the surface in this block seems to maximize denitrification.
• Redox potentials ranged from -400 mV to 600 mV. Redox probes averaged 104 mV, 97 mV, and 168 mV at the field edge, mid-buffer, and stream edge respectively. These values fall within the threshold for denitrification to occur (<250 mV).
• The dissolved organic carbon (DOC), a critical component of denitrification, ranged from about 2 mg/L to 11 mg/L. All monitoring blocks in the buffer displayed similar DOC concentrations with a mean DOC concentration of about 5 mg/L, which should support the microbial carbon requirements for denitrification.

Future Work

• Detailed groundwater topographic maps will be used to help determine preferential flow paths through the buffer and possible areas where dilution could occur.
• The heavy isotopes N¹⁵ and O¹⁸ will also be monitored to additionally determine if denitrification is occurring. Microbes responsible for denitrification prefer lighter NO₃ molecules that do not have the heavy isotopes. If groundwater samples become enriched with N¹⁵ from field edge to the stream edge, this may indicate microbial denitrification.
• The residence time of groundwater in the buffer will be estimated using hydraulic gradients and Darcy’s law. Longer residence times allow for more denitrification to occur, which should lower NO₃-N concentrations at the stream edge.
• Nitrogen loads entering and exiting the buffer will be estimated using NO₃-N concentration data and hydraulic gradients. This will indicate denitrification is occurring if the load exiting the buffer is much less than the load entering the buffer.
  

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