Evaluation of Onsite Wastewater System Performance and Nutrient Trading Policy - Falls Lake Strategy


  • M. Sadler - Hazen and Sawyer
  • K. Waldroup, E. Buchan, D. McLawhorn - City of Raleigh, NC

The Falls Lake reservoir was categorized by the Environmental Management Commission (EMC) as nutrient sensitive waters (NSW) in 1983. The NSW classification imposed phosphorus limits in National Pollutant Discharge Elimination System (NPDES) permits for dischargers to the watershed. Furthermore, the 1993 Neuse Basinwide Water Quality Plan identified additional eutrophication in the Neuse River watershed, particularly the Neuse Estuary. A Total Maximum Daily Load (TMDL) was subsequently imposed on dischargers downstream of Falls Lake Dam in 1998 with limits effective in 2002. In July 2005, the North Carolina General Assembly passed Session Law 190 that required the EMC to develop nutrient control criteria to manage drinking water supply sources, including the Falls Lake Reservoir. Additionally, the NC Department of Environment and Natural Resources (NCDENR) Division of Water Quality (DWQ) found sufficient violations for turbidity and chlorophyll-a to place the Lake on the 2008 draft 303(d) list of impaired waters.

The Falls Lake Rules consist of a staged and adaptive implementation plan with several regulatory compliance dates. Rule 15A NCAC 2B .0278 (Stormwater Management for Existing Development) requires that stakeholders reduce or offset nutrient contributions from existing development. Stakeholders must start implementing and tracking Stage 1 load reductions from existing development by January 15, 2014 and Stage 2 load reductions by January 15, 2021. The Rule also includes a framework for offsetting nutrient credits. Rule 15A NCAC 02B .0282 outlines the requirements for entities who wish to buy credit for nutrient load-reducing activities (credit buyers) from other entities who are implementing nutrient load reduction activities (credit sellers). The Rule specifies that parties who achieve excess nutrient load reductions under the Falls Nutrient Strategy may recover costs by selling credits and provides opportunity for third parties to produce reductions and sell credits.

On-site wastewater treatment systems (OWTS) have been designated as a nonpoint source contributor to the lake from existing development. The contribution of OWTS to the Falls Lake Reservoir was modeled in the Watershed Analysis Risk Management Framework (WARMF) to support the development and implementation of the TMDL. The Stage 1 and 2 TMDL goals are significant, so the reduction of OWTS in the watershed may provide an opportunity to meet the TMDL goals. However, there is much controversy and disagreement over the fate and effect of nutrients from OWTS to surface waters.

There were several objective of this study. The literature review included a summary of effluent quality from various OWTS, a discussion on nutrient attenuation in soil, and a discussion of nutrients and groundwater movement to receiving streams. Other objectives included an analysis of various water quality trading and credit policies in other states specific to nitrogen and/or phosphorus. The water quality trading strategies that were selected for review included an overall framework for point to nonpoint source trading or had specific guidelines for trading between point sources and septic systems.

For more information, please contact the author at msadler@hazenandsawyer.com.

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