Evaluation of Wastewater System and Nutrient Trading for Falls Lake Nutrient Strategy Development


  • Mary E. Sadler, PE - Hazen and Sawyer
  • Kenny Waldroup, PE, Dan McLawhorn, Ed Buchan

The Falls Lake reservoir was categorized by the Environmental Management Commission (EMC) as nutrient sensitive waters (NSW) in 1983. 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. In 2008, the 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. DWQ initiated a stakeholder process for the Falls Lake nutrient management strategy in August 2008.

The Falls Lake Rules consist of a staged and adaptive implementation plan with several regulatory compliance dates. Rule 15A NCAC 02B .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.

There is much controversy and disagreement over the fate and effect of nutrients from OWTS to surface waters. OWTS have been designated as a nonpoint source contributor to the lake from existing development. The contribution of OWTS to the Falls Lake Reservoir has been modeled in the DWQ’s Watershed Analysis Risk Management Framework (WARMF) to support the development and implementation of the nutrient management strategy. The Phase 1 and 2 goals are significant, so the reduction of OWTS in the watershed may provide an opportunity to meet the goals.

This paper will summarize the highlights and conclusions of a study commissioned by the City of Raleigh to evaluate the OWTS issue with respect to the Falls Lake Rule. The 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 policies in other states specific to nitrogen and/or phosphorus. The water quality trading strategies that were selected for review included an interesting overall framework for point to nonpoint source trading or had specific guidelines for trading between point sources and septic systems. The City wished to provide a balanced perspective on the OWTS issue coupled with an analysis of nutrient point to nonpoint source trading strategies that could help facilitate the development of a trading policy framework for the Falls Lake watershed.

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

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