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N.C. Department of Environment and Natural Resources

NC Department of Environment and Natural Resources
Water Quality - Buy Down Calculations

Water Quality

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Nutrient Offset Buy Down Calculation

 
In nutrient strategy areas, a developer not meeting the loading rate targets on their site has the option to ‘buy down’ a portion of their nutrient load from a DWR approved nutrient offset provider to meet loading rate targets. 
 
The developer’s nutrient loading is calculated using the development plan and a nutrient accounting tool.  The developer must only use the accounting tool that is approved for their watershed/basin.  The following are the tools and the areas for which they are approved:
 
 
Using the appropriate accounting tool, the developer calculates their new development nutrient loading.  For the development to proceed, it must meet the loading rate targets or, at a minimum, the offsite threshold requirements for their watershed [see Table].  Once that obligation is met, they may buy down the remainder of their nutrient loads to meet the loading rate targets.  The following calculation procedures are a guide to help illustrate the buy-down calculation method.
 
Also, a reporting form also needs to be submitted to the local government by the developer to characterize their development project.
 
Sample Buy Down Calculation (No Delivery Factor): Neuse, Tar-Pamlico, and Falls (Reporting form link)
For the Neuse, Tar-Pamlico, or Falls nutrient strategy areas, the amount of offset needed to purchase is the difference between the nutrient load from the development and the loading rate target.  This amount is then multiplied by 30 years.  The following is an example illustrating the calculation process:
  •  A 10 acre residential community in the Neuse has a stormwater plan that meets the offsite thresholds by yielding an average load of 6 nitrogen (N) pounds per acre per year (N lbs/ac/yr).
  • The developer needs an additional 2.4 N lbs/ac/yr to meet the loading rate targets for the Neuse (6 N lbs lbs/ac/yr – 3.6 N lbs/ac/yr).
  • The developer’s total nutrient offset obligation is 720 N lbs (2.4 N lbs/ac/yr x 10 ac x 30 yrs).
Sample Buy Down Calculation (Delivery Factor): Jordan Watershed (Reporting form link)
Buy down amounts in the Jordan Watershed must include a factor to account for the amount of nutrients being delivered to the Lake.  This factor can be found on the Division’s map of Jordan Delivery Factors.  The following is an example of the offset calculation for the Jordan Watershed:
  •  A 10 acre residential community in the Upper New Hope Jordan (UNH) subwatershed has a stormwater plan that yields an average of 6 N lbs/ac/yr and 1 P lb/ac/yr meeting the offsite thresholds of 6 N lb/ac/yr for single family residential development.
  • The developer needs an additional 3.8 N lbs/ac/yr and 0.18 phosphorus (P) lbs/ac/yr to meet the loading rate targets for the UNH subwatershed (6 N lbs/ac/yr – 2.2 N lbs/ac/yr; 1 P lb/ac/yr – 0.82 P lbs/ac/yr).
  • The delivery factor used to account for nutrient loading from the development that are transported to Jordan Lake is 61% for N and 58% P based on the development being located in watershed 060110.
  • The developer’s nutrient offset needed for purchase are 695.4 lbs N  (3.8 N lbs/ac/yr x 10 ac x 30 yrs x 61%) and 31.32 P lbs (0.18 P lbs/ac/yr x 10 ac x 30 yrs x 58%).
 
 
Nutrient Strategy Area
Nutrient Reduction Goal
Offsite Thresholds
(see individual new development rules for a full description of the requirements)
3.6 N lb/ac/yr & No P goal
6 N lbs/ac - Single-family, detached, duplex dev’t (1 acre minimum);
10 N lbs/ac – Commercial, multi-family residential, industrial (1/2 acre minimum)
 
4.0 N lb/ac/yr & 0.4 P lb/ac/yr
Upper New Hope
Lower New Hope
Haw River
2.2 N lb/ac/yr &  0.82 P lb/ac/yr
4.4 N lb/ac/yr &  0.78 P lb/ac/yr
3.8 N lb/ac/yr & 1.43 P lb/ac/yr
2.2 N lb/ac/yr & 0.33 P lb/ac/yr
30% of N & P reduction need onsite for projects less than one acre;
50% of N & P reduction need onsite for projects over one acre

 

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