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NC Nutrient Export & Offset Calculator

Post-development total nitrogen (TN) and total phosphorus (TP) export for a North Carolina site in the Tar-Pamlico, Neuse, Falls Lake or Jordan Lake watershed. It uses the land-cover coefficients and Simple Method from NC DEQ's SNAP v4.2.0 workbook. Enter onsite land cover and you get export in lb/ac/yr, the excess over target, and an estimated DMS offset payment if the site is not treated.

in/yr
ac
ac
ac
ac
ac
lb/ac/yr
lb/ac/yr
$/lb
$/lb
—ac
—%
—lb/yr · lb/ac/yr
—lb/yr · lb/ac/yr
—lb/yr
—lb/yr
—lb/yr
—USD

This is the untreated post-development export for the regulated project area. Offsite and existing-to-remain land covers are excluded, as in SNAP's Project Area. Targets are pre-filled from SNAP's Subwatershed Loading Rate Table, but the workbook's formulas never use that table. Your reviewing jurisdiction sets the target; enter its number. DMS rates are the term-credit rates posted for 7/1/2026–9/30/2026. DMS re-evaluates them quarterly, so check the current rate schedule. The payment assumes excess lb/yr × 30 years × rate (15A NCAC 02B .0703). DMS notes that permanent-credit rates are still under development, so confirm the credit basis with DMS before quoting a total.

$$ L = \sum_i \frac{P}{12}\,P_j\,R_{v,i}\,C_i\,A_i \times 2.72, \qquad E = \max\!\left(0,\; L - T\,A\right) $$
$$ \text{Buy} = E\,F^2, \qquad \$ \approx \text{Buy} \times 30 \times \text{rate} $$
P average annual precipitation (in) · Pj = 0.9, the fraction of storms producing runoff · Rv runoff coefficient · C event mean concentration (mg/L) · A area (ac) · 2.72 converts mg/L·ac-ft to lb · T target export rate (lb/ac/yr) · E excess load (lb/yr) · F delivery factor (Jordan only; 1.0 elsewhere).

SNAP v4.2.0 land-cover coefficients

Simple Method inputs by land cover (NC DEQ SNAP v4.2.0)
Land coverRvTN EMC (mg/L)TP EMC (mg/L)
Roof0.951.180.11
Roadway0.951.640.34
Parking / driveway / sidewalk0.951.420.18
Managed pervious / landscaping0.052.481.07
Protected forest0.050.970.03

Lawn has the highest concentration but runs off only 5% of its rainfall, so per acre it exports far less than pavement. On a typical site most of the nitrogen comes from impervious cover. Of the impervious covers, roadway has the highest phosphorus concentration.

What differs between the four rules

Tar-Pamlico and Falls Lake regulate both nitrogen and phosphorus. The Neuse outside Falls Lake regulates nitrogen only, so SNAP computes no TP offset there. In Jordan Lake, each 12-digit subwatershed has a delivery factor, the fraction of the load that reaches the lake, and the offset is scaled by it. As of the 7/1/2026 schedule, DMS has temporarily suspended new nutrient in-lieu-fee applications in Jordan's Upper and Lower New Hope areas, so there is no DMS rate to quote there. Private mitigation banks may still sell credits.

The DEQ Nutrient Offset form applies the delivery factor once to get the offset required (line G), and again on the purchase line (line I). This calculator reproduces the form as written. The two lines are the same wherever F = 1.0.

Worked example

Example — 5-acre commercial site, Lower Tar River

Given: Greenville station, P = 49.56 in/yr. 1.0 ac roof, 0.5 ac roadway, 1.5 ac parking, 2.0 ac lawn. Targets 4.0 lb TN and 0.8 lb TP per ac/yr.
Impervious runoff depth = (49.56/12)(0.9)(0.95) = 3.531 ft; lawn = (49.56/12)(0.9)(0.05) = 0.186 ft
TN = 2.72 × [3.531(1.0·1.18 + 0.5·1.64 + 1.5·1.42) + 0.186(2.0·2.48)] = 11.33 + 7.88 + 20.46 + 2.51 = 42.17 lb/yr → 8.43 lb/ac/yr
TP = 1.06 + 1.63 + 2.59 + 1.08 = 6.36 lb/yr → 1.27 lb/ac/yr
Excess: TN 42.17 − 4.0(5) = 22.17 lb/yr; TP 6.36 − 0.8(5) = 2.36 lb/yr
Untreated DMS estimate at the Q3 2026 Tar-Pamlico rates (13.82 and 198.92 USD/lb): 22.17 × 30 × 13.82 + 2.36 × 30 × 198.92 ≈ 9,194 + 14,110 = 23,303 USD. That is the cost of offsetting the whole excess. SCMs usually bring it down, and the least-cost mix of treatment and buy-down is what the full SNAP workbook is for.

References: NC DEQ. Stormwater Nitrogen and Phosphorus (SNAP) Tool, Version 4.2.0, Project Calcs and Nutrient Offset worksheets. NC DEQ Nutrient Practices and Crediting. NC DEQ Division of Mitigation Services, Current Rate Schedules (nutrient offset term-credit rates effective 7/1/2026–9/30/2026, retrieved 9/25/2026). 15A NCAC 02B .0703, Nutrient Offset Credit Trading. Schueler, T. (1987). Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs (the Simple Method).

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