Nitrogen Management Strategies & Tools

Background

A detailed synopsis of the origins and changes to the BBNEP strategy are found on our nitrogen management origins page. Please report errors or send comments about this page to Joe Costa.

In brief, in 1990, the Buzzards Bay NEP, a participant in the U.S. Environmental Protection Agency's National Estuary Program, developed a Total Maximum Daily Loads (TMDLs) strategy to manage anthropogenic nitrogen inputs to coastal embayments in Buzzards Bay. This strategy was contained in the 1991 Buzzards Bay Comprehensive Conservation and Management Plan. The proposed approach was based on relating empirical information about water quality and living resources (like eelgrass) to different measures of nitrogen loading. While the 1991 CCMP recognized that embayment-specific models were desirable for setting nitrogen loading limits, these were viewed as too expensive and difficult to apply to 30 Buzzards bay embayments in a timely way. The BBNEP's methodology provided a quick yardstick to measure watershed loadings and establish watershed limits.

The BBNEP methodology compared watershed nitrogen loading estimates (using the accepted methods of the time) to observations of ecosystem response. By the mid 1990s, as new scientific information became available, and as the NEP began evaluating results of the citizen water quality monitoring program, we began modifying some of our nitrogen loading model assumptions, such as adding nitrogen contributions from natural landscapes. In 1999, we also recommended more stringent loading limits based on our approach (see our 1999 TMDL page).

In 2001, the Department of Environmental Protection established the Massachusetts Estuaries Project with the goal of developing nitrogen TMDLs for 89 embayments in Massachusetts. The MEP methodology supersedes the tiered loading limit strategy proposed by the BBNEP in 1991 for setting loading limits standards for coastal embayments, and fulfils one of the key goals of the Buzzards Bay CCMP: to identify embayment specific nitrogen loading limits based on sound science. The focus of the Buzzards Bay NEP now is to assist municipalities in complying with with recommended nitrogen loading limits.

The MEP approach is based on nitrogen loading model linked to an embayment specific circulation modeling that could predict total nitrogen concentrations based on different loading scenarios (an ecosystem response model).

The nitrogen loading methodologies used by the MEP are quite similar to that of the BBNEP's and Cape Cod Commission's, and these loading models use similar assumptions. However in 2001, the MEP updated these loading approaches based on new studies and information. The MEP's parcel loading spreadsheets are also more comprehensive in that every Assessor's office parcel area is also assigned an area for impervious, lawn, natural landscapes, and so forth. These areas are multiplied times loading coefficients. The Buzzards Bay NEP's GIS and parcel analysis spreadsheets have been updated to use all MEP loading coefficients, except septic system loadings are based on numbers of residential units and assumed annual occupancy (this could be approximated by Mass GIS land use, US Census GIS information, town parcel data, and so forth. MEP uses parcel data and parcel specific water use records.

Nitrogen loading pie chart, residential development, MEP Model, rounding to nearest percent. This septic loading rates used in this pie chart are comparable to the MEP estimates for the Wareham River estuary watershed. The MEP wastewater loading rates are based on local water use. Got to our nitrogen loading assumptions page to see examples of actual MEP residential wastewater loading rates.

2009 Updates to the BBNEP nitrogen loading spreadsheets

Because of the level of effort needed to develop parcel level data nitrogen loading spreadsheets for an entire watershed, many managers and consultants still use the BBNEP nitrogen loading spreadsheets as a good first approximation of nitrogen loading. To ensure that these spreadsheets are consistent with the assumed nitrogen loadings adopted by the MEP, the Buzzards Bay NEP has has revised all the loading coefficients used by the MEP loading model as listed below. The only difference between our loading model spreadsheets with the MEP is the calculation of septic system loading. Because parcel level water use data is not readily available to planners calculating loading, our spreadsheets retain calculations based on septic system number and presumed average annual occupancy.

		Old BBNEP rate		MEP Loading Coefficient		The BBNEP has adopted the MEP loading rates in this table.
Category	MassGIS land Use #	(kg/ha unless spec.)	(lb/ac unless spec.)	(kg/ha unless spec.)	(lb/ac unless spec.)	comments
cranberry bogs	21 or 23	18.6	16.6	22.93	20.46	MEP assumes 31 lb/ac application and 66% leaching
cropland, various (except Pasture)	1	20.0	17.8	10.20	9.10	MEP assumes 30.33 lb/ac application and 30% leaching
Pasture	2	10.0	8.9	5.00	4.46	MEP assumes 4.46 lb/ac application and 100% leaching
Nurseries	inc in 21, or 36	10.1	9.1	5.00	4.46	Nurseries use fertilizer, and this value may be too low. However, nurseries occupy negligible amounts of land area in any watershed, so the MEP value is adopted without objection.
Fertilized Lawns, per 5,000 sq, ft lawn	use parcel data	3.4	3.0	1.21	1.08	The original BBNEP loading rate was 3 lbs per lawn effective leaching rate based on assumptions from the early 1990s. Based on interviews on Cape Cod studies, MEP assumed only 5.40 lbs of N were applied annually to each 5000 sq ft lawn with 20% leaching to groundwater.
Lawn fertilizer per acre, effective based on use	na	29.3	26.1	5.27	4.70	The MEP assumptions result in a loading rate of 9.41 lbs/ac (10.5 Kg/ha), but they assume only 1/2 of all lawns are fertilized, hence the lower rate. The original BBNEP method conservatively assumed all lawns were fertilized with a higher application rate. The new lawn assumptions make lawns a less important nitrogen source than roof runoff and and nearly equal to pasture lands per unit area.
Golf Courses	7	29.3	26.1	26.70	23.83	MEP uses different rates for greens, fairways, and roughs, and this value is the average rate for several courses, but fertilizer rates can vary slightly from course to course.
Spectator Recreation	8	29.3	26.1	29.29	26.14	Playing fields, parks, etc.
Impervious, road run off	18, and sewered 16 and roads	15.8	14.1	15.13	13.5	MEP assumes impervious recharge is 90% of annual precipitation (site specific, this example 44.4) x1.5 ppm N. (see note 1).
Impervious, roof run off	na	7.3	6.5	7.57	6.76	Same assumption as for roads, except nitrogen concentration assumed to be 0.75
Deposition on embayments and FW ponds	20	7.3	6.5	10.9	9.73	The BBNEP ignored atmospheric deposition on freshwater ponds, presuming that little reached coastal waters, thus the original BBNEP value is for embayment deposition. The MEP uses an atmospheric N concentration of 1.08 for both.
Natural Landscapes	3,4	0.17	0.15	0.5	0.45	Originally the BBNEP ignored this contribution, then in the 1990s we adopted the values shown. The BBNEP also assumed no contributions from wetlands. The MEP estimate (for wetlands and forested uplands) assumes 27.25 in. annual recharge and 0.072 ppm N.
1/2 acre lot, 3 person occupancy		23.9	21.3	22.1	19.7	This example is comparable to the MEP estimates for the Wareham River estuary watershed where 15.26 lbs per septic unit was used for the entire watershed, with an estimated 2.48 persons per unit (this value includes, average lawn, roof, and driveway size, but does not add natural landscapes). In other watersheds, the MEP used rates appreciably lower. The BBNEP formerly used 5.86 pounds per person. Go to our nitrogen loading assumptions page to see examples of actual MEP residential wastewater loading rates.
Notes: 1) Annual precipitation decreases by several inches moving west to east across the Buzzards Bay watershed.

Loading and Land Use Spreadsheets

Below we provide the BBNEP's loading spreadsheets here to evaluate both sites and watersheds. The BBNEP's nitrogen loading calculators can also help evaluate and guide land use decisions until better information is available through the MEP in the coming years. Town's can also consider our nitrogen model bylaws to implement MEP recommended loading limits.

Single Parcel Spreadsheet: Calculate loading with this single parcel loading spreadsheet

Calculate N loading to a bay using MassGIS or comparable land use data with this Excel spreadsheet (revised 7/2009).

Excel Spreadsheet "n-subdivision.xls": Calculate nitrogen loading from a subdivision with this spreadsheet
(right click to save to your hard disk)

Interactive Nitrogen Calculators

We have created a webpage with Interactive calculators so that town officials, managers, engineers, and the public can quickly estimate and validate nitrogen loading contributions from various sources. Go to our Interactive Nitrogen Calculators. When the page opens, you may have to tell your web browser to allow scripts to run on that page. These calculators are just spreadsheets where you can enter data, and then even save on your own computer.

Related Spreadsheets

Calculate your own Eutrophication Index with this Excel spreadsheet

Buzzards Bay sub-basin land use statistics and embayment areas (Excel file)

MEP septic loading estimates as compared to other approaches

Because most watershed nitrogen loads are dominated by septic systems, the most important element of any nitrogen loading model is to correctly estimate the number of septic systems in the watershed, and beyond that, estimating the number of individuals using those septic systems. The Buzzards Bay NEP's approach was simply to assume per capita wastewater discharge to groundwater was 2.7 kg (5.94 lb). To calculate annual loadings, today the BBNEP would use GIS town parcel data and either US census statistics and town statistics to get average annual occupancy for the watershed being studied. One disadvantage of using US Census data alone is that it fails to capture high summer seasonal use which occurs in many coastal village areas. Generally it is best to use town information based on summer population to calculate annual average occupancy (= .75 year-round + .25 seasonal populations, for example).

The MEP took a very different approach to the problem by examining annual water use based on municipal water records. The advantage of this process is that it better captured seasonal increased occupancy. The disadvantage of this approach was that it was confounded by the fact that water use during the summer may double at at any particular residence because of lawn and garden irrigation, or by water consumption for filling pools.

Another issue that arises with water use data is that an assumed wastewater concentration must be adopted. Actual septic system loading is calculated by flow times concentration. In reviewing household wastewater studies, the BBNEP recognized that wastewater nitrogen concentrations from residences with similar occupancies can vary dramatically with water use. That is to say, wastewater nitrogen concentrations will be much higher in homes constructed with low flow devices (toilets and showers) or where water conservation is practiced.