Massachusetts Estuaries Project Information

Related Pages:   Nitrogen Management Origins  |  N-Loading Assumptions

Massachusetts Estuaries Project

The Massachusetts Estuaries Project was established as a collaboration between the Massachusetts Department of Environmental Protection and UMass Dartmouth in December 2001 with the goal of developing nitrogen TMDLs for 89 embayments in Massachusetts. The effort, which cost several millions of dollars, was originally scheduled to be complete by 2010, but delays in completion of studies and budget shortfalls have slowed progress. Most work is now expected to be complete by 2013. Additional information on the MEP effort can be found on the DEP MEP studies page and at EPA Region 1 Approved TMDLs.

The MEP represents fulfillment of one of the goals of the original Buzzards Bay CCMP to identify embayment specific nitrogen loading limits based on sound science. The 1991 CCMP recommended embayment specific modeling and use of parcel level data (and the BBNEP used a parcel level nitrogen loading analysis for the 1991 Buttermilk Bay overlay district), however GIS parcel coverages were not readily available for most towns in the early 1990s, embayment specific modeling was quite challenging, and site specific water quality data to evaluate water quality modeling was completely lacking. Consequently, the BBNEP adopted a simple tiered system of recommended loadings based on the empirical response between nitrogen loading and various ecosystem measures like eelgrass cover. Our approach was used for setting watershed loading targets in several estuaries during the 1990s, including the upgrade of the Wareham wastewater treatment facility.

In 2001, scientists at UMass Dartmouth developed a more sophisticated approach linking embayment specific circulation modeling to land use loading estimates to better predict total nitrogen at a sentinel station in an estuary with changes in watershed nitrogen loading. The MEP runs these water quality models to determine what amount of watershed nitrogen loading would result in a concentration at a sentinel station of around 0.4 ppm total nitrogen, if the water quality goal is to restore or protect eelgrass, and 0.5 ppm, if the goal is only to protect or restore benthic habitat like shellfish beds. The Buzzards Bay NEP had proposed similar water quality standards in this 2000 report.

The actual selection of the exact TN at a sentinel station in an estuary, and the location of that station, are subjective decisions based on best professional judgment, and involves consideration of conditions and loading in nearby estuaries that are used as reference systems. Theses decisions affect the estimated nitrogen total maximum daily load (TMDL) for that estuary, and define how much watershed nitrogen loading must be reduced.

MEP Hydraulic Model

The water quality and loading software used by the MEP were proprietary customizations of the SMS, WMS, RMS software packages produced by BOSS International. These software packages allow the user to import GIS shapefiles, bathymetric data, and site-specific tidal elevation data to model the embayment’s circulation patterns, and to predict concentrations of compounds that act conservatively with mixing (e.g. salinity). It is the linkage between the water quality and circulation models and the watershed nitrogen loading model (including groundwater and surface flow pathways and attenuation estimates) that represent the core predictive modeling approach used for setting TMDLs.

Watershed Loading

Conceptually, the nitrogen loading methodologies used by the MEP in their models are quite similar to that of the BBNEP’s and Cape Cod Commission’s approach during the 1990s, and these loading models use similar assumptions. However, MEP updated the values of some of the most important based on their review of new studies and information. Most notably, the MEP uses a considerably lower estimate of loading from lawn fertilization, slightly lower per capita nitrogen contributions from septic systems, higher loadings for cranberry bogs (until 2011), lower for other agriculture like corn fields, and higher loading for natural landscapes. 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, although because these are based on fixed assumptions or percentages, they are not site specific, and generally equivalent to multiplying the number of residential units in a watershed by presumed lawn, roof, and other area types. The exception to this rule of thumb occurs when a town GIS data set has actual building footprints, in which case these values are used. These parcel areas are multiplied times loading coefficients. When available, municipal water use for each parcel is quantified for each subwatershed.

To approximate this loading methodology in a simplified form, the Buzzards Bay NEP’s GIS and parcel analysis spreadsheets have been updated to use all MEP loading coefficients, except septic system loadings, which can employ either numbers of residential units and assumed annual occupancy, or MEP reported water use averages for single family homes. This is somewhat of a simplification in that the MEP uses parcel data and parcel specific water use records to predict seasonal changes in occupancy, although for some studies, when water use data are not available, they may apply average residential water use data from neighboring towns. The water use method has its own set of confounding factors (e.g. residential water use often doubles in summer because of lawn watering and pool use), but these issues can be addressed in a loading model if desired. To use the BBNEP’s spreadsheets, go to our Interactive Loading Spreadsheets page.

Septic Loading Issues

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) per year. To calculate annual loadings the BBNEP would then use US census statistics and GIS coverages to get average occupancy for the watershed being studied. One disadvantage of this process was that it failed to capture high summer seasonal use which occurred in some Cape Cod coastal areas.

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 any particular residence because of lawn and garden irrigation, and 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.

The annual per capita loading rate adopted by the BBNEP (2.7 kg) in the 1990s was based on a review of studies of human waste and wastewater. The value adopted was also comparable to the assumed 35 ppm and 56 gpd per capita discharge assumed by the Cape Cod Commission in their loading models in the 1990s. This static loading rate does not account for seasonal or intermittent use of properties, particularly in areas with high seasonal use and swings in population, and this problem is discussed elsewhere on our website. Many households use more than 55 gallons and actual septic system effluent leaving the leaching field may be higher than 35 ppm, but there is considerable variability among communities.

The MEP addressed the variable and interment occupancy of homes in Cape Cod watersheds in their loading models by using actual municipal water use by properties when this data was available. Their calculation is based on multiplying 90% of the annual water use (10% was presumed to be used on average for outdoor uses like pools and lawn watering), times an assumed groundwater discharge concentration of 26.25 ppm (in their reports and loading spreadsheets the 90% coefficient is applied nitrogen concentration, so loading is 23.625 mg per liter times total water use). Technically the MEP does not calculate per capita water use, they simply use the water use per property, which for single family homes may typically range from 140 to 180 gallons per day as an annual average. We discuss this approach in greater detail on our Nitrogen Loading Assumptions page.

Lagoon Pond Study posted on the MV Times in 2011

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