Note: For use in calculating septic system density allowances applicable to: a) the Planning Area, generally; b) the developed portion of a cluster development project in the Preservation Area, where overall yield is determined by NJDEP Preservation Area Rules; and c) non-Major Highlands Development in the Preservation Area where not exempt from the Highlands Act.

The model Highlands Area Land Use Ordinance provides two methods by which the septic system yield may be calculated for a development project in the Planning Area of a conforming municipality (see Section 5.3). The default method relies on Highlands Council calculations of the maximum septic system density (i.e., minimum average lot size per septic system), based on the HUC14 subwatershed drought recharge and the applicable nitrate target for the Land Use Capability Map (LUCM) Zone (referred to as "Highlands Zones and Sub-Zones" in the model Land Use Ordinance). Under this method, all development project lands in each LUCM Zone of each HUC14 subwatershed are assigned the same maximum septic system density. A chart is available at the following link providing the minimum average lot size required per septic system, by HUC14 for each LUCM Zone.

The alternative method allows for a parcel-specific analysis of septic system density for conventional subdivisions in the Planning Area only. This alternative method cannot be used to calculate the septic system yield for conventional subdivisions in the Preservation Area; applicants must use the standards described in N.J.A.C. 7:38-3.4(b).

The alternative method must be used to determine the maximum septic system density of residential cluster developments, either in the Planning Area or the Preservation Area. The Highlands Council, with the permission of the New Jersey Geological Survey, has created a version of their "A Recharge-Based Nitrate-Dilution Model for New Jersey V6.2" for use in the Planning Area of the Highlands Region. This new model is "A Recharge-Based Nitrate-Dilution Model for the Highlands Region V1.0."

(NOTE: Regardless of the method used, septic system density is only one potential determinant of development yield. The Highlands Land Use Ordinance addresses additional requirements that may further limit development potential to less than the septic system yield. The approvable development yield is a combined result of all Ordinance requirements. Development yield may also be further limited by insufficient developable sites or septic system locations, as regulated by underlying municipal ordinances and Board of Health requirements.)

Please use the link below to download "A Recharge-Based Nitrate-Dilution Model for the Highlands Region V1.0." The following restrictions and notes apply to use of the model:

  • The Highlands Council developed two versions of the model, one of which runs on Excel® 2007 and the other runs on Excel® 2003. Each model runs a solver macro distinct to that version of MS Excel, and so is not usable on the other version of Excel. Any issues with the operation of these files should be brought to the attention of the Highlands Council, not NJGS, which is not responsible for this version.
  • The model should be downloaded and saved to the user's computer. DO NOT run it from the Web link.
  • Read the "Read Me" file! This file can be found within the Excel workbook. It contains critical instructions for activation and use of the model.

Calculation of Septic System Density

  • To use this model for the alternative method, the user must know the total acreage of each soil type by LUC zone and HUC14 subwatershed on the subject property. Where a project extends over municipal or HUC14 subwatershed boundaries, the septic system density and yield for each area must calculated separately.
  • The model user will select the soil type, municipality and applicable LUCM zone. The model will automatically select the appropriate RMP nitrate target for the LUCM zone (e.g., 2 mg/L for the Existing Community Zone, 1.87 mg/L for the Conservation Zone, and 0.72 mg/L for the Protection Zone) to solve for acres per septic system. (Note: To calculate septic density for residential cluster development, the appropriate nitrate standard is 10 mg/L, regardless of LUCM Zone, as discussed below, which gives a density result that only applies to the cluster development area itself - the overall development yield is calculated in the manner defined below.) All the cells requiring user inputs are highlighted. Do not change the model default conditions of 4 persons per household and 10 pounds of nitrate loading per year per person, as these are standards used by the Highlands RMP to determine septic system density. The result is the allowable density of residential septic systems. See below for information regarding translation to non-residential development.

Calculation of Septic System Yield

  • Users can tabulate the results for each zone/municipality/HUC14 combination in the table titled Septic System Computation Table at the link below. The tab Computation Sheet can serve as a template for user calculations. Note that the table should be printed and the calculations performed manually according the instructions. The spreadsheet does not calculate the yield automatically because several user inputs are required. Instructions are listed to assist the user in the calculations.
  • As discussed above, if your area of study boundary encompasses more than one LUCM zone, municipality, or HUC14 subwatershed, then you must perform separate density calculations for each unique combination of LUCM zone/municipality/HUC14. RMP policies do not allow for the septic system yield to be "transferred" between LUCM zones or between HUC14 subwatersheds, so they must be calculated independently. (Note: although HUC14 subwatershed is not a user input in the model, it must be identified and tracked separately).
  • Utilizing the results of the Nitrate Dilution model, the user will then divide the acres of each soil type within the LUCM zone/municipality/HUC14 combination by the septic system density (acres per septic system) resulting from the model for that unique combination. This is the septic system yield for that combination. Do not round the results down or up yet – use the results as they are reported in the next step.
  • Finally, the user will sum the septic system yield for all soil types in each LUCM Zone/HUC14 combination (and municipality, where applicable). Round down to the nearest integer for each result. This is the septic system yield for the subject parcel, for each LUCM Zone and HUC14 subwatershed; it may not be transferred to another LUCM Zone or subwatershed.
  • The result is the total allowable number of residential septic systems. For non-residential development, part or all of the total septic system yield may be apportioned to non-residential uses by the applicant using the standards from the Highlands Area Land Use Ordinance. Again, the results may not be transferred to another LUCM Zone or subwatershed.

    • Calculation of Septic System Density for Cluster Developments

  • To use this model for the calculation of septic system density in the developed portion of a cluster development, the user must know the total acreage of each soil type by LUC zone and HUC14 subwatershed on the portion of the property where the cluster development is proposed. Where a cluster development extends over municipal or HUC14 subwatershed boundaries, the septic system density and yield for each area must calculated separately.
  • The septic system yield will be determined through the method established under Section 5.3 of the model Highlands Land Use Ordinance (or local equivalent) for the Planning Area, or N.J.A.C. 7:38-3.4 (NJDEP Highlands Water Protection and Planning Act Rules) in the Preservation Area.
  • The maximum density for septic systems shall be established through application of the alternative method, as described above, to the intended location of the cluster development using a nitrate target of 10 mg/L.

A Recharge-Based Nitrate-Dilution Model for the Highlands Region V1.0 for use in MS Excel® 2003 (Download)

A Recharge-Based Nitrate-Dilution Model for the Highlands Region V1.0 for use in MS Excel® 2007 (Download)

Septic System Computation Table for MS Excel® (Download)