Department of
Environmental Quality

On-site subsurface wastewater disposal systems (e.g., septic systems) have the potential to transport pollutants from sewage to ground water. To help prevent this, "nutrient-pathogen evaluations" (N-P evaluations) are required for certain proposed on-site wastewater disposal systems. Where required, conducting an N-P evaluation is part of the application process to receive a permit to install a subsurface sewage disposal system. 

The general term "nutrient-pathogen evaluation" refers to a scientific evaluation of the water quality impacts of on-site subsurface sewage disposal (septic systems). The evaluation predicts whether the effluent from sewage disposal systems will be diluted enough to prevent ground water contamination. The evaluation includes compiling existing information, collecting site-specific information, and completing predictive contaminant modeling for ground water. The results depend on many variables, including aquifer characteristics, treatment system density and configuration, and ground water flow direction.

Purpose of the N-P Evaluation

The main purpose of an N-P evaluation is to protect the quality of Idaho's ground water, as directed by the Ground Water Quality Rule (IDAPA 58.01.11.006).  More specifically, N-P evaluations are designed to:

• Determine the appropriate number of on-site wastewater treatment systems for a given parcel of land, and
• Direct the placement of the on-site wastewater treatment systems in a way that will not significantly degrade the quality of ground water or surface water resources.

When An Evaluation is Required

DEQ requires an N-P evaluation for proposed central septic systems located in nitrate priority areas or over "sensitive resource aquifers" and for all proposed large soil absorption systems (LSAS).  In addition, some of Idaho's Public Health Districts and counties require N-P evaluations for smaller on-site wastewater treatment systems. (A central system is any system that receives wastewater from two or more homes or buildings under separate ownership or receives more than 2,500 gallons per day of wastewater. A LSAS is a subsurface sewage disposal system designed to receive 2,500 gallons per day or more.)

DEQ or the Public Health District or counties may also require N-P evaluations on parcels of land where unusual conditions or circumstances cause concern about surface or ground water quality. These types of conditions include areas where:

• Nutrient and/or pathogen contamination already exists and has the potential to create a health risk
• Soil depth is shallow
• There is a predominance of gravel or coarse-grained sediment
• Ground water is close to the surface (10 feet or less)
• Fractured bedrock is close to the surface (10 feet or less)

Whether an N-P evaluation is performed or not, all developments using on-site wastewater treatment systems are subject to the rules governing on-site wastewater treatment systems (IDAPA 58.01.03) and the associated Technical Guidance Manual for Individual and Subsurface Sewage Disposal Systems. 

Types of N-P Evaluations

There are two types, or levels, of N-P evaluations: Level 1 and Level 2. Depending on circumstances and the location of the proposed system, a developer may be required to perform a Level 1 evaluation, a Level 2 evaluation, or both.

Level 1

The district health departments or DEQ may allow an abbreviated N-P evaluation (termed "Level 1") when site conditions or design factors warrant a review of preliminary information prior to determining the need for a more complete Level 2 N-P evaluation. Level 1 N-P evaluations may be considered under the following circumstances:

• Proposed lot sizes are unusually large,
• Site conditions warrant a review of the "area of concern" designation, or
• A Level 2 N-P evaluation has been performed within 0.5 mile radius of the proposed development, and site and design conditions are sufficiently similar

DEQ has provided a nitrogen mass-balance spreadsheet to assess the expected nitrogen load. This is a simplified screening tool used during the Level 1 evaluation to determine whether a more detailed Level 2 evaluation is needed. The mass-balance spreadsheet allows the N-P professional to adjust lot sizes, orientation with respect to ground water flow, and wastewater treatment options to minimize ground water impacts.

A Level 1 N-P evaluation may suffice if the results of the mass-balance spreadsheet indicate a sufficiently low impact to ground water from nitrate, or data demonstrate that site conditions do not warrant the "area of concern" designation.

Level 2

A Level 2 N-P evaluation is more detailed than a Level 1 evaluation and is required whenever a Level 1 evaluation is determined to not be sufficient or when a Level 1 evaluation does not indicate sufficient protection of ground water quality. In this case, the developer may choose to conduct a Level 2 evaluation or may choose to add additional treatment measures to the system and conduct another Level 1 evaluation.

Outcomes of an N-P Evaluation

An N-P evaluation may show that effluent from the treatment system evaluated will not adversely impact surface or ground water. In this case, the system is approved and the permit application process may continue.

In other instances, the N-P evaluation may show that the effluent may contaminate ground or surface water. In these cases, additional measures may need to be taken to receive approval. These may include providing advanced nutrient treatment systems or reconfiguring lots to maximize the dilution capability of the aquifer.

The ultimate outcome is providing wastewater treatment while protecting, or even restoring, the high quality of Idaho's water.