What is an N-P Evaluation?

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.

What is the Purpose of an 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 is an N-P Evaluation 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 District Health Departments 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 District Health Departments 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.

_{Source: Idaho DEQ, Idaho Department of Environmental Quality}