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in the analysis are overly conservative, causing the resultant nitrate-nitrogen concentration estimate of 15.9 mg-N/L to be significantly higher than I would expect or calculate for this project. The two factors of most concern are the estimates for wastewater flow and rainfall- recharge. 1. Wastewater Flow. Terracon assumed full 2-person occupancy of all four bedrooms, with a unit flow of 70 gpd (gallons per day)per person, giving a total daily wastewater flow from the OWTS of 560 gpd. This flow would be a valid estimate of maximum daily wastewater flow, which is a common and appropriate approach for OWTS sizing. However,in estimating long-term nitrate impacts per the Hantzsche/Finnemore equation,the value for wastewater flow is intended to represent the long-term average discharge of wastewater from the residence/OWTS. In my experience reviewing and monitoring wastewater flows for individual residential and small conununity wastewater systems, the average flow is significantly less than the system design flow, commonly as little as one-third to one-half. The lower average flows are generally a result of(a) smaller occupancy rates(3 per household is common per census data), and(b) lower actual wastewater generation rates when averaged over time. In my opinion, an average wastewater flow of 150 gpd would be a reasonable and safe value for a typical 3-bedroom residence; 175 to 200 gpd would reasonable for the 4-bedroom residence proposed in this project. 2. Rainfall-Recharge.Terracon developed an estimate of annual onsite rainfall-recharge ("deep percolation")by a simple water balance, comparing average monthly rainfall vs evapotranspiration(reference ETo) for the project area. The net surplus(rainfall>ETo) during the months of November through February were summed to arrive at an average annual recharge value of 4.49 inches per year. I agree with the basic procedure; however, using the ETo value overestimates the amount of actual evapotranspiration(ET) for a developed residential site. The published ETo values represent the estimated ET(water demand) for lawns and other uniform turf areas,which are generally much higher than the ET for most types of vegetation,e.g., trees, shrubs,ground cover,perennials, etc. The ET associated with buildings and paved areas is even less (e.g., <10%of ETo). Since a residential site will commonly have a mix of vegetation and landscape development,the water balance-recharge calculations will be more accurate by adjusting the ETo (downward), according to the mix of developed landscape conditions. I suggest calculating a"composite" ET adjustment factor. I've attached an example calculation sheet and link to a recommended University of California reference to obtain appropriate adjustment factors ("plant factors"). In the attached"hypothetical" I calculated a composite adjustment factor of 0.58, and applied this to the rainfall and ETo water balance data from the Terracon report to arrive at an estimated average annual deep percolation/recharge value of 6.97 inches per year. By my analysis,revising the wastewater flow assumptions and rainfall-recharge calculations per the above will significantly reduce the resultant nitrate-nitrogen concentration for his project to a level below 10 mg-N/L. This will bring the project within the 10 mg-N/L criterion and avoid the need for a supplemental treatment system for nitrogen reduction. Since my attached calculations are hypothetical, I recommend that Terracon consider these suggested changes to the nitrate loading analysis and submit revised project calculations for final review. 2