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Page 2 Mr. Meleyco March. 19, 1997 f nitrate loading impact on groundwater quality to be significantly underestimated. Additionally, the Kleinfelder report does not clarify whether the reference nitrate values and caieulations are for nitrate measured"as NO,"or"as N'; thea is a significant di Terence which appears to beove_looked in the report. My specific points of disagreement are as follows: • Nitrate "as N" vs Nitrate ,as NO3". The Meinfelder report does not identify the nitrate MI asurcment units and appears to use values for nitrate "as N"and -'as Npy"interchangeably. The values are not interchangeable. The drinking water standard for nitrate is 10 mgn(as N)and 45 mg/1 (as NO3). The Kleinfelder nitrate loading calculations use values for nitate as N, but conclude their analysis by comparing the results to the drinking water standard of 45 mgll, which is the standard expressed "as NO3". The results should, instead, be compared to the 10 mg/I r standard. This is the convention used in the original EIR and in the Chico study that Kleinfelder s makes reference to and utilizes data from. • Septic System Nitrogen Content Kleinfelder assumes a concentration of 30 rrg/1 for the nitrate content of septic tank effluent discharged to the leachfields. This is an extremely liberal assumption.the this is intended to be nitrate "as NO,"; it would correspond to a value nitrate- nitrogen value of about 6.8 mg/l. Even if the 30 mg/I is intended to be nitrate as N, the (77� assumption is still much too low and is not supported by any literature that I am aware of.They cite the fact that they sampled septic systems for nitrate,finding the concentrations to be [1.0 mg/l. The problem with this is that the septic tank effluent rarely contains nitrogen in the nitrate form; it is nearly all in the form of arxmonia or organic nitrogen corning out of the septic tank (an anaerobic environment). It is only after the effluent passes through the aerated soil zone below the leaching trenches that the nitrogen is transformed to nirrate (i.e., by"nitrification"), and this is generally recognized to be a total, 100%conversion. The discussion wid assumptions in the Kleinfelder report appear to completely miss this critical point. Literature data show septic tank effluent to have total nitrogen concentrations generally in the range of 40 to 60 mg/l; and tnis•should be used in the nitrate loading calculations in place of the 30 risg/1 used by KJeinfelder. • Denitrification. The Kleiafelder study assumes a value of 40% for denitrification of the percolating septic effluent. Denitrification is the biological process in tete soil whereby certain bacteria utilize: [he oxygen in the nitrate (NO;) and release nitrogen gas (N2) to the atmusphere. Denitrification is expressed as a percentage, and the normally accepted range is 10 to 25%. 1t is not something that can be measured, but must be estimated. .Denitrification is generally highest where the sols are wet, fine-textured (i.e., clayey), and have high amounts of organic matter;well-drained,permeable and sandy soils with a deep water table generally do not provide favorable conditions for high rates of denitrification. In my opinion, the 40%rate used in the Kleinfelder study significantly overestimates the likely denitrification potential at the site. The main rationale for a 40% rate cited is the soil texture. This ignores the other factors that are critical to denitrification,namely wet,poorly drained and high organic conditions. Their analysis