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Finding existing studies with adequate ortho-P data from OWTS proved to be a lim- iting factor in validating the MEANSS per- formance for ortho-P. Fortunately, the SWAT watershed model provided an adequate assess- ment method for ortho-P. The results showed a good match between MEANSS and measured ortho-P loads, with the MEANSS estimate 38% higher than the SWAT model. The TN com- parison was similar, with the MEANSS esti- mate 37% higher than the SWAT model. Conclusion MEANSS was developed as an easy-to-use and cost-e¬ective tool to estimate nitrogen
and phosphorus loadings to groundwater and surface waters from OWTS. These results show that MEANSS generally provides good estimates of nutrient loading to groundwater and surface water when compared with sev- eral studies and other methods for estimat- ing nutrient loads from OWTS. The data requirements for MEANSS are relatively min- imal compared with other methods, and the parameters are readily available using exist- ing soil databases and GIS tools. Future work on MEANSS could include the evaluation of how to better distribute loads seasonally to match the variation in groundwater flows that control the delivery
of nutrients to surface water. As sites with adequate data become available, additional comparisons against measured data could be conducted. Acknowledgement: The author thanks Michael W. Suplee, PhD, for his reviews and comments that greatly improved this manuscript. Corresponding Author: Eric Regensburger, Hydrogeologist, Water Quality Planning Bureau, Montana Department of Environ- mental Quality, 1520 East Sixth Avenue, Hel- ena, MT 59601. Email: eregensburger@mt.gov.
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