The buildup of nitrates in upper ground-water zones is a potential cumulative effect of on-site sewage disposal practices which is not addressed by standard siting and design criteria. Literature concerning the contribution and fate of nitrogen beneath septic tank disposal fields is reviewed. From these findings, convenient, simplified methods are developed for estimating long-term ground-water nitrate increases on an area-wide basis. The methods are presented in a manner useful to engineers, planners, and regulatory agencies for routine evaluation of existing and proposed land developments and for design of large, common disposal systems. Typical solutions are shown graphically to illustrate the relative importance of various factors, including development density, rainfall recharge, and soil denitrification. Predicted values are compared with actual monitoring data for three California communities to verify the reasonableness of the suggested methods. Several possible regulatory applications are suggested.