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Predicting Ground -Water Nitrate -Nitrogen Impacts by Norman N. Hantzschea and E. John Finnemoreb Abstract 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. Introduction The use of on-site subsurface sewage disposal systems, in particular septic tank disposal fields, has long been recog- nized as one of the most effective means of dealing with domestic waste -water problems in rural settings. Many soils have a high capacity to accept, filter, and assimilate sewage effluent. Also, in sparsely populated areas, the availability of large amounts of open land tends to minimize possible water quality or public health effects associated with such sewage disposal practices. There is now, however, a growing trend to make permanent use of on-site systems for large-scale urban fringe, rural residential, and recreational develop- ments. Small, unsewered communities are also tending more and more to maintain and continue with the use of septic tanks rather than embarking on major sewerage con- struction projects. During the past several years, water quality and public health agencies and researchers throughout the country a Questa Engineering Corporation, 1220 Brickyard Cove Road, Suite #206, Richmond, California 94807. bDepartment of Civil Engineering, Santa Clara University, Santa Clara, California 95053. Received April 1991, revised September 1991, accepted September 1991. Discussion open until January 1, 1993. have worked to develop guidelines and criteria to improve on-site sewage disposal practices. The aim has been to mini- mize potential health and water quality problems associated with the siting, design, construction, and maintenance of such systems. The main concern is the protection of water supplies and general public health from the standpoint of bacterial contamination and disease transmission. Protec- tion of ground -water quality, for example, is achieved by requiring a specified vertical separation distance between the disposal system and the highest expected rise of the water table. This provides an unsaturated soil zone wherein high degrees of physical, biological, and chemical treatment occur. Surface waters are similarly protected by the estab- lishment of lateral setback requirements. An important water quality issue that previously has not been addressed in guidelines and regulations is that of the persistent or increasing effect of large numbers of sys- tems in concentrated areas. For example, many substances contained in sewage are soluble and may move relatively unaffected through the soil to accumulate in underlying ground waters or discharge to adjacent surface waters. Also, under certain conditions, the total volume of sewage dis- charged from many systems may alter local ground -water levels to the point of affecting the performance of individual systems or the degree of treatment provided by the soil system (Finnemore and Hantzsche, 1983). The buildup of nitrate in ground water is potentially one of the most significant long-term consequences of on - 490 Vol. 30, No. 4—GROUND WATER—July-August 1992