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