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a ti -Water Predicting Ground Nitrate-Nitrogen Impacts r by Norman N. Hantzschea and E. John Finnemoreb T - h M Abstract The buildup of nitrates in upper ground-water zones is a potential cumulative effect of on-site sewage disposal practic which is not addressed by standard siting and design criteria. Literature concerning the contribution and fate of nitrog beneath septic tank disposal fields is reviewed. From these findings, convenient, simplified methods are developed f estimating long-term ground-water nitrate increases on an area-wide basis.The methods are presented in a manner useful engineers, planners, and regulatory agencies for routine evaluation of existing and proposed land developments and f Y' design of large,common disposal systems. Typical solutions are shown graphically to illustrate the relative importance various factors,including development density,rainfall recharge,and soil denitrification.Predicted values are compared wi actual monitoring data for three California communities to verify the reasonableness of the suggested methods. Seve• possible regulatory applications are suggested. In#roduction have worked to develop guidelines and criteria to inrprr The use of on-site subsurface sewage disposal systems, on-site sewage disposal practices.The aim has been to mi in particular septic tank disposal fields,has long been recog- mize potential health and water quality problems associat nized as one of the most effective means of dealing with with the siting, design, construction, and maintenance domestic waste-water problems in rural settings.Many soils such systems. The main concern is the protection of wn- ,j have a high capacity to accept,filter,and assimilate sewage supplies and general public health from the standpoint effluent.Also,in sparsely populated areas,the availability of bacterial contamination and disease transmission. Prot large amounts of open land tends to minimize possible water tion of ground-water quality, for example, is achieved quality or public health effects associated with such sewage requiring a specified vertical separation distance betwe disposal practices. There is now, however, a growing trend the disposal system and the highest expected rise of t to make permanent use of on-site systems for large-scale water table.This provides an unsaturated soil zone whey: urban fringe, rural residential, and recreational develop- high degrees of physical,biological,and chemical treatme -`' occur. Surface waters are similarly protected by the carr ments. Small, unsewered communities are also tending 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 I struction projects. not been addressed in guidelines and regulations is that �= During the past several years,watet quality and public the persistent or increasing effect of large numbers of s, health agencies and researchers throughout the country tems in concentrated areas. For example,many substanc contained in sewage are soluble and may move relativ, s unaffected through the soil to accumulate in underlyi ' ground waters or discharge to adjacent surface waters.Al, under certain conditions, the total volume of sewage f- ay 'Questa Engineering Corporation, 1220 Brickyard Cove charged from many systems may alter local ground-wa Road,Suite#206, Richmond,California 94807. levels to the point of affecting the performance of individi bDepartment of Civil Engineering, Santa Clara University, systems or the degree of treatment provided by the Santa Clara,California 95053. system (Finnernore and Hantzsche, 1983). Received April 1991, revised September 1991, accepted The buildup of nitrate in ground water is potentiL September 1991. Discussion open until January 1, 1993. one of the most significant long-term consequences of c •'z ._N� 490 Vol. 30, No. 4—GROUND N/1TER—July-August I