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5. The curves of Figures I and 2 show the strong influence of the rainfall recharge component on the resul- tant nitrate -nitrogen concentration. The application of the methods presented in this paper and the reasonableness of the results are, therefore, limited by the accuracy with which the rainfall recharge fraction can be estimated or determined by the user. For best results, the user should perform a thorough water balance analysis using techniques such as those developed by the U.S.D.A. Soil Conservation Service (1964) or Thornthwaite and Mather (1955), or other infor- mation based on local studies. Planning Applications The nitrate assessment procedures outlined in this paper may have a number of land use and environmental planning applications. A principal advantage is the minimal requirement for data. Zoning and Subdivision Proposals Preliminary evaluation of potential water quality impacts is useful when broad land use planning decisions are being made. Computation of A or use of Figure 2 can provide an initial basis for determining appropriate develop- ment densities to assure protection of areal ground -water quality. Residential subdivision proposals can be screened for potential long-term nitrate impacts by applying equations (1) and (2). The North Coast Regional Water Quality Control Board of California and several local health departments have adopted these procedures for this pur- pose. An indication of no potentially excessive nitrate build- up in ground water according to the analyses presented in this paper would obviate the need for further study. In the event that preliminary analyses indicate possible problems (e.g., planned development density exceeds 1/A), further analyses might be required to define the ground -water system and potential effects more specifically. Also, mitiga- tion measures and ground -water monitoring requirements may be formulated based on the preliminary nitrate predic- tions. Possible mitigation measures might include reducing development and sewage loading densities, incorporating nitrogen removal systems (Laak, 1982), or modifying the disposal system locations or design (Harkin et al., 1979). Buildout in Existing Unsewered Areas Continued buildout of certain existing development areas using on-site sewage disposal systems may pose signifi- cant long-term ground -water nitrate concerns. In cases where development density is approaching critical levels predicted by equation (2), then further analysis of possible localized problems and more complete study of the ground- water system is warranted. Ground -water monitoring may be used to verify the water quality concerns indicated by the predictive equations. The preliminary analyses using equa- tions (1) and (2) provide a rational basis for the design of field monitoring programs. Specific mitigation measures, including modified design standards, might be appropriate for any additional development that would tend to aggra- vate observed ground -water quality problems. 498 Conclusions The accumulation of nitrate in the upper saturated zone is a cumulative effect of on-site sewage disposal prac- tices which has not been addressed by standard siting and design criteria. This paper presents a convenient method for estimating long-term increases in ground -water nitrate - nitrogen caused by on-site sewage disposal. The method is useful to practicing engineers and regulatory agencies for the general planning and evaluation of residential develop- ments as well as for the site-specific design of on-site sewage disposal systems. This is evidenced by their adoption in parts of California. The greatest potential for ground -water nitrate -nitrogen problems arises in areas of low rainfall recharge and high development density. The situation may be critical if local ground waters are used for domestic water supply. Existing communities and cluster developments using large, com- mon septic tank disposal fields are also likely to be of significant concern because of the high concentration of waste -water disposal in a limited area. In newer develop- ments, mandatory space requirements for roads, buildings, open space, etc., will sometimes keep the overall intensity of development and waste -water application below critical levels. Comparison of predicted values with actual field sampling data for several case study locations in California confirms that the methods provide reasonable first approx- imations of nitrate -nitrogen effects in ground water from septic tank disposal fields. The agreement between predicted and observed values is sufficient to enable potential areas of concern to be identified, thus making the method an effec- tive planning tool. A promising application of these nitrate assessment procedures is for regulatory purposes. The limited data requirements and straightforward computations make the approach widely suitable for evaluation of zoning and land use plans, subdivision proposals, and continued develop- ment in unsewered areas. The need for mitigation measures, long-term monitoring, or more detailed site investigations can also be readily determined by use of these procedures. Acknowledgments This paper was developed from a study supported by United States Environmental Protection Agency Grant No. P0092 110 10 and by California North Coast Regional Water Quality Control Board Contract No. 0-092-118-0. References Alexander, M. 1965. Nitrification. Soil Nitrogen. W. V. Bartholomew and F. E. Clark, eds. American Society of Agronomy, Inc., Madison, Wl. Agronomy Series, No. 10. pp. 307-343. Broadbent, F. E. and F. Clark. 1967. Denitrification. Soil Nitro- gen. W. V. Bartholomew and F. E. Clark, eds. American Society of Agronomy, Inc., Madison, Wl. Agronomy Series, No. 10. pp. 344-359. California Regional Water Quality Control Board, Central Coast Region (CRWQCB). 1983. Los Osos/Baywood Park area basin plan amendment. Staff Report. 75 pp. California Regional Water Quality Control Board, Central Coast