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5.The curves of Figures I and 2 show the strong Conclusions influence of the rainfall recharge component on the resul- The accumulation of nitrate in the upper saturated tant nitrate-nitrogen concentration.The application of the zone is a cumulative effect of on-site sewage disposal prac- methods presented in this paper and the reasonableness of tices which has not been addressed by standard siting and the results are,therefore,limited by the accuracy with which design criteria.This paper presents a convenient method for the rainfall recharge fraction can be estimated or determined estimating long-term increases in ground-water nitrate- by the user. For best results,the user should perform a nitrogen caused by on-site sewage disposal.The method is thorough water balance analysis using techniques such as useful to practicing engineers and regulatory agencies for those developed by the U.S.D.A.Soil Conservation Service the general planning and evaluation of residential develop- (1964)or Thornthwaite and Mather(1955),or other infor- ments as well as for the site-specific design of on-site sewage mation based on local studies. disposal systems. This is evidenced by their adoption in parts of California. Planning Applications The greatest potential for ground-water nitrate-nitrogen The nitrate assessment procedures outlined in this problems arises in areas of low rainfall recharge and high paper may have a number of land use and environmental development density.The situation may be critical if local planning applications.A principal advantage is the minimal ground waters are used for domestic water supply.Existing requirement for data. communities and cluster developments using large, com- mon septic tank disposal fields are also likely to be of Zoning and Subdivision Proposals significant concern because of the high concentration of Preliminary evaluation of potential water quality waste-water disposal in a limited area.In newer develop- impacts is useful when broad land use planning decisions are ments,mandatory space requirements for roads,buildings, being made. Computation of A or use of Figure 2 can open space,etc.,will sometimes keep the overall intensity of provide an initial basis for determining appropriate develop- development and waste-water application below critical ment densities to assure protection of areal ground-water levels. quality. Comparison of predicted values with actual field Residential subdivision proposals can be screened for sampling data for several case study locations in California potential long-term nitrate impacts by applying equations confirms that the methods provide reasonable first approx- (1) and.(2). The North Coast Regional Water Quality imations of nitrate-nitrogen effects in ground water from Control Board of California and several local health septic tank disposal fields.The agreement between predicted departments have adopted these procedures for this pur- and observed values is sufficient to enable potential areas of pose.An indication of no potentially excessive nitrate build- concern to be identified,thus making the method an effec- up in ground water according to the analyses presented in tive planning tool. this paper would obviate the need for further study.In the A promising application of these nitrate assessment event that preliminary analyses indicate possible problems procedures is for regulatory purposes. The limited data (e.g., planned development density exceeds I/A),further requirements and straightforward computations make the analyses might be required to define the ground-water approach widely suitable for evaluation of zoning and land system and potential effects more specifically.Also,mitiga- use plans, subdivision proposals, and continued develop- tion measures and ground-water monitoring requirements ment in unsewered areas.The need for mitigation measures, may be formulated based on the preliminary nitrate predic- long-term monitoring,or more detailed site investigations tions.Possible mitigation measures might include reducing can also be readily determined by use of these procedures. development and sewage loading densities,incorporating nitrogen removal systems(Laak, 1982), or modifying the Acknowledgments disposal system locations or design(Harkin et al.,1979). This paper was developed from a study supported by Buildout in Existing Unsewered Areas United States Environmental Protection Agency Grant No. P009211010 and by California North Coast Regional Water Continued buildout of certain existing development Quality Control Board Contract No.0-092-118-0. 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 References predicted by equation(2),then further analysis of possible Alexander, M. 1965. Nitrification. Soil Nitrogen. W. V. localized problems and more complete study of the ground- Bartholomew and F.E.Clark,eds.American Society of Agronomy,Inc.,Madison,WI.Agronomy Series,No.10. water system is warranted.Ground-water monitoring may pp.307-343. be used to verify the water quality concerns indicated by the Broadbent,F.E.and F.Clark.1967.Denitrification.Soil Nitro- predictive equations.The preliminary analyses using equa- gen.W.V.Bartholomew and F.E.Clark,eds.American tions(1)and(2)provide a rational basis for the design of Society of Agronomy, Inc., Madison, WI. Agronomy field monitoring programs. Specific mitigation measures, Series,No.10.pp.344-359. including modified design standards,might be appropriate California Regional Water Quality Control Board,Central Coast Region(CRWQCB).1983.Los Osos/Baywood Park area for any additional development that would tend to aggra- basin plan amendment.Staff Report.75 pp. vate observed ground-water quality problems. California Regional Water Quality Control Board,Central Coast 498