ISDS 



Nitrogen concentrations in ISDS effluent depend upon level of use, but concentrations 

 average 63 mg/1, ranging from 30 to 80 mg/1 (Tilchin et al., 1978; Andreoli et al., 1979; 

 Haridn et al., 1979; EPA, 1980). Because natural background levels of nitrate are quite low 

 (below 100 ppb in undeveloped areas), levels of no more that 0.3 mg/1 total soluble 

 nitrogen have been found sufficient to induce eutrophicarion (Viets and Hageman, 1971). 

 Other authors have recommended a nitrogen criterion of 0.10 as necessary to limit 

 eutrophication in pristine waters (Briggs and Feiffer, 1986). Samples of Rhode Island 

 river waters have yielded nitrate levels ranging firom 0.30 mg/1 on the Branch River, 

 Forestdale to 1.1 mg/1 on the Blackstone at Manville. 



A similar literature review addressing phosphorus migration (Joubert, 1987) indicated 

 that under most conditions phosphorus tends to be attenuated quickly and efficientiy by soil 

 processes. Except in sensitive waterbodies (including fresh waters and some fresher 

 inshore sectors of estuaries), phosphorus presents less hazard as a transportable nutrient 

 than does nitrate. In sensitive phosphorus-limited waterbodies, however, extremely low 

 phosphorus concentrations can induce eutrophication, and very serious concern is 

 warranted. 



Although drinking water criteria have not been developed for phosphorus, which is not 

 normally considered a public health threat, limits have been established for control of 

 eutrophication and protection of aquatic life. EPA proposed phosphorus criteria of 0.05 

 mgA for mouths of streams entering lakes or reservoirs, and set a 0.025 mg/1 within 

 impoundments to control eutrophication (Briggs and Feiffer, 1986). A 1976 aquatic life 

 protection criterion of 0. 1 mg/1 was established by CEQ, although concentrations as low as 

 0.01 mg/1 have been found to promote eutrophication of open fresh waters (Fetter, 1980). 



Ironically, anaerobic conditions in septic tanks convert organic phosphorus and 

 phosphate (which enter ISDS from human waste and detergent sources in equal 

 proportions) to orthophosphate, the form most available for plant uptake (Magdoff et al., 

 1974). Attenuation of phosphorus in soil is high, but not unlimited, efficiency being based 

 on soil and effluent characteristics. Continued loading can lead to "saturation" of a soil and 

 eventual failure with respect to effluent renovation (Novak and Adriano, 1975). In sandy 

 soils with very fast percolation rates, Sikora and Corey (1976) calculated that soil 

 phosphorus saturation under septic tanks should occur to a depth of 3.41 feet in one year. 

 In contrast, penetration in a slow percolation rate silt loam could be as shallow as 6 inches, 

 according to the same authors. 



In general, Sikora and Corey concluded that phosphorus contamination of groundwater 

 could be anticipated primarily in sandy soils with low organic matter content, soils having 

 high water table, and shallow soils over creviced bedrock. Systems in sandy soils near 

 surface water bodies, therefore, are most likely to contribute phosphorus loading to 

 receiving waters. Evidence of contamination would appear after a period of operation, 

 when the capacity of the soil to adsorb phosphorus had been reached. These factors 

 illustrate the importance of setting buffer distances which adequately account for long-term 

 nutrient transport. 



Joubert's review of the literature suggested that, in summary, phosphorus removal from 

 ISDS effluent is most efficient in fine textured soils having slow percolation rates and 

 sufficient surface area for cation exchange. Retention capability in coarse textured soils and 

 under saturated conditions would be reduced (Joubert, 1987). 



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