ISDS 



SITE EVALUATION 



Findings and Concerns 



Although the simplicity and convenience of the percolation test have made it the most 

 commonly used technique for evaluating site suitability, a number of flaws limit its 

 reliability in application to ISDS siting: 



Results may vary by 90% or more in the same soils (US EPA, 1980); 



The test is not capable of measuring any fundamental soil properties, such as 



hydraulic conductivity, which are of key importance in predicting potential system 



failure. 



The percolation test cannot be reUably reproduced (Bauma et al., 1972); 



Limiting use of percolation tests to certain seasons fails to capture long term trends 



in water table variability; 



"Dosing" of a site prior to the performance of a percolation test can produce 



misleading results; and 



Test accuracy may be vulnerable to drainage alterations which have previously 



affected the site. 



In view of the percolation's unreliability, RI ISDS regulations require that soil test pits 

 be dug and examined by licensed engineers, and that descriptions of soil layers, ground 

 water separation, occurrence of impervious layers and other pertinent information be 

 evaluated as part of the permit evaluation. ISDS staff "observance" of the soil trench 

 examination is required. 



In practice, the ISDS Section is insufTiciently stafTed to observe soil pit 

 examinations consistently, except in known problem areas. Therefore, the 

 staff must frequentiy rely on percolation results, and the reports of contract engineers who 

 are generally not trained in soil science to the degree necessary in order to evaluate many 

 site parameters indicative of potential concerns. Additional staff engineers, soil 

 scientists, hydrogeologists, and system designers are critically needed. 



Certain key aspects of soil drainage can contribute to system failure and/or resource 

 degradation. For example, groundwater mounding frequentiy occurs in "tight" soils, while 

 very fast percolation through coarse soils may cause rapid development of a biological 

 "mat" which inhibits leachfield permeability. 



The soil pit analyses required by the regulations, even where performed, are not capable 

 of fully evaluating the potential for failure introduced by system design under specific 

 hydrological conditions. Where permitted designs rely on percolation tests, potential 

 overloading of natural soil capabiUties becomes virtually impossible to evaluate. 



Recommendations 



A. Site Evaluation Procedure 



**Re-examine site evaluation procedures, in light of best available research 

 results. Begin process of converting to comprehensive soil evaluation 

 (currently used successfully in several New England states) as site 

 evaluation method. 



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