ISDS 



***Require two sufficiently spaced percolation tests on all sites where 

 percolation results are utilized in the site evaluation, and use the 

 percolation test only in combination with other soil tests showing soil 

 profile characteristics accurately. 



***Require full soil examination where percolation rates are excessively 

 fast or slow, or where depth to water table or impervious layer is less than 

 4' and 6' respectively. Establish minimum design rate on rapid percolation rate soils 

 to ensure that a system is large enough to accommodate slower irrfiltration following natural 

 formation of a biological "mat." 



*Reqiiirc that mounding analysis be performed by a certified engineer where ISDS flow 

 rates exceed 5000 gpd. 



♦Require submission of the newest applicable RI Soils Survey map with all ISDS permit 

 applications. The soils survey data is currendy being recompiled by the SCS to improve 

 photogrammetric accuracy. In that regard, the data will become increasingly useful in 

 improving the accuracy of site suitability evaluation and sizing, in tracking system 

 performance and failure, in relating performance to soil characteristics, in targeting ISDS 

 rehabilitation grants to areas of greatest need, and in assisting towns in addressing 

 sewering requirements. 



B. Site Suitability Formula 



*With the assistance of the URI Department of Engineering and Natural 

 Resource Sciences, develop a point system for site suitability analysis: 



• A clear formula should be specified in the regulatory language as establishing firm 

 numerical criteria to be used in system sizing and design and in establishing 

 variance conditions and inspection schedules. The formula should assign a 

 specified number of points for soil type, depth to groundwater and impervious 

 layer, slope, lot size, buffers to waterbodies, type of development and flows, 

 existing ckainage alterations, availability of public water and location of lot with 

 respect to groundwater aquifer, water supply watershed, coastal watershed, or 

 other vulnerable areas. 



• Using the point formula, update design flow requirements and siting stipulations to 

 provide adequate system size, establish adequate buffer distances, account for 

 cumulative impacts, and provide for protection of sensitive areas as outlined in 

 other sections of these findings and recommendations. The formula would not 

 provide relief from minimum sizing, separation, and buffer requirements, but 

 would be used to increase predictability of expected requirements. 



• Require two percolation tests on all sites where percolations are used. 



• Establish minimum design rate on rapid percolation rate soils to ensure that a 

 system is large enough to accommodate wastewater flow following formation of 

 biobgical "mat." 



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