The Board of Appeals finds that similar such conversion of all seasonal residences in the general area having lots similar (or less limited) in lot size and land and soil type characteristics would not result in substantial danger of contamination of the groundwater supply or of any pond or stream. In making such finding, the Board shall consider the ability of the soil to absorb expected quantities of sewage disposal effluent, the degree of filtration of effluent before entering bedrock fissures or other groundwater supply and other characteristics of the land and soil types.