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C <br />' surface throughout the study area. The presence of the hard limestone <br />layer and the high groundwater level are conditions that prohibit the <br />application of inexpensive or conventional methods in which to treat and <br />' dispose of domestic wastewater from homes in Rockridge. <br />' <br />The Rockridge area of Indian River County is a single-family lot <br />subdivision consisting of single -story, masonry dwelling units on building <br />' <br />lots generally 57 feet wide by 97 feet deep. The study area contains 422 <br />dwelling units. Each of the dwelling units is served by the Indian River <br />' <br />County Utilities Department water supply system. The water lines are <br />typically small diameter pipelines laid within existing easements along the <br />tcurrently <br />back lot lines in each of the. blocks. Each of the 422 dwelling units is <br />served by an onsite septic system consisting of a septic tank and <br />an effluent disposal field. <br />Environmental Protection Agency methodology requires that the study area be <br />' <br />evaluated on a block -by -block basis when determining substantial human <br />habitation and population density requirements. Figure 4-1 illustrates the <br />' <br />block boundaries for the five blocks within Rockridge. The boundaries were <br />drawn roughly along city block lines, using street rights-of-way as a <br />' <br />guideline. Each block was assigned an arbitrary reference number in order <br />to <br />tabulate data. <br />' Table 4-2 demonstrates that all the blocks in the Rockridge study area meet <br />the substantial human habitation rule, which states that the block must <br />have had some inhabitants before October 18, 1972. Table 4-2 also <br />illustrates the present population density per net acre on a block -by -block <br />' basis. None of the block densities fall below 1.7 persons/acre, which <br />would have indicated to the EPA that a central collection system was not <br />t cost-effective for the block. Four blocks in the study area had a density <br />for greater than 10 persons/acre, indicating to the EPA that a central <br />collection system is cost-effective for those blocks. All blocks had <br />' densities greater than 1.7 persons/acre, and no block had a density less <br />' IRC.VB4 <br />6/4/87 <br />4-8 <br />