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M, .- M Geohydrology In Iridian River County there is a shallow aquifer supporting a water table at an average depth of twelve feet below land surface, and the deeper Floridan Aquifer at 300-600 feet depths. Floridan Aquifer water has elevated chloride levels and thus is used primarily for irrigation purposes. This situation leaves the shallow aquifer as the single source for good quality ground water. Water levels in the shallow zone rise and fall in response to rainfall illustrating that the infiltration of rain water recharges the aquifer. Soil material above the water table is a natural filtration medium screening out some pollutants and providing a degree of water quality protection for the shallow aquifer. Since all surface water is potential recharge, not only rainwater, this system is important to ground water quality protection, and it functions without public expense. Height of Capillary Zone The subsurface occurrence of ground water is divided into zones of saturation and aeration. In the zone of saturation all the interstices are filled with ground water; the water table represents the top of this zone. A capillary zone extends above the visible water table to the limit of capillary rise of water; the thickness of the zone varies with soil or rock texture. From this capillary limit to the ground surface is the zone of aeration wherein the interstices are occupied partially by water and partially by air. This zone provides vertical recharge/storage for the water table aquifer. Water transmitted through the zone of aeration maintains the water table, and excess water causes the water table to rise. The thickness of this zone, along with the soil permeability rate, affects the storage of infiltrating water. Surface waters -run off if rainfall exceeds the soil percolation rate, and when the soil is saturated. To estimate the limit of a capillary zone, I referred to a chart showing the distribution of moisture above the water table in experi- ments in a fine sand (Ground Water Hydrology, D. K. Todd). The capillary zone appears to terminate within two (2) feet above the visible water table. A method for approximating typical capillary fringe heights is: hc. _ .3 ,d is the pore diameter, mm. Typical d pore diameter for fine sands is .lmm, yielding he = 10 inches. This estimate compares favorably with another table showing he = one foot for fine sand (Applied Hydrogeology, C. W. Fetter, Jr.). You described the sand ridge to be generally composed of fine sands overlying medium sands. The maximum capillary rise for fine sands should not exceed two feet. If medium sands are the predominant "lithologic texture at the water table depth, then the maximum capil- lary rise should be less than one foot. Comments Now, with an objective to protect the water table and capillary fringe, you can merely incorporate these minimum figures into the ordinance. However, I suggest some additional footage for two reasons: JUN 31981 1) Surface waters recharge the shallow aquifer through the soil cover and some minimum additional thickness should be maintained to provide for filtration of the recharging water (Water Quality), and 51 BOOK 4U PACE6�fi