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3.1.3 Bedrock Storage Coefficient <br /> The storage coefficient is related to the interconnected porosity of a rock and is defined as the <br /> volume of water an aquifer releases from or takes into storage per unit surface area of the <br /> aquifer per unit change in the hydraulic head (Driscoll, 1986). The storage coefficient is <br /> dimensionless. <br /> Crystalline volcanic rocks have very low primary porosity, but can develop moderate secondary <br /> porosity due to development of fractures (Freeze and Cherry, 1979). For unconfined conditions, <br /> storage coefficients are approximately equal to the effective porosity and range from 0.01 to 0.3. <br /> For confined conditions storage coefficients range from 0.001 to 0.00001 (Driscoll, 1986). The <br /> upper part of the zone of saturation is expected to be unconfined, and the deeper part of the flow <br /> system is expected to be confined by poorly welded zones of low permeability. <br /> The storage coefficient can also be estimated using the relationship that storage coefficient varies <br /> directly with the thickness of the aquifer. The rule-of-thumb relationship from Todd (1980, p. <br /> 46) is as follows: <br /> Storage coefficient = 3 x 10-6 x aquifer thickness <br /> Based on the above relationships and past experience in volcanic terrains, the storage coefficient <br /> probably ranges from 0.01 to 0.005. <br /> smijuan\sunny\110361\oct9l.Rpt 17 <br /> n N simon WOW-URRNCH <br />