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• 3.2.2 Characteristics of Water Bearing Strata <br />3.2.2.1 Shallow-colluvial Strata <br />Deposits of unconsolidated sediments are well developed in <br />the East Roatcap Creek drainage. The thickness of these <br />colluvial and landslide sediments range from a few feet to 85 <br />feet (DH-86). At the proposed construction site the thickness of <br />colluvium reaches approximately 50 feet (Dti-84). Colluvium is <br />composed of clays, and silty clays with scattered boulders of <br />volcanic origin. Small lenses of sand and gravel were also <br />encountered during the drilling investigation. A portion of <br />these sediments, particularly near the stream bed, are saturated <br />and water levels are close to the surface (springs and ponds) or <br />within 40 feet from the ground surface. However, the high clay <br />content reduces transmissivity of ground water through, these <br />sediments. Hydraulic conductivity of the colluvial strata was <br />measured in five of the installed monitoring wells. A total of <br />five falling head tests and two raising head tests were <br />performed. Results of testing indicated a range of hydraulic <br />conductivity from 4.0 x 10-4 to 1.8 x 10-1 ft/d y and a range of <br />transmissivity from 5.2 x 10-3 to 6.8 x 10-~ ft2/day. The <br />average values for hydraulic condu tivity and transmissivity are <br />7.2 x 10-2 ft/day and 2.6 x 10-1 ft~/day, respectively. Recharge <br />of the colluvial strata is provided from the surface streams and <br />by direct infiltration of runoff and precipitation on the valley <br />slopes. Water level measurements in monitoring wells SM-10 and <br />SM-11 and in East Roatcap Creek between these two monitoring <br />wells (see Cross-section of Figure H-7) indicates that the stream <br />is losing water into the colluvial strata. This observation was <br />confirmed by a comparison of flow data at stations SW-6A <br />(upstream of the construction) and SW-6 (downstream of the <br />construction). water level measurements in piezometers installed <br />on the western slope of East Rcatcap Creek indicate a small <br />amount of recharge and flow toward the creek. However, this <br />small amount of flow, due to low transmissivity of the strata on <br />the slope does not change the losing character of the stream. <br />The depth of water table in the colluvial sediments in the study <br />area is shown on Figure H-4. Ground water in the colluvial <br />sediments is of unconfined character. <br />Underflow in the saturated part of the colluvial sediments <br />was calculated for the cross-sectional area as shown on Figure H- <br />7. This calculation indicated a very low flow through the <br />sediments equal to 33.12 cubic feet/day, or 0.17 gpm. The <br />velocity of flow was calculated as 0.06 ft/day. <br />Ground water in the colluvial strata is generally of poor <br />quality. Samples of water collected from wells SM-5 and SM-6 <br />indicated a range of total dissolved solids from 492 to 714 mg/1. <br />water is of bicarbonate, sodium, sulfate type with a pH between <br />7.6 and 8.6. <br />14 <br />