SPDSS_Task34-3_Phase 3_StreambedConductanceTesting

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remove the larger cobbles and facilitate grain-size testing. It is likely that groundwater movement between the stream and underlying aquifer is inhibited by t11e armor>11g of the streambed surface. Sediment materials were described at the sites by t11e field geologist. Up to two representative core intervals from each corellole were subnutted for geotechnical testing to deterniuze the streambed sediment grauz-size distribution. A total of 175 core samples were subnutted to Hepworth-Pawlak Geotechnical Inc. of Parker, Colorado, for geotechnical grain-size testing by the wet-sieve method following ASTM D-422 and ASTM D-2217 (ASTM 2002, ASTM 1998). 2.3 streambed Vertical Hydraulic Gradient Testing The vertical hydraulic gradient across t11e streambed was measured at eac11 test site at a location near the center of the active charulel at each transect. Incases of a narrow streambed, the vertical hydraulic gradient was measured upstream or downstream of the core-11o1e testing locations to prevent interference by other testing and sampling. A schematic diagram of the streambed vertical hydraulic gradient testing procedure is provided in Figure 3. At each location a piezometer was driven 3 ft into t11e streambed. This was done immediately upon arrival at the site to allow the aquifer's water level to equilibrate as long as possible before measurement (approximately 2 to 4 hours), which was typically performed just before leaving t11e site. T11e piezometer consisted of a 1-inc11 diameter steel pipe with a 4-inch wire-wrapped stainless steel screen and a drive point at the bottom. The water level difference between the stream and aquifer was determined by measuring the depth to water inside and outside t11e piezometer with an electric water level probe suspended from the same measuring point. To deternune the vertical hydraulic gradient across the streambed, the depth to water inside the piezometer was t11en subtracted from t11e depth to water outside the piezometer and t11e difference divided by 3 ft, the depth the piezometer screen extend below the streambed. At t11e four locations with DWR/ USGS gaging stations and adjacent SPDSS monitoring wells (Henderson, Kersey, Balzac, and Julesburg), the aquifer water levels and the stream stage height at the gages were recorded to supplement stream-aquifer 1ydraulic gradient measurements. Dur>11g Phase 2 of the SPDSS Task 35, monitoring wells were installed and instrumented wit11 water level monitors adjacent to these gaging stations. The measuring points for the gage and the monitoring well at each of these locations were surveyed by a licensed surveyor. Groundwater level data were collected using automated transducers and data loggers, while stream stage data were obtained from the DWR. For t11e period of record during t11e Task 34 field activities, approximately October and November of 2005, a head difference between the aquifer and stream stage was calculated as t11e groundwater level elevation niulus the stream stage elevation for each site. The results are presented in Section 3.4. Fnlal T~zsl: 3~TM 06-28-07.doc