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depth to groundwater will be measured in the nearby monitor well to determine whether a sample can be collected. If so,the drilling crew will collect the sample prior to advancing the CPT probe to the underlying Riverbank aquifer. The sample will be collected in a 1- liter amber bottle and preserved for laboratory analysis by EPA method 8015. All borings will be sampled in the Riverbank aquifer. The sample will be collected when the probe reaches the Modesto-Riverbank contact or when groundwater is encountered, if the soil-water interface is below the contact. In most of the existing borings,the contact is at approximately 65 feet below grade. The CPT equipment will be washed between borings to prevent cross-contamination. Task 5. Interval Pumping Tests EHD has also requested that pumping tests of each stratigraphic interval be conducted to characterize the hydrologic properties of each and to measure groundwater seepage velocities. As previously explained in written communications in November 1999 and May 2000, pumping tests of the Modesto aquifer are only feasible when the water level is at 45 feet or less in GT-10. Other wells, where the aquifer is thinner and the base is shallower, such as GT-5, could not be tested unless the water level is less than 40 feet below grade. Therefore, it is unlikely that a pumping test of this interval could be conducted in the foreseeable future unless there is a dramatic increase in rainfall that results in a rising water table. It is known from purging during previous monitoring events, as well as from hydraulic conductivity tests performed on samples from the Modesto aquitard, that this zone does not yield water readily and is quickly pumped dry in those wells that have a limited length of screen in the underlying Riverbank aquifer(GT-5, GT-6, and GT-9). Two samples from this zone in 1996 yielded hydraulic conductivities of 1.3 x 10-5 and 4.4 x 10"8. These results demonstrate that this zone is not an aquifer. Pumping this interval would cause the pumping well to go dry long before any effect is felt in any of the other monitoring wells, which are much too far away to be affected by the limited pumping that is possible from this zone. We ask EHD to reconsider the benefit of such a test. Conducting a pumping test of the Riverbank aquifer is also problematic at this time, because the three wells that are screened within this zone are 2" in diameter. In our experience,tests conducted on wells that are less than 4" in diameter are of limited effect. In addition, it is necessary to have at least one observation well that is screened within the pumping zone. The farther this well is from the pumping well,the more water must be pumped and the longer it will take for drawdown to be observed at the observation well. Wells GT-I and GT-3 are more than 30 feet apart, and GT-2 is farther still. It is possible that pumping of GT-3 might cause drawdown in GT-1, but it would be preferable to install a 4-inch or 6-inch diameter well between these wells so that they could both be