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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, Cry- Page II Considering only the groundwater elevations in wells near the southern perimeter of the well field and those at the northern end of the well field, on February 24, 2005, the mean groundwater gradient in the shallow, near-surface aquifer beneath the site was j approximately 0.003 ft/ft. Similarly, at the scale of the complete well field, the general direction of groundwater flow was north-northeast. However, as is discussed below, locally within the well field the development of a groundwater mound radically modified that flow regime, which otherwise would have been typical of that seen since groundwater-quality monitoring was initiated at the Navarra Site in May 2000. ! : 2.1 Groundwater Mound f An unexpected change in the groundwater flow regime that had developed since the October 2004 monitoring round and that became evident from the February 24, 2005 depth to groundwater measurements was a groundwater mound centered on Monitoring Well MW-16. :1 Existence of the mound was apparent from the shallow depth to the water table in that well (2.60 ft., measured on February 24, 2005). The resulting distribution of groundwater - contours, due to the groundwater mounding that affected Monitoring Well MW-16 and adjacent wells is shown on Figure 2 and is also apparent on cross-sections A-A' and E-E', ii shown on Figures 4 and 8,respectively. It is, of course, unlikely that a groundwater mound would develop in a natural groundwater regime. Such mounds are invariably caused by a source of water being discharged on or close to the ground surface in the area where the groundwater table is elevated. There are no water supply lines or other water-supply or other utility lines in the area around MW-16 that might have leaked and supplied the water that has accumulated ={ in the mound on the groundwater table. However, from our extensive portfolio of engineering projects for installation of potable water-supply wells and sewerage treatment systems for sites in the area around the 7500 West Eleventh Street property, SJC is aware that the enclosed yard on the east side of the Capitol Furniture store at 7501 West Eleventh Street is the location of a septic tank leach field that provides for the discharge of sewage from that property. Experience has shown that when a leach field is failing, usually due to the accumulation of sewage solids in the pore space of the underlying soil, sewage water fills the discharge =' Iines and may rise well above the bed of the leach field. Such a condition may pass unnoticed if the increased head induced by the rise in the water is sufficient to generate enough flow through the reduced-permeability soils, or combined with that drainage, there is sufficient loss of liquids by evaporation and vegetative evapotranspiration from a shallow soil cover over the leach field. Unfortunately, the majority of the area of the f .} leach field at 7501 West Eleventh Street is paved with bituminous macadam. Such paving-over of a leach field is not normally permitted and, in this case, may be sufficient to reduce evaporative losses of the sewage water to render the leach field incapable of 1_ . processing the volume of sewage water that is currently flowing into it. SJC .a