The horizon is a level of the fill material, which I assume is fill dirt.

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Note added at 6 hrs (2014-11-21 00:25:40 GMT)
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5. Geologic And Hydrogeologic Site Conditions
Hydrologic data collected during the 2001 and 2002 rounds were reviewed to provide a more
complete understanding of hydrological conditions of the Investigation Area. In addition,
analysis benefited from data collected during earlier time frames. An analysis of the data indicated that two primary stratigraphic layers exist at the site, referred to as the Lower and Fill Horizons. The Fill Horizon, as its name implies, was formed as a result of placement of fill. Water level data suggest that the Fill and the Lower Horizons generally behave as one heterogeneous unconfined aquifer, and that there is a generally consistent radial groundwaterflow pattern beneath the Investigation Area, primarily toward the Chehalis River. Data alsosuggest the Lower Horizon likely represents a significant barrier to the downward migration ofpotential contaminants through the Lower Horizon into the deeper subsurface. Although tidesinfluence surface waters of the area, no significant tidal influence was measured in site wells.

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