CHARACTERIZATION OF AN APPALACHIAN COLLUVIUAL DEPOSIT AND ITS IMPACT UPON GROUNDWATER FLOW, CENTRE COUNTY, PENNSYLVANIA

A subsurface investigation of a 30-acre site located along the northwest-facing slope of Bald Eagle Mountain in Centre County, Pennsylvania facilitated a three-dimensional investigation of colluvium. Because this investigation included numerous 3+ meter test pits and deep borings (in which continuous sections of colluvium were recovered), we have an unusual opportunity to analyze the genesis of these deposits and to describe how the colluvium influences groundwater flow.

The site is located in an unglaciated portion of Pennsylvania, along the northwestern limb of the Nittany Anticlinorium and is underlain by Middle Devonian shale and siltstone. Steeply dipping Lower Devonian through Upper Ordovician siliciclastic and carbonate bedrock is upslope of the site. Topography across the site slopes approximately 8° to the northwest.

The site is underlain by colluvium ranging from less than 12.5 to over 33 meters in thickness. This colluvium consists of highly compacted mud, sand, and rock fragments. Distribution of cobbles and boulders within the colluvium indicates temporal variance in sediment transport and/or weathering. Although there are intervals of the colluvium in which unweathered sulfide minerals are present, the overall compositional and textural characteristics of the colluvium suggest that most colluvial sediment was exposed to significant weathering prior to final deposition.

Groundwater flow within the colluvium is two-tiered with a low permeability layer separating the two flows. Physical and chemical characteristics of deep and shallow groundwater indicate that there is little mixing of the two flows. It seems likely that there are distinct up-slope recharge areas for each of these flows.

The colluvium is overlain by soils derived predominantly from the underlying colluvium. On average, composite soil samples from test pits ranged from very gravelly loam to very gravelly clay loam. A weakly-developed fragipan was identified in test pits excavated at the site and is similar. This fragipan forms a dense, low permeability layer that limit downward movement of infiltrating surface waters. The material acts as a leaky upper confining layer to the shallow groundwater flow within the colluvium and results in artesian conditions across much of the site.