ASSESSING THE ROLE OF TERRACE DEPOSITS IN SINKHOLE AND KARST DEVELOPMENT USING SOIL AND PERMEABILITY ANALYSIS IN AREAS UNDERLAIN BY CARBONATE BEDROCK IN SHENANDOAH COUNTY, VIRGINIA

Karst features including sinkholes are common within the carbonate deposits of rural northern Virginia. These features have local effects on the landscape and water movement that are relevant to agricultural and rural landowners. This study attempts to further understand the role that unconsolidated terrace deposits play in the karst and sinkhole development by characterizing the texture and permeability of the deposits at various levels above the modern floodplain. The study area was selected because four distinct levels of deposits representing modern and ancient floodplains are present and underlain by carbonate bedrock of the Beekmantown Group and Conococheague Formation.

GIS analyses were performed to identify suitable areas with a Johnson Permeameter to test the permeability of the soil in three terrace deposits ranked from youngest (level 1) to oldest (level 3) and in a modern alluvial deposits.

Twelve permeability tests were conducted and the Glover solution was used to determine an average ksat value and soil texture which were calculated for each of the four sites. It was determined that T1 was the most permeable and had the highest percentage of sand and the lowest percentage of clay, and abundant cobbles were present in this deposit. The modern alluvial deposit had lower ksat values than T1 and the average permeability decreased on the higher terraces. The soil composition test for all sites showed to be silt loam, making the soil fertile and well-drained. When compared with the permeability rates of agricultural soils, it was determined that these rates ranged from slow to moderately slow in their overall permeability classes.

The results were consistent with the hypothesis that these terrace deposits are somewhat permeable and contribute to the development of sinkholes in the underlying bedrock which increases the rate of chemical weathering and is greatest during the time that deposits are exposed at the T1 and T2 levels. Levels T2 and T3 did decrease in permeability but do not correlate with the soil textures that would further limit permeability such as containing more clay than sand. This suggests that there are other factors at play such as the presence of cobbles and coarse sediments resurrecting permeability at the bottom of borings that warrants further testing in the future.