1 HYDROPEDOLOGY OF PROBLEMATIC INTERFLUVE TRANSPORTED SOILS IN THE CENTRAL PIEDMONT OF VIRGINIA Erik D. Severson 1 ABSTRACT Transported soils have been mapped and observed in upland landform positions throughout Virginia’s central and western Piedmont. Over 145,000 ha of transported soils on interfluves have been recognized in just the central Virginia Piedmont. The soils of focus in this study were the Appomattox and the Mattaponi soil series that both contain reticulate mottling patterns and form perched water tables (PWT). These problematic soils occur throughout the mid-Atlantic Piedmont and therefore their associated interpretations are a province-wide phenomenon and not a localized occurrence. They are problematic because the soil morphology is difficult to interpret with respect to their depth of seasonal saturation and septic systems installed into these soils may fail prematurely. Understanding the water table dynamics of these soils would aid in the interpretation process used by onsite septic system evaluators to make sound land use decisions. INTRODUCTION Onsite wastewater disposal systems (OWDS) are commonly referred to as septic systems and are designed to treat and dispose of wastewater. It is estimated that over 1.1 million OWDSs are installed in Virginia alone (Marcellis, 2009). Between 25 and 40 % of Virginians rely on OWDSs (EPA, 2002). Basic components of a typical OWDS are the source of wastewater, the septic tank, the dispersal system, and the soil. Wastewater produced by the home flows into the septic tank. Solids settle to the bottom of the tank and the liquid portion of the waste is termed septic tank effluent (STE). The STE flows into the drainfield and ultimately into the soil. Onsite wastewater disposal systems in the Central Virginia Piedmont which are installed into soils with restrictions may aid to the premature OWDS failure. Failure of OWDS is defined in Virginia in several ways including sewage backup into the household, surface seepage of septic tank effluent (STE) and contamination of groundwater. Failures of OWDS occur for multiple reasons, but often due to their installation into unsuitable soils. Impeded internal drainage may result in a perched water table (PWT) which, at the time of its presence would preclude additional hydraulic load of STE and would cause STE to encroach the surface. Perched Water Tables (PWT) A PWT is defined as a layer of saturated soil separated from the underlying local water table by a layer of unsaturated soil (Soil. Sci. Soc. Am., 2008). Soil saturation is usually defined by the presence of free water which is not under measureable tension (Soil Survey Staff, 1999). The upper level in the ground where water pressure is equal to atmospheric pressure is referred to as the water table (Soil Sci. Soc. Am., 2008). The water table can be envisioned by a below ground surface where the soil is saturated. The water table usually moves down gradient. A PWT formed as a result of a subsurface limiting layer and produces local episaturation conditions similar to those shown by Figure 1-1. 1 Erik D. Severson, Soil Scientist, Virginia Tech, 1502 Williamson Rd. 2 nd Floor, Roanoke VA 24012 [email protected]
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HYDROPEDOLOGY OF PROBLEMATIC … HYDROPEDOLOGY OF PROBLEMATIC INTERFLUVE TRANSPORTED SOILS IN THE CENTRAL PIEDMONT OF VIRGINIA Erik D. Severson1 ABSTRACT Transported soils have been
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HYDROPEDOLOGY OF PROBLEMATIC INTERFLUVE TRANSPORTED SOILS IN
THE CENTRAL PIEDMONT OF VIRGINIA
Erik D. Severson1
ABSTRACT
Transported soils have been mapped and observed in upland landform positions throughout
Virginia’s central and western Piedmont. Over 145,000 ha of transported soils on interfluves have
been recognized in just the central Virginia Piedmont. The soils of focus in this study were the
Appomattox and the Mattaponi soil series that both contain reticulate mottling patterns and form
perched water tables (PWT). These problematic soils occur throughout the mid-Atlantic Piedmont
and therefore their associated interpretations are a province-wide phenomenon and not a localized
occurrence. They are problematic because the soil morphology is difficult to interpret with respect
to their depth of seasonal saturation and septic systems installed into these soils may fail
prematurely. Understanding the water table dynamics of these soils would aid in the interpretation
process used by onsite septic system evaluators to make sound land use decisions.
INTRODUCTION
Onsite wastewater disposal systems (OWDS) are commonly referred to as septic systems and are
designed to treat and dispose of wastewater. It is estimated that over 1.1 million OWDSs are
installed in Virginia alone (Marcellis, 2009). Between 25 and 40 % of Virginians rely on OWDSs
(EPA, 2002). Basic components of a typical OWDS are the source of wastewater, the septic tank,
the dispersal system, and the soil. Wastewater produced by the home flows into the septic tank.
Solids settle to the bottom of the tank and the liquid portion of the waste is termed septic tank
effluent (STE). The STE flows into the drainfield and ultimately into the soil.
Onsite wastewater disposal systems in the Central Virginia Piedmont which are installed
into soils with restrictions may aid to the premature OWDS failure. Failure of OWDS is defined
in Virginia in several ways including sewage backup into the household, surface seepage of septic
tank effluent (STE) and contamination of groundwater. Failures of OWDS occur for multiple
reasons, but often due to their installation into unsuitable soils. Impeded internal drainage may
result in a perched water table (PWT) which, at the time of its presence would preclude additional
hydraulic load of STE and would cause STE to encroach the surface.
Perched Water Tables (PWT)
A PWT is defined as a layer of saturated soil separated from the underlying local water table by a
layer of unsaturated soil (Soil. Sci. Soc. Am., 2008). Soil saturation is usually defined by the
presence of free water which is not under measureable tension (Soil Survey Staff, 1999). The
upper level in the ground where water pressure is equal to atmospheric pressure is referred to as
the water table (Soil Sci. Soc. Am., 2008). The water table can be envisioned by a below ground
surface where the soil is saturated. The water table usually moves down gradient. A PWT formed
as a result of a subsurface limiting layer and produces local episaturation conditions similar to
those shown by Figure 1-1.
1 Erik D. Severson, Soil Scientist, Virginia Tech, 1502 Williamson Rd. 2nd Floor, Roanoke VA 24012 [email protected]
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Figure 1-1. Example of a red problematic Piedmont soil with perched water above a hydraulically
limiting layer at 75 to 105cm. Photo by: Jarred Hubbard
Perched zones of soil saturation are ordinarily only observed seasonally, developing in the cooler
autumn months and persisting through the winter and into the spring. This is in response to
decreased evapotranspiration rates when air temperatures are low and plant growth and associated
water uptake slows or ceases. These events may be important in the transport of dissolved or
suspended chemicals. Soils with PWTs usually have moderately permeable horizons overlying
layers with slow or very slow permeability. Permeability is the relative ease to which gasses,
liquids, and plant roots pass through a layer of soil. The flow of downward percolating water is
retarded by the restrictive horizon, thus causing water to mound at the restriction interface with
the overlying permeable materials (Daniels and Fritton, 1994), and the water is then forced to move
laterally (Reuter, 1998; Shaw et al., 2001). The rate of lateral flow is primarily controlled by
gradient of the land surface, the topography of the restrictive layer (Jenkinson and Franzmeier,
1996), and the permeability of the materials above the restricting layer. Lateral flow results in a
range of potential fates of STE on the landscape (Figure 1-2). Soils without a restrictive horizon
allow for the unimpeded downward percolation of liquids from the OWDS trench bottoms into the
regional groundwater table.
Seasonal High Water Table Determinations
Determination of the depth to a seasonal high water table (SHWT) is a regulatory requirement of
an on-site septic system soil evaluation in Virginia according to the 2012 Sewage Handling and
Disposal Regulations (SDHR, 2012). The minimum required vertical distance from the bottom of
the drainfield trenches to a soil limitation such as an impermeable layer, bedrock, or groundwater
is 46 cm (SDHR, 2012). The SHDR define the “seasonal water table” as that portion of the soil
profile where a color change has occurred in the soil as a result of saturated soil conditions.
Determining the characteristics which indicate soil saturation is difficult to ascertain in the
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problematic transported soils in the central Piedmont. Figure 1-2 shows the aerobic soil
underneath drainfield trenches needed to renovate wastewater before it reaches groundwater