Progress 07/01/07 to 12/31/10
Outputs
OUTPUTS: Soils in the Piedmont of South Carolina were evaluated for their potential loading rates as components of on-site wastewater disposal systems. All experiments were conducted within Cecil sandy loams (Fine, kaolinitic, thermic Typic Kanhapludults) or similar soils. The work within this project amassed the following information that will be useful to personnel in agencies who determine loading rates: 1) Saturated soil hydraulic conductivities can be reduced by two orders of magnitude along trench walls when a backhoe is used to dig the trench and the soil water content is at or near field capacity. The mean Ksat in the Bt1 horizon (clay) of the Cecil soils we used was about 0.3 cm/hr. During excavation the Ksat of soil along the trench was commonly in the 0.003 cm/hr range. As soil water content of the trench face decreased (from approximately 0.4 to 0.20 cm3 cm-3), the Ksat increased. However, our K sat values with the Bt horizon of the excavated trench were always lower than those of undisturbed soil. We excavated trenches, refilled them, and left them for a year before we dug out the trench and took samples to measure Ksat in the smeared sidewall. We found no significant change in Ksat measurements. We scoured the smeared sidewall with rakes and hoes to reduce the effect of the backhoe, but our results were not consistent; we concluded that raking gives minimum benefits. Slowly picking away smeared and compacted soil with a pocket knife resulted in sidewall Ksat values that were the same as undisturbed soil; however, such effort would not be practical. 2) While Bt horizons of the Cecil soil are usually described as having moderate subangular block structure, great ranges in structure were found in the soils we studied. Within our experimental sites we found soils with strong, moderate, and weak subangular blocky as well as sites where we deemed great masses of the trench wall to consist of nearly structureless soil. It was not uncommon to find platy structure in some zone of the trench. Our results demonstrated the well-know relationship: as soil structure improves (become stronger), the saturated hydraulic of the soil increases. In undisturbed soil cores of similar texture, the Ksat often varied by more than an order of magnitude between strong and very weak or non-existent structure. Cores taken in platy material often had Ksats of zero. The data clearly show that soil structure should be a soil characteristics used to determine loading rates. 3) Interflow of common on many sloping sites in the Piedmont. This interflow can be from upslope when water (potentially damaging amounts) flows into the drainfield, or it can be downslope from the drainfield when effluent is transported out of the area. Curtain drains are commonly recommend at the site, but are often not installed because of expense. Our data suggest that a curtain drain should be installed when the Ksat of the Bt horizon is one-third or less of that of the Ap horizon, the Ap horizon is at least 15 cm thick, and the slope is 4% or greater. PARTICIPANTS: This work was conducted by Virgil Quisenberry, soil scientist, and Philip Brown, graduate student in soil physics. Collaborations with personnel in the South Carolina Department of Health and Environmental Control and the National Resources Conservation Service. TARGET AUDIENCES: The target audience was personnel in state agencies and private consultants who designed on-site waste water disposal systems. Data generated in this project will be of help to those who do design work in the South Piedmont area of SC. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Proper sizing of septic drainfields is essential if on-site wastewater disposal systems are to function properly. Proper sizing demands that soils be correctly evaluated. Relating drainfield size and soil characteristics has been even more critical as trench lengths have been decreased when alternative drainline products (non-gravel systems) are used. The soil physical measured in this project can be combined with other data (historical and current) and used by personnel who design drainfields for the thousands of septic tanks that are installed each year.
Publications
- No publications reported this period
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Progress 01/01/09 to 12/31/09
Outputs
OUTPUTS: In many counties in the Piedmont of South Carolina septic drain lines are sized based on a loading rate of 0.3 gal/ft2/day (1.22 mL/cm2/day) for a Cecil-type soil (fine, kaolinitic, thermic Typic Kanhapludults). If the Bt horizon has a clay content greater than a typical Cecil, the loading rate is often lowered. If the clay content is less than the typical Cecil, the loading rate is often increased. Those who evaluate and rate a site often speak of using structure in combination with texture in determining loading rates, but structure evaluations from auger boring are difficult, if not impossible. The assigned loading rates of most areas are deemed to be acceptable as failure rates are about 10% or less. It is not known what percentage of the failures can be attributed to the assigned loading rate as so many factors can lead to non-functioning systems. If one assumes that the correct loading rate, or long termed acceptance rate (LTAR) of a soil is 10 times the saturated hydraulic conductivity, then a LTAR of 1.22 mL/cm2/day would mean that Ksat should be about 5 cm/day. We selected five areas of Cecil soils. Initial inspections of the sites indicated a significant range in clay contents and possibly structure within the Bt horizons of the five sites. At each location a 3-m by 3-m area was flooded until the soil was saturated throughout the Bt horizon as indicated by tensiometers. Ksats were calculated based on a constant flux density and measured gradients. After the in situ measurements were made, the soil was allowed to drain and dry for several weeks. An Amoozemeter later was used to measure Ksat at 3 depths within the Bt at 6 locations within the same area. Finally, 10 undisturbed soil cores (7.5 cm diameter, 7.5 cm height) were taken at each of the same three depths, and Ksat was measured in the laboratory by the constant head method. The results are in agreement with previous findings: Ksat increases as the sample volume increases. When Ksat was measured within the 3-m by 3-m areas, the results for each of the three depths were always greater than 5 cm/day. Thirteen of the 18 (72%) measurements by the Amoozemeter were greater than 5 cm/day. Three of the measurements indicated a Ksat very nearly zero. Of the 30 soil cores, 19 (63%) had Ksat greater than 5 cm/day. Five of the cores had Ksat values very nearly zero. PARTICIPANTS: For this reporting period, the investigator was the only one who was involved in the project. TARGET AUDIENCES: The target audience is those employed by the state to do on-site wastewater loading rate determinations. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The results from this project will help to improve our ability to properly size septic drainline systems for homeowners. A survey conducted in a nearby state four year ago suggested that accurate assessment of soil properties can lower failure rates to less than 5%. These data and others we will collected can help us recognize those soils conditions that lead to system failure.
Publications
- No publications reported this period
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Progress 01/01/08 to 12/31/08
Outputs
OUTPUTS: Saturated hydraulic conductivities were measured every 10-cm along 10-meter transects at two depths in two soils in the Piedmont of South Carolina. The soils were Cecil sandy loam (Fine kaolinitic, thermic Typic Kanhapludults) and Pacolet sandy loam (Fine, kaolinitic, thermic Typic Kanhapludults). The soils often occur on the same landscapes with thickness of the argillic horizon as the primary difference between the two. Both are commonly used for on-site waterwater systems. Transect locations were established by extensive borings to determine that soils within the 10-meter transects were consistent with the series of interest. For each transect the upper sampling depth was established within the Bt1 horizon (50 cm for Cecil and 45 cm for Pacolet) and the lower depth was established in the BC horizon (110 cm for Cecil and 90 cm for Pacolet). Along the transects soil structure was evaluate and assigned a number from 5 to 1 with 5 being moderate block and 1 being massive. Undisturbed cores (331 cm3) were taken every 10 cm along each transect to determine saturated hydraulic conductivity and bulk density. At each core location a structural value (1-5) was assigned. The textures for the two soils were nearly identical (clay in the Bt1 and clay loam in the BC). Saturated hydraulic conductivities were greater in the Bt1 than in the BC of each soil. Bulk densities were greater in the BC horizon of each soil. Variations in saturated hydraulic conductivity along the transects were much more related to strength of structure than to clay content or bulk density. These data suggest that soil structure needs to be considered in designing loading rates for on-site wastewater areas. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: The audience for this work is South Carolina Department of Health and Environmenal Control (DHEC) personnel and soil scientists who regularly make site evaluations for on-site waterwater systems. The data suggest that auger borings may be insufficient to make accurate site appraisals because of the need to evaluate soil structure in the Bt and BC horizons. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
More data need to be collected before we suggest that South Carolina require soil evaluation to be made in pits and not from auger borings.
Publications
- No publications reported this period
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Progress 01/01/07 to 12/31/07
Outputs
OUTPUTS: The soil water content at the time septic drainline trenches are dug greatly affects the degree of smearing along the trench wall. Smearing results in increased soil bulk density and decreased hydraulic conductivity. Measurements within a Bt horizon of a Cecil soil (fine, kaolinitic, thermic Typic Kanhapludults) with a clay texture showed that opening a trench with a backhoe when the soil water content was about field capacity increased the bulk density within portions of the outer 1 cm of the wall from 1.35 g cm-3 to 1.55 g cm-3. The saturated hydraulic conductivity decreased by three to four orders of magnitude within the compacted zone.
TARGET AUDIENCES: The direct audience for our findings are the various state agencies that permit septic systems. The indirect audience are homeowners who rely on septic systems.
Impacts
Increasing our understanding of the hydraulics of septic drainline trenches can lead to the development of improved protocols for installation of septic systems. These improvements can decrease environmental concerns while increasing systems performance.
Publications
- No publications reported this period
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