Hydrologic parameters describe the way water moves through soil and are critical components in siting on-site systems.
Saturated Hydraulic Conductivity
Saturated hydraulic conductivity is the rate at which water moves through a saturated soil. In a saturated soil, all of the pore voids are filled with and transmitting water. The rate of flow through a soil is dependent on the sizes, number, and interconnectedness of the voids in the soil. The number of voids, their size, and spacing depend on numerous soil characteristics such as texture, mineral content, structure, biological activity, and horizon placement. Saturated hydraulic. conductivity will vary between soil horizons within the same profile and from location to location.
Saturated hydraulic conductivity is important in on-site systems. This parameter can help the designer determine the volume of wastewater that the site can transmit in a given time when the soil is saturated. On-site treatment and disposal fields are not supposed to become saturated, but under some conditions, such as during wet winter months, the ground may become saturated. Thus, the saturated hydraulic conductivity can determine how a system will perform under worst-case conditions.
The measure of water movement through a saturated soil has the units of distance/time and is given the symbol Ksat.
Each soil horizon has its own individual Ksat, which normally varies greatly from the Ksat of other horizons in the same profile.
Based on measurements of saturated hydraulic conductivity, soils are grouped into six classes, from very low to very high hydraulic conductivity. See Table 4.4.11 for a complete listing of saturated hydraulic conductivity classes.
If saturated hydraulic conductivity measurements are not available, saturated hydraulic conductivity can be estimated from certain soil properties, as shown in Table 4.4.11.
Saturated hydraulic conductivity is not typically used in North Carolina as an evaluation tool for determining wastewater flow through a soil for on-site systems with flows less than 1,500 gallons per day. To accurately measure Ksat, each horizon in a profile must be tested, four or five measurements for each horizon must be taken, and the test must be done seasonally (winter and summer). This requires more time, money, and effort than most people want to expend. Additionally, Ksat tests are conducted using clean water. Wastewater, with its particulate matter, causes water to move more slowly through a soil and thus Ksat measurements for wastewater differ. Because of these difficulties determining saturated flow, North Carolina does not use Ksat to evaluate a site's potential for an on-site treatment and disposal system; a site evaluation is performed instead. (For more information see Section 4.5).
Unsaturated Hydraulic Conductivity
Water can move through a soil even if the soil is not saturated. The unsaturated hydraulic conductivity is a measurement of the rate at which water moves in unsaturated soils.
Unsaturated hydraulic conductivity can help the designer determine the volume of wastewater that the site can transmit in a specific time when the soil is unsaturated. On-site treatment and disposal fields are supposed to have an unsaturated zone under the trenches to allow the wastewater to be treated aerobically.
Under unsaturated soil conditions, voids are not completely filled with water. Unsaturated hydraulic conductivity depends on the water content of the soils as well as soil characteristics such as texture, mineral content, structure, biological activity, and horizon placement. At the same moisture status, unsaturated hydraulic conductivity, like saturated hydraulic conductivity, will vary between soil horizons within the same profile and from location to location.
In an unsaturated soil, the driving force of water movement is a gradient potential that is caused by suction. Matric suction is the affinity of water molecules to other water molecules or to capillary voids. Water moves from higher to lower matric suction potentials. In other words, water moves from voids that are full to voids that are unfilled. The geometric shape and size of the voids can affect the matric potential. For example, the large voids in sandy soils empty quickly. Once the soil is desaturated, water is trapped in capillary wedges that do not contact other capillaries and water movement stops.
Once the biomat has formed in the treatment and disposal trenches, it often becomes the limiting layer or controlling layer for water flow. In those cases, the biomat is the least conductive layer to water flow. If there is no restrictive soil layer beneath the trenches, then wastewater moves through the soil in unsaturated flow after it passes through the biomat. Since unsaturated flow is slower than saturated flow, the wastewater flows through the soil at a slower flow rate. The slower flow rates give the wastewater more contact time in the soil profile and more time to be cleansed by biochemical and chemical processes before it becomes part of the ground water. Be aware, however, that there may be soil layers under the trench that have such slow permeability that these soil layers, rather than the biomat, control the rate of wastewater flow through the soil.
Recall that any site rated as a SUIT ABLE or PROVISIONALLY SUIT ABLE site must have at least 12 to 18 inches of separation between the trench bottom and soil wetness conditions or other restrictive horizons.
Two specific soil characteristics that affect unsaturated hydraulic conductivity are soil texture and depth.
As soils dry from saturated conditions to lower moisture levels, the hydraulic conductivity decreases. This decrease of hydraulic conductivity varies with soil texture. In sandy soils, hydraulic conductivity decreases very quickly as the moisture content drops; in clay soils the decrease is much slower. This decline in hydraulic conductivity is shown in Figure 4.4.23.
From the North Carolina Onsite Guidance Manual