5 Septic System Plume Formation and Dispersion

When the infiltrating wastewater encounters the water table, it enters the local groundwater flow system, forming a plume that is distinct from the background groundwater. Examples of two well-characterized plumes from septic systems in unconfined sand aquifers of Ontario, Canada are shown in Figures 2 and 3. Figure 2b and Figure 3 are repeated here for the readers convenience.

Repeat of Figure 2b and Figure 3 – Repeated for the reader’s convenience. Figure 2b shows a household septic system plume in Cambridge, Ontario, Canada, and was adapted from Robertson and others (1991). Figure 3 shows a campground septic system plume in Long Point, Ontario, Canada, and was adapted from Robertson and others (2013).

Figure 2 shows a plume from a household septic system that was mapped using Na⁺ as the plume tracer. Plume boundaries remain sharp throughout the 200 m length of the plume that was mapped, and Na⁺ concentrations remain essentially undiluted in the core of the plume. Model sensitivity analyses as shown in Figure 4, indicate that the persistence of sharp plume boundaries, with an absence of core zone dilution, requires low values for both horizontal and vertical transverse dispersivity (< 1 cm). The modeled values are consistent with low dispersivity values determined from natural-gradient tracer tests in other sand aquifers (Sudicky et al., 1986; Moltyaner and Killey, 1988; Garabedian et al., 1991).

Figure 4 – Cambridge septic system plume showing model calibrations for: a) horizontal transverse dispersivity and b) vertical transverse dispersivity. Dots indicate Na⁺ concentrations measured: a) in a monitoring fence located 95 m from the drainfield and aligned transverse to the direction of groundwater flow and b) in a multilevel sampling bundle located 30 m from the drainfield. Dashed and solid lines indicate Na⁺ concentrations predicted using a three-dimensional analytical advection-dispersion model with differing values for horizontal (α_th) and vertical (α_tv) transverse dispersivity (from Robertson et al., 1991).

The second plume example as was shown in Figure 3 is from a septic system at a large campground (Long Point site) and shows a groundwater plume that again, has sharp boundaries, and an absence of core dilution throughout the 200 m mapped length of the plume. In this case, the plume was traced using an artificial sweetener (acesulfame) that is currently used in a wide range of calorie-reduced food and beverage products. Concentrations throughout the plume core are > 18 µg/L, which is about 10,000 times higher than the background detection limit value at this site (< 0.01 µg/L). Such an unusually high contrast between background and effluent concentrations allows very small inputs from septic system wastewater to be detected as discussed further next.

An important factor to consider when monitoring septic system behavior is the thickness of the septic system plume. Plume thickness is governed by the wastewater loading rate, drainfield size and the ambient average linear groundwater velocity. It can be estimated using the relationships shown in Equation 11 and Equation 12.

where:

For example, a typical household septic system receiving 1 m³/day of wastewater discharged to a drainfield that is 10 m by 10 m in plan view, would have a wastewater loading rate of 0.01 m/day. For an average linear groundwater velocity of 0.2 m/day, groundwater would reside under the drainfield for 50 days, because a drop of water moving at 0.2 m/day would require 50 days to traverse the 10 m length of the drainfield. For an aquifer porosity of 0.33, the plume would be 1.5 m thick (ignoring dispersion). This requires careful vertical placement of sampling locations for effective monitoring. Both the Cambridge and Long Point sites shown in Figures 2 and 3, are located close to groundwater flow divides, where groundwater velocities are relatively low (~0.07 to 0.08 m/day, Robertson et al., 2019). These low velocities, combined with a higher wastewater loading rate at Long Point (0.06 m/day, Robertson et al., 2019), have resulted in the formation of plumes that are relatively thick (~3 to 5 m), compared to plumes at many other sites.

The low capacity for dispersion in the Cambridge and Long Point aquifers, as well as in many other aquifers (e.g., Weiskel and Howes, 1991), allows relatively high concentrations of wastewater constituents to persist in the plume core zones. Thus, unless attenuation reactions occur, relatively long distances of travel may be required before contaminants are reduced to acceptable levels by dilution with background groundwater.