Gravel Effect on Wastewater Infiltration from Septic System Trenches

Septic systems have been developed that use a chamber rather than gravel in drain line trenches. Gravel is thought to impede infiltration due to a masking effect, a reduction in biomat hydraulic conductivity when the gravel is embedded, or due to fine particles that wash off the gravel and form a low‐conductivity layer, but results from studies on the effect of gravel have varied. Our objective was to determine the effect of gravel masking and embedded gravel on water flow in septic system trenches. We used the finite‐element numerical model HYDRUS‐2D for our analysis. We simulated water movement from the trench bottom into the Bt1 and BC horizons of a Cecil soil. Gravel masking generally had little effect. In the BC horizon where the biomat hydraulic conductivity was one order of magnitude less than that of the soil, embedded gravel produced a chamber‐to‐gravel system infiltration ratio of 1.50. When sidewall flow was included in the BC horizon, the effect of embedded gravel was lessened and the infiltration ratio dropped to 1.33. In the Bt1 horizon where the biomat hydraulic conductivity was four orders of magnitude less than that of the soil, embedded gravel had more of an effect, producing an infiltration ratio of 1.93, close to the manufacturer's claim that chamber systems have twice the infiltration rate of gravel systems. When sidewall flow was included in the Bt1 horizon, the effect of gravel was diminished with an infiltration ratio of 1.70. Gravel had less of an effect than claimed by the manufacturer of the chamber system because lateral gradients pulling water into the areas beneath gravel particles compensated, in part, for the reduced cross‐sectional area available for infiltration in the gravel systems.