Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2008
Publication Date: September 1, 2008
Citation: Hawkins, C.L., Shipitalo, M.J., Rutledge, E.M., Savin, M.C., Brye, K.R. 2008. Earthworm populations in septic system filter fields and potential effects on wastewater renovation. Applied Soil Ecology. 40(1):195-200.
Interpretive Summary: Most homes in rural areas use septic systems to process domestic waste water. A system of pipes and gravel-filled trenches referred to as the filter field is an important component of a septic system that disperses the waste water and allows it to infiltrate into the soil. If infiltration is too fast the waste water can move to ground water before it is adequately renovated by natural processes in the soil. If infiltration is too slow the effluent can overflow onto the soil surface. High infiltration rates in the burrows of deep burrowing species of earthworms may decrease the effectiveness of filter fields while the mainly horizontal burrows of shallow burrowing species may improve renovation of waste water by spreading it out over a larger area. In order to assess the potential effects of earthworms on septic system filter fields, we measured the numbers and types of earthworms close to trenches and compared these to nearby areas not affected by effluent addition. In the five systems investigated we found an average of six times more earthworms near the trenches than in the background areas. Thus, the supply of organic wastes and nutrients in the effluent combined with moist soil conditions near the trenches was favorable to earthworm survival. Only shallow burrow earthworms were noted at these five sites, so they should have a positive effect on septic system performance. If increased numbers of deep burrowing earthworms are found near the trenches in other areas, septic system designers and installers may need to devise techniques to discourage their activity.
Wastewater renovation in septic-system filter fields can be affected by preferential flow through soil macropores. Earthworm burrows may contribute to this concern by penetrating the infiltrative surface of soil-treatment trenches. Additionally, the moist, nutrient-rich environment surrounding trenches may increase earthworm populations by enhancing earthworm survival. Potential effects of earthworms in filter fields will likely depend on the species present. Endogeic earthworms make largely horizontal burrows that may enhance wastewater renovation by distributing the effluent over a larger area. In contrast, anecic species make deep vertical burrows that may reduce renovation by acting as pathways for preferential flow that decrease effluent contact with the soil matrix. Our objectives were to determine relative earthworm numbers and biomass with distance from soil treatment trenches, and identify species present to estimate potential effects of earthworms on wastewater renovation. Five septic systems were selected for investigation. At each site earthworm populations were measured using formalin extraction at 10 locations along each of three 7-m long transects perpendicular to the trenches. Results indicated that earthworms were attracted to septic system trenches, with an average of 6.4 times more earthworms and 5.4 times more earthworm biomass within 1 m of the trench than in the background (3.5 to 7.0 m from the trenches) in 13 of the 15 transects. This suggests that earthworms may have a significant effect on the movement of effluent. Because only epigeic and endogeic species were observed, the potential for reduced renovation and ground water contamination at these sites is likely low. This may not be the case in areas with large numbers of anecic earthworms.