Short communication Earthworm populations in septic system filter fields and potential effects on wastewater renovation Carrie L. Hawkins a, *, Martin J. Shipitalo b , E. Moye Rutledge a , Mary C. Savin a , Kristofor R. Brye a a University of Arkansas, Department of Crop, Soil, and Environmental Sciences, 115 Plant Sciences Building, Fayetteville, AR 72701, United States b USDA-Agricultural Research Service, North Appalachian Experimental Watershed, P.O. Box 488, Coshocton, OH 43812, United States 1. Introduction In the United States, 21% of homes use onsite wastewater treatment systems (US Census Bureau, 2004). The majority of these systems are soil-treatment systems, which utilize soil to remove pollutants from wastewater (US EPA, 2002). After the wastewater is pre-treated in a septic tank, the effluent is transmitted to gravel-filled trenches where it infiltrates into the soil. The area occupied by the trenches is usually referred to as the filter field. Natural processes, including biologically mediated oxidation, chemical sorption, and physical filtration, remove pollutants as the effluent percolates through the soil (Miller and Wolf, 1976). To ensure adequate renovation, the effluent must contact a great enough volume of soil for a applied soil ecology 40 (2008) 195–200 article info Article history: Received 12 June 2007 Received in revised form 26 February 2008 Accepted 3 March 2008 Keywords: Earthworms Septic systems Wastewater abstract Wastewater renovation in septic-system filter fields can be affected by preferential flow through soil macropores. Anecic earthworm species make deep vertical burrows that may reduce renovation by acting as preferential flow paths that decrease effluent contact with the soil matrix. On the other hand, endogeic earthworms make largely horizontal burrows that may enhance wastewater renovation by distributing the effluent over a larger area. Additionally, the moist, nutrient-rich environment in filter fields may increase earthworm populations by enhancing their survival. Therefore, our objectives were to determine earthworm numbers and biomass with distance from soil treatment trenches, and identify species present to estimate potential effects on wastewater renovation. Five septic systems were investigated. 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. There were an average of 6.4 times more earthworms and 5.4 times more earth- worm biomass within 1 m of the trench than in the background (3.5–7.0 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 groundwater contamination at these sites is likely low. This may not be the case in areas with large numbers of anecic earth- worms. # 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 479 575 5737; fax: +1 479 575 7465. E-mail address: clhawki@uark.edu (C.L. Hawkins). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/apsoil 0929-1393/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2008.03.001