Frontiers in assessing septic systems vulnerability in coastal Georgia, USA: Modeling approach and management implications

Threats to public health and environmental quality from septic systems are more prevalent in areas with poorly draining soils, high water tables, or frequent flooding. Significant research gaps exist in assessing these systems’ vulnerability and evaluating factors associated with higher rates of septic systems replacement and repair. We developed a novel GIS-based framework for assessing septic system vulnerability using a database of known septic system specifications and a modified Soil Topographic Index (STI) that incorporates seasonal high groundwater elevation to assess risks posed to septic systems in coastal Georgia. We tested the hypothesis that both the modified STI and septic system specifications such as tank capacity per bedroom and drainfield type would explain most of the variance in septic system repair and replacement using classification inference tree and generalized logistic regression models. Our modeling results indicate that drainfield type (level vs. mounded) is the most significant variable (p-value < 0.001) in predicting septic systems functionality followed by septic tank capacity per bedroom (p-value < 0.01). These show the importance of septic system design regulations such as a minimum requirement for horizontal separation distance between the bottom of trenches and seasonal water table, and adequate tank capacity design. However, for septic systems with a mounded drainfield and a larger tank capacity per bedroom, the modified STI representing hydrological characteristics of septic system location is a significant predictor of a high septic system repair and replacement rate. The methodology developed in this study can have important implications for managing existing septic systems and planning for future development in coastal areas, especially in an environment of rapid climatic change.

Bio-Tower Application for Improvement of a Decentralized Waste Water Treatment System for Residential Applications–Reduction of Nonpoint Source Pollution by Nitrogen

The application of decentralized wastewater treatment system, also known as septic system is very common in suburban and rural areas with no access to centralized sewage treatment plants. Minimizing water pollution and the effects on wildlife and humans is of specific concern in rural and urban areas. A packed bio-tower addition to a 1000 gallon septic tank was tested under pilot conditions using municipal residential sewage. The septic tank packed bio-tower pilot system is able to reduce the NH3-N influent level of 16.5 mg/l to 24.0 mg/l by 77.3% to 96.7% at influent flow levels between1060 l/d (280 gal/d) and 3997 l/d (1056 gal/d). Biochemical oxygen demand levels reduction was 97.0% from 280 mg/l to 8.5 mg/l. for a flow rate of 1060 l/d (280 gal/d). Research showed that a bio-tower addition to a septic system has the potential to improve the systems overall performance.

Estimation of Saprolite Thickness Needed to Remove E. coli from Wastewater

Saprolite, weathered bedrock, is being used to dispose of domestic sewage through septic system drainfields, but the thickness of saprolite needed to remove biological contaminants is unknown for most saprolites. This study developed and tested a simple method for estimating the thickness of saprolite needed below septic drainlines to filter E. coli from wastewater using estimates of the volume of pores that are smaller than the length of the coliform (≤10 μm). Particle size distribution (texture) and water retention data were obtained for 12 different saprolites from the Piedmont and Mountain regions of North Carolina (N.C.). Saprolite textures ranged from clay loam to coarse sand. The volume of pores with diameters ≤10 μm were determined by water retention measurements for each saprolite. The data were used in an equation to estimate the saprolite thickness needed to filter E. coli. The estimated saprolite thicknesses ranged from 36 cm in the clay loam to 113 cm for the coarse sand. The average thickness across all samples was 58 cm. Saprolite thickness estimates increased as silt percentage decreased and as sand percentage and in situ saturated hydraulic conductivity increased. Silt percentage may be most useful for estimating appropriate saprolite thicknesses in the field.

Efficiency of saprolite for removing E. coli from simulated wastewater

Saprolite, weathered bedrock, is being used to dispose of domestic sewage through septic system drainfields, but its ability to remove coliforms is unknown. This study determined if Escherichia coli could be removed by a sandy loam saprolite material. Triplicate columns containing saprolite were prepared with lengths of 30, 45, and 60 cm. A 215-mL solution containing 1 × 105 CFU/100 mL of non-toxic E. coli was applied to the top of each column for 5 days/week for 13 weeks, and selected outflow samples were analyzed for E. coli. Control columns had only tap water applied to them at the same time. Significantly higher (p ≤ 0.10 compared to controls) E. coli concentrations were only detected in samples collected at the end of week 3 for the 30-cm columns and week 4 for the 45-cm columns. E. coli concentrations were small and ranged from approximately 2 to 3 MPN/100 mL. No E. coli were detected in any outflow from the 60-cm columns. From weeks 5 to 13, E. coli concentrations from all columns were either undetectable or not significantly different from the control. The results showed that 60 cm of sandy loam saprolite was sufficient for the removal of E. coli from simulated wastewater.

Wastewater and Septic System Management for the COVID-19 Virus: Frequently Asked Questions

This new 3-page publication of the UF/IFAS Department of Soil and Water Sciences answers common questions about the potential role of wastewater and septic systems in transmission of COVID-19. It is intended as guidance for the general public. Written by Mary G. Lusk. https://edis.ifas.ufl.edu/ss692

Fecal Indicator Bacteria Transport from Watersheds with Differing Wastewater Technologies and Septic System Densities

Wastewater contains elevated concentrations of fecal indicator bacteria (FIB). The type of wastewater treatment technology and septic system density may influence the FIB concentration and exports at the watershed scale. The goal of this study was to gain a better understanding of FIB concentrations and exports from watersheds served by conventional septic (CS) systems, sand filter (SF) septic systems, and a municipal sewer (SEW) system. Seven watersheds (3 CS, 3 SF, and 1 SEW) were monitored to quantify FIB concentration and export monthly from April 2015 to March 2016. The type of wastewater treatment did not yield significant differences in FIB concentration or exports when pooling watersheds using similar wastewater treatment. Watersheds with the highest septic densities (approximately 0.4 systems ha−1) contained greater FIB concentrations and exports than watersheds with the lowest (approximately 0.1–0.2 systems ha−1), but only FIB concentrations significantly differed. These findings suggest that when the septic system density exceeds 0.4 systems ha−1, water quality degradation from septic leachate may be observable at the watershed scale, especially in watersheds dominated by residential development. More research is recommended to determine if this density threshold is similar for other water pollutants and/or in watersheds with differing hydrogeological, land use, and wastewater characteristics.

Landscaping on or near Septic Drain Fields

Septic systems are common throughout most rural areas, and their care and maintenance are essential to the health of people, wildlife, livestock, agricultural commodities, and water resources. One way to ensure optimal performance of your septic system is to landscape appropriately near the drain field. The purpose of this new 3-page publication of the UF/IFAS Department of Soil and Water Sciences is to provide landscape management guidance for septic system drain fields. Information presented here will be useful for homeowners, landscape management professionals, and Extension agents who work in horticulture, natural resources, agriculture, and family services. Written by Whitney C. Elmore, William Lester, James Moll, Andrea Albertin, and Mary Lusk.https://edis.ifas.ufl.edu/ss687