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Concept: Septic drain field


Onsite wastewater systems, or septic tanks, serve approximately 25% of the United States population; they are therefore a critical component of the total carbon balance for natural water bodies. Septic tanks operate under strictly anaerobic conditions, and fermentation is the dominant process driving carbon transformation. Nitrate, Fe(III), and sulfate reduction may be operating to a limited extent in any given septic tank. Electron acceptor amendments will increase carbon oxidation, but nitrate is toxic and sulfate generates corrosive sulfides, which may damage septic system infrastructure. Fe(III) reducing microorganisms transform all major classes of organic carbon that are dominant in septic wastewater: low molecular weight organic acids, carbohydrate monomers and polymers, and lipids. Fe(III) is not toxic, and the reduction product Fe(II) is minimally disruptive if the starting Fe(III) is added at 50-150mgL(-1). We used (14)C radiolabeled acetate, lactate, propionate, butyrate, glucose, starch, and oleic acid to demonstrate that short and long-term carbon oxidation is increased when different forms of Fe(III) are amended to septic wastewater. The rates of carbon mineralization to (14)CO(2) increased 2-5times (relative to unamended systems) in the presence of Fe(III). The extent of mineralization reached 90% for some carbon compounds when Fe(III) was present, compared to levels of 50-60% in the absence of Fe(III). (14)CH(4) was not generated when Fe(III) was added, demonstrating that this strategy can limit methane emissions from septic systems. Amplified 16S rDNA restriction analysis indicated that unique Fe(III)-reducing microbial communities increased significantly in Fe(III)-amended incubations, with Fe(III)-reducers becoming the dominant microbial community in several incubations. The form of Fe(III) added had a significant impact on the rate and extent of mineralization; ferrihydrite and lepidocrocite were favored as solid phase Fe(III) and chelated Fe(III) (with nitrilotriacetic acid or EDTA) as soluble Fe(III) forms.

Concepts: Redox, Soil, Sewage treatment, Wastewater, Nitrate, Septic tank, Imhoff tank, Septic drain field


The past three decades' data on outbreaks in the United States indicate that homes dependent on untreated groundwater (e.g. wells) for household drinking water that are also reliant on onsite treatment of household wastewater (e.g. septic systems) may be at greater risk for waterborne disease. While groundwater quality monitoring to protect public health has traditionally focused on the detection of fecal indicator bacteria, the application of emerging source tracking strategies may offer a more efficient means to identify pollution sources and effective means of remediation. This study compares the movement of common fecal indicator bacteria (E. coli and enterococci) with a chemical (optical brighteners, OB) and a molecular (Bacteroides HF183) source tracking (ST) target in small scale septic drainfield models in order to evaluate their potential utility in groundwater monitoring. Nine PVC column drainfield models received synchronized doses of primary-treated wastewater twice daily, with influent and effluent monitored bi-weekly over a 7-month period for all targets. Results indicate that E. coli and enterococci concentrations were strongly associated (Spearman’s rank, p<0.05), and correlations between enterococci and optical brighteners were moderately strong. Bacteroides HF183 was significantly, but not strongly, associated with optical brighteners and both indicator bacteria (Point-biserial correlation, p<0.05), most likely due to its sporadic detection. Application of human ST marker monitoring in groundwaters at risk of contamination by human sewage is recommended, although consistent interpretation of results will rely on more detailed evaluation of HF183 incidence in source contamination waters.

Concepts: Water, Spearman's rank correlation coefficient, Water pollution, Sewage treatment, Correlation and dependence, Pearson product-moment correlation coefficient, Wastewater, Septic drain field


Septic systems may contribute micropollutants to shallow groundwater and surface water. We constructed two in situ conventional drainfields (drip dispersal and gravel trench) and an advanced drainfield of septic systems to investigate the fate and transport of micropollutants to shallow groundwater. Unsaturated soil-water and groundwater samples were collected, over 32 sampling events (January 2013 to June 2014), from the drainfields (0.31-1.07 m deep) and piezometers (3.1-3.4 m deep). In addition to soil-water and groundwater, effluent samples collected from the septic tank were also analyzed for 20 selected micropollutants, including wastewater markers, hormones, pharmaceuticals and personal care products (PPCPs), a plasticizer, and their transformation products. The removal efficiencies of micropollutants from septic tank effluent to groundwater were similar among three septic systems and were 51-89% for sucralose and 53->99% for other micropollutants. Even with high removal rates within the drainfields, six PPCPs and sucralose with concentrations ranging from <0.3 to 154 ng/L and 121 to 32,000 ng/L reached shallow groundwater, respectively. The human health risk assessment showed that the risk to human health due to consumption of groundwater is negligible for the micropollutants monitored in the study. A better understanding of ecotoxicological effects of micropollutant mixtures from septic systems to ecosystem and human health is warranted for the long-term sustainability of septic systems.

Concepts: Human, Water, Soil, Sewage treatment, Wastewater, Hygiene, Septic tank, Septic drain field


Onsite wastewater treatment systems, such as septic systems, serve 20% of U.S. households and are common in areas not served by wastewater treatment plants (WWTPs) globally. They can be sources of nutrients and pathogen pollution and have been linked to health effects in communities where they contaminate drinking water. However, few studies have evaluated their ability to remove organic wastewater compounds (OWCs) such as pharmaceuticals, hormones, and detergents. We synthesized results from 20 studies of 45 OWCs in conventional drainfield-based and alternative onsite wastewater treatment systems to characterize concentrations and removal. For comparison, we synthesized 31 studies of these same OWCs in activated sludge WWTPs. OWC concentrations and removal in drainfields varied widely and depended on wastewater sources and compound-specific removal processes, primarily sorption and biotransformation. Compared to drainfields, alternative systems had similar median and higher maximum concentrations, reflecting a wider range of system designs and redox conditions. OWC concentrations and removal in drainfields were generally similar to those in conventional WWTPs. Persistent OWCs in groundwater and surface water can indicate the overall extent of septic system impact, while the presence of well-removed OWCs, such as caffeine and acetaminophen, may indicate discharges of poorly treated wastewater from failing or outdated septic systems.

Concepts: Water, Water pollution, Sewage treatment, Wastewater, Industrial wastewater treatment, Septic tank, Septic drain field, Onsite sewage facility


Up-flow column percolation tests are used at laboratory scale to assess the leaching behavior of hazardous substance from contaminated soils in a specific condition as a function of time. Monitoring the quality of these test results inter or within laboratory is crucial, especially if used for Environment-related legal policy or for routine testing purposes. We tested three different sandy loam type soils (Soils I, II and III) to determine the reproducibility (variability inter laboratory) of test results and to evaluate the difference in the test results within laboratory. Up-flow column percolation tests were performed following the procedure described in the ISO/TS 21268-3. This procedure consists of percolating solution (calcium chloride 1 mM) from bottom to top at a flow rate of 12 mL/h through softly compacted soil contained in a column of 5 cm diameter and 30 ± 5 cm height. Eluate samples were collected at liquid-to-solid ratio of 0.1, 0.2, 0.5, 1, 2, 5 and 10 L/kg and analyzed for quantification of the target elements (Cu, As, Se, Cl, Ca, F, Mg, DOC and B in this research). For Soil I, 17 institutions in Japan joined this validation test. The up-flow column experiments were conducted in duplicate, after 48 h of equilibration time and at a flow rate of 12 mL/h. Column percolation test results from Soils II and III were used to evaluate the difference in test results from the experiments conducted in duplicate in a single laboratory, after 16 h of equilibration time and at a flow rate of 36 mL/h. Overall results showed good reproducibility (expressed in terms of the coefficient of variation, CV, calculated by dividing the standard deviation by the mean), as the CV was lower than 30% in more than 90% of the test results associated with Soil I. Moreover, low variability (expressed in terms of difference between the two test results divided by the mean) was observed in the test results related to Soils II and III, with a variability lower than 30% in more than 88% of the cases for Soil II and in more than 96% of the cases for Soil III. We also discussed the possible factors that affect the reproducibility and variability in the test results from the up-flow column percolation tests. The low variability inter and within laboratory obtained in this research indicates that the ISO/TS 21268-3 can be successfully upgraded to a fully validated ISO standard.

Concepts: Statistics, Soil, Experiment, Arithmetic mean, Mean, Standard deviation, Standard, Septic drain field


Septic systems can be a potential source of phosphorus (P) in groundwater and contribute to eutrophication in aquatic systems. Our objective was to investigate P transport from two conventional septic systems (drip dispersal and gravel trench) to shallow groundwater. Two new in-situ drainfields (6.1 m long by 0.61 m wide) with a 3.72 m2 infiltrative surface were constructed. The drip dispersal drainfield was constructed by placing 30.5 cm commercial sand on top of natural soil and the gravel trench drainfield was constructed by placing 30.5 cm of gravel on top of 30.5 cm commercial sand and natural soil. Suction cup lysimeters were installed in the drainfields (at 30.5, 61, 106.7 cm below infiltrative surface) and piezometers were installed in the groundwater (>300 cm below infiltrative surface) to capture P dynamics from the continuum of unsaturated to saturated zones in the septic systems. Septic tank effluent (STE), soil-water, and groundwater samples were collected for 64 events (May 2012-Dec 2013) at 2 to 3 days (n = 13), weekly (n = 29), biweekly (n = 17), and monthly (n = 5) intervals. One piezometer was installed up-gradient of the drainfields to monitor background groundwater (n = 15). Samples were analyzed for total P (TP), orthophosphate-P (PO4-P), and other-P (TP-PO4-P). The gravel trench drainfield removed significantly (p<0.0001) greater TP (~20%) than the drip dispersal in the first 30.5 cm of the drainfield. However, when STE reached >300 cm in the groundwater, both systems had similar TP reductions of >97%. After 18 months of STE application, there was no significant increase in groundwater TP concentrations in both systems. We conclude that both drainfield designs are effective at reducing P transport to shallow groundwater.

Concepts: Soil, Sewage treatment, Wastewater, Nitrate, Septic tank, Sphagnum, Septic drain field


Column percolation tests may be suitable for prediction of chemical leaching from soil and soil materials. However, compared with batch leaching tests, they are time-consuming. It is therefore important to investigate ways to shorten the tests without affecting the quality of results. In this study, we evaluate the feasibility of decreasing testing time by increasing flow rate and decreasing equilibration time compared to the conditions specified in ISO/TS 21268-3, with equilibration periods of 48h and flow rate of 12mL/h. We tested three equilibration periods (0, 12-16, and 48h) and two flow rates (12 and 36mL/h) on four different soils and compared the inorganic constituent releases. For soils A and D, we observed similar values for all conditions except for the 0h-36mL/h case. For soil B, we observed no appreciable differences between the tested conditions, while for soil C there were no consistent trends probably due to the difference in ongoing oxidation reactions between soil samples. These results suggest that column percolation tests can be shortened from 20 to 30days to 7-9days by decreasing the equilibration time to 12-16h and increasing the flow rate to 36mL/h for inorganic substances.

Concepts: Time, Monotonic function, Future, Soil, Nitrogen, Difference, Probability theory, Septic drain field


Quantitative assessment of nitrogen (N) loading from septic systems is needed to protect groundwater contamination. We determined the mass balance of water and N in the mounded drainfield of a drip-dispersal septic system. Three lysimeters (152.4 cm long, 91.4 cm wide, 91.4 cm high, with 1:1 side slope) were constructed using pressure-treated wood to mimic mounded drainfields. Of total water inputs, septic tank effluent (STE) added 57% water and natural rainfall added 43% water from January 2013 to January 2014. Outputs included leached water (46%) from the lysimeters over 67 sampling events ( = 15 daily and = 52 weekly flow-weighted), potential evapotranspiration (28%), and water stored in the drainfields (26%). Over 13 mo, each drainfield received 227 g of total N (STE, 99%; rainfall, 1%), of which 33% leached, 23% accumulated in the drainfield, and 6% was taken up by grass, with the remainder (38%) estimated to be gaseous N loss. Using these data, the leaching of water from 2.5 million drip-dispersal drainfields in the state of Florida was estimated to be 2.29 × 10 L yr, which would transport 2.4 × 10 kg of total N yr from the drainfields to shallow groundwater. Further reduction of N below drainfields in the soil profile could be expected before STE reaches groundwater. Our results provide quantitative information on the water and N loading and can be used to optimize drainfield conditions to attenuate N and protect groundwater quality.

Concepts: Water, Soil, Water cycle, Sewage treatment, Irrigation, Wastewater, Septic tank, Septic drain field


Septic systems, a common type of onsite wastewater treatment systems, can be an important source of micropollutants in the environment. We investigated the fate and mass balance of 17 micropollutants, including wastewater markers, hormones, pharmaceuticals and personal care products (PPCPs) in the drainfield of a septic system. Drainfields were replicated in lysimeters (1.5m length, 0.9m width, 0.9m height) and managed similar to the field practice. In each lysimeter, a drip line dispersed 9L of septic tank effluent (STE) per day (equivalent to 32.29L/m(2) per day). Fourteen micropollutants in the STE and 12 in the leachate from drainfields were detected over eight months. Concentrations of most micropollutants in the leachate were low (<200ng/L) when compared to STE because >85% of the added micropollutants except for sucralose were attenuated in the drainfield. We discovered that sorption was the key mechanism for retention of carbamazepine and partially for sulfamethoxazole, whereas microbial degradation likely attenuated acetaminophen in the drainfield. This data suggests that sorption and microbial degradation limited transport of micropollutants from the drainfields. However, the leaching of small amounts of micropollutants indicate that septic systems are hot-spots of micropollutants in the environment and a better understanding of micropollutants in septic systems is needed to protect groundwater quality.

Concepts: Water pollution, Soil, Sewage treatment, Wastewater, Septic tank, Septic drain field, Sewerage infrastructure, Onsite sewage facility


Septic systems can be a potential source of phosphorus (P) in shallow groundwater. Our objective was to investigate the fate, mass balance, and transport of P in the drainfield of a drip-dispersal septic system. Drainfields were replicated in lysimeters (152.4 cm long, 91.4 cm wide, and 91.4 cm high). Leachate and effluent samples were collected over 67 events (n = 15 daily; n = 52 weekly flow-weighted) and analyzed for total P (TP), orthophosphate (PO4P), and other P (TP - PO4P). Mean TP was 15 mg L(-1) (84% PO4P; 16% other P) in the effluent and 0.16 mg L(-1) (47% PO4P, 53% other P) in the leachate. After one year, 46.8 g of TP was added with effluent and rainfall to each drainfield, of which, <1% leached, 3.8% was taken up by St. Augustine grass, leaving >95% in the drainfield. Effluent dispersal increased water extractable P (WEP) in the drainfield from <5 to >10 mg kg(-1). Using the P sorption maxima of sand (118 mg kg(-1)) and soil (260 mg kg(-1)), we estimated that ∼18% of the drainfield P sorption capacity was saturated after one year of effluent dispersal. We conclude that despite the low leaching potential of P dispersed with effluent in the first year of drainfield operation, a growing WEP pool in the drainfield and low P sorption capacity of Florida’s sandy soils may have the potential to transport P to shallow groundwater in long-running septic systems.

Concepts: Soil, Sewage treatment, Wastewater, Septic tank, Sphagnum, Septic drain field