Abstract Septic systems are considered a source of groundwater contamination. In the study described in this article, the fate of microbes applied to a sandy loam soil from
Approximately 20% of the U.S. and 50% of the
The seasonal high water table (SHWT), identified using morphological properties with a chroma 2 or less (i.e., redoximorphic features) (Vepraskas, 1992), or by direct monitoring, is considered a restrictive layer.
The fate and transport of E. coli and other bacteria through soil have been studied for decades (Ausland, Stevik, Hanssen, Kohler, & Jenssen, 2002; Bolster &
Mathematical models have been used to describe transport of bacteria through soil (Abu-Ashour, Joy, Lee, Whiteley, & Zehn, 1994; Hijnen, Brouwer-Hanzens, Charles, & Medema, 2005; Hornberger, Mills, & Herman, 1992;McGechan & VintÉn, 2003). In recent years, the HYDRUS model (Simunek,
One of the problems for selecting an appropriate separation distance is related to water flow from the septic system trenches. Water flow in coarse-textured soils is through interparticle pores, while in wellstructured loamy and clayey soils water flow is mainly through macropores (root channels, animal borings, and interped faces) (Amoozegar, Niewoehner, & Lindbo, 2008; Vepraskas, Jongmans, Hoover, & Bouma, 1991). Also, in areas with shallow groundwater CF may play an important role in vertical and horizontal water flow (
The coastal plain region of
Materials and Methods
The soil was collected from a
The soil texture (sandy loam with 64% sand, 30% silt, 6% clay, and 0.3% organic matter) was determined by the hydrometer method (Gee & Or, 2002), and the soil water retention was determined (Dane & Hopmans, 2002) using eight 7.5-cm diameter, 7.5-cm long cores, packed at an average bulk density of 1.49 g/cm3. The pore size distribution was calculated using the average water content at different pressure heads (Figure 2). Approximately 45% of the pores in the columns were smaller than 0.06 mm (60 |xm) and 90% of the pores were smaller than 0.15 mm (150 |xm).
The columns were initially saturated from the bottom to prevent air entrapment in the soil. The columns were then drained and the water levels in them were regulated at 30, 45, and 60 cm below the soil surface by adjusting the Tygon tubing outlets (Figure 1). Five replications were used for each water table treatment and two columns were used as control with water table at 30 cm depth. Once the water table was established at the desired depth, the drainage outlets were fixed, and the columns were left stagnant for three weeks to form anaerobic conditions.
Wastewater Characteristics and Application
E. coli isolated from human urine (
The long-term application rate (LTAR) for sandy loam textured soils in
Seventeen 200-mL aliquots of AWW were autoclaved, and 15 of them were inoculated with three to four colonies of E. coli each day. The inoculated aliquots were then incubated for 24 hours at 37°C. Each 200mL dose of AWW had between 9.4xl04 and 9.7xl06 CFU of bacteria/100 mL, which is typical of actual septic tank effluent (Ausland et al., 2002; Prasad, Rajput, & Chopra, 2006). The spread plate technique was used to enumerate the E. coli concentration in 100-pL aliquots from AWW serial dilutions (10 4 to 10'6) on mFC agar plates. The inoculated plates were incubated in a water bath at 44.5°C for 24 hours before the dark blue colonies were counted. These plates also served as the source of E. coli for subsequent inoculation of AWW.
Once a day for 65 days, 200 mL of inoculated AWW were applied to the top of each of the 15 test columns, and 200 mL of sterilized AWW were applied to each of the two control columns. The sampling port at the designated water table level on the side of the respective column was then opened to allow a 100-mL soil solution sample from top of the water table to drain freely into a dark colored (light-blocking) sample bottle. To maintain a constant depth to water table, another 100mL sample was removed from an outlet at the bottom of the column and discarded. All the applied wastewater infiltrated the soil after application, giving an average rate of 1.1 cm/ day for water flow through the unsaturated zone. The soil water profile above the water table in each column fluctuated between consecutive wastewater applications, but by maintaining a water table at a given depth the average rate of vertical flow and water content fluctuations remained relatively uniform during our study.
The 100-mL samples collected at the water table were enumerated for E. coli within 30 minutes of collection by the Colilert procedure (
The safe drinking water standard for total coliform is zero (
After nine days, the E. coli concentrations at the water table for all five replications for the 30-cm separation treatment were equal to the inflow concentration, indicating a complete breakthrough. The water table for this treatment was then dropped to 60 cm depth by lowering the outflow tubes 5 cm every hour. This treatment simulated a falling water table, although the rate of the water table drop was faster than what occurs under natural conditions. The second part of the experiment was continued to day 65 using the same procedure as previously described.
Statistical analysis was performed with a nonparametric one-way Kruskal-Wallis Test (Hollander & Wolfe, 1973). This test makes no assumptions about the normality of the data. The overall comparisons were made with the NPAR1WAY procedure of SAS (
Results and Discussion
Three days after the wastewater application, the E. coli counts at the water table in four of the replications for the 30-cm water table treatment were above the 200 CFU/100 mL (Figure 3a). Within nine days, microbial counts at the water table for all five replications (2.2 x 106 CFUs/100 mL) were statistically on the same order of magnitude as the AWW inflow concentration of 3.9 x 106 CFU/100 mL (Figure 3a). With virtually no decrease in concentration of E. coli within 30 cm of unsaturated zone, this was deemed a treatment failure. The volume of water held against free drainage in the upper 30 cm of the column calculated using the average soil water characteristic curve (see Figure 2) was approximately 2,100 cm3. Based on this, the cumulative volume of water applied to the columns in nine days was equivalent to 0.85 pore volume, indicating that microbes were transported to the water table with little to no retention. Also, approximately 90% of the pores of this soil were filled with water at 17 cm of tension, indicating that the CF in this soil is approximately 17 cm thick. Since CF is almost saturated, little microbial attenuation may take place, leading to rapid contamination of the top of the saturated zone. Overall, we can say with certainty that 30 cm of unsaturated flow above a water table is insufficient for microbial treatment.
A high degree of variability occurred overall with E. coli concentrations ranging from 101 to 105 on day 65 for the 45 cm water table treatment (Figure 4). Even though E. coli concentrations for three replications did not exceed the standard, the geometric mean of E. coli concentration was close to or exceeded the 200 CFU/100 mL during 65 days of wastewater application. Therefore, 45 cm of unsaturated flow in this soil may not be sufficient for adequate treatment of bacterial contamination. We believe this may be due to the presence of the CF (assumed to be 17 cm thick). Considering the CF, only 28 cm of unsaturated flow was present for this treatment.
Sixty centimeters of separation may be the critical threshold for removal of enteric bacterial contamination during 65 days of wastewater application because the E. coli concentration at the water table never reached the 200 CFU/100 mL limit selected as standard (Figure 5). Three of the five replicates never showed E. coli concentration, one had a maximum of 2 CFU/100 mL, and the last one had a maximum concentration of 34 CFU/100 mL. Excluding the CF, 60 cm of separation is equivalent to approximately 43 cm of unsaturated flow in this soil.
The pairwise comparison confirmed that, at a probability level of .05, the CFU for both 30- and 45-cm water table treatments were greater than 60 cm of separation. No statistically significant difference existed, however, between 30- and 45-cm treatments, even though the numerical average concentration of E. coli for 30-cm treatment was substantially greater than the 45-cm separation distance. At the .075 probability level, all treatment comparisons were statistically significant.
Water Table Fluctuation
As described earlier, the water table at 30 cm depth was lowered to 60 cm to ascertain if microbial contamination survives and travels with the falling water table. As the water table receded, concentrations of E. coli at the water table decreased substantially, indicating that a limited number of live microbes move downward with the falling water table (Figure 3b). After lowering the water table to 60 cm, the average concentration of bacteria on day 10 was
The overall Kruskal-Wallis nonparametric statistical test and the pairwise comparison test indicated that a significant difference existed among water table treatments. Treatment that began with 30 cm of separation and was increased to 60 cm of separation (deeper water table) had higher CFU values than the treatments where water table remained at 60 cm of separation. Treatments with 45 cm of separation had greater values of CFU than both of these treatments after day 10.
The mechanisms for bacteria attenuation in soil include physical filtration, adsorption to soil particles, and die-off (Gannon,
Sixty centimeters of unsaturated flow was most efficient at removing E. coli, while 30 and 45 cm of unsaturated flow were inadequate for treating microbes. Within nine days, the E. coli in wastewater reached the water table at 30 cm below the soil surface. Forty-five centimeters of unsaturated flow decreased the microbial counts reaching the water table, but it was very close or exceeded the 200 CFU/100 mL standard during 65 days of wastewater application. With 60 cm of unsaturated flow, E. coli concentrations were reduced to an acceptable level during the study period. Based on the results for individual replications of the treatments, we estimated that the probability of groundwater contamination to be 100% when the separation distance above a water table is 30 cm or less, greater than 40% when the distance is 45 cm, and less than 10% when the distance is 60 cm or more. In areas with SHWT, pretreatment of wastewater prior to distribution within the drainfield is a viable option for minimizing groundwater contamination by microbes present in septic system effluent (Duncan, Reneau, & Hagedorn, 1994).
The results of our study show that the longer the distance of the unsaturated flow under septic system trenches, the more effective the soil is in removing anaerobic bacteria from wastewater. Dropping the water table depth, which increases the length of unsaturated flow path, increases the efficacy of the soil to treat AWW for bacterial contamination. In our study, however, E. coli concentrations after dropping the water table from 30 to 60 cm were still greater than the concentrations of E. coli detected at the water table in the treatment with 60 cm of continuous unsaturated condition for the duration of the experiment. This may be a result of the previously saturated soil acting as a source of contamination. Also, indigenous aerobic bacteria may have been eliminated by the saturated and anaerobic conditions when the water table was high, making the soil ineffective in removing the incoming bacteria. Overall, it does not appear that a significant amount of microbial contamination travels with the descending water table. M
Acknowledgements: This research was supported in parts by the
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