We confirm that funding for construction was provided by a commercial source, Hazen and Sawyer Inc; however, this does not alter our adherence to all PLOS ONE policies on sharing data and materials.
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
Septic systems can effectively treat wastewater when correctly sited, operated, and maintained. However, there is increasing evidence of phosphorus (P) transport from septic systems located in areas with sandy soil and high groundwater tables [
In the U.S., the two most common conventional drainfield designs are the drip dispersal and gravel trench systems. The gravel trench system utilizes a gravel layer below the drip line, allowing the sides and bottom of the trench to act as infiltrative surfaces. The drip dispersal system omits the gravel layer and utilizes only the bottom of the bed as the infiltrative surface [
Phosphorus attenuation in septic system drainfields utilizes a combination of biotic and abiotic processes including sorption/precipitation reactions, plant uptake, and mineralization/ immobilization by microbes [
Limited research has been conducted to investigate P dynamics in conventional drainfields in areas with sandy soils and shallow groundwater. The objectives of this study were to (1) investigate the P dynamics in the two conventional drainfield types in areas with shallow water tables and sandy soils, (2) identify which P forms are more mobile in the drainfields, and (3) determine if seasonality (wet or dry) play a role in P transport to shallow groundwater.
The study site was located at the Gulf Coast Research and Education Center of the University of Florida in Wimauma, Florida, USA. At the site, the soil is loamy sand and is classified as a Spodosol, zolfo fine series (sandy siliceous, Hyperthermic Oxyaquic Alorthods). During the study period (May 2012 to Dec 2013), the mean monthly wet season temperature (June–September) was 25°C. Total annual rainfall during 2012–2013 was 115–131 cm, with mean monthly rainfall ranging from 0.3 to 41.9 cm [
Two
The drip dispersal system was constructed by placing 30.5 cm of commercial sand on top of natural soil, whereas the gravel trench system had an additional 30.5 cm of gravel on top of the sand layer.
Data for soil texture, pH, and EC from De and Toor [
Parameters | Natural Soil (n = 5) | Commercial Sand (n = 5) |
---|---|---|
Sand (%) | 86.5±1.4 | 100±0 |
Silt (%) | 8.5±1.4 | 0 |
Clay (%) | 5.0±1.8 | 0 |
Texture | Loamy sand | Commercial sand |
pH | 6.4±0.09 | 7.2±0.13 |
EC (dS m-1) | 0.06±0.01 | 0.06±0.01 |
Water-soluble P (mg kg-1) | 4.99±0.63 | 4.92±0.37 |
Total Ca (mg kg-1) | 993±129.8 | 734.8±58.8 |
Total Mg (mg kg-1) | 122.5±14.5 | 41.8±14.4 |
In both systems, a drip line was placed on top of the drainfield and covered with 15 cm of commercial sand before planting St. Augustine grass (
Each drainfield was instrumented with a total of four suction cup lysimeters (5.1 cm diameter; Soil Moisture Equipment Corporation, Santa Barbara, CA). One suction lysimeter (L2) was located in the center of the drainfield at a depth of 61 cm below infiltrative surface with the remaining lysimeters were placed at the south end of the drainfield at a depth of 30.5 cm (L1), 61 cm (L3) and 106.7 cm (L4) (
A total of 64 sampling events were conducted from May 2012 to Dec 2013. Initially, STE, soil-water, and groundwater were collected every 2 to 3 days (n = 13), then at weekly (n = 29), biweekly (n = 17), and finally at monthly (n = 5) intervals. This sampling regime was used to capture the temporal changes in treatment performance throughout the study period. Background groundwater samples were collected over 15 sampling events from April 2013 to Dec 2013 (n = 12 biweekly, n = 3 monthly) from a piezometer installed up-gradient of the drainfields. Soil-water samples from each suction cup lysimeter were collected after applying 50 kPa of vacuum pressure for 48 hr using a peristaltic pump. Groundwater samples were collected from the piezometers following purging of three equipment volumes following Florida Department of Environmental Protection standard operation procedures [
Unfiltered and filtered samples (STE, soil-water, and groundwater) were analyzed for total P (TP) and PO4–P, respectively, on a Seal AA3 Auto Analyzer (Seal Analytical, Mequon, WI, USA) using US EPA Method 365.1. For TP analysis, unfiltered samples were first digested using persulfate [
Due to similar measurement results in the center and south end of the drainfields, mean data from groundwater monitoring instruments P1–P2 from 5/4/12 to 3/14/13 (events 1–48) are presented along with P3–P5 from 3/28/13 to 12/12/13 (events 49–64) as a combined series of 64 events and represented as P1–P5 (>300 cm below the infiltrative surface). Similar data were observed for lysimeters installed 61 cm in the center and south end of the drainfield, thus, data from L2 and L3 are combined and hereafter presented as L2–L3 (61 cm). Mean, median, and range were calculated in Microsoft Excel 2007.
A two-way ANOVA was conducted in JMP Pro 11 [
Concentrations of TP, PO4–P, and other–P significantly decreased from STE to 30.5 cm depth below the infiltrative surface in both the drip dispersal and gravel trench drainfields (
Parameter (n = 64) | Cl | Total P | PO4–P | Other–P | |||||
---|---|---|---|---|---|---|---|---|---|
–––––––––––––––––––––––––––––––mg L−1––––––––––––––––––––––––––––––– | |||||||||
Septic tank effluent | Mean | 109 | 13.0 | A | 9.8 (75%) |
A | 3.3 (25%) |
A | |
Median | 94.5 | 12.8 | 8.5 | 1.75 | |||||
Range | 67–196 | 5–35 | 3.9–26.0 | 0.2–11.0 | |||||
Background groundwater (n = 15) | Mean | 26 | 0.11 | FG | 0.033 (30%) |
E | 0.077 (70%) |
DEF | |
Median | 24 | 0.08 | 0.01 | 0.06 | |||||
Range | 13–50 | 0.03–0.11 | 0.01–0.06 | 0.10–0.11 | |||||
Drip Dispersal | L1 (30.5 cm) | Mean | 90 | 4.20 | B | 3.60 | B | 0.60 | B |
Range | 22–187 | 0.1–17.0 | 0.02–16.4 | 0.01–4.5 | |||||
L2-L3 (61 cm) | Mean | 88 | 0.40 | D | 0.30 | C | 0.09 | DE | |
Range | 21–187 | 0.2–1.3 | 0.2–1.1 | 0.01–0.3 | |||||
L4 (106.7 cm) | Mean | 85 | 0.14 | FG | 0.07 | D | 0.07 | F | |
Range | 18–172 | 0.03–0.6 | 0.02–0.6 | 0.0–0.2 | |||||
P1-P5 (>300 cm) | Mean | 40 | 0.18 | F | 0.04 | E | 0.14 | CD | |
Range | 14–82 | 0.03–0.6 | 0.004–0.2 | 0.009–0.6 | |||||
Gravel Trench | L1 (30.5 cm) | Mean | 89 | 1.60 | C | 1.30 | B | 0.30 | BC |
Range | 23–180 | 0.2–4.5 | 0.06–3.9 | 0.01–1.8 | |||||
L2-L3 (61 cm) | Mean | 84 | 0.30 | DE | 0.18 | C | 0.12 | DE | |
Range | 30–177 | 0.09–0.8 | 0.07–0.5 | 0.008–0.6 | |||||
L4 (106.7 cm) | Mean | 92 | 0.12 | G | 0.04 | E | 0.08 | EF | |
Range | 34–182 | 0.01–0.7 | 0.005–0.2 | 0.003–0.5 | |||||
P1-P5 (>300 cm) | Mean | 53 | 0.30 | EF | 0.07 | E | 0.23 | CD | |
Range | 13–119 | 0.02–1.5 | 0.002–0.5 | 0.01–1.3 |
a Values in the parentheses are percent of total P
Mean values followed by the same letter in the same column are not significantly different.
ANOVA was performed on the log-transformed data.
Within the drainfields, concentrations of both P forms decreased resulting in <0.14 mg L–1 TP at 106.7 cm below the infiltrative surface, which was equivalent to >98% reduction in TP from the STE. A slightly higher PO4–P in the drip dispersal (0.07 mg L–1) than gravel trench (0.04 mg L–1) at 106.7 cm depth is attributed to the smaller infiltrative surface of drip dispersal as compared to the gravel trench system.
Concentrations of PO4–P continued to decline from 106.7 cm to >300 cm, with no significant differences observed between groundwater beneath either drainfield and background groundwater (
Overall, our data indicates that effective and efficient removal of P can be accomplished by both drainfield designs as after STE passed through both drainfields and reached groundwater, TP, PO4–P, and other–P were reduced by >98%, 99%, and 93–96%, respectively.
The proportion of PO4–P was 75% of TP in the STE, which increased to 81–86% of TP at 30.5 cm and then gradually decreased to <23% of TP at >300 cm below the infiltrative surface in both drainfields (
The proportion of other–P showed an inverse relationship to PO4–P. For example, other–P was 25% of TP in STE, and decreased to 14–19% of TP at 30.5 cm before increasing to 77–78% of TP at >300 cm in both drainfields (
The results of the two-way ANOVA showed a seasonal effect for other–P and no seasonal effect for PO4–P or TP (
Source | Degrees of Freedom | |||
---|---|---|---|---|
Total P | PO4–P | Other–P | ||
Season (wet/dry) | 1 | 0.07 | 0.19 | 0.0062 |
System/Depth | 9 | < .0001 |
< .0001 |
< .0001 |
Season*System/Depth | 9 | 0.03 | 0.11 | 0.8 |
a Indicates F-test used in the hypothesis testing.
* Indicates a significant difference at α = 0.05.
Concentrations of P significantly decreased from STE to 30.5 cm depth in both drainfields, with the gravel trench removing significantly more TP (~20%) than the drip dispersal. The higher TP removal in gravel trench system was likely due to the additional 30.5 cm of gravel layer, which could retain more STE in the gravel pores and provide more side-wall infiltration area than sand layer alone in the drip dispersal system [
Our data showed that TP concentrations at 30.5 cm depth increased over time with higher concentrations particularly during the second wet season. This was most evident in the drip dispersal system, which was likely caused by a decrease in the residence time due to the increased rainfall in the second wet season. It is important to note the gravel trench drainfield showed less of an increase over time at this depth, suggesting that the gravel layer may be storing additional STE within the gravel pores, which aided in buffering the system from the seasonal elevated rainfall. For these reasons, the gravel trench system may be more suitable for attenuating P, particularly in Florida where 60–70% of total rainfall in a year occurs during the four-months wet season of June to September [
Phosphorus concentrations significantly decreased from 30.5 to 61 cm depth, which contributed to additional TP attenuation of 8.5% in the gravel trench and 28% in the drip dispersal, with cumulative TP reductions of 97% in both systems upto 61 cm depth. Additional 2% TP was attentuated from 61 to 106.7 cm depth, with cumulative TP reduction of approximately 98% in the drip dispersal and 99% in the gravel trench drainfields upto 106.7 cm. Overall, the data indicated that the greatest reductions occurred within the first 61 cm of the drip line and P attenuation slowed down with depth. This high attenutation was likely caused by our newly constructed drainfields with favorable biogeochemical conditions and availability of sorption sites in the surface horizons resulting in limited P transport to lower depths and groundwater. In our earlier study [
Previous research has shown that once P enters the groundwater zone, P attenuation ceases and sorption and precipitation reactions only retard P transport [
The two-way ANOVA showed that there was no significant seasonal effect on PO4–P or TP, however, other–P was significantly greater during the dry season as compared to the wet season (
More than 97% of TP attenuation occurred in the first 61 cm of two newly constructed conventional drainfields (drip dispersal and gravel trench). There was a little difference in the overall performance between the two systems, with the exception of gravel trench drainfield that significantly removed 20% more TP within the first 30.5 cm and had significantly lower PO4–P concentrations than the drip dispersal drainfield at 106.7 cm. We observed that other–P concentrations significantly increased in both systems during the dry season and were likely due to the absence of rainfall that diluted other–P during the wet season. The gravel trench drainfield buffered against wet sesaon P increase and did not show overall increasing P trends as was observed in the drip dispersal drainfield. For these reasons, the gravel trench drainfield may be a more reliable design for Florida soils particular in coastal areas where P removal is critical and sandy soils and shallow groundwater limit the unsaturated effluent treatment depth. Overall, the P concentrations in the groundwater below both drainfields systems were not significantly different than background groundwater P concentrations. Thus, we can conclude that after 18 months of STE application, both systems were effective at limiting P transport to groundwater. We suggest that partitioning the contribution of various pollutants from septic systems to groundwater and connected surface waters in coastal areas such as Florida is critical to devise the best drainfield designs that are not only effective at attentuating P, but also other nutrients such as nitrogen and organic contaminants.
Funding for the construction of drainfields was provided by Florida Department of Health via Hazen and Sawyer Inc. We thank Hazen and Sawyer Inc. personnel for their cooperation and support. Funding for the research work was provided by Soil & Water Quality Laboratory located at the Gulf Coast Research and Education of the University of Florida–IFAS.