Septic Systems Contribution to Phosphorus in Shallow Groundwater: Field-Scale Studies Using Conventional Drainfield Designs.

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 (p300 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

Concentrations and Loads of Dissolved and Particulate Organic Carbon in Urban Stormwater Runoff

Urban landscapes are significant contributors of organic carbon (OC) in receiving waters, where elevated levels of OC limit the light availability, increase the transport of pollutants, and result in high costs of potable water treatment. Our objective in this study was to investigate the concentrations, fractions (dissolved and particulate), and loads of OC in a residential catchment (3.89 ha drainage area) located in Florida, United States. The outlet of the stormwater pipe draining the residential catchment was instrumented with an automated sampler, a flowmeter, and a rain gauge. The rainfall and runoff samples collected over 25 storm events during the 2016 wet season (June to September) were analyzed for dissolved organic carbon (DOC) and total organic carbon (TOC), with particulate OC (POC) calculated as the difference between TOC and DOC. Mean concentration of DOC was 2.3 ± 1.7 mg L−1 and POC was 0.3 ± 0.3 mg L−1 in the rainfall, whereas DOC was 10.5 ± 6.20 mg L−1 and POC was 2.00 ± 4.05 mg L−1 in the stormwater runoff. Concentrations of DOC were higher during the rising limb of the hydrograph in 15 out of 25 storm events, suggesting flushing of DOC, with an increase in the amount of runoff, from the landscape sources in the residential catchment. The estimated total export of OC during the 2016 wet season was 66.0 kg ha−1, of which DOC was 56.9 kg ha−1 (86.2% of TOC), and POC was 9.1 kg ha−1 (13.8% of TOC). High concentrations and loads of OC, especially DOC, in the stormwater runoff imply that residential catchments in urban watersheds are hot-spots of DOC influx to water bodies. Reducing DOC transport in the urban landscapes is complex and require identifying the origin of DOC and then using site-specific targeted approaches to mitigate DOC loss.https://doi.org/10.3390/w1204103

Composition, sources, and bioavailability of nitrogen in a longitudinal gradient from freshwater to estuarine waters

Nitrogen (N) transport from land to water is a dominant contributor of N in estuarine waters leading to eutrophication, harmful algal blooms, and hypoxia. Our objectives were to (1) investigate the composition of inorganic and organic N forms, (2) distinguish the sources and biogeochemical mechanisms of nitrate–N (NO3–N) transport using stable isotopes of NO3 − and Bayesian mixing model, and (3) determine the dissolved organic N (DON) bioavailability using bioassays in a longitudinal gradient from freshwater to estuarine ecosystem located in the Tampa Bay, Florida, United States. We found that DON was the most dominant N form (mean: 64%, range: 46–83%) followed by particulate organic N (PON, mean: 22%, range: 14–37%), whereas inorganic N forms (NOx–N: 7%, NH4–N: 7%) were 14% of total N in freshwater and estuarine waters. Stable isotope data of NO3 − revealed that nitrification was the main contributor (36.4%), followed by soil and organic N sources (25.5%), NO3 − fertilizers (22.4%), and NH4 + fertilizers (15.7%). Bioassays showed that 14 to 65% of DON concentrations decreased after 5-days of incubation indicating utilization of DON by microbes in freshwater and estuarine waters. These results suggest that despite low proportion of inorganic N forms, the higher concentrations and bioavailability of DON can be a potential source of N for algae and bacteria leading to water quality degradation in the estuarine waters

Evidence of Phosphate Mining and Agriculture Influence on Concentrations, Forms, and Ratios of Nitrogen and Phosphorus in a Florida River

Florida has a long history of phosphate-mining, but less is known about how mining affects nutrient exports to coastal waters. Here, we investigated the transport of inorganic and organic forms of nitrogen (N) and phosphorus (P) over 23 sampling events during a wet season (June–September) in primary tributaries and mainstem of Alafia River that drains into the Tampa Bay Estuary. Results showed that a tributary draining the largest phosphate-mining area (South Prong) had less flashy peaks, and nutrients were more evenly exported relative to an adjacent tributary (North Prong), highlighting the effectiveness of the mining reclamation on stream hydrology. Tributaries draining > 10% phosphate-mining area had significantly higher specific conductance (SC), pH, dissolved reactive P (DRP), and total P (TP) than tributaries without phosphate-mining. Further, mean SC, pH, and particulate reactive P were positively correlated with the percent phosphate-mining area. As phosphate-mining occurred in the upper part of the watershed, the SC, pH, DRP, and TP concentrations increased downstream along the mainstem. For example, the upper watershed contributed 91% of TP compared to 59% water discharge to the Alafia River. In contrast to P, the highest concentrations of total N (TN), especially nitrate + nitrite (NOx–N) occurred in agricultural tributaries, where the mean NOx–N was positively correlated with the percent agricultural land. Dissolved organic N was dominant in all streamwaters and showed minor variability across sites. As a result of N depletion and P enrichment, the phosphate-mining tributaries had significantly lower molar ratios of TN:TP and NOx–N:DRP than other tributaries. Bi-weekly monitoring data showed consistent increases in SC and DRP and a decrease in NOx–N at the South Prong tributary (highest phosphate-mining area) throughout the wet season, and different responses of dissolved inorganic nutrients (negative) and particulate nutrients (positive) to water discharge. We conclude that (1) watersheds with active and reclaimed phosphate-mining and agriculture lands are important sources of streamwater P and N, respectively, and (2) elevated P inputs from the phosphate-mining areas altered the N:P ratios in streamwaters of the Alafia River.https://doi.org/10.3390/w1308106

Physiochemical properties of drainfield sand and soil (mean±standard deviation).

<p>Data for soil texture, pH, and EC from De and Toor [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0170304#pone.0170304.ref014" target="_blank">14</a>].</p

Fixed effects test for total P, orthophosphate–P (PO₄–P), and other–P.

<p>Fixed effects test for total P, orthophosphate–P (PO₄–P), and other–P.</p

Mean (n = 64) percentages of PO₄–P and other–P in the effluent, unsaturated zone lysimeters located at 30.5 cm (L1), 61 cm (average of L2 and L3), and 106.7 cm (L4) below the infiltrative surface, and saturated zone piezometers (average of P1 to P5) located at >300 cm depth in the groundwater in the (A) drip dispersal and (B) gravel trench drainfields.

<p>Mean (n = 64) percentages of PO₄–P and other–P in the effluent, unsaturated zone lysimeters located at 30.5 cm (L1), 61 cm (average of L2 and L3), and 106.7 cm (L4) below the infiltrative surface, and saturated zone piezometers (average of P1 to P5) located at >300 cm depth in the groundwater in the (A) drip dispersal and (B) gravel trench drainfields.</p

A north-south longitudinal cross-section (not to scale) of the drainfields showing vadose zone and groundwater monitoring instruments: (A) drip dispersal and (B) gravel trench systems.

<p>A north-south longitudinal cross-section (not to scale) of the drainfields showing vadose zone and groundwater monitoring instruments: (A) drip dispersal and (B) gravel trench systems.</p