Review of phosphorus attenuation in groundwater plumes from 24 septic systems

Phosphorus attenuation and mobilization in sand filters treating onsite wastewater

The effectiveness of phosphorus (P) removal by sand filters is limited during septic tank effluent (STE) treatment. The elevated effluent P concentrations pose threats to drinking water quality and contribute to eutrophication. The concern of P leaching from sand filters is further exacerbated by the increased frequency of flooding and natural precipitation due to climate change. This study aimed to understand P attenuation and leaching dynamics, as well as the removal mechanisms in sand filters treating STE, offering insights into the design and implementation of P removal/recovery modules to onsite wastewater treatment systems. P attenuation and leaching during STE treatment and rainfall were studied in bench-scale columns (new vs. aged sand). At standard STE loading (1.2 gallon d-1 ft-2), 24-32% removal of total phosphorus (TP) was achieved, while increased P removal efficiency (35-53%) was observed at low loading (0.6 gallon d-1 ft-2) with influent containing 10.3-20.0 mg P L-1. Complete breakthroughs were observed in both aged (12-70 days) and new columns (27-73 days) at test hydraulic loadings. The maximum TP attenuation level was 20.6-45.3 mg P kg-1 and 25.3-33.0 mg P kg-1, in aged and new sand columns, respectively. When simulated rain was applied (15-60 mm h-1), 80-97% of the attenuated P leached out and the leaching dynamics were impacted by rainfall duration rather than the intensity. The highest concentrations of TP (15.6-15.9 mg L-1) were leached out from both columns within the first 2-6 h. Orthophosphate was the dominant P species in treated effluent (83-84%) and leachate (69-88%), demonstrating its significance as the major P form in the discharge. In addition, aged sand (>5 years) accumulated higher levels of Mg, Al, Ca, and Fe, thus enhancing the P attenuation level during STE treatment. Collectively, this study underscored the importance of frequent field monitoring for reliable long-term P removal estimates.

Characterizing Spatiotemporal Variability in Phosphorus Export across the United States through Bayesian Hierarchical Modeling

Phosphorus inputs from anthropogenic activities are subject to hydrologic (riverine) export, causing water quality problems in downstream lakes and coastal systems. Nutrient budgets have been developed to quantify the amount of nutrients imported to and exported from various watersheds. However, at large spatial scales, estimates of hydrologic phosphorus export are usually unavailable. This study develops a Bayesian hierarchical model to estimate annual phosphorus export across the contiguous United States, considering agricultural inputs, urban inputs, and geogenic sources under varying precipitation conditions. The Bayesian framework allows for a systematic updating of prior information on export rates using an extensive calibration data set of riverine loadings. Furthermore, the hierarchical approach allows for spatial variation in export rates across major watersheds and ecoregions. Applying the model, we map hotspots of phosphorus loss across the United States and characterize the primary factors driving these losses. Results emphasize the importance of precipitation in determining hydrologic export rates for various anthropogenic inputs, especially agriculture. Our findings also emphasize the importance of phosphorus from geogenic sources in overall river export.

Evaluation of Phosphorus Enrichment in Groundwater by Legacy Phosphorus in Orchard Soils with High Phosphorus Adsorption Capacity Using Phosphate Oxygen Isotope Analysis

Long-term phosphorus (P) fertilization results in P accumulation in agricultural soil and increases the risk of P leaching into water bodies. However, evaluating P leaching into groundwater is challenging, especially in clay soil with a high P sorption capacity. This study examined whether the combination of PO4 oxygen isotope (δ18OPO4) analysis and the P saturation ratio (PSR) was useful to identify P enrichment mechanisms in groundwater. We investigated the groundwater and possible P sources in Kubi, western Japan, with intensive citrus cultivation. Shallow groundwater had oxic conditions with high PO4 concentrations, and orchard soil P accumulation was high compared with forest soil. Although the soil had a high P sorption capacity, the PSR was above the threshold, indicating a high risk of P leaching from the surface orchard soil. The shallow groundwater δ18OPO4 values were higher than the expected isotopic equilibrium with pyrophosphatase. The high PSR and δ18OPO4 orchard soil values indicated that P leaching from orchard soil was the major P enrichment mechanism. The Bayesian mixing model estimated that 76.6% of the P supplied from the orchard soil was recycled by microorganisms. This demonstrates the utility of δ18OPO4 and the PSR to evaluate the P source and biological recycling in groundwater.

Nitrogen and Phosphorus Treatment Can Be Sustainable During on-Site Wastewater Disposal

Monitoring of a seasonal-use, on-site wastewater disposal system (septic system) in Canada, over a 33-year period from 1988 to 2021, showed that during recent sampling the groundwater plume had TIN (total inorganic nitrogen) averaging 12.2 mg/L that was not significantly different than early values, representing 80% removal, whereas SRP (soluble reactive phosphate), although higher than early values averaging 0.08 mg/L, was still 99% lower than the effluent concentration. Evidence suggests that the anammox reaction and possibly also denitrification contribute to TIN removal, whereas SRP removal is primarily the result of mineral precipitation. Most of the removal occurs in close proximity to the drainfield infiltration pipes (within about 1 m) demonstrating that reaction rates are relatively fast in the context of typical groundwater plume residence times. This long-term consistency demonstrates that sustainable nutrient treatment can be achieved with conventional on-site wastewater disposal systems that have low capital costs and require minimal energy input and maintenance.

Modeling multi-year phosphorus dynamics in a bioretention cell: Phosphorus partitioning, accumulation, and export

Phosphorus (P) export from urban areas via stormwater runoff contributes to eutrophication of downstream aquatic ecosystems. Bioretention cells are a Low Impact Development (LID) technology promoted as a green solution to attenuate urban peak flow discharge, as well as the export of excess nutrients and other contaminants. Despite their rapidly growing implementation worldwide, a predictive understanding of the efficiency of bioretention cells in reducing urban P loadings remains limited. Here, we present a reaction-transport model to simulate the fate and transport of P in a bioretention cell facility in the greater Toronto metropolitan area. The model incorporates a representation of the biogeochemical reaction network that controls P cycling within the cell. We used the model as a diagnostic tool to determine the relative importance of processes immobilizing P in the bioretention cell. The model predictions were compared to multi-year observational data on 1) the outflow loads of total P (TP) and soluble reactive P (SRP) during the 2012-2017 period, 2) TP depth profiles collected at 4 time points during the 2012-2019 period, and 3) sequential chemical P extractions performed on core samples from the filter media layer obtained in 2019. Results indicate that exfiltration to underlying native soil was principally responsible for decreasing the surface water discharge from the bioretention cell (63 % runoff reduction). From 2012 to 2017, the cumulative outflow export loads of TP and SRP only accounted for 1 % and 2 % of the corresponding inflow loads, respectively, hence demonstrating the extremely high P reduction efficiency of this bioretention cell. Accumulation in the filter media layer was the predominant mechanism responsible for the reduction in P outflow loading (57 % retention of TP inflow load) followed by plant uptake (21 % TP retention). Of the P retained within the filter media layer, 48 % occurred in stable, 41 % in potentially mobilizable, and 11 % in easily mobilizable forms. There were no signs that the P retention capacity of the bioretention cell was approaching saturation after 7 years of operation. The reactive transport modeling approach developed here can in principle be transferred and adapted to fit other bioretention cell designs and hydrological regimes to estimate P surface loading reductions at a range of temporal scales, from a single precipitation event to long-term (i.e., multi-year) operation.

Chemical and biological tracers to identify source and transport pathways of septic system contamination to streams in areas with low permeability soils

Septic systems are widely used in rural areas that lack centralized sewage treatment systems. Incomplete removal of domestic wastewater contaminants in septic systems can lead to leaching of nutrients (P and N), bacteria/viruses, and trace contaminants to surrounding groundwater and surface water. This study focuses on delineating the fate of wastewater contaminants in localities where septic systems are installed in moderate to fine-grained overburden materials to assess potential impacts on groundwater and surface water quality in these settings. Nutrients and a suite of anthropogenic tracers, including host-specific fecal indicator bacteria (bovine- and human-specific Bacteroides), pharmaceutical compounds (caffeine, carbamazepine, gemfibrozil, ibuprofen, naproxen, and sulfamethoxazole), and an artificial sweetener (acesulfame-K), were selected to evaluate differences in transport properties. Surface water samples (n = 103) were collected from streams upstream (US) and downstream (DS) of three rural hamlets up to two times monthly over one year. Results indicate the presence of wastewater indicators in the streams, with DS locations showing significantly elevated concentrations of both chemical and biological anthropogenic tracers. Human-specific Bacteroides, caffeine, and acesulfame-K were consistently observed at elevated concentrations at all DS sites. Nutrients exhibited varied concentrations between US and DS locations at three study sites. The occurrence of human-specific Bacteroides in the surface water samples suggests the presence of preferential flow pathways within the silt/clay overburden. These results demonstrate the advantages of using a combined tracer approach, involving a conservative tracer such as acesulfame-K coupled with the human-specific biological indicator Bacteroides (BacHum), to understand not only impacting sources but also potential transport pathways of septic system contamination to nearby streams. Septic systems may be an underappreciated contaminant source in rural hamlets located in fine-grained overburden materials; although, a distinction of specific nutrient sources (septic systems vs. agriculture) remains challenging.

Groundwater Quality and Health: Making the Invisible Visible

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Variation in septic system effluent inputs to tributaries in multiple subwatersheds and approaches to distinguish contributing pathways and areas

Quantifying the contribution of septic systems to contaminant, including nutrient, loading to streams is needed in many watersheds to inform water quality management programs. However, this quantification is challenging due to the distributed locations of septic systems and uncertainties regarding the pathways delivering effluent from septic systems (functioning and failing) to a stream. The objectives of this study were firstly to evaluate how septic effluent inputs to streams vary with stream discharge conditions for multiple subwatersheds with different characteristics (i.e., geology, septic system density, and typical age), and secondly to examine new approaches for distinguishing the pathways and the contributing areas delivering septic effluent to streams. These approaches use the artificial sweetener acesulfame as a conservative tracer for septic effluent in applications of: (i) stream concentration-discharge (C-Q) relationships using low frequency sampling data, (ii) hysteresis behavior in event-based C-Q relationships, and (iii) longitudinal stream sampling. For all nine subwatersheds studied, the amount of septic effluent reaching the subwatershed outlets was considerably higher during high stream discharge (event) conditions compared to low discharge (baseflow) conditions, suggesting pathways other than groundwater may also be important. Generally, the percentage of septic effluent reaching the outlets was less for subwatersheds with newer households compared to those with older households. The combined interpretation of low frequency and event-based C-Q relationships indicate that complex pathways control the delivery of septic effluent to the subwatershed outlets. The interpretations suggest that groundwater pathways may dominate in some subwatersheds, while more rapid pathways associated with failing septic systems (e.g., overland runoff) may be important in others. Finally, longitudinal stream sampling illustrate the potential of acesulfame data to identify key areas contributing septic effluent to the stream. The novel approaches used here can be applied to guide future investigations aiming to quantify and manage water quality impairment from septic systems.

Nitrogen Attenuation in Septic System Plumes

The persistence of inorganic nitrogen is assessed in a set of 21 septic system plumes located in Ontario, Canada, that were studied over a 31-year period from 1988 to 2019. In the plume zones underlying the drainfields, site mean NO₃ ^- values averaged 34 ± 27 mg N/L and exceeded the nitrate drinking water limit (DWL) of 10 mg N/L at 16 of 21 sites. In plume zones extending up to 30 m downgradient from the drainfields, site mean NO₃ ^- values averaged 24 ± 20 mg N/L and exceeded the DWL at 9 of 13 sites. Site mean total inorganic nitrogen (TIN; NH₄ ⁺  + NO₃ ^-  - N) removal averaged 34 ± 26% in the drainfield zones and 36 ± 44% in the downgradient plume zones, indicating that much of the removal occurred within the drainfields. Removal was much higher at nine sites where drainfield TIN included >10% NH₄ ⁺ (62 ± 25% removal). TIN removal was not correlated with wastewater loading rate, system age, or sediment carbonate mineral content, but was correlated with water table depth, where shallower water table sites had generally less complete wastewater oxidation. At many of these sites, both NO₃ ^- and NH₄ ⁺ were present together in the plumes and were lost concomitantly, suggesting that the anammox reaction was making an important contribution to the observed TIN loss. When groundwater nitrate contamination is a concern, considering on-site treatment system designs that lead to a lesser degree of wastewater oxidation, could be a useful approach for enhancing N removal.

Spatiotemporal controls on septic system derived nutrients in a nearshore aquifer and their discharge to a large lake

This study evaluates spatiotemporal variability in the behavior of septic system derived nutrients in a sandy nearshore aquifer and their discharge to a large lake. A groundwater nutrient-rich plume was monitored over a two-year period with the septic system origin of the plume confirmed using artificial sweeteners. High temporal variability in NO₃-N attenuation in the nearshore aquifer prior to discharge to the lake (42-96%) reveals the complex behavior of NO₃-N and potential importance of changing hydrological and geochemical conditions in controlling NO₃-N discharge to the lake. While PO₄-P was retarded in the nearshore aquifer, the PO₄-P plume extended over 90 m downgradient of the septic system. It was estimated that the PO₄-P plume may reach the lake within 10 years and represents a legacy issue whereby PO₄-P loads to the lake may increase over time. To provide broader assessment of the contribution of septic systems to P and N loads to a large lake, a regional scale geospatial model was developed that considers the locations of individual septic systems along the Canadian Lake Erie shoreline. The estimated P and N loads indicate that septic systems along the shoreline are only a minor contributor to the annual P and N loads to Lake Erie. However, it is possible that nutrients from septic systems may contribute to localized algal blooms in shoreline areas with high septic system density. In addition, disproportionate P and N loads in discharging groundwater may change the N:P ratio in nearshore waters and promote growth of harmful cyanobacteria. The study provides new insights into factors controlling the function of the reaction zone near the groundwater-lake interface including its impact on groundwater-derived nutrient inputs to large lakes. Further, the study findings are needed to inform septic system and nutrient management programs aimed at reducing lake eutrophication.

Species and potential sources of phosphorus in groundwater in and around Mataram City, Lombok Island, Indonesia

Geochemical evaluation of the species and potential sources of P in groundwater in and around Mataram City, Lombok Island, Indonesia can aid in the initial research on evaluating the fate of P when detailed geological information is unavailable. The results of ion chromatography and inductively coupled plasma-optical emission spectroscopy show that the concentrations of PO43– and total dissolved P (TDP) in groundwater range from approximately 0.1 to 8.5 mg l–1 and from 0.04 to 2.95 mg l–1, respectively. Dissolved inorganic P accounted for 86%, on average, of the TDP concentration, and PO43– represented the predominant P species in this groundwater. The potential sources for most of this dissolved PO43–, according to indices developed based on the PHREEQC software and groundwater quality data, could be the dissolution of hydroxyapatite and/or vivianite minerals. However, the potential sources of dissolved PO43– in groundwater with a TDP concentration of ≥ 1 mg l–1 is likely to be the reduction of Fe(III)–(hydro)oxides, the initial decomposition of organic matter, or the dissolution of carbonate-rich fluorapatite considering the Fe2+, dissolved organic carbon, Ca2+ and F− concentrations measured. In addition, as several groundwater samples had a TDP concentration of ≤ 1 mg l–1 and comparatively high concentration of NO3–, other potential sources of dissolved PO43– in this groundwater could be anthropogenic.

Assessing the Socio-Environmental Risk of Onsite Wastewater Treatment Systems to Inform Management Decisions

Quantifying the risk that failing onsite waste treatment systems (OWTS), such as septic systems, present to human health and the environment is a key component in natural resource management. We integrated environmental and socio-demographic data to assess the potential environmental risk and environmental justice concerns related to septic infrastructure. We used this process to develop a framework that can be applied in other jurisdictions. We found only 8% of the registered OWTS presented potential environmental risk due to the topographic, hydrologic, or edaphic characteristics of their placement. In contrast, almost 70% of the OWTS presented potential environmental risk due to their age (25 years or older). Approximately 60% of the OWTS we estimated to be at risk from age or placement were found in census blocks with more than 30% of the population living below the poverty line, had a population that was more than 50% nonwhite, or was predominantly nonwhite and impoverished. Our work suggests that jurisdictions with limited information about septic infrastructure may be able to use geospatial data that they do have to predict the parcel-level locations of OWTS. These locations can then be used to inform environmental monitoring to proactively address environmental justice concerns.

Simultaneous denitrification, phosphorus recovery and low sulfate production in a recirculated pyrite-packed biofilter (RPPB)

The simultaneous removal of nitrate (15 mg N-NO₃^- L^-1) and phosphate (12 mg P-PO₄^3- L^-1) from nutrient-polluted synthetic water was investigated in a recirculated pyrite-packed biofilter (RPPB) under hydraulic retention time (HRT) ranging from 2 to 11 h. HRT values ≥ 8 h resulted in nitrate and phosphate average removal efficiency (RE) higher than 90% and 70%, respectively. Decrease of HRT to 2 h significantly reduced the RE of both nitrogen and phosphorus. The RPPB showed high resiliency as reactor performance recovered immediately after HRT increase to 5 h. Solid-phase characterization of pyrite granules and backwashing material collected from the RPPB at the end of the study revealed that iron-phosphate, -hydroxide and -sulfate precipitated in the bioreactor. Thermodynamic modeling predicted the formation of S⁰ during the study. Residence time distribution tests showed semi-complete mixing hydrodynamic flow conditions in the RPPB. The RPPB can be considered an elegant and low-cost technology coupling biological nitrogen removal to the recovery of phosphorus, iron and sulfur via chemical precipitation.