REDUCED WATER QUALITY TREATMENT FUNCTIONS OF SOIL-BASED ONSITE WASTEWATER TREATMENT SYSTEMS DUE TO CLIMATE CHANGE

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1 REDUCED WATER QUALITY TREATMENT FUNCTIONS OF SOIL-BASED ONSITE WASTEWATER TREATMENT SYSTEMS DUE TO CLIMATE CHANGE Jennifer A. Cooper 1,*, George W. Loomis and Jose A. Amador 1 ABSTRACT The effects of climate change are expected to reduce the ability of soil-based onsite wastewater treatment systems (OWTS) to treat domestic wastewater. In the northeastern U.S., the projected increase in atmospheric temperature, elevation of water tables from rising sea levels, and heightened precipitation will reduce the volume of unsaturated soil and oxygen available for treatment. Incomplete removal of contaminants may lead to transport of pathogens, nutrients, and biochemical oxygen demand (BOD5) to groundwater, increasing the risk to public health and likelihood of eutrophying aquatic ecosystems. Shallow narrow drainfield OWTS, which include prior advanced treatment steps and provide unsaturated drainfields with greater groundwater separation distances relative to conventional OWTS, are expected to be more resilient to climate change. We used intact soil mesocosms to quantify water quality functions for two advanced shallow narrow drainfield types and a conventional drainfield under a current climate scenario and a moderate climate change scenario of 3 cm rise in water table and 5 C increase in soil temperature. While no fecal coliform bacteria (FCB) was released under the current climate scenario, up to 18 CFU FCB/mL (conventional) and up to CFU FCB/mL (shallow narrow) were released under the climate change scenario. Total P removal rates dropped from 1% to 66% (conventional) and 71% (shallow narrow) under the climate change scenario. Total N removal increased from 14% to 19% under climate change scenario in the conventional, but dropped from 5.6% to less than % in the shallow narrow under the climate change scenario, with additional leaching of N in excess of inputs indicating release of previously held N. No significant difference was observed between scenarios for BOD removal. The data indicate that all three drainfield types experience some diminished treatment capacity. 1 Laboratory of Soil Ecology and Microbiology and New England Onsite Wastewater Training Center, University of Rhode Island, Kingston, RI. *jen_cooper@my.uri.edu INTRODUCTION The ability of onsite wastewater treatment system (OWTS) soil treatment areas (STAs; drainfields) to renovate wastewater relies on the vertical separation distance between the infiltrative surface of the STA and the water table. Predicted climate change (increased temperature, sea level rise) threaten to reduce both the unsaturated zone required for complete wastewater renovation, and the amount of oxygen available for treatment. The Intergovernmental Panel on Climate Change (IPCC, 13) predicts a temperature increase of 3-5 C over the next 1 years, with sea level projected to rise by 9-1 cm over the same time period, resulting in higher water tables in coastal regions. These factors will affect coastal communities that rely on

2 OWTS for wastewater renovation. Since 4% of the US population resides in coastal communities (NOAA, 11), the impact of climate change on OWTS function and water quality is likely to be significant. Reduced oxygen conditions are expected to lead to diminished OWTS contaminant treatment. Elevated atmospheric temperature will reduce O solubility and increase O consumption by microbial processes, resulting in less O available for aerobic treatment processes. Additionally, as sea levels continue to rise, the associated elevation of the water table will decrease the vertical separation between the STA infiltrative surface and the groundwater, where unsaturated conditions facilitate removal of BOD5, N, P and pathogens by physical, chemical, and biological processes. This will severely impact OWTS in coastal communities, as well as systems in shallow water table areas that were installed decades ago, and where a rising water table has slowly reduced vertical separation distances. Shallow narrow drainfields may be more resilient to climate change effects in comparison to a conventional STA. By design, the shallow narrow STAs receive effluent that has passed through advanced treatment technologies, resulting in reduced BOD5 ( 3 mg/l), particulates ( 3 mg/l TSS) (RIDEM, 13), prior to dispersal to a shallowly placed infiltrative surface. In contrast, the conventional STA receives septic tank effluent, without further pre-treatment, gravity dispersed to a deeper infiltrative surface in comparison to the shallow narrow STAs. Shallower dosing of wastewater in the soil profile is thought to provide better oxygenation of the soil and wastewater, mitigating some of the reducing effects of climate change. Enhanced filtration through finer soil particles in the upper portion of the soil profile in the shallow narrow STAs may aid in contaminant removal. In addition, the shallow narrow STAs incorporate timeddosing, with a relatively small volume of wastewater dosed at frequent, time controlled intervals providing a relatively constant level of moisture in the unsaturated treatment area of the soil. By contrast, the conventional STAs experience temporary saturation during the typical twice-daily periods of large volume use. Together, these factors may result in more reliable treatment of wastewater in the shallow narrow STAs than in the conventional STA under climate change conditions, although this has not been tested experimentally. Several hypotheses were formed about the performance of conventional and shallow narrow STAs under a moderate climate change scenario of 5 ºC increase in temperature and 3 cm elevation in water table. The climate change scenario was predicted to result in lower oxygen availability in all three STAs. Biochemical oxygen demand was expected to be better removed in all STA types under the climate change scenario due to higher temperatures increasing microbial activity. Soil microbial communities are carbon limited, thus input C will likely be readily consumed (Schimel and Schaeffer, 1). Shallow narrow STAs were anticipated to retain more renovation functions for pathogenic microorganisms and P due to higher O levels in the oxygenated advanced treatment step and shallow placement of the infiltrative area, which allows greater O diffusion. However, the conventional STA was expected to have diminished water quality function for these contaminants because septic tank effluent has no dissolved O and the drainfield soil has lower opportunity for diffusion of O relative to the shallow narrow STAs. Greater pathogenic bacteria and virus survival has been noted in wetter (more saturated) soils (Campbell, 1976; Quanrud, 3). Generally, microbial pathogens in soil experience increased mortality with increased temperatures (Gerba, 1975; Nasser 1; Morales et al., 15). However, loss of unsaturated soil volume was expected to be more important than

3 elevated temperature, especially in the conventional STA. Likewise, anaerobic conditions can lead to reduction of iron oxides to ferrous iron (Fe 3+ Fe + ), which is more soluble in water (Brady and Weil, ). Hence, phosphate bound to iron oxides was anticipated to be released into the dissolved phase from the conventional STAs. By contrast, it was expected that the shift towards anaerobic conditions would favor N removal in all three drainfield types by increasing loss as N/NO from denitrification, which is enhanced by anoxic conditions and the presence of reduced C, N, and S compounds as electron donors. Overall, we anticipated that the shallow narrow drainfields will be more resilient to climate change. We used intact soil mesocosms (15- cm tall 15-cm-dia.) to represent three STA types: (i) conventional pipe and stone (), (ii) shallow narrow drainfield (SND) and (iii) Geomat (GEO), a SND variation (Fig. 1). The was dosed with ml of septic tank effluent (STE) every 1 h over 1.5 h, corresponding to 4 ml d -1 (.6 L m - d -1 ). The SND and GEO received wastewater that had passed through a single-pass sand filter (SFE). They were dosed with.5 ml SFE every 3 min over 15 min, corresponding to ml d -1 (113 L m - d -1 ). Septic tank effluent and SFE were collected weekly from the same treatment train at a residence in South Kingstown, RI, USA. Characteristics of wastewater inputs can be found in Table 1. Further details of the experimental design, sampling, and analytical methods can be found in Cooper et al. (15). The mesocosms were receiving wastewater for 4 months prior to the climate change experiment. The soil infiltrative area was established at cm below the ground surface for SND (Fig. 1), at 5 cm for GEO, and at 84 cm for MATERIALS AND METHODS Fig. 1. (A) Schematic diagram of soil mesocosms representing a shallow narrow drainfield (SND), GeoMat (GEO), and pipe and stone () drainfield. The wastewater input to SND and GEO was sand filter effluent (SFE), whereas the received septic tank effluent (STE). The approximate location of soil horizons, ports for gas sampling, and moisture and temperature probes are indicated. Water exits the mesocosms through a hanging water column device used to adjust the height of the water table. The atmosphere in the infiltrative area is connected to a 3-cm soil column. (B) Detailed schematic diagram of the SND, GEO and delivery devices. Diagrams are not to scale.. The water table was controlled using a hanging water column (Fig. 1) and was set at 1 cm below the infiltrative surface for SND and GEO, and at 56 cm for. This established the water table at 1-14 cm from the ground surface for all drainfield types under the present 3

4 climate scenario and reduced to 9-11 cm under the climate change scenario. Columns were maintained at. ±.7 ºC under the present climate scenario and warmed with 115V heating cable (Hydrokable, Sacramento, CA) to keep temperatures at 5 ±.7 ºC under the climate change scenario. Water outputs from the mesocosms were collected at the bottom of the mesocosms under both climate scenarios, in N purged 1-L Nalgene bottles fitted with an airlock. Water ph was determined using an Ultrabasic 1 ph meter (Denver Instruments). Five-day biochemical oxygen demand (BOD5) was determined using Oxitop BOD pressure sensor heads (WTW, College Station, TX) at ± 3 C. Fecal coliform bacteria was enumerated by the membrane filtration method (APHA, 1998). Electrical conductivity was measured using a model probe (Control Company). Samples for TN and TP analysis were digested using the persulfate oxidation method (APHA, 1998). Colorimetric methods were used to determine NO3 (Doane and Horwath, 3), NH4 (Weatherburn, 1967), and PO4 (Murphy and Riley, 196) concentrations using a Bio-Tek microplate reader (Powerwave 34, Winooski, VT). Sulfate was measured turbidimetrically (APHA, 1998) using a model UV16U UV-visible spectrophotometer (Shimadzu Corp., Columbia, MD). Oxygen samples were collected with an air-tight, -ml syringe and injected immediately to a flow-through cell connected to an O probe (model O-BTA, Vernier, Beaverton, OR). RESULTS AND DISCUSSION Input wastewater characteristics for STE and SFE used in the present climate and climate change scenarios are listed in Table 1. Reported chemical and biological properties are within the range observed by others (Siegrist, 1; Loomis et al., 1; Potts et al., 4) Table 1. Characteristics of septic tank effluent (STE) and sand filter effluent (SFE) used in our study under the present climate (n=8) and climate change (n=11) scenarios. Values are means ± standard deviation. Property STE SFE Present Climate Present Climate Climate Change Climate Change ph 6.3 ±. 6.5 ±. 3.9 ± ±.5 Dissolved O, mg L -1. ±.. ±..4 ±.6 3. ± 1.3 BOD 5, mg L ± ± ± ± 8.6 Electrical conductivity, µs 786 ± 47 6 ± ± 85 4 ± 1 Fecal coliform bacteria, CFU 1 ml ± ± ± ± Total N, mg L ± 8. 5 ± ± ± 11 NH4-N, mg L -1 5 ± ± 15 1 ± ±.9 NO3-N, mg L -1. ±.4. ±. 4 ± 8. 4 ± 8.7 Total P, mg L ± ± ± ± 1.6 PO4-P, mg L ± ± ± ±1. SO4-S, mg L ± ±.4 15 ±.8 13 ±3.9 Collection temperature, C ±. 1 ± 6.1 ±. 11 ± 6.8 4

5 Volumetric soil moisture content increased under the climate change scenario, as expected (Fig. ), for both conventional and shallow narrow STA types. Less oxygen was detected in soil pores in the climate change scenario in comparison to the present climate scenario (Fig. 3). Increased moisture and decreased available oxygen under the climate change scenario confirm our first hypothesis that conditions will be less oxic under climate change conditions. Depth Below Infiltrative Surface (cm) Present climate Climate change Moisture Content (v/v) Depth Below Infiltrative Surface (cm) Moisture Content (v/v) SND/GEO Fig.. Volumetric soil moisture content for the conventional () and shallow narrow (SND/GEO) soil treatment areas under the present climate and climate change scenarios. Values are a 4 h average at each depth after steady state was attained under each scenario Current Future Our hypothesis concerning BOD5 was confirmed, with median concentrations of BOD5 in output water lower for the climate change scenario than for the present climate scenario for all three Depth (cm) SND GEO Depth (cm) Fig. 3. Soil pore oxygen content under present climate and climate change for the shallow narrow (SND/GEO) and conventional soil treatment areas. Values are means (n = 8-11) and error bars represent one calculated standard deviation for each mean. STA types (Fig. 4). There was more variability in the BOD5 output under climate change in comparison to the present climate scenario. By contrast, SND and variability in output BOD5 concentrations remained relatively constant between the two climate scenarios. The lower median BOD5 concentrations in output water from all three STA types was likely due to increased microbial activity driven by the higher soil temperature and rapid consumption of organic carbon. No fecal coliform bacteria (FCB) was released under the present climate scenario. In contrast, FCB was released from all three STA types under the climate change scenario (Fig. 5). This partially confirms our hypothesis, since we expected FCB to increase in output water from ; however, the presence of FCB in output water from SND and GEO was unexpected. The present climate - climate change O (%) O O (%) (%) Depth (cm) 5

6 3..5 SND 3..5 GEO 3..5 BOD 5 (mg/l) Present 1Climate Climate Change Present Climate Climate Change 1 Present 1 Climate Climate Change Fig. 4. Concentration of biochemical oxygen demand (BOD5) under present climate and climate change scenarios for shallow narrow (SND/GEO) and conventional () soil treatment areas. Boxes represent the median and interquartile range, whiskers represent the 1 th and 9 th percentiles, with dots representing outliers beyond the 1 th and 9 th percentiles. presence of FCB in output water was highly variable under the climate change scenario, and more so in SND and GEO STAs. The number of fecal coliform bacteria released in SND and GEO output water was greater than inputs from SFE for multiple weeks, suggesting bacterial growth within these STAs under climate change conditions. In addition, the presence of FCB may have been due to increased moisture content. Unsaturated conditions are known to favor FCB removal in the STA (Powelson and Gerba, 1994; Beal et al., 5) and increased soil moisture may have been more important than increased temperature. The combination of high temperature promoting growth and wetter soil aiding bacterial survival may be responsible for the diminished removal rates under climate change 6 SND 6 GEO 6 FC (CFU/1mL) Present 1 Climate Climate Change Present 1 Climate Climate Change 1 Present Climate Climate Change Fig. 5. Concentration of fecal coliform bacteria (FC) under present climate and climate change scenarios for shallow narrow (SND/GEO) and conventional () soil treatment areas. Boxes represent the median and interquartile range, whiskers represent the 1 th and 9 th percentiles, with dots representing outliers beyond the 1 th and 9 th percentiles. 6

7 Our hypothesis for total nitrogen (TN) removal under climate change, which predicted better removal, was not supported by the data. The median removal rate of TN was lower for SND and GEO under climate change (Fig. 6). The SND STA was actually a net source of nitrogen to output water, which appears as a negative removal rate. In contrast, the median TN removal for was slightly higher, partially confirming the hypothesis. In all cases TN removal was more variable under climate change. 6 4 SND 6 4 GEO 6 4 TN removal (%) Present Climate Climate Change Present 1 Climate Climate Change 1 Present Climate Climate Change Fig. 6. Removal of total nitrogen (TN) under present climate and climate change scenarios for shallow narrow (SND/GEO) and conventional () soil treatment areas. Boxes represent the median and interquartile range, whiskers represent the 1 th and 9 th percentiles, with dots representing outliers beyond the 1 th and 9 th percentiles. There were more events of either no TN removal or negative TN removal under the climate change scenario. Under present climate conditions 5% (SND) and 1% (GEO and ) of the observations reflected a positive removal of TN. However, only 7% (SND) and 63% (GEO and ) of climate change observations resulted in removal of TN. Under climate change conditions 63% (SND) and 7% (GEO and ) of the observations were determined that soil was a source of TN to the output water. For, organic carbon is likely the controlling factor for TN removal, with C inputs limiting heterotrophic denitrification. In contrast, little carbon is present in wastewater inputs (sand filter effluent) to SND and GEO (Table 1), with the higher soil temperatures under climate change driving quick consumption of an already limited amount of C. Our hypothesis for total phosphorus (TP) removal, that SND and GEO would remain unchanged, and would release TP, was largely incorrect. Median TP removal was lower under climate change for all three STA types, and was also more variable under the climate change scenario (Fig. 7). The SND and GEO STAs maintained near complete removal of TP under present climate conditions and, while removal was not complete under the present climate scenario, TP removal was lower under climate change for all three STA types. Potential mechanisms of TP 7

8 SND GEO TP removal (%) Present 1 Climate Climate Change Present 1 Climate Climate Change Present 1 Climate Climate Change Fig. 7. Removal of total phosphorus (TP) under present climate and climate change scenarios for shallow narrow (SND/GEO) and conventional () soil treatment areas. Boxes represent the median and interquartile range, whiskers represent the 1 th and 9 th percentiles, with dots representing outliers beyond the 1 th and 9 th percentiles. release include Fe reduction from the shift to lower O conditions, or increased mortality of poly phosphate accumulating microorganisms under climate change. CONCLUSION As hypothesized, lower O availability and wetter soils were observed in the STA under climate change conditions. Removal of fecal coliform bacterial, total nitrogen and total phosphorus were less effective under climate change, regardless of the type of STA. In contrast removal of BOD5 increased under climate change. Our results suggest that climate change has the potential to reduce removal of contaminants from wastewater in the STA. ACKNOWLDEGEMENTS This study was funded by grants from Rhode Island Sea Grant, the Rhode Island Agricultural Experiment Station, by a grant from University of Rhode Island Enhancement of Graduate Research Program to J.A.C., and by personal funds of the authors. We thank Alissa Becker, Rachel Naylor, Faith Anderson, Ivan Morales, Tom Boving and Dave Potts for technical and field assistance. We are especially grateful to the homeowners that provided us access to their onsite wastewater treatment system. REFERENCES APHA Standard methods for the examination of water and wastewater, th ed. American Public Health Association, Washington, DC. Beal, C.D., Gardner, E.A., Kirchhof, G., and Menzies, N.W. 6. Long-term flow rates and biomat zone hydrology in soil columns receiving septic tank effluent. Water Resources. 4(1):

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