Onsite Wastewater System Nutrient Transport in the North Carolina Piedmont

Transcription

1 Onsite Wastewater System Nutrient Transport in the North Carolina Piedmont Charlie Humphrey, PhD Associate Professor of Environmental Health Sciences East Carolina University Co-authors: Jordan Jernigan, Brent Serozi, Dr. Mike O Driscoll, Guy Iverson, Dr. Sushama Pradhan, and Dr. Eban Bean What is Next? Overview of nitrogen and phosphorus treatment by onsite systems Overview of prior research related to nutrient treatment by onsite systems Project goal and objectives Methods Results and data interpretation Summary Questions and discussion 1

2 Onsite Systems and Nitrogen Treatment TKN NH 4 NH 4 NO 3 ON N 2 ON N 2 NH 4 NO 3 (Diagram: Crighton, 2011) Ammonification, adsorption, nitrification, immobilization, denitrification Wastewater TN typically between 25 and 100 mg/l ; 4kg/TN/person/yr (10 lbs) Onsite Systems and Phosphorus Treatment Phosphorus (P) Treatment Adsorption, immobilization, precipitation, dilution, dispersion OWS disperse wastewater with elevated P (2-12 mg/l) P concentrations exceeding 0.03 mg/l may stimulate algae growth in some waters Compound α-feooh 7.8 γ-alooh 8.2 Fe(OH)3 8.5 Al(OH)3 8.9 α-al2o3 9.1 ph pzc 2

3 Conventional and Single Pass Sand Filter Systems Prior NC Studies: OWS Nutrient Treatment Carteret County Study (16 OWS) Group I soils: DIN = 20.9 mg/l (22% reduction) PO 4 = 2.46 mg/l (58% reduction) Group II soils: DIN = 17.1 mg/l (75% reduction) PO 4 = 0.39 mg/l (95% reduction) Group III soils: DIN = 4.0 mg/l (91% reduction) PO 4 = 0.04 mg/l (99% reduction) Humphrey et al. (2010) Humphrey et al (2011) Beaufort County Study (2 OWS in Group I-II soils) Groundwater TDN near OWS: 12 to 34 mg/l or 59-80% lower than tank effluent Groundwater > 15 m down-gradient under 4 mg/l TDN > 93% concentration reduction (Humphrey et al 2013) Second year at Site 1 indicated 20% mass reduction TDN near drainfield and up to 85% mass reduction 35m down-gradient (O Driscoll et al 2014) Groundwater PO 4 (site 1, year 2) = 3.0 mg/l or 0% concentration reduction near drainfield and 0.84 mg/l at 30 m down-gradient (72% reduction) (Humphrey et al 2014) 3

4 Prior NC Studies: OWS Nutrient Treatment Pitt County Study (4 OWS in Group III and 1 in Group IV soils) (Paired Watersheds 3 OWS and 4 Sewer) Groundwater near drainfields: TDN = 2.3 to 34 mg/l (47 to 95% reduction) Groundwater 15+ m down gradient TDN = 0.5 to 8.3 mg/l (87 to 99% reduction (Iverson et al 2015) Groundwater near drainfield : PO 4 -P = 1.1 mg/l (85% reduction); PO 4 -P = 1.0 mg/l at 15m+ (86% reduction) (Humphrey et al 2015) Watershed-scale influences: TDN and TDP concentrations and loads were higher for streams draining watersheds with OWS relative to sewer Project Goal and Objectives Goal: gain a better understanding of OWS nitrogen and phosphorus treatment in piedmont soils Objectives Instrument and monitor at least 2 OWS (20-35 piezometers) Sample 4-6 times for nutrients and physical, chemical parameters Determine the nitrogen and phosphorus treatment efficiency of the OWS 4

5 Methods (Site Selection) Piedmont county Use OWS Adjacent to surface water Lick Creek area priority Funds to potentially enhance performance of systems via a different grant Consulted with EHS from Durham and Wake Counties Reviewed topographic maps, soil maps, to select potential volunteered sites Obtained well permits Piezometer Installations 5

6 Survey Work Groundwater flow direction, hydraulic gradient, and water table dynamics Piezometer Installations and Soil Characterization Install piezometers within the groundwater flow path of the OWS plume. Collect soil samples during installations for particle size analyses, ph, ECEC, and for profile descriptions. 6

7 Site Characteristics (Site 100) Conventional OWS Installed occupants Cecil Series near OWS Groundwater (13.8 ft) 1000 gallon tank trenches Wake County Site Characteristics (Site 200) 900 gallon tank ~60 x 5 bed Installed 1930 s 2-3 occupants Georgeville Series Groundwater (6.2 ft) Northern Durham 7

8 Site Characteristics (Site 400) Single Pass Sand Filter Installed 1968 and gallon tank 2 occupants Chewacla Series Groundwater (2.1 ft) Durham County Site Characteristics (Site 500) Single Pass Sand Filter Installed occupants 2 tanks each 1,000 gallons Chewacla Series Soil Groundwater (3 ft) 8

9 Water Quality Sampling Points Tanks Piezometers Surface waters Well water Filter effluent Total dissolved nitrogen, NH 4, NO 3, Cl, PO 4, isotopic analyses of δ 15 N and δ 18 O, ph, dissolved oxygen, ORP, temperature, depth water Data Analyses Treatment efficiency (concentration reductions) with distance from OWS Chloride to nutrient ratios to determine potential mass losses of nutrients from septic tanks to groundwater down-gradient from the OWS Two component mixing model using Cl concentrations from septic effluent and background groundwater to determine % piezometer samples that are groundwater and wastewater Isotopic analysis to determine sources of NO 3 and potential NO 3 reduction processes 9

10 Results- Nitrogen Concentration Reductions 93% 95% 97% Septic effluent > Drainfield (p = ) Drainfield > Well (p = ) Drainfield > 35 m (p = ) 15 m > Well (p = ) Drainfield > 15 m (p = ) 35 m > Well (p = ) Site 100 Nitrogen Treatment (Mass Reduction) 10

11 Nitrogen Mass Removal Evidence NO 3 concentration decline & δ 15 N enrichment along a groundwater flow path indicates denitrification Site 100 PO 4 -P Treatment Tank Drainfield > 99% reduction in concentration and > 98% reduction in mass Most sampling points less than 0.03 mg/l Cecil soil (65% clay) > 9 ft vadose zone beneath trenches 11

12 Site 200 TDN Concentration Reductions Septic tank to drainfield (86% overall; varied between 65 and 96%) Differences significant at p = 0.02 for piezometer with highest concentration Groundwater 35 m down-gradient (1.32 mg/l) lower than drainfield (p = 0.016) Groundwater 5 m down-gradient (3.45 mg/l) not sig different than drainfield (p = 0.143) Site 200 TDN Mass Reduction

13 Site 200 TDN Mass Reduction NO 3 concentration decline & δ 15 N enrichment along a groundwater flow path indicates denitrification Site 200 PO 4 -P Treatment Tank to drainfield > 99% reduction in concentration and > 93% mass Most locations < 0.05 mg/l PO 4 -P Decline in groundwater PO 4 -P with distance from system Drainfield 0.06 mg/l (200 and 201s) 15 m = mg/l; 35 m = mg/l Piezometer 207/208 not in flow path had mean PO 4 -P < 0.01 mg/l Creek PO 4 -P = mg/l Statistically significant PO 4 -P concentration near drainfield relative to 35 m, 207/208 and the creek 13

14 Site 400 TDN Reduction TDN concentration Reductions 75% septic tank to overflow 91% septic tank to groundwater near filter 93% tank to filter effluent No mass reduction of TDN between tank and overflow 50% mass reduction of TDN between tank and filter effluent Tank and Overflow mostly NH 4 85%+ Filter effluent mostly NO 3 (90%) Site 400 PO 4 Treatment % PO 4 -P concentration reduction from tank to overflow 98% PO 4 -Pconcentration reduction from tank to filter effluent 83% mass reduction of PO 4 -P from tank to filter effluent 14

15 Site 500 TDN Reduction Concentration Reductions Tank to Filter = 67% Tank to Ditch = 63% Tank to GW (501) = 97% Tank to filter p = 0.04 Most filter effluent NH 4 so filter not efficient at nitrifying wastewater Site 500 PO 4 -Treatment Filter effluent PO 4 -P was 81% lower than septic tank effluent Ditch receiving filter effluent had PO 4 -P concentrations 86% lower than tank Groundwater near sand filter at piezometer 501 had PO 4 -P concentrations significantly higher than at piezometers away from the filter (p < 0.04) 15

16 Site 400 and 500 Isotope Data Groundwater NO 3 near the sand filter at site 400 was from wastewater source and NO 3 in water supply wells at the sites most likely from wastewater source (animal or human) Summary Conventional, subsurface discharge OWS had mean TDN and PO 4 concentration reductions of 98% and 99% at 35 m downgradient from the systems and mass load reductions of 70 and 96% Isotope, Cl/TDN ratios and NO 3 concentrations point towards denitrification as a mass removal mechanism for N Surface discharge systems had mean TDN and PO 4 concentration reductions of 80 and 84% and mass load reductions of 50 and 83% (Site 400) Surface overflow from malfunctioning OWS had TDN and PO 4 concentrations of 12.9 and mg/l respectively (75-87% concentration reduction) but mass reductions were not observed using 2 component mix model 16

17 References Humphrey, C. P., O'Driscoll, M. A., & Zarate, M. A. (2010). Controls on Groundwater Nitrogen Contributions from On-site Wastewater Systems in Coastal North Carolina. Journal of Water Science and Technology 62 (6), Humphrey, C.P. & O Driscoll, M.A. (2011). Biogeochemistry of Groundwater Beneath On-site Wastewater Systems in a Coastal Watershed. Universal Journal of Environmental Research and Technology, 1(3) Humphrey, C.P., O Driscoll, M.A., Deal, N., Lindbo, D., Zarate-Bermudez, M.A., and Thieme, S. (2013). On-site Wastewater System Nitrogen Contributions to Groundwater in Coastal North Carolina. Journal of Environmental Health, 76 (5) O Driscoll, M.A., Humphrey Jr, C.P., Deal, N.E., Lindbo, D.L., and Zarate-Bermudez, M.A. (2014). Meteorological Influences on Nitrogen Dynamics of a Coastal Onsite Wastewater Treatment System. Journal of Environmental Quality. DOI: /jeq Humphrey, C.P., O Driscoll, M.A., Deal, N., and Lindbo, D. (2014). Fate and Transport of Phosphate from an Onsite Wastewater System in Beaufort County, North Carolina. Journal of Environmental Health, 76 (6) Iverson, G., O Driscoll, M.A., Humphrey Jr, C.P, Manda, A.K., and Anderson-Evans, E. (2015). Wastewater Nitrogen Contributions to Coastal Plain Watersheds, NC, USA. Water, Air and Soil Pollution. 226 (10) 355. DOI: /sll Humphrey, C., Anderson-Evans, E., O Driscoll, M., Manda, A., and Iverson, G. (2015). Comparison of Phosphorus Concentrations in Coastal Plain Watersheds Served by Onsite Wastewater Treatment Systems and a Municipal Sewer Treatment System. Water Air and Soil Pollution. DOI /s Acknowledgements NC DHHS Home/system owners Durham County Public Health Wake County Environmental Services Jonathan Harris, Christa Sanderford, John Woods, Jim Watson, Adam Trevisan, Colleen Rochelle and others 17