QUANTIFYING SUBSURFACE WASTEWATERMIGRATION IN COASTAL SURFICIAL AQUIFERS: A GEOELECTRICAL APPROACH BACKGROUND: ONSITE WASTEWATERDISPOSAL INCOASTALNC

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1 QUANTIFYING SUBSURFACE WASTEWATERMIGRATION IN COASTAL SURFICIAL AQUIFERS: A GEOELECTRICAL APPROACH Michael O Driscoll, Associate Professor, Geological Sciences/ Director, Coastal Water Resources Center Charles Humphrey, Assistant Professor, Environmental Health Sciences Program David Mallinson, Professor, Dept. of Geological Sciences Matthew Smith, Graduate Student, Dept. of Geological Sciences Sarah Hardison, Graduate Student, Dept. of Geological Sciences BACKGROUND: ONSITE WASTEWATERDISPOSAL INCOASTALNC There were 1.4 million septic systems in North Carolina (US Census, 1990) when population was 6.6 million. ~ 60% of Coastal NC residences use septic systems to dispose of their wastewater to the shallow groundwater system (NCNERR 2001). Some coastal watersheds have systems/ sq. mile (Pradhan et al. 2007). (Pradhan et al. 2007) 1

2 BACKGROUND: WATER QUALITY ISSUES ASSOCIATED WITH RESIDENTIAL WASTEWATER Nutrients Pathogenic Microorganisms Pharmaceuticals/Personal Care Products Surface failures or backups EPA (2003) estimates ~ 10 20% of systems are failing THE DEFINITION OF FAILURE Current NC regulations defines system malfunctioning as wastewater at surface/residence or direct effluent release to groundwater failing not failing What if the system does not adequately treat the wastewater in the subsurface? 2

3 CAN WE DETECT INADEQUATE SUBSURFACE WASTEWATER TREATMENT? Beaufort Co., CDC Study Drainfield To estuary To estuary Drainfield CAN WE DETECT INADEQUATE SUBSURFACE WASTEWATER TREATMENT? YES, BUT.. Example: Groundwater Total Dissolved Nitrogen (TDN) Transport to Pamlico River Estuary, Beaufort County ( ) Drainfield NC 15 m setback NC 30 m setback Groundwater flowpath 3

4 CAN WE POSSIBLY MONITOR ~ 2 MILLION SYSTEMS? WOULD NEED AT LEAST 6 MILLION WELLS/ PIEZOMETERS!!!!!!! Groundwater monitoring to determine if inadequate treatment occurs. But there are limitations: Labor and resource intensive Intrusive Need property owner permission Wells/piezometers may be misplaced & may not intercept contaminant plumes In the Coastal Plain, surficial aquifer is typically well connected to surface water network. Inadequate treatment can affect sw and gw. and teacher power 2 OBJECTIVE: DETERMINE IF RESISTIVITY SURVEYS CAN DETECT WASTEWATER AFFECTED GROUNDWATER Hand auger Geoprobe Electrical resistivity 4

5 STUDY SITES Scale=100km N Greenville 1. Site 1 Residential/Estuary (Beaufort Co.) 2. JW Smith School Large system/river (Craven Co.) 3. West Craven High School Large system (Craven Co.) 4. ATFS Science Education Center/Ponds, River (Pitt Co.) 5. Site 2 Residential/River (Pitt Co.) Island (Carteret County) Three Investigations (2009 present) 1. Beaufort Co. (CDC) 2. Craven and Carteret Co. (RDG, WRRI) 3. Pitt Co. (WRRI) METHODS: CAPACITIVELY COUPLED RESISTIVITY Resistivity defined by the resistance to flow of electric current in a material. Measured by injecting electric current into the subsurface with current electrodes and measuring voltage difference between potential electrodes. Using Ohm s Law and a geometric factor based on electrode geometry and spacing, the apparent resistivity can be determined. OhmMapper (Geometrics, Inc., 2001) 5

6 METHODS: CAPACITIVELYCOUPLED RESISTIVITY Increased spacing between transmitter and receiver increases depth of resistivity measurement. Since apparent resistivity measurements represent an average value to a specified depth, inverse modeling is required to determine actual resistivity at a specified depth. Geometrics Inc., 2001 INTERPRETATION: WASTEWATER INPUTS SHOULD DECREASE RESISTIVITY Controls Wastewater on subsurface inputs typically resistivity increase dissolved ions in groundwater increase Moisture pore content water conductivity decrease Water quality subsurface (total resistivity dissolved solids) Clay content wastewater 6

7 RESULTS: CAN RESISTIVITY SURVEYS DETECT THE WASTEWATER INFLUENCE ON GROUNDWATER CONDUCTIVITY? YES (IN COASTAL PLAIN SURFICIAL AQUIFERS WITH SHALLOW WATER TABLES) CAN RESISTIVITY SURVEYS DETECT ELEVATED GROUNDWATER NUTRIENT CONCENTRATIONS? MAYBE (AT LEASTASASCREENING TOOL) RESULTS: RESIDENTIAL SITE/BEAUFORT CO. Median Groundwater Specific Conductivity ( ; n=9) 7

8 RESULTS: RESIDENTIAL SITE/BEAUFORT CO. RESISTIVITY AND GROUNDWATER CONDUCTIVITY A A A A Survey, February 2, 2011 RESULTS: JWS SITE/CRAVEN CO. RESISTIVITY AND GROUNDWATER CONDUCTIVITY Drainfield Piezometers 3 D Geophysical Survey Area 2 Dimensional Geophysical Transect (Red Line A A ) Smith,

9 2 D Resistivity Transects (Craven Co., JWS Site) Drainfield A A Low resistivity under the drainfield (green and blue; resistivity is < ~100 ohm m) RDG, 2011 JWS Site Comparison of Resistivity vs GW Conductivity Drainfield Resistivity (ohm m) R² = Groundwater Specific Conductivity (us/cm) JWS Elementary School, T1 (175 m long), ER survey collected on Sept. 10, 2012 Low resistivity under drainfield corresponds with zone of elevated groundwater conductivity, Indicates wastewater affected groundwater

10 CANRESISTIVITYSURVEYS DETECT ELEVATED GROUNDWATER NUTRIENT CONCENTRATIONS? Earth Resistivity (ohm m) resistivity tdn Groundwater Specific Conductivity (us/cm) Groundwater Total Dissolved Nitrogen (mg/l) 9/10/2012 JWS CANELEVATED GROUNDWATER NUTRIENT CONCENTRATIONS BE DETECTED WITHSHALLOWEARTH RESISTIVITY? Groundwater TDN (mg/l) May help screen areas with elevated gw nutrients /pathogens But. can have elevated gw conductivity/reduced resistivity and low N concentrations if N retention/loss occurs, e.g. denitrification Maybe. Elevated resistivity; Low tdn Resistivity (Ohm m) 10

11 Detection of surface water impairment JWS 3D resistivity; 4 m depth (9/10/2012) Dark green and blueresistivity < ~ 320 ohm m In plume area groundwater conductivity is > 300 us/cm Groundwater conductivity at spring also indicates approximate N S orientation of ww plume Humphrey et al Elevated Conductivity and Nitrogen Concentrations in Groundwater Indicators of Wastewater Influence * Mean data for 3 sampling dates (Sept., Nov. 2012, Mar. 2013) 50 Groundwater Mean Total Dissolved Nitrogen (mg/l) Background (1, 3, 9) Spring Tank (Wastewater) Groundwater Mean Specific Conductivity (us/cm) 6 10 Core of Plume

12 Resistivity surveys can help with piezometer siting Craven Co. WCH Mapview 1.3 m depth 11/14/2012 Low resistivity under the drainfield (dark green; resistivity is < ~500 ohm m) Initial study at WCH Craven County 2 D Survey 7/13/2011 Depth (m) RDG Study, M. Smith Thesis Resistivity declined with depth, suggested more conductive water at depth and that we needed more piezometers if plume was diving But funding for that pilot project was all gone! 12

13 Resistivity data from previous study helped us site nested piezometer 25 and confirm the presence of wastewater affected groundwater at depth GEOPHYSICAL AND WATER QUALITY CHARACTERIZATION OF ON SITE WASTEWATER PLUMES Water Resources Research Institute Project Number: W GROUNDWATER CONDUCTIVITY VS RESISTIVITY AT ALL 4 SITES (WRRI STUDY ) Surficial aquifer resistivity < ~ 250 ohm m can indicate the presence of wastewater affected groundwater. Resistivity (Ohm m) Wastewater affected groundwater Groundwater Conductivity (us/cm) 13

14 Ideal Sites shallow groundwater, sandy soils, large groundwater conductivity contrasts Core at WCH (4m depth) Specific Conductivity (us/cm) WC-BG WCH JWS Science Center Residential WC-T WC-DF JWS-BG JWS-T JWS-DF ATFS-BG ATFS-T ATFS-DF R-BG R-T R-DF T= tank (wastewater) DF=drainfield groundwater BG= background groundwater when contrast between background gw and drainfield gw conductivity is low, resistivity contrasts will be low Summary Resistivity surveys can detect areas of wastewater affected groundwater when drainfield groundwater specific conductivity contrasts with the background groundwater. May be a useful screening tool for finding groundwater with potentially elevated nutrient/pathogen concentrations, but false positives are possible if nutrient loss/retention occurs. Interpretation may be complicated by heterogeneous geology, clays, salinity, water use, and infrastructure. 14

15 Future Work Ground Penetrating Radar Images of Drainlines (Smith, 2013) Nondestructive imaging of onsite systems in various geological settings (Ground Penetrating Radar, Electromagnetic Induction, and Resistivity Mapping) Broader scale surveys that determine risk factors for sw contamination. Acknowledgments STUDENTS Matt Smith Sarah Hardison Dustin DeLoatch FUNDING Centers for Disease Control and Prevention NC WRRI NC DENR 319 COLLABORATORS Max Zarate (CDC) Dave Lindbo (NCSU) Nancy Deal (DHHS) Alex Manda (ECU) TECHNICAL SUPPORT Jim Watson John Woods And Homeowners and Facilities Managers! Thanks for your attention! 15