Natural Source Zone Depletion at a Fuel Release Site

Transcription

1 Natural Source Zone Depletion at a Fuel Release Site Doug Mackay Adjunct Professor, Emeritus University of California, Davis For Presentation at 2018 CUPA Conference, San Francisco, February 8, 2018

2 Acknowledgments Coauthors Chevron Energy Technology Company: Tim Buscheck, Eric Daniels, Natasha Sihota UC Davis: Charlie Paradis, Nick de Sieyes, Emily Hathaway, Ehsan Rasa, Rad Schmidt, Sherry Peng, Southwest Jiaotong University, Chengdu, China: Prof. Han Zhang Collaborators Dave Patten, Chevron Environmental Management Company Travis Flora and colleagues, Stantec Regulatory support Gerald O Regan, Santa Clara Department of Environmental Health Discussions and advice/review Dr. Barbara Bekins (USGS), Prof. Bruce Honeyman (Colo. Sch. Mines, emeritus), Dr. John Wilson (Scissortail Environmental), Chuck Newell (GSI) Funding Chevron Environmental Management Company

3 Outline Natural Source Zone Depletion (NSZD) of Petroleum Hydrocarbons (PHCs) Depletion of PHC source by vertical migration through vadose zone total flux of mass from source (grams per day) migrating in vapor: efflux Depletion of PHC source by horizontal migration through saturated zone total flux of mass from source (grams per day) migrating in groundwater Estimation of NSZD at a PHC release site Site characteristics Comparison of NSZD via vadose and saturated zones Summary and Implications

4 LNAPL release to subsurface A typical cartoon (this from Garg et al., GWMR, 2017) Let s convert this to a conceptual diagram more useful for our discussions of NSZD

5 Natural Source Zone Depletion (NSZD) (based on chemical species ) V-NSZD in vadose zone yields gaseous petroleum hydrocarbon (PHC) degradation products (DPs) detectable at surface GW-NSZD in saturated zone results in transport of PHCs & DPs dissolved in groundwater (GW), and immobilization of some DPs as solids Ground surface Vadose zone Saturated zone V-NSZD Total flux of vapor phase PHC and DPs due to gas diffusion and gas flow PHC & DPs LNAPL LNAPL Immobilized DPs PHC & DPs GW-NSZD Total flux of dissolved PHC and DPs due to GW flow Note: All PHCs and their DPs contain CARBON

6 Natural Source Zone Depletion (NSZD) (based on CARBON CONTENT of chemical species ) V-NSZD Local fluxes of vapor phase carbon: Background: CO 2 Vapor Plume: CO 2, CH 4 (rarely PHC) Ground surface Vadose zone Saturated zone Immobilized Carbonates GW-NSZD Local fluxes of dissolved carbon: GW Plume: PHC, DIC, DOC, CH 4 Background: DIC, DOC Goal is determining RED arrows, but measure TOTAL so have to subtract BACKGROUND

7 Importance of Quantifying NSZD (Natural Source Zone Depletion) Assess monitored natural attenuation as a remediation strategy Quantify and document contaminant mass loss Reduce costs by using results in site management decisions Compare NSZD rates with typical expected engineered remediation depletion rates Define baseline depletion to assess actual results of engineered remediation

8 Site history 1930s-1993: Site operated, dispensing primarily gasoline Groundwater level dropped dramatically during the 1950s and 1960s, apparently leading to deep penetration of LNAPL Groundwater rebounded in 1970s leading to entrapment of LNAPL as much as 45 feet below water table April 1993: service station ceased operations and all above and below ground facilities were removed : Groundwater extraction and treatment 1993: Over-excavation and disposal of 840 yd 3 of soil : Soil vapor extraction 1996: Oxygen release compound : Biosparge system : Low-flow ozone sparge system None of these technologies was effective : Monitored Natural Attenuation, the period addressed in this study of NSZD

9 Study site Max extent of source zone in vadose and saturated zones

10 Vadose and Saturated Zone Monitoring Locations All unpaved Shallow Middle Deep More efflux locations than wells; total efflux area greater than total GW discharge area

11 Weather and Depth to Water Temperature pretty consistent Rainfall lower in 2012 and 2014 Depth to water variable, decreasing during latter half of efflux monitoring

12 Groundwater Monitoring

13 Concentration (mg/l) Concentration (mg/l) Groundwater Monitoring Results U-7 (a) U-7 DIC calc DIC DOC TPHg BACKGROUND (d) U-15 middle 1 TPHd-sgc Methane 0.1 Ca O 3 sparge Monitoring of natural attenuation 1/14/04 1/13/07 1/12/10 1/11/13 1/11/16 (b) UV-2 shallow Date (e) U-18 middle O 3 sparge Monitoring of natural attenuation O 3 sparge Monitoring of natural attenuation (c) UV-3 shallow (f) U-4 deep O 3 sparge Monitoring of natural attenuation O 3 sparge Monitoring of natural attenuation Date

14 Surface Efflux Monitoring Chamber Offset Install soil collars Measure chamber offset Allow minimum of 2 hours for equilibration Conduct real time surficial CO 2 efflux measurement using the LI-8100A Measure CH 4 efflux by either Manually collecting gas samples over time through an in-line sampling port as shown, or Collecting real-time CH 4 measurement with LGR GGA plumbed in-line with LI-8100A Bottom line: CH 4 efflux was insignificant

15 Total Soil Respiration (TSR) Total amount of CO 2 migrating through soil surface This is the raw result of monitoring, but includes natural soil respiration (NSR) Efflux Monitoring Results Contaminant Soil Respiration (CSR) Total amount of CO 2 migrating through soil surface which originates from PHC degradation CSR = TSR-NSR

16 Overview of Results During the period of MNA: TOTAL NSZD averaged 1930 g/d as carbon 6% GW-NSZD 107 ± 53 g/day as carbon (high uncertainty) 94% V-NSZD 1823 ± 96 g/day as carbon (lower uncertainty) V-NSZD often expressed as gallons per acre per year This small site is similar to some much larger sites Site Area (hectares) V-NSZD (gallons/acre/yr) Reference Former service station This study Former Guadalupe Oil Field, CA; multiple diluent releases Lundegard and Johnson, 2006 Former Midwest Refinery Eichart etal., 2017 Crude oil spill, Bemidji, MN 6.5 >1400 Sihota et al., 2011, 2016

17 Implications, 1 To estimate V-NSZD Need to measure CO 2 (CH 4 likely much less important) Need to carefully define relevant background Easiest if site is unpaved Pay attention to depth to groundwater (DTW) as V-NSZD may vary as DTW changes To estimate GW-NSZD Need wells across transect downgradient of source zone Need to measure DIC and DOC (primary components) Need to carefully define relevant background

18 Implications, 2 V-NSZD and GW-NZSD comparisons can inform remediation decisions regarding media to target and potential to enhance biodegradation (i.e., vadose zone vs. saturated zone). V-NSZD and GW-NSZD rates measured prior to vadose zone or groundwater remediation can serve as a baseline and measurements can be repeated during and following remediation to quantify progress. V-NSZD and/or GW-NSZD baseline rates can be compared to engineered remediation depletion rates to determine if remediation has reached its limit of effectiveness. V-NSZD and GW-NSZD can be measured periodically to monitor remediation progress

19 Implications, 3 Achievement of remediation goals for groundwater may not be strongly related to V-NSZD especially where there are LNAPL smear zones extending far into the saturated zone, such as at our study site