1. Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison 1

2.  U.S. Environmental Protection Agency U.S. Environmental Protection Agency  Monitored natural attenuation as the "reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The 'natural attenuation processes' that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants." (EPA, OSWER Directive 9200.4-17P)biodegradationEPA, OSWER Directive 9200.4-17P  American Society for Testing and Materials (ASTM) American Society for Testing and Materials (ASTM)  Natural attenuation as the "reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes, such as; biodegradation, dispersion, dilution, adsorption, and volatilization." (ASTM, E1943-98)ASTM, E1943-98 2

3.  Adsorption: contaminants attached to clay particles, e.g., TCE adsorbed to organic matter in clay particles  Biodegradation: breakdown by microorganisms, e.g., leachate (organics) decomposed to CO 2  Ion exchange: Replacement of monovalent irons with divalent irons, e.g., Ca 2+, heavy metals, etc.  Dilution: mixing with large quantities of groundwater  Filtration: trap particulates in pores in the soil  Precipitation: change of the phase from liquid to solids, e.g., Fe 2+ + S 2- → FeS ↓ 3

4.  Attenuation: reduce contaminant concentrations during transport through the soil environment  Dilution by uncontaminated, infiltrating water  Physico-chemical interactions that fix, or retard, contaminent movement through the soil materials  High risk because of uncertainty in predicting contaminant loading and quantifying leachate attenuation mechanisms  Regulatory trend: maximize containment and removal of the leachate before release to the environment  A natural attenuation site permits the slow migration of liquids, allowing the natural processes of attenuation and dispersion in and beyond the site to reduce concentrations of the pollutants to safe levels.  Physical, chemical, and biological processes 4

5.  Use the natural, in-place soils and the groundwater flow system beneath the site to attenuate or cleanse contaminants leached from the solid wastes  Occurs primarily in the unsaturated zone beneath the base of the landfill Q L, Leakage Q 1 Groundwater Leachate plume Q 2 Precipitation Q 3 Groundwater Aquitard Aquifer 5 Q 4 Seepage Refuse Natural Attenuation

6.  Filtration: remove SS, precipitates  clogging  Dilution: decrease leachate concentration via upgradient groundwater and infiltration downgradient from the landfill.  Hydraulic dispersion: mechanical dispersion; velocity gradient, pore size difference, tortuosity of pore channels  Volatilization: evaporate to the vapor phase; occurs primarily in the unsaturated zone and at the water table surface; function of Henry’s law constant.  Physical sorption: attach to the surface of the soil particles by van der Waals forces; controlled by organic content of a soil  Diffusive processes: molecular diffusion in response to concentration gradients; significant primarily in situations with slow-moving, small contaminant pulses; major driving force in low-permeability soils 6

7.  Precipitation: Form insoluble salts of multivalent metallic ions; CO 3 2-, OH -, H 3 SiO 4 -, PO 4 3- High capacity and low reversibility; ion exchange and chemical precipitation Removal of Pb 2+, Zn 2+, Hg 2+, Cd 2+, Cu 2+, and Cr(IV) pH   precipitation  Fast rate  Sorption Adsorption, absorption, and desorption; remove organic compounds Polarity log K ow 7

8.  Ion exchange Taking-up and giving-off of positively charged ions by a soil Remove heavy metals Organic acid  IE  Cation exchange capacity (CEC): number of milliequivalents of cations that 100 g of soil can adsorb Na + < Li + < K + < < Mg 2+ < Ca 2+ < Ba 2+ < Cu 2+ < Al 3+ < Fe 3+ PO 4 3-, CrO 4 2-, Cr 2 O 7 2-, and AsO 4 3- : good removal Cl -, SO 4 2-, and NO 3 - : poor removal  Solubilization: CO 2 dissolution into water 8

9.  Aerobic and anaerobic biodegradation  Irreversible process  Major attenuation mechanism of organic matter  Long term  Enhanced by addition of nutrients and electron acceptor  Sorption negligible  Despite high organic matter in leachate, incidences of groundwater contamination by leachate have been identified only rarely.  Hg 2+ ion is readily converted to the highly volatile methyl forms in both aerobic and anaerobic sediments. 9

10.  Natural attenuation is generally unsuitable for all but small MSW landfills.  Inward gradient landfills: developed by excavating into a saturated clay soil environment; avoid disposal of liquid waste and avoid clay in daily cover material; require 1 ft thick granular drainage blankets  Chloride, sodium, sol. COD: unattenuated  K, NH 4 +, Mg, SiO 2, and Fe: moderately attenuated  Pb, Cd, Hg, and Zn: strongly attenuated  Ca, Br, and Mn: markedly higher in the effluent  Use mathematical model to understand the movement of landfill generated leachates into the subsurface 10

11. 11 % of groundwater RODs RODs: Record of Decisions

12. Sound scientific documentation (laboratory measurements or literature describing such measurements) that the mechanism claimed as responsible for contaminant destruction or control is scientifically feasible in the type of environment at the site 12 Cause  Documentation that the proposed mechanism is actually occurring at the site Effect

13.  Aerobic biodegradation of petroleum hydrocarbons 13 Electron acceptor: O 2 CO 2 HC  Biodegradation of organic contaminants N2N2 NO 3 - /NO 2 - H2SH2SSO 4 2- EA: HCCO 2  Reductive dechlorination of solvents Alkalinity TCE/1,1,1-TCA Cl - + Strong acid  Precipitation of uranium as UO 2(s) UO 2 2+ UO 2(s) Electron donor (COD/BOD) Strong acid Alkalinity

14.  General natural attenuation information  http://toxics.usgs.gov/highlights/nas_2.2.0/ http://toxics.usgs.gov/highlights/nas_2.2.0/  Natural Attenuation Software (NAS) Download  http://www.nas.cee.vt.edu/index.php http://www.nas.cee.vt.edu/index.php 14