Processes Controlling Attenuation of Dissolved Organic Matter in the Subsurface

By Jerry A. Leenheer
U.S. Geological Survey, P.O. Box 25046, MS 408, Denver Federal Center, Lakewood, Colorado 80225

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Abstract

The four major processes that directly control attenuation of dissolved organic matter (DOM) in the subsurface are adsorption and/or coprecipitation, biodegradation, and dilution.  Factors that indirectly control DOM attenuation are groundwater redox state and pH (affects adsorption/precipitation and biodegradation); nutrient availability (affects biodegradation and redox); and rainfall, snowmelt, recharge, and evapotranspiration (affects dilution and precipitation).

DOM attenuation by adsorption (reaction with soil and aquifer surfaces) and/or precipitation (with groundwater cations such as aluminum, calcium and iron) is most obviously evidenced by soil podzolization processes in which DOM derived from oxidation of surface vegetation leaches aluminum, iron, manganese, and clay colloids from the albic horizon followed by precipitation (caused by pH and Eh changes) of DOM/metal complexes and clays in the underlying spodic horizon (Soil Conservation Service, 1975). Soil podzolization occurs in humid environments whereas in more arid environments, calcic horizons tend to form that coprecipitate DOM. A study (Alsmadi and Fox,2000) of soil aquifer treatment (SAT) of reclaimed water (tertiary-treated wastewater) in Phoenix, Arizona found that SAT resulted in doubling the amount of organic matter (from 14 to about 28%) that was associated with iron, manganese, and carbonate coatings on aquifer surfaces that formed during SAT treatment.  Adsorption and precipitation processes reduced DOM by greater than 90% during infiltration of "black waters" in northern Florida through karst sinkholes into the Upper Floridian limestone aquifer (Rostad et al., 2000).

DOM components, such as fulvic acid derived from tannins and lignins, are removed by ligand-exchange adsorption on aluminum, iron, and manganese sesquioxide coatings on mineral surfaces (Leenheer, 1991) whereas fulvic acid derived from terpenoid resins are not removed by sorption on these sesquioxides (unpublished research, Leenheer, 2002).  The presence of clustered carboxylic acids in fulvic acid structures is necessary for fulvic acid adsorption/precipitation with calcium, iron, and manganese because of the formation of polydentate chelate complexes. These structures are present in fulvic acid derived from tannins and lignin (Leenheer et al., 1998), but not in fulvic acid derived from terpenoid resins such as was found in the Norman landfill (unpublished research, Leenheer, 2002).

Peptidoglycan colloids derived from bacterial cell walls comprise 30-40% of the DOM in many natural surface-waters (Leenheer et al., 2000) and 40% in reclaimed water (Leenheer et al., 2001).  When this reclaimed water was basin-infiltrated in Los Angeles County, California., this colloidal DOM was quantitatively removed in the first 1-foot of infiltration.  Similar results were observed during infiltration of Santa Ana River in Anaheim Lake in Orange County, California (Leenheer, unpublished research, 2002). The mechanism of removal of these peptidoglycan colloids is believed to be adsorption on mineral surfaces as biodegradable algal cells were observed to infiltrate over 200 ft below Anaheim Lake.  The sorption mechanism for these colloids is hypothesized to be a combination of electrostatic interactions (basic positively-charged lysine unit on peptide chain with negatively-charged mineral surfaces) and hydrogen bonding of amide structures (peptides and N-acetyl amino sugars) with mineral oxides and hydroxides (Leenheer, 1991).

Biodegradation processes can also be subdivided into colloidal DOM biodegradation and truly dissolved DOM biodegradation. The colloidal DOM that adsorbs at the bottom of basins used to infiltrated reclaimed water is a component of the biologically active "schmutzdecke" organic layer where aerobic biodegradation removes about 25 % of the DOC during the first 10 cm of infiltration (Quanrud et al., 1996).  Additional reductions in truly dissolved DOC over longer flow-paths and time periods result in overall DOC reductions from 48 to 90 % (Bower et al., 1974; Idelovitch and Michail, 1984; Wilson et al., 1995). These reductions in DOM result from a combination of adsorption and biodegradation.

A combination of low dissolved oxygen concentrations in groundwater and organic structures resistant to biodegradation has resulted in sulfonated anionic surfactant metabolites of LAS being stable to biodegradation over long time periods and flow paths in infiltrated sewage effluents at Cape Cod, Ma. (Field et al., 1992) and Los Angeles, Ca (Leenheer et al., 2001).  The greater DO concentrations in groundwater infiltrated from the Santa Ana River, Orange County, California is probably responsible for the more efficient biodegradation of surfactant metabolites and DOC at this site (Ding et al., 1999, Leenheer, unpublished research, 2002).  Aliphatic alicyclic rings are structural components of DOM that are most resistant to biodegradation (Trudgill, 1984) and are found in LAS metabolites such as dialkytetralin sulfonates (Field et al., 1992) and terpenoid resin acids.  However, both DOC attenuation data and spectral characterization of DOM fractions isolated from infiltrated Santa Ana River water at the Orange County study site indicate aerobic biological degradation of aliphatic alicyclic rings of both DATS and terpenoid acids (Leenheer, unpublished research, 2002).

DOM attenuation by dilution can be assessed by comparing various conservative tracers (bromide, chloride, sulfate, boron, tritium, conductivity) that occur in infiltrating water versus receiving groundwater (Anders and Schroeder, 1998; Leenheer et al., 2001).  A unique tracer such as sulfur hexafluoride can also be added to infiltrating water (Gamlin et al., 2001).  The conservative component of DOM in infiltrated water can be identified and differentiated from native ground-water DOM at the compound-class level of identification by a combination of infrared and ¹³C-nuclear magnetic resonance spectral analyses, ¹⁴C age dating of DOM isolates, and ¹³δC isotope analyses of DOM isolates (Leenheer et al., 2001).  This comprehensive study of DOM attenuation at various points and times in the infiltration flow-path was able to differentiate the various DOM attenuation processes by assessing variations of dissolved constituents with time in the source water, assessing dissolved constituents in upgradient and downgradient uncontaminated groundwaters, and by applying multiple, independent analytical techniques to characterize DOM and various inorganic tracers.

References

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In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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