Biochemical Indicators for the Bioavailability of Organic Carbon in Ground Water

Abstract

The bioavailability of total organic carbon (TOC) was examined in ground water from two hydrologically distinct aquifers using biochemical indicators widely employed in chemical oceanography. Concentrations of total hydrolyzable neutral sugars (THNS), total hydrolyzable amino acids (THAA), and carbon-normalized percentages of TOC present as THNS and THAA (referred to as ''yields'') were assessed as indicators of bioavailability. A shallow coastal plain aquifer in Kings Bay, Georgia, was characterized by relatively high concentrations (425 to 1492 lM; 5.1 to 17.9 mg/L) of TOC but relatively low THNS and THAA yields (~0.2%-1.0%). These low yields are consistent with the highly biodegraded nature of TOC mobilized from relatively ancient (Pleistocene) sediments overlying the aquifer. In contrast, a shallow fractured rock aquifer in West Trenton, New Jersey, exhibited lower TOC concentrations (47 to 325 lM; 0.6 to 3.9 mg/L) but higher THNS and THAA yields (~1% to 4%). These higher yields were consistent with the younger, and thus more bioavailable, TOC being mobilized from modern soils overlying the aquifer. Consistent with these apparent differences in TOC bioavailability, no significant correlation between TOC and dissolved inorganic carbon (DIC), a product of organic carbon mineralization, was observed at Kings Bay, whereas a strong correlation was observed at West Trenton. In contrast to TOC, THNS and THAA concentrations were observed to correlate with DIC at the Kings Bay site. These observations suggest that biochemical indicators such as THNS and THAA may provide information concerning the bioavailability of organic carbon present in ground water that is not available from TOC measurements alone.

Origins and bioavailability of dissolved organic matter in groundwater

“…Bioavailable DOM is subjected to microbial decomposition, resulting in a reduction in size and molecular weight. Novel molecules are synthesized by soil microbes, and some of these metabolites enter the DOM reservoir in groundwater of systems (Chapelle et al 2009(Chapelle et al , 2011. In comparison, substantially higher percentages of BDOC were measured in surface water (29 ± 1 %; this study), which is consistent when compared to average literature values for streams (11-27 %), lakes (14 %), rivers (19 %), and seawater (19 %) (Qualls and Haines 1992;Søndergaard and Middelboe 1995;Volk et al 1997;Stutter et al 2013).…”

“…Overall, DOM in groundwater is a mixture of components derived from various sources and altered by multiple processes, making it difficult to predict the composition and bioreactivity of DOM solely from bulk DOM analyses (Chapelle et al 2009;Shen et al 2012a).The regional chromatography model is presented as a conceptual framework for understanding the sources and processes influencing the properties of DOM during its transport from the surface litter layer to groundwater. Additional studies of water transit times and pathways through soils would provide further insights about the processes shaping the concentration, composition, and bioavailability of DOM in groundwater.…”

“…The chemical composition and bioavailability of DOM, which are key factors affecting water quality and diagenetic processes in groundwater, are altered during its transport through the soil column. The bioavailability of DOM is tightly linked to its origins and its utilization by microorganisms and is not readily reflected in the concentrations of DOC (Benner 2003;Chapelle et al 2009). …”

“…Molecular analyses of specific biochemical components of DOM can provide qualitative and quantitative insights about the bioavailability and diagenetic alteration of DOM without conducting bioassay experiments (Benner 2003;Davis and Benner 2007;Shen et al 2012a). Amino acids are bioreactive components of DOM and have been used as molecular indicators of bioavailable DOM in marine environments (Benner 2003;Davis et al 2009) and have been applied recently to groundwater (Chapelle et al 2009). The composition of hydrolysable amino acids in DOM can be informative for revealing the extent of diagenetic processing of DOM.…”

“…Several studies have examined the bioavailability of DOM in groundwater using amino acids and other bioreactive compounds (e.g., carbohydrates) (Routh et al 2001;Chapelle et al 2009;Peter et al 2012). These studies have provided a snapshot of the relatively low bioavailability of DOM in groundwater.…”