3.3 Anthropogenic Sources of DOC

One of the defining characteristics of groundwater in pristine aquifer systems (those that have not been chemically affected by human activities) is that DOC concentrations are relatively low, in the 0.4 to 4.0 mg/L (~30 to ~330 µmol/L) range (Regan et al., 2017). In contrast, groundwater that has been chemically affected by human activities frequently exhibits DOC concentrations in the 4.0 to 120 mg/L (~330 to ~10,000 µmol/L) range (Regen et al., 2017). Because of that observation, DOC concentrations greater than 4.0 mg/L (~330 µmol/L) have historically been used as a qualitative indicator of chemical contamination by human activity (Barcelona, 1984). A variety of human activities including sewage disposal, tilled agriculture, feedlot operations, petroleum hydrocarbon storage facilities, and industrial solvent disposal have the potential for delivering dissolved organic compounds to groundwater.

3.3.1 Tilled Agriculture

An example of tilled agriculture affecting DOC concentrations in groundwater was described by Thayalakumaran and others (2015). The study site is in Queensland, Australia, in some of that country’s most productive irrigated farmland. In particular, the area is noted for some of the highest yields and highest quality of sugarcane in Australia. The agricultural land is underlain by a sandy aquifer of alluvial and marginal marine origin. The tilled farmland is heavily irrigated with water derived from wells and the nearby Burdekin River. The combination of the principal crop (sugarcane) and rapid recharge of percolated irrigation water produces an abundance of both particulate and dissolved organic carbon. The resulting DOC provides significant reducing potential that has had a dramatic effect on the groundwater chemistry of the shallow aquifer. Concentrations of DOC range from 5 to 50 mg/L (~40 to ~410 µmol/L), concentrations of dissolved oxygen and nitrate are generally less than 1 mg/L, and concentrations of dissolved iron often exceed 1 mg/L. These reducing conditions have both positive and negative effects on groundwater quality. On the positive side, the high concentration of bioavailable DOC produces anoxic conditions that foster nitrate reduction, effectively preventing excessive concentrations of nitrate from accumulating in the groundwater. On the negative side, those conditions also promote Fe(III) reduction leading to high concentrations of dissolved iron that can lead to problems such as clogging of irrigation wells.

3.3.2 Feedlot Operations

An example of cattle feedlot operations affecting DOC concentrations and groundwater chemistry was reported by Coote and Hore (1979). The study site was in Ontario, Canada, and had been used as a feedlot for 6 years. Before the feedlot operation, the site had been a barnyard for the previous 80 years. The feedlot was approximately 24 m × 34 m in size and was underlain by a shallow sandy aquifer with the water table approximately 2 m below land surface. A system of shallow, depth-nested sampling wells was installed adjacent to the feedlot extending 130 m downgradient. Not surprisingly, DOC concentrations just two meters downgradient of the feedlot and just below the water table were high (~700 mg/L; ~60 mmol/L), and DOC concentrations 130 m downgradient decreased only to 79 mg/L (6.5 mmol/L). Chloride concentrations along that same flow path decreased from 664 mg/L to 79 mg/L, virtually identical to the change in DOC concentrations. That, in turn, suggests that the concentration decreases reflect mixing with uncontaminated recharge water and that biodegradation was less important. However, the high levels of DOC near the feedlot show that feedlots can be a significant source of DOC to groundwater.

3.3.3 Petroleum Hydrocarbons

An example of petroleum hydrocarbons affecting concentrations of DOC in groundwater was described by Petkewich and others (1997). The study site was a jet fuel storage facility in Hanahan, South Carolina where an estimated 83,000 gallons (~314,000 L) of Jet Propellant-4 fuel leaked from a tank in 1975 (Vroblesky et al., 1997). A plume of petroleum hydrocarbon-contaminated groundwater developed over time and migrated beneath an adjacent housing development. A series of strategies were employed to remediate the site, and in 1990 a pump-and-treat system was installed to collect contaminated water. For the next four years, groundwater chemistry from each of 18 extraction wells was monitored for a variety of analytes, including DOC and total petroleum hydrocarbons (TPH). Measurements of DOC would include the contribution of TPH compounds. However, concentrations of TPH compounds would not reflect the influence of naturally occurring DOC generated from plant material at land surface.

Concentrations of DOC and TPH hydrocarbons measured in groundwater collected from one of the extraction wells (EW-8) are shown over a four-year period in Figure 11. Initially, groundwater produced from the well had relatively low DOC concentrations (3.0 mg/L; 250 µmol/L) and TPH concentrations below the detection limit (11 mg/L; ~85 µmol/L). This reflects the fact that well EW-8 was located outside the existing plume of contaminated groundwater. After initiation of pumping, however, petroleum hydrocarbon-contaminated water was drawn to the well and concentrations of both TPH and DOC initially increased (Figure 11). Over time, however, DOC concentrations continued to increase whereas TPH concentrations remained static at about 1.5 mg/L (125 µmol/L). By the end of the monitoring period (1996), DOC concentrations had more than doubled from their initial levels (~8 mg/L; ~670 µmol/L) whereas TPH concentrations were once again near or below the detection level (1 mg/L).

Graph showing concentrations of dissolved organic carbon and total petroleum hydrocarbons measured in groundwater

Figure 11  Concentrations of dissolved organic carbon (DOC) and total petroleum hydrocarbons (TPH) measured in groundwater from extraction well EW-8 between 1991 and 1996 at the Hanahan SC, fuel storage facility. Data are from Petkewich and others (1997).

The data in Figure 11 show some interesting trends that are probably representative of processes occurring at other petroleum hydrocarbon-contaminated sites as well. The increasing DOC concentrations reflect, in part, drawing TPH to well EW-8 (Figure 11) as soon as pumping commenced. But the relatively low TPH concentrations (1 to 2 mg/L) sustained over time cannot account for the entire observed increase of DOC concentrations (to ~8 mg/L; ~670 µmol/L). The source of DOC present in the initially uncontaminated groundwater were plants growing at land surface. But as pumping drew TPH to the well, the availability of a new source of metabolizable carbon (TPH) seems to have stimulated TPH biodegradation. This new carbon source (TPH) may have stimulated microbial activity, increased production of microbial metabolites (Figure 8), and those metabolites subsequently increased overall DOC concentrations (Figure 11).

3.3.4 Chlorinated Solvents

Dissolved organic carbon that is either naturally occurring or is artificially added to groundwater, can serve as an electron donor driving the reductive biodegradation of chlorinated solvents in groundwater (Chapelle et al., 2012). An example of naturally occurring DOC driving reductive dechlorination (Bradley et al. 2009) with artificially added DOC enhancing the process was described by Bradley and others (2012). The site is a decommissioned Naval Air Warfare Center (NAWC) located in West Trenton, New Jersey that is underlain by fractured, Triassic-age mudstone shales and sandstones. Large quantities of trichloroethene (TCE) were used at this site as a refrigerant to simulate high-altitude temperatures for testing jet engines between 1956 and 1998. Over the years, accidental spills of TCE contaminated the underlying bedrock aquifer, and efforts to remediate that contamination were initiated in 1995 (Lacombe, 2000).

The NAWC site is one of only a few where concentrations of DOC and chlorinated ethenes were monitored under first naturally occurring conditions, and then after adding artificial DOC to further enhance biodegradation. Natural DOC at this site is generated in the soil zone overlying the bedrock and is transported to the fractured aquifer by percolating recharge. The artificial DOC added to enhance reductive dichlorination consisted of an emulsified vegetable oil solution (EOS) that included sodium lactate (Borden et al. 2007). Some results of the NAWC study are shown in Figure 12.


Figure 12  Concentrations of a) TCE, cis-DCE, and DOC and b) TCE, VC, and DOC at the NAWC site in well BR-36 before and after bioremediation using emulsified vegetable oil.

Prior to October 2008, naturally occurring DOC generated in the soil zone was the sole electron donor supporting the degradation of the chlorinated ethenes. That natural DOC (~5 to 10 mg/L; ~400 to ~800 µmol/L) supported reductive dichlorination of TCE (~5,000 µg/L) as indicated by the high concentrations (~4,000 µg/L) of cis-dichloroethene (cis-DCE), the first degradation product of TCE (Figure 12a). Concentrations of vinyl chloride (VC), the degradation product of cis-DCE, however, were less than 20 µg/L. This suggested that natural DOC, while capable of driving reductive dichlorination of TCE, was less able to drive cis-DCE dichlorination. After addition of the emulsified vegetable oil/sodium lactate solution, the higher concentrations of DOC (~40 to 60 mg/L; ~3,300 to 5,000 µmol/L) led to increased concentrations of both cis-DCE (Figure 12a) and VC (Figure 12b) while lowering concentrations of TCE. These studies show that both natural and artificially added DOC can drive reductive dichlorination in chlorinated-ethene contaminated groundwater.


Dissolved Organic Carbon in Groundwater Systems Copyright © 2022 by Francis H. Chapelle. All Rights Reserved.