Groundwater flow

Redox Zone Groundwater
FIGURE 4.14b One year later, methane isopleths show that the methane enhanced zone has become larger, as the concentrations of alternative electron acceptors decreased.

Field Test for Occurrence of Redox Reactions

• Map groundwater redox potentials at the site. Include at least one location upgradient of the plume. It is important to avoid aeration of well samples for these measurements.

• Locations where groundwater redox potentials are lower than background are where electron acceptor species are being reduced, a sign of biodegradation.

• Redox potentials within the plume can help indicate which electron acceptors are active in different locations.

• In regions of biodegradation activity, the zone of low oxidation potential (reducing zone) will become larger with time, as diminishing DO concentrations move farther and farther from the spill region.

Using Alkalinity to Locate Anaerobic Biodegradation Within the Plume

Groundwater alkalinity increases during aerobic respiration, denitrification, iron (III) reduction, and sulfate reduction and is unchanged during methanogenesis. The two main processes that increase alkalinity are

1. Production of carbon dioxide in the biodegradation of fuel hydrocarbons, which increases the total carbonate and alkalinity of the groundwater.

2. Redox reactions involving nitrate, iron (III), and sulfate as electron acceptors all consume acidity as H+ (see Equations 4.37, 4.39, and 4.40). Thus, alkalinity increases in groundwater where these reactions are taking place.

A measurement of alkalinity within a hydrocarbon plume can be used to infer the amount of petroleum hydrocarbons destroyed. For every 1 mg/L of alkalinity (as CaC03) produced, 0.13 mg/L of BTEX is destroyed.13

Field Test for Using Alkalinity as an Indicator of Biodegradation

1. Map groundwater alkalinity concentrations at the site. Include at least one location upgradient of the plume.

2. Locations where groundwater alkalinity is higher than background are where C02 is being produced and H+ is being consumed, which are signs of biodegradation.

3. Alkalinity levels within the plume can help indicate the amount of petroleum hydrocarbons that have been destroyed.

4. In regions of biodegradation activity, the zone of higher alkalinity will become larger with time.

5. Increased alkalinity levels within the plume can be used to infer the extent of biodegradation occurring.

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