The term Eh, which is the oxidation-reduction potential (often referred to as redox potential), is an expression of the tendency of a reversible redox system to be oxidized or reduced. It is especially significant in its influence on biodegradation processes. The energy of oxidation (electron-escaping tendency) present in a reversible oxidation-reduction system (in volts [V] or millivolts [mV]) is measured as the potential difference between a standard hydrogen electrode and the system being measured. Large positive values (up to ca. + 800 mV) indicate an oxidizing tendency, and large negative values (down to ca. -500 mV) indicate a strong reducing tendency. Eh values of + 200 mV and lower indicate reducing conditions in near-surface soils and sediments.16
The Eh of connate waters (water entrapped in the interstices of sediment at the time of deposition) ranges from 0 to -200 mV. For example, formation water from two monitoring wells in the lower limestone of the Florida aquifer near Pensacola ranged from + 23 to -32 mV,67 and formation fluids from a Devonian limestone in Illinois used for injection at a depth of about 3200 ft had an Eh of -154 mV.16
Several measures of organic pollutant loading to waters have been developed to indicate the redox status of a system:
1. Biochemical oxygen demand (BOD)
2. Chemical oxygen demand (COD)
3. Total organic carbon (TOC)
4. Dissolved organic carbon (DOC)
5. Suspended organic carbon (SOC)
When values for any of these parameters are high, oxygen is rapidly depleted in groundwaters and reducing conditions will develop. BOD and COD were designed to measure oxygen consumption during the microbial degradation of municipal sewage. They are only semiquantitative indicators of organic loading because measurement procedures for these parameters have no direct geochemical significance.65 Malcolm and Leenheer68 recommend the use of DOC and SOC, which are independent of microbial effects, toxic substance, and variability with diverse organic constituents. TOC, when measured as a single parameter (rather than as the sum of DOC and SOC), provides less information for geochemical interpretation.
Reducing conditions predominate in the deep-well environment for several reasons:
1. No source of oxygen replenishment exists.
2. Higher temperatures in the deep-well environment are associated with decreases in Eh.
3. Neutral to slightly alkaline water in the deep-well environment favors lower Eh values.
Deep-well injection of wastes can change, at least temporarily, the Eh of the injection zone. For example, Ragone and coleagues69 observed a change from reducing to oxidizing conditions when tertiary-treated sewage (reclaimed water) was injected into the Magothy aquifer, Long Island, NY, at a depth of 400 ft. The reclaimed water had 6.6 mg/L dissolved oxygen compared with no dissolved oxygen in the formation water. On the other hand, the Eh of an acidic waste dropped dramatically, from +800 mV to ca. +100 mV, when mixed with siltstone under conditions of low oxygen and simulated deep-well temperature and pressure.67 Similarly, the Eh of an alkaline waste dropped from + 600 mV to ca. +200 mV.67
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