ESTRATEGIAS PARA EL COBRO DE LOS TRIBUTOS EN LA ILUSTRE MUNICIPALIDAD DE CUENCA

1.1 Intr

1.1 Intr1.1 Intr

1.1 Intr1.1 Introductionoductionoductionoductionoduction

The objective of this session was to evaluate the value and potential role of redox parameter measurements in petroleum hydrocarbon site characterization and remediation. (Redox parameter measurements are herein defined as any parameter measurement that provides insight into the electron transfer status of an aquifer system or the propensity of an aquifer system to donate or accept electrons.) General conclusions of the session included:

1. The need to measure redox parameters at petroleum hydrocarbon impacted sites will depend on site-specific questions that need to be answered and the remedial strategies that are under consideration. Knowledge of the electron transfer (redox) status within an aquifer will not always be required.

2. Collection of at least some redox parameters such as dissolved oxygen, dissolved (ferrous) iron, and dissolved manganese can be very beneficial in the initial stages of site characterization to help develop a conceptual model for the site and to aid in screening potential remedial strategies. The success or failure of some remedial strategies can be impacted by the electron transfer status of the aquifer.

3. Additional redox parameter measurements including dissolved nitrate/nitrite, dissolved sulfate, methane, dissolved carbon dioxide (as measured by acidimetric titration), and oxidation-reduction potential (e.g., electrode-based ORP measurements) may be useful particularly in cases where a natural or enhanced bioremediation strategy is under consideration.

4. In cases where an electrode-based redox potential (ORP) measurement is made, the measurements should be used for qualitative purposes only to support other redox parameter measurements in the development of the site conceptual model. No direct effort should be made to link electrode-based redox potential (ORP) measurements to specific electron acceptor pathways or to the speciation of redox sensitive species in the aquifer.

5. Redox parameter measurements should not be used as surrogate measurements to reduce the number of hydrocarbon monitoring locations or reduce the frequency of hydrocarbon sample collection. Redox parameter measurements should be used strictly in support of direct hydrocarbon measurements to better understand (conceptualize) subsurface processes and plume behavior at the site.

1.2 Role of Redox Parameter

1.2 Role of Redox Parameter1.2 Role of Redox Parameter

1.2 Role of Redox Parameter1.2 Role of Redox Parameter Measur Measur Measur Measur Measurementsementsementsementsements

Extensive research addressing petroleum hydrocarbon behavior and fate in ground water has been conducted over the past twenty years. Various in situ and ex situ technologies for remediating petroleum hydrocarbon impacted sites have been developed and utilized with varying degrees of success. Evaluation of the feasibility of using one or more remedial technologies at a particular site may in some cases benefit significantly from knowledge of the redox conditions in the subsurface. Knowledge of specific redox parameters and conditions at a site can aid in establishing baseline conditions from which the existing behavior or anticipated future behavior of a hydrocarbon plume can be evaluated. In some cases, redox parameters may also be used as a cost-effective means of evaluating the performance of specific remedial technologies.

Three key questions will generally need to be answered in assessing the role of redox parameter measurements at petroleum hydrocarbon impacted sites.

1. Will knowledge of the redox status in the aquifer be of any value in evaluating the feasibility of potential remedial strategies at the site?

2. If so, which redox parameter measurements might be useful?

3. Will the redox parameter measurements be used for qualitative or quantitative purposes?

Knowledge of the redox status of a hydrocarbon impacted aquifer system could certainly be important should in-situ bioremediation technologies be under consideration. Specifically, the presence/absence and distribution of electron acceptors can have important implications with regard to biodegradation pathways and plume behavior. Petroleum hydrocarbons (aromatics, PAHs) as well as coal-derived hydrocarbons (e.g., cresols, phenols) generally undergo much more rapid and efficient biodegradation under aerobic (oxidizing) condi- tions. Thus, knowledge of the presence/absence of oxygen (and other potential electron acceptors) provides insight into existing electron and biological activity and the means by which bioactivity can potentially be enhanced. Knowledge of redox conditions can also be important in evaluating the biodegradation potential of gasoline additives such as methyl-tert butyl ether (see Session 2 Summary).

The potential use of redox parameter measurements in the evaluation of technologies other than bioremediation for hydrocarbon impacted aquifer systems will not be so obvious and will generally need to be considered on a case by case basis. For example, in the case where a thermal based remediation technology might be the preferred remedy, redox parameter measurements are not likely to be of any value. Alternatively, if a remedy involving delivery of oxygen to the subsurface is to be considered, then knowledge of specific redox parameters (e.g., dissolved iron and manganese concentrations) in the ground water and their potential impact on system performance can be important.

The use of redox data for qualitative or quantitative purposes will depend on how the redox data are to be utilized. If the objective is simply to determine which redox processes might be dominant at a particular location, then establishment of the relative presence/absence of selected electron acceptors within and outside a hydrocarbon plume will generally suffice. However, if an effort is being made to answer questions regarding electron acceptor capacity and longevity (e.g., for a monitored natural attenuation remedy), then a quantitative approach may be needed. This would require a more rigorous and defensible (and generally more sophisti- cated) data collection and analyses process than would be required for a qualitative analysis. The significant spatial and temporal variability of many redox parameters can make quantitative analyses at sites extremely challenging.

1.3 Site Characterization

1.3 Site Characterization 1.3 Site Characterization

1.3 Site Characterization 1.3 Site Characterization ApplicationsApplicationsApplicationsApplicationsApplications

The need to collect redox parameter measurements at petroleum hydrocarbon impacted sites will depend on the site-specific questions that need to be answered. The most important information regarding petroleum hydrocarbon plume migration and behavior will generally be gained from direct knowledge of both the temporal and spatial distribution of specific contaminants of concern (e.g., BTEX compounds). Redox parameter measurements are best used as secondary lines of evidence for locating residual source areas and for better understanding, predicting, and/or confirming dissolved hydrocarbon plume migration and behavior. It is known that a strong correlation generally exists between petroleum hydrocarbon concentrations and the relative distribution of various electron acceptors and reduction products including dissolved oxygen, ferrous iron, manganese, nitrate, sulfate, and methane. This correlation results from the utilization of electron acceptors in a favored sequential order by microorganisms during oxidation (metabolism) of hydrocarbon compounds. The use of oxygen as an electron acceptor allows for the greatest energy gain (and thus the most efficient and rapid degradation) during oxidation of hydrocarbons by microorganisms. Thus, as long as oxygen is available, oxygen will be the predominant and preferred electron acceptor utilized by microorganisms. However, once oxygen is depleted, microorganisms must turn to less favorable electron acceptors, which results in the oxidation process becoming increasingly less efficient. The relative relationships between petroleum hydrocarbon concentrations, electron acceptors, and redox potential are illustrated in Figure 1.1. It should be noted that the relative relationships are equilibrium tendencies and may or may not be correlated directly.

Collection of selected redox parameter measurements in the initial stages of site characterization may aid in establishing the feasibility of one or more technologies at a given site as well as evaluating the stability of the

area of affected ground water. For example, the absence of oxygen, presence of high levels of ferrous iron, and low ORP readings in the core of a BTEX plume at a site (relative to background locations) can suggest strong bioactivity and therefore indicate conditions that might be conducive to an aerobic bioremediation-based remedial strategy. Low Eh ground water with a high dissolved iron and manganese content (which can lead to clogging of pumping wells and above ground treatment systems) could signify greater maintenance attention should an ex-situ remedial strategy such as pump and treat be considered. Low Eh ground water with a high dissolved iron and manganese content could also present problems for in situ systems such as air sparging. The collection of redox information can thus be important in contributing to the initial screening and evaluation of candidate remedial technologies.

Redox parameters that should be measured both within and outside the plume in the initial stages of site characterization should at a minimum include dissolved oxygen, ferrous iron, and manganese. ORP measure- ments may also be considered, but should be used solely as a secondary line of evidence in support of these more reliable redox parameter measurements. If bioremediation is to be considered as a candidate remedy, then the redox parameter measurements should be expanded to include nitrate/nitrite, sulfate, and in some cases, alkalinity.

Measurement of dissolved oxygen, dissolved (ferrous) iron, and manganese within a hydrocarbon plume relative to background concentrations at the site will provide an indication of whether biodegradation processes are in effect and consequently whether the aquifer system is amenable to natural or enhanced bioattenuation. Measurement of these parameters will also provide an indication of design features that may need to be incorporated into candidate in situ and ex situ treatment systems to avoid potentially adverse reactions (e.g., precipitation). Measurement of nitrate/nitrite and sulfate/sulfide will provide an indication of additional electron acceptor availability/capacity within the aquifer while alkalinity measurements can be used as a

TPH- BTEX Oxygen O2→H2O Nitrate NO3-→N2 Manganese Mn4+_→Mn2+ Iron Fe3+_→Fe2+ Sulfate SO42-→H2S CO2 CO2→ CH4 Redox Potential

Low High High Low Low High Low High