STEPWISE APPROACH SELECTS MOST EFFECTIVE REMEDIATION ALTERNATIVE

Storage tank owners and operators are increasingly faced with remediating the subsurface beneath tanks that have leaked petroleum products. Often these releases have occurred over several years and the hydrocarbons have migrated and contaminated large volumes of soil and groundwater.

Determining the most-effective, lowest cost way to perform this remediation is vital to minimizing environmental expenditures and maintaining profit margins.

Eric Deaver, national oil and gas program manager for Environmental Science & Engineering Inc., Herndon, Va., has developed a technique for selecting the most appropriate remediation alternative for a particular site. An unpublished report outlining the process selection technique includes an expandable matrix of treatment technologies to facilitate the selection process (Table 2).

REMEDIATION PROCESS

The remediation process typically begins with a site characterization. The goals of this phase are:

• To determine the vertical and horizontal extent of contamination

• To characterize site hydrogeology

• To identify the contamination source

• To identify any threats to human health, aquatic life, or water supplies

• To collect data applicable to determining site closure status or designing and evaluating remedial systems.

Site conditions may indicate that immediate interim remedial action is necessary to alleviate potential threat to human health or to remove a source of continuing contamination.

Typical circumstances that indicate such immediate action are:

• Vapor accumulations or explosion hazards

• Exposure of local third parties, particularly to hydrocarbon vapors

• Imminent threat to public or private water supplies

• Identification of leaking piping or tanks

• Large accumulations of phase-separated hydrocarbons on the groundwater surface.

Interim remedial activities usually fall into four categories: vapor control, groundwater contamination control, tank closure and soil removal, and phase-separated hydrocarbon recovery.

Vapor control usually entails the installation of vacuum pumps, positive-pressure blowers, vapor-interception trenches, and vapor-discharge treatment systems. Examples of vapor-discharge treatment systems are vapor-phase activated carbon, thermal incinerators, catalytic convertors, and internal combustion engines.

Groundwater control entails removing groundwater, via pumping, from one or more groundwater extraction wells or interception trenches. The extraction of this groundwater will reverse its natural or static gradient, thereby ceasing, and often reversing, migration of the contaminant.

Tank closure and soil removal eliminate the continuing source of contamination. This technique typically involves emptying and cleaning the tank, then devaporizing it. The tank is then either filled with an inert material (sand, grout, polyethylene foam) and abandoned or, more frequently, removed.

A removed tank usually is disposed of at a scrap metal yard. Contaminated soil is excavated and treated or disposed of at an approved facility.

The presence of phase-separated hydrocarbons is especially, dangerous if accumulations are present on surface waters, in trenches, in excavations, or in monitor wells. Vacuum trenches often are the most expeditious method of removing phase-separated hydrocarbons. Frequently floating skimmers and sorbent booms also are used, in conjunction with vacuum trucks, to collect phase-separated hydrocarbons from surface water.

Small-diameter skimming pumps and automatic bailing systems usually are used to collect phase-separated hydrocarbons from monitor wells. Trenches and groundwater extraction systems can be added to collect these hydrocarbons before they migrate to more sensitive areas.

CLEANUP ALTERNATIVES

Upon completion of site characterization and interim remedial action, a report outlining site conditions and the work performed to date is filed with the appropriate regulatory agency. The agency will then determine the extent to which the soils and groundwater require remediation.

In some states, this level is preset and nonnegotiable. In others, it can be negotiated based on a model of the fate and transport of contaminants. Such a risk assessment often is effective in obtaining project closure without remediation, sometimes even if hydrocarbons are present at concentrations greater than normally preset levels.

Assuming that the site must be remediated, a remedial evaluation and alternative analysis must be performed. The following procedure should ensure that all feasible alternatives are evaluated and the most effective option is chosen.

• Summarize site conditions. Site conditions include geology; aquifer conditions; contaminant phases to be remediated; concentrations of contaminants in vapor, dissolved, and absorbed phases; vertical and horizontal extent of contamination; remediation goals; and any other aspects of the site conditions that may affect the selection of alternatives.

• Develop list of remedial techniques. These alternatives should be selected based on their ability to remediate the phases and contaminants present. Because it is not feasible to evaluate every possible alternative in detail, Deaver recommends selecting 5-10 alternatives known to be effective for the phases and types of contaminants present.

  Many frequently used equipment components are listed in Table 2, along with the phases and types of contaminants for which they are suitable. If an appropriate type of contaminant or component is not listed, this matrix can be expanded to include additional information.

• Eliminate infeasible alternatives. The list of alternatives can be reduced to two or three by eliminating those that, because of site-specific conditions, are not appropriate. Examples of such conditions include insufficient fuel supply, complex permitting requirements, insufficient open space, and, for some sites, even a high public profile. At this stage, alternatives should not be eliminated simply because they will be difficult or require specialized design.

• Compare site conditions to applications. The list of alternatives should be evaluated in terms of: limitations imposed on system effectiveness by site-specific conditions; ability of system to attain cleanup goals; regulatory acceptance, permitting requirements; expected operation time; preliminary pilot tests or feasibility studies; reliability; simplicity; operation and monitoring (O&M) requirements; waste disposal requirements; long-term liability; and amount of excavation and disruption to site operations,

  Based on these criteria, the alternatives should be ranked according to effectiveness, regulatory acceptance, operational requirements, and ease of operation.

• Select system. Once the list has been narrowed, the best system should be selected based on cost. A site-specific cost estimate for the selected alternative therefore must be developed.

  In developing capital costs, major remedial equipment can be separated from the other capital costs and can be either rented or leased-to-own. The cost estimate should also include O&M charges, which will be incurred on a monthly basis.

  These charges include: monthly samples; site inspections and system adjustments (at least weekly); water disposal and discharge (S/gal vs. flow rate/month); waste disposal and replacement costs or regeneration costs for activated carbon (based on lb/month removed and carbon capacities); groundwater monitoring; soil borings; monthly updates; and quarterly reports. An example of a cost estimate is shown in Table 1.

• Estimate time required for remediation. The following information is compiled and used in computer models to calculate the project length: groundwater velocity determinations; projected removal or destruction rates; projected pumping or extraction rates; calculations of volume of contaminated material (soil or groundwater); average concentrations of hydrocarbons and porosity of soil; air velocities, loading rates, and projected concentration of vacuum system effluent stream; and product baildown or recovery tests. Keep in mind that the range of time frames usually does not span more than 6 months from best to worst case.

• Develop final cost estimate. Add the capital cost to the sum of the O&M costs for the shortest and longest time-frames. This produces a budgetary range.

  IMPLEMENTATION

  After selecting the most effective alternative, a remedial action plan, or corrective action plan, is developed to present the details of the remedial system to the regulatory agency. Detailed plans, specifications, O&M requirements, installation procedures, permitting procedures, and critical time schedules are included in this plan.

  Many states have developed outlines or checklists that can be used to ensure that the plan will meet regulatory specifications and approval. Once the plan is approved, the project may be installed and started up. Keep in mind that the projected time for complete remediation may be fine-tuned from time to time.

  After the remediation is complete, a closure report typically, is developed to document compliance with the remediation goals and to request project closure from the regulatory agency.

  A period of post-closure monitoring, which may entail well gauging, groundwater sampling, or soil sampling, is customarily required as part of the closure process. This requires about 6 months to 1 year to ensure that at least one seasonal cycle of groundwater fluctuation has been monitored.

  Copyright 1993 Oil & Gas Journal. All Rights Reserved.