Contaminants cleanup time

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. 

Removal Rate of Contaminants and Required Cleanup Time... 

The mass of contaminant that must be removed during the second stage is divided into two or three equal parts, corresponding to successive cleanup time intervals. 

Pulsed or variable pump-and-treat Varied pumping rate allowing for contaminants to dissolve, desorb, and/or diffuse from stagnant areas Same as pump-and-treat May increase cleanup time... 

The technology is not applicable for contaminants that are not biodegradable, such as chlorinated solvents, pesticides, and herbicides, polychlorinated biphenyls (PCBs), metals, and certain inorganics. Also, the cleanup time can take up to several months to complete, depending on the type of contaminant. 

The cost of a vertical Lasagna system was evaluated by DuPont using a cost optimization model. For remediation of TCE to a depth of 40 to 50 ft (12 to 15 m) in clay on a 1-acre (4047-m ) site, costs were estimated to range from 40 to 90/yd ( 52 to 117/m ). Soil properties, depth of contamination, cost of emplacing electrodes and treatment zones, required purge water volume, cleanup time, and cost of electrical power were all included in the estimate (D12500Y, p. 10). 

When one considers the approaches that can be utilized to biologically treat PCB-contaminated sites, a hierarchy is apparent based upon ease of cleanup, time frame, and cost, and this hierarchy can be summarized as follows. 

Recovery (To verify cleanup time) All classes 24 months Test to establish time that a clean room takes to return from a contaminated condition to the specified clean room condition. This should not take more than 15 min. In accordance with ISO 14644-3 Annex B13 ... 

Although designs that provide adequate protection of public health and safety also protect the environment, low-level contamination limits, as specified by various nations, differ greatly, so there is no common international approach for land contamination issues. The trend in some countries is to establish limits on land contamination in time and area. Other countries may rely on clean-up and recovery actions. A related issue is consideration for long-term cleanup and recovery following an accident. These issues are addressed in TECDOC-801 through the concept of the Complementary Design Objective. 

Soil. The first reported field trial of the use of hyperaccumulating plants to remove metals from a soil contaminated by sludge appHcations has been reported (103). The results were positive, but the rates of metal uptake suggest a time scale of decades for complete cleanup. Trials with higher biomass plants, such as B.juncea, are underway at several chromium and lead contaminated sites (88), but data are not yet available. 

Field measurements provide savings in sample handling and analysis time, and they eliminate costly delays when re-sampling is required. In addition, they permit important real-time decisions by the on-scene cleanup coordinator regarding removal of sufficient contaminated soil to effect the desired cleanup while avoiding the removal of low-level contamination beyond that required. 

Soil Cleanup, or remediation, of hazardous waste sites will often produce contaminated soil. Contaminated soil must be handled as hazardous waste if it contains a listed hazardous waste or if it exhibits a characteristic of hazardous waste. As with hazardous waste, land disposal of hazardous soil is prohibited until the soil has been treated to meet LDR standards. These contaminated soils, due to either their large volume or unique properties, are not always amenable to the waste codespecific treatment standards. Because of this, U.S. EPA promulgated alternative soil treatment standards in 268.49 in May 1998. The alternative soil treatment standards mandate reduction of hazardous constituents in the soil by 90% or 10 times UTS, whichever is higher. Removal of the characteristic is also required if the soil is ignitable, corrosive, or reactive. 

Limitations need to be carefully considered before selecting this method for site remediation. These include the depth of contamination, the total length of time required for cleanup to below accepted limits, potential contamination of vegetation and the food chain, and difficulty in establishing and maintaining vegetation at some polluted sites.8... 

The design of a phytoremediation system is determined by several factors associated with the contaminants (type, concentration, and depth), the conditions at the site, the plants, the level of cleanup required and the available time. Extraction techniques have different design requirements than immobilization or degradation methods. Nevertheless, it is possible to specify a few design factors that are a part of most phytoremediation efforts. 

Lead at a lightly contaminated brownfield site has a concentration in soil of 600 mg/kg to a depth of 1 ft. The cleanup standard has been set at 400 mg/kg. Indian mustard, Bmssica juncea, will be planted, fertilized, and harvested three times each year for phytoextraction. Using small doses of EDTA, it is possible to achieve concentrations in the plant of 5000 mg/kg (dry weight basis), and harvestable densities of 2.721 (3 short tons) dry matter per crop. Estimate the time required for cleanup ... 

U.S. EPA defines MNA as the reliance on natural processes, within the context of a carefully controlled and monitored site cleanup approach, to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, stabilization, and transformation. These processes reduce site risk by transforming contaminants to less toxic forms, reducing contaminant concentrations, and reducing contaminant mobility and bioavailability. Other terms for natural attenuation in the literature include intrinsic remediation, intrinsic bio-remediation, passive bioremediation natural recovery, and natural assimilation. 30... 

Despite appropriate PPE and procedures, however, there will remain a potential for breaches of protection and contamination of the workers. Furthermore, there is potential that such a breach or contamination will not be recognized at the time of occurrence. Finally, while it may be appropriate to conduct medical surveillance of cleanup workers for epidemiologic monitoring of the effectiveness of the protective measures, monitoring may not be reliable enough or timely enough to rely on for clinical decisions regarding the need for antimicrobial prophylaxis on an individual basis. 

CDC recommends that decontamination/cleanup workers receive antimicrobial prophylaxis, using standard regimens starting in conjunction with or prior to the time of first entry into a contaminated location and continuing for 60 days after final opportunity for exposure. 

A number of techniques have been developed to directly volatilize soil components and contaminants and introduce them into the carrier gas of a gas chromatograph. Such procedures avoid the time and cost of extraction, cleanup, and concentration and also avoid the introduction of contaminants during the extraction process. However, these methods are not universally applicable and caution must be used when applying new or untested analyses or analytical procedures [2-5],... 

During remediation, contaminant levels decrease until they achieve an asymptotic level. Once asymptotic conditions are reached for several successive sampling periods, continuing remediation activities generally result in little further decrease in contaminant reduction. However, frequently when active remediation is halted, levels of dissolved contaminants abruptly increase (rebound). This increase is the result of the diffusion into solution of contaminants that were previously adsorbed onto the surface of the aquifer media. Sometimes more efficient cleanup is achieved by operating the remediation system on a cycle of several days on and several days off. Cyclic operation allows the operator to time the remediation to treatment of the higher rebound concentrations. 

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