Guidelines on remediation of contaminated sites
10. Operation and evaluation
10.1 Introduction and objectives
The nature of the evaluation phase depends on the type of remedial measures. With excavation, the evaluation phase is short term and takes place more or less simultaneously with the remediation phase. With in-situ remediation of soil, air or groundwater, the operation and evaluation phase can be longer term. Therefore, the evaluation it is often divided into an ongoing monitoring/operation phase, followed by an evaluation of compliance at the conclusion of the remediation. There is often a difference between the effect during operation and the lasting effect of remedial measures, and in these cases there must be a sharp distinction between evaluation during operation and evaluation of the lasting effects. Under circumstances where there is no immediate need for remedial measures, simple monitoring may be necessary. This applies in particular to groundwater aquifers. Evaluation measures are closely related to the individual media and remedial methods, and therefore the descriptions below are very method-specific.
The objectives of the evaluation phase is to evaluate and document the effect of completed remedial measures, cf. the guidelines presented in the planning phase. The objective of monitoring in cases where no remedial measures are carried out is to check that an unacceptable spreading of contaminants does not take place.
Stop criteria and monitoring programmes must always be set before starting a remedial measure. Stop criteria are based on the risk assessment, and are usually set during preparation of the outline project. When there is an operation phase, a monitoring programme must be prepared along with the outline project. During running-in of the technical equipment, an operating manual should also be prepared which contains all necessary information about the system, a description of the equipment, activities, and descriptions of work /40/.
10.2 Evaluating excavation
Excavation removes soil contamination either partly or completely. Evaluations are carried out simultaneously with the excavation in order to ensure that the soil is sorted into contaminated soil and clean soil (possibly in several classes of contamination). Evaluation should also ensure that residual contamination is in compliance with the excavation criteria (e.g. that the contamination level in the sides and bottom of the excavation are sufficiently low). Guidelines for managing the contaminated soil should be clearly stated in the project description, and before commencing work, they should be designed as instructions/action plans for personnel directly involved.
In order to ensure that requirements are complied with, excavation must take place under environmental supervision. The tasks of the environmental supervision personnel are described in Section 8.5.
There are three types of evaluations for excavation.
 | Evaluation of the excavated soil. |
| Evaluation and documentation of residual contamination after excavation. |
| Evaluation of remediated soil. |
10.2.1 Evaluating excavated soil
In order to optimise remediation of contamination, a clear excavation strategy should be agreed upon. The excavation strategy primarily depends on the results of contamination mapping in the investigation phase, in particular the homogeneity of the contamination. A strategy for sampling must always be established, outlining where samples are to be collected from, how often they are to be taken, and how samples are to be taken for field measurements and laboratory analyses.
Furthermore, attention should be directed to how the excavation is to be physically carried out. This is important with regard to geotechnical considerations, and is also often important with regard to evaluating the excavation. For example, it may be appropriate to describe how large an area should be removed at each level, the depth of soil removal at each level, the maximal extent of the excavation, what size the bucket on the excavator should have, etc.
The method and price of soil treatment depend on the type of contamination and its concentration. It is therefore necessary to document the contamination in the excavated soil by taking soil samples for analysis. The number of samples which should be taken for laboratory analysis depends on several factors, including:
| How homogeneously the contamination is distributed. |
| The type of contamination (can the contamination be detected using simple methods, for example field measurements or visually). |
| Whether the contamination is to be divided into several classes of contamination for different methods of disposal. |
| How the soil is to be disposed of (soil for reuse or landfilling may require more analyses than soil for treatment). |
| The total amount of contamination (small amounts require relatively more analyses than large amounts). |
| How many investigations have been carried out before excavation started (for example, has the contamination been well-defined?). |
In the design phase, a plan for the collection of samples is established. The plan may include the number of samples and the systematic pattern of distribution to be used in the area in question and divided into each layer of soil to be removed. In addition, the plan can describe collection of samples from an interim soil storage location, receiver facilities, or directly from the excavator bucket.
Requirements can either be defined as a number of samples per unit of weight, or per volume of excavated soil. In general, a conversion factor of 1.8 tonnes/m3 can be used.
Soil treatment prices depend on the degree of contamination. In cases of very heterogeneous contamination, it can often be financially advantageous to collect many samples so that the soil can be sorted into contamination categories. Financial optimisation must provide the basis for how many samples should be taken in these cases. The supervision inspection journal and plan should always include details of where in the excavation a soil sample originates.
When taking samples with volatile substances, especially in interim soil storage location, it is necessary to be aware of losses of contaminants. Therefore, appropriate sample containers (diffusion-proof), sampling methods (not surface samples) and handling (store cool and dark, deliver to laboratory within 24 hours and rapid extraction at the lab) should be used.
For excavation of organic contamination to be delivered to soil treatment plants, the number of required samples primarily depends on the need to sort the soil into different categories with a view to achieving financial advantages. Therefore, the number depends on the specific case (amount, homogeneity, type of contamination, price differences between categories). Different clients, environmental authorities, and soil treatment plants can have their own requirements for the number of analyses. The final environmental documentation should be carried out when the soil is deposited after treatment.
Assigning soil directly to a landfill requires greater certainty and therefore a larger number of samples than soil assigned to a treatment plant. The number of samples and methods of analysis depend on the specific case (amount, homogeneity, type of contamination, final landfill), and the environmental authorities’ requirements for documentation. The number of samples will typically be 1 sample per 30 tonnes when the soil is excavated due to contamination.
Requirements for the selection of analysis parameters and methods are the same as in the investigation phase and depend on the type of contamination. These are described in Appendix 4.9. Analyses must be carried out at an accredited laboratory. Detection limits must, as a rule, be 1/10 of the quality criteria.
For organic contamination, it should be noted that the content of heavy metals in the excavated soil (particularly lead) should also be known, since a high content of metals may be crucial for the treatment and the price.
At the receiving treatment facility, samples are often routinely analysed as part of the receiving and registration procedure. There may be variances in analysis results compared with those carried out by the supervision authorities. Therefore, it is important to note where samples have been collected.
Contamination investigations endeavour to determine the extent of the contamination so that the amount of soil to be excavated can be ascertained. The detailed project almost always provides an estimate of the amount of contaminated soil. There are often inconsistencies between the estimated amounts and the actual amounts because investigations are carried out based on spot checks. The actual amount excavated is obtained from weight notes completed when the soil is transported. In order to avoid misunderstandings, the supervisory inspection prepares forms that specify the load. The form should be signed by the driver, the supervision personnel, and the receiving facility, and then returned to the supervision personnel.
10.2.2 Documentation of residual contamination
Excavation is stopped temporarily when it is estimated by an inspection that adequate contamination has been excavated to have reached compliance with the previously set stop criteria. This compliance with the stop criteria should be documented by collecting an appropriate number of samples for chemical analysis from the sides and bottom of the excavation. These samples may be supplemented by field measurements. The number of samples should be agreed upon in advance with the supervision personnel. The focus should be on the most critical areas. If remediation is carried out due to risks involving outdoor areas and indoor air, most of the samples should be taken from the uppermost metres, while for risks involving the groundwater, there should be more documentation from the bottom of the excavation. The density of samples also depends on the nature of the contamination (is it visible, is it detectable using field methods, is the soil homogenous, etc.?).
Under the description of acceptance criteria in the detailed project, a minimum number of analyses must be stated, as well as the number of analyses per area unit. As a rule, samples should always be collected from all sides and the bottom of the excavation. Normally, as a minimum, sampling should correspond to level 3 as described in the guidelines on sampling and analysis of soil /3/. In cases where there are visual indications in the open excavation that the contamination distribution is inhomogeneous, for example through the distribution of geological layers and/or colour, more samples should be taken for residual contamination. The samples should be handled as described in the section above.
If contaminated soil remains in areas not accessible by the excavator, for example under buildings, a risk assessment should determine which precautions should be taken. For further details, refer to Chapter 5.
To document completion of the project, a report is prepared which documents that agreements have been complied with, including excavation, handling, and analysis procedures, and concentration levels in both excavated soil and remaining soil. A risk assessment should be carried out to determine the consequences of allowing residual contamination to remain.
10.3 Evaluation of in-situ remediation of soil contamination
Running-in and operation of remediation systems are described in Chapter 9. The following is a description of the concepts of evaluating the operation and final compliance. Evaluation of operation takes place at regular intervals during operation with a view to ascertaining the progress of remediation and whether the technical equipment is working optimally. The final compliance takes place when an evaluation of operation shows that it is likely that the acceptance criteria have been reached.
10.3.1 Evaluation of the operating period
The following describes possible evaluations for in-situ remediation methods used for soil contamination, including active methods such as soil vapour extraction and bioventilation, and passive methods such as immobilisation methods.
During the operating period, the contamination should be monitored so that changes in the contamination can be documented. In soil vapour extraction, operation should initially be followed closely (samples should be collected at least one week after the start), and subsequently with increasing time intervals, for example after 1, 3, 6, 9, and 12 months. Further evaluation of operation after this period can be set according to the results of the first year’s operation /32/. This will typically be about 2-4 times each year.
During the operating period, measurements will primarily be of the contaminants in the discharged air. Furthermore, the air flow and the air pressure should be monitored. As completion of the remediation approaches, monitor wells for soil gas/groundwater may be included in the evaluation. In addition to measuring contaminants, it is possible to measure for oxygen, carbon dioxide, and temperature in the discharged air. The final evaluation of the success of the remediation is described in Section 10.3.2.
In bioventilation, evaluation of the operating period is best carried out by measuring oxygen and carbon dioxide consumption using bio-activity tests. By comparing with earlier measurements, an indication of changes in activity is obtained. In bio-activity tests, a specific quantity of oxygen is injected into the contaminated layer. Changes in the oxygen and carbon dioxide content are subsequently measured in the nearby monitoring wells. As a minimum, this should be carried out twice a year.
In addition, contaminants in the soil gas should be analysed for in existing monitor wells at the end of the operating period. Monitoring groundwater/soil can also be advantageous with regard to checking whether the water/soil is cleaner. Air measurements will typically be performed if the stop criteria consists of air concentrations. The same applies to water/soil. In monitoring groundwater, the redox conditions should be checked in order to determine the degradation potential.
In forced leaching, evaluation of the operating period is best performed by analysing water samples from the inlet and outlet of the water-treatment device. Analyses should be carried out an appropriate number of times at increasing time intervals. This should be done about once a week in the first month and subsequently about once a month, extending to a minimum of once every six months.
In addition, samples from monitor wells should be analysed at appropriate intervals during the operating period for content of contaminants in the groundwater aquifer.
In immobilisation methods (fixing/capping methods) the clean side of the cut-off system should initially be monitored twice a year, falling to once a year. For volatile contamination, it is normal to carry out soil gas measurements, while for water soluble substances, the groundwater aquifer should be monitored downgradient of the cut-off system. For extra security and to achieve optimal monitoring, it may be relevant to construct double walls, with monitor wells placed between the walls.
10.3.2 Stop criteria and the final compliance
Stop criteria should be set before commencement of remedial measures. The following parameters should be included in the decision process:
| Sample medium for final compliance (air/soil/water, possible combination) |
| Procedures for evaluating final compliance |
| Strategy of sampling to determine the lasting effect of remediation |
| Measurement and analysis parameters |
| Measurement and analysis procedures |
| Permissible variation in results |
Contamination may be in the water, soil, and/or air phase. Therefore, it is possible to determine changes in the contamination in a single medium, or in a combination of several media. For example, if the indoor air is threatened, a soil gas criterion could be set. If remediation takes place due to risks for outdoor areas, a soil criterion may be appropriate, and a groundwater criterion could be set if drinking water is threatened. It is often necessary to establish new wells/boreholes between existing wells/boreholes.
The normal procedure for evaluating final compliance is to obtain a concentration in the media (soil, water, air) which relates to the limit values for individual substances. In a few cases, the obtained ratios between individual contaminants have been used. In these methods, substances which are quickly removed are compared with substances which are difficult to degrade /46/. This method can only be used in remediation where the substances which degrade rapidly are the most critical, usually for indoor air problems.
Finally, stop criteria may be interpreted pragmatically in relation to the rate of remediation. When the remediation process is sufficiently slow, remedial measures can be stopped temporarily. a subsequent risk assessment forms the basis for deciding whether remedial measures should be stopped permanently, or whether it is necessary to continue using another technique. Several remediations have followed this process in practice.
There is a difference between the effect during operation and the lasting effect for many in-situ remedial methods. In some cases, contaminants will reappear/flow back after equipment is shut off, causing the remediation to fail to comply with original stop criteria (rebound effect). The final compliance evaluation should therefore establish a sampling strategy which will document the lasting effect (that the remediation is satisfactory). Thus, before commencing remedial measures, decisions should be made on which sample medium will provide the best evidence of the lasting effect, the number of samples required for sound decision-making (when it absolutely certain that the remediation effect is lasting), the length of time between sampling, and whether individual samples or mixed samples (soil) should be taken. Typically, there should be assessments of how long it will take before contamination is transported into the water phase and further into compliance wells.
When the evaluations during the operating period reveal adequately low concentrations in air discharge (where this is the only requirement for operation), samples must be collected from the sample medium selected for the final compliance. Samples must be collected from places other than the air discharge for ventilation methods. As a minimum, two consecutive analyses of the air discharge should show no measurable contamination. The samples should be collected with about a two month interval, where the pump has been stopped for a period. Following this, samples of soil/water/air can be collected from new wells in order to check for compliance with stop criteria.
In forced leaching, the quality criteria for compliance are fixed concentrations either in soil or in groundwater. However, in practice, these pre-set criteria have not been used. For in-situ remediation completed up to now, risk assessment of residual contamination has provided the basis for stopping remediation.
For most passive in-situ methods, e.g. immobilisation, there is no distinction between final compliance and evaluating the operating period. Monitoring corresponding to evaluating the operating period is continued (although as time goes by, longer intervals between monitoring rounds are used).
Decisions must be made on which contaminants are to be quantified through analyses. These may be individual substances or mixtures of substances. It is important to define the evaluation procedures to be used, including methods of analysis. Methods of analysis are described in the Guidelines on sampling and analysis of soil /3/. If a specific correlation is to be demonstrated, it may also be relevant to use field measurements as part of the stop criteria.
Before starting remedial measures, rules for interpreting variations in analysis results must be established. For example, there may be requirements that a specific percentage of the results must comply with criteria, while at the same time establishing a maximum concentration which no single analysis may exceed.
In in-situ remediation in Denmark, both air and water have been used as media for stop criteria. For soil vapour extraction, a number of cases have been completed where the stop criteria have been fulfilled. In forced leaching, there are examples of full remediation of both soil and water. A few remediations have been completed because the stop criteria have been achieved, but several remediations have been stopped due to operational problems.
10.4 Evaluating groundwater remediation
There are various methods/principles for remediation of groundwater, cf. descriptions in Chapter 9. The methods may be roughly divided into pump-and-treat and other in-situ techniques.
10.4.1 Evaluating pump-and-treat
When pumping is started, it is necessary to check whether the contamination is under hydraulic control. A monitoring programme is usually established when the plant is constructed, and this is revised once the installation has been run in. The monitoring programme lays down where the potentiometric surface should be measured and where measurements of pump yield are to be made, as well as how often the yield is to be measured.
Wells for monitoring hydraulic control are usually located within and near the borders of the capture zone of the well. Therefore, observations on both sides of the groundwater divide should document that the contamination plume is on the right side of the divide.
Evaluation of whether contamination has been remediated as planned includes sampling and analyses of contaminants in the pumping well and monitor wells. Wells for monitoring remediation should be located within the contamination plume, at the source of contamination, and possibly in minor upper aquifers above the contamination plume, downgradient of the source.
The monitoring programme establishes where samples are to be collected, and how often water samples are to be collected, as well as which analyses are to be conducted.
In pump-and-treat, there is often a difference between the remediation effects during operation and the lasting effects. When the pump is turned off, contaminants will often be released/flow back, so that remediation does not comply with the stop criteria as expected (rebound effect). For example, contaminating substances can be released into groundwater when the groundwater table rises after pumping is slowed down. It is therefore very important that the lasting effect is measured after pumping has been stopped. There should be estimates of when it will be possible to ascertain any rebound effects in wells. If the lasting effect is deemed to have been met, but control measurements after a period of 3 months or so indicate that stop criteria have been exceeded, pumping must be recommenced. This procedure should be repeated until the stop criteria have been achieved.
The desired remediation level is established on the basis of the risk assessment conducted in the investigation phase. The stop criteria should contain requirements that values below the remediation level are achieved for several consecutive monitoring rounds. In addition, samples should be analysed from several monitor wells as well as from the pump well. Stop criteria may be varied according to the location of the well from which the sample was collected.
Experience with the pump-and-treat method is great. Many projects have been completed. However, many projects have demonstrated difficulties in achieving the stop criteria. On the other hand, using hydraulic control, it is possible to prevent groundwater contamination from spreading towards water abstraction wells etc., and at the same time remove some of the contamination.
There are examples where successful remediation in relation to land use has occurred in aquifers near the surface. On the other hand, there are only a few examples of completed pump-and-treat projects in aquifers with high yields where the goal is to remediate to drinking-water standards.
Requirements for discharge are laid down in a discharge permit by supervision authorities for treatment plants, surface water recipients etc. In addition to requirements for parameters to be analysed, analysis procedures to be used, compliance concentrations, and permitted discharge amounts, a discharge permit also contains requirements on frequency of sampling and analyses. Therefore, in accordance with the required frequency of analysis, water samples must be collected prior to discharge into the sewer mains in order to check for compliance.
When discharging contaminated groundwater, evaluations should be made to ensure that the treatment processes are running satisfactorily. For example, in filter technology, all filters must be regularly backwashed, cleaned, or replaced. The effect of the filter is reduced over time as the filter material slowly loses its ability to ad/absorb components, or it becomes clogged. Therefore, a certain amount of monitoring and evaluation of operation must be expected in connection with backwashing, cleaning, and replacement of filters.
The extent of monitoring water treatment is very method specific, and should therefore be described in the monitoring programme. For example, separators must be emptied at appropriate intervals.
For water treatment with activated carbon, the system usually comprises two filters in series. The treatment effect of the system is best measured between the filters so that the filters can regularly be replaced one at a time, and contaminants never break through the final filter.
10.4.2 Evaluating in-situ remedial methods
Evaluation of the remediation effect of air sparging, as with pump-and-treat, is primarily done through monitoring the groundwater. This implies analysis of groundwater samples from monitor wells located centrally and on the periphery of the contaminated area. The frequency of sampling can for example be after 1, 2, 3, 6, 12, 18, and 24 months /32/. It should be noted that air sparging can cause significant spreading of contamination by transport in the sparged air in the saturated zone, probably as a result of low-permeability horizontal zones. The existence of such low-permeable zones should be examined in the design phase. If these zones are present, monitoring should also be carried out further away from the sparge area, in regard to possible indoor air problems as well as to groundwater. At the same time, it is also important to regularly measure the potentiometric surface of the groundwater in order to monitor mounding of the water table.
Concurrently with air sparging, it is normal to remove stripped contaminants from the unsaturated zone using soil vapour extraction. Measurements of these air emissions should be included in the monitoring programme.
For air sparging, the pragmatic view will generally apply with regard to stop criteria. Remedial measures can be stopped when concentrations of contaminants are low and there are no notable changes in the contamination pattern (even after taking possible rebound effects into account). In addition, a specific risk analysis should conclude that remedial measures can be stopped. Sparging may be replaced by monitoring or pump-and-treat if the risk analysis deems this necessary.
Reactive permeable barriers allow the passage of groundwater, while degrading or removing contamination from the groundwater. Compliance is evaluated in the groundwater zone and should include samples collected before inflow, in the barrier itself, and after the barrier. Furthermore, contamination should be monitored upgradient and downgradient, as well as before and after any cut-off walls in order to examine effectiveness. To ensure the required flow direction, and in order to enable commencement of possible measures against mounding problems, the groundwater table should also be monitored.
Impermeable barriers should primarily be monitored downgradient of the contamination. As an extra measure to achieve optimal evaluation, it may be relevant to establish double barriers with monitoring between the barriers.
With methods where oxidising agents are added to the groundwater zone, effectiveness should be evaluated downgradient and in pump wells where the purpose is to produce an ‘oxygen barrier’. In addition to contaminants, monitoring should identify when the oxidising agent should be replaced or recharged.
10.5 Evaluating soil gas remediation
In buildings where there is a risk of indoor air problems, and where remedial measures have been started as a result of this risk, the effect of these remedial measures should be evaluated at regular intervals. This should usually be carried out by taking measurements under the floor, and not in the building itself, so that false sources can be ignored. If measurements are conducted of the indoor air, and not of the soil gas, reference samples should always be taken in rooms which are not affected by the contamination. Background measurements should also be taken outside.
In cases where passive or active ventilation has been established, for example ventilated drains in the capillary-breaking drainage layer, evaluation can be carried out by measuring emissions to the air or by taking measurements in ventilation pipes under the floor. If contamination cannot be detected in the air discharged, the equipment can be stopped and final evaluation can subsequently be carried out by measurements in the soil gas (possibly in drainage pipes). Active ventilation can be stopped when contents less than the stop criteria have been recorded at least twice at intervals of two months. It should be noted that after stopping pumps, rebound effects may occur in drainage pipes, and therefore it is never sufficient to conduct a single round of measurements.
10.6 Controlling measures for landfill gas
A monitoring programme should be established at landfills where gas generation from rubbish presents a risk to people or the environment. A suggested monitoring programme for landfill gas is described below. For further information, refer to a report from the Danish Environmental Protection Agency on landfill gas /17/.
The objective of monitoring is to check changes in the amount of gas in order to determine whether remedial measures should be started or changed, or whether remedial measures in progress continue to be satisfactory.
Landfill gas can be monitored at several locations, including:
| On the surface |
| Underground with soil lances |
| In monitoring screens installed in excavated ditches |
| In monitor wells |
| In existing wells (leachate collection wells, etc.) |
| In buildings equipped with alarm systems |
The most common method is measurement in monitor wells or soil lances. Location and intervals of lances or wells are determined from surveys of the risks of gas generation and gas flow. Location depends on the methane content, the amount of gas, gas penetration through the rubbish, the extent of the landfill, the surrounding geological strata, and the distance to buildings, pipe systems, and sewers. Finally, the location depends on the design of the remedial installation.
There should always be measuring points outside the extent of the landfill, particularly between the landfill and any nearby buildings. A more detailed description of the design of wells and their location appears in a report from the Danish Environmental Protection Agency /17/. An example of the design of a stationary measuring point appears in Appendix 4.6.
Excavated ditches can be utilised at shallow landfills. Direct push or hammered lances are mostly used for point sources due to their limited range. Measurements at existing water supply wells and leachate monitoring wells can be used to supplement, but not replace specially constructed gas-measuring points.
Where remedial measures have been completed for landfill gas, measurements should if possible be made between the preventive installations and buildings using the above monitoring sites. This is possible if gas barriers, permeable ditches, ventilation drains, or wells are used as remedial measures between the source of gas and the building. If landfill gas is not recorded from repeated monitoring in wells, the frequency of monitoring can be reduced. In cases where gas penetration is cut-off using technical construction measures or by changing the pressure gradient in the building, monitoring should primarily be carried out under the floor or inside the building.
In buildings in high-risk areas, systems for measuring gas concentrations and gas alarms should be installed as an extra check to ensure the effectiveness of remedial measures. In cases of a methane content of more than 1 per cent by volume, defined as 20 per cent of the lower explosion limit of 5 per cent methane by volume, or in cases where there is a carbon dioxide content of more than 5 per cent by volume, the building should be evacuated /17/. There should always be an action plan for the safety of people in the building, and everyone should be acquainted with this plan.
Monitoring should continue until the risk to people and the environment has abated; that is, until gas concentrations are not explosive (methane) or toxic (carbon dioxide). Concentrations of methane must be less than 1 per cent by volume, and concentrations of carbon dioxide from decaying waste must be less than 1.5 per cent by volume, as measured inside the landfill over a longer period. For example, this may be a two-year period where measurements are taken under different weather conditions at least every six months (warm weather in summer and frozen soil in winter, as well as during periods with decreasing atmospheric pressures while below an absolute value of 1,000 millibars). Alternatively, an actual investigation of the waste could be performed and provide statistical certainty that the degradable waste has been degraded /17/.
Consideration should be given to the safety of people who are employed to establish evaluation and monitoring. Therefore instructions for procedures regarding safety at work should always be available.