2 The 4 Technology Solutions
SITE ASSESSMENT AND REMEDIATION FOCUS ON THE POSSIBLE
An On-Line Version of a Column First Published in the:
National Environmental Journal May/June 1995 Vol. 5 No. 3
By: David B. Vance
Groundwater and saturated soils impacted by hydrocarbon (or other) contaminants are typically assessed through the installation of soil borings and monitor wells. The information gathered through that activity is then used to design subsequent remediation systems. This column will discuss that process and at what point returns on assessment and remediation expenditures begin to significantly diminish.
The goals of any subsurface assessment are to determine the vertical and horizontal extent of contamination, the nature of the geological matrix (Fractures) (Granular) , and the configuration of the groundwater table. Once those goals have been accomplished there is little value in continued evaluation using intrusive methods. Furthermore, it is impossible to completely physically delineate the subsurface at a site, with the possible exception of complete excavation, which is an alternative that is common due to the certitude it provides. However, in many cases excavation is not possible, certitude is impossible to achieve, and the cost of its misguided pursuit grows exponentially.
The greater the degree of compositional and grain size heterogeneity of the subsurface soils at a site the more acute the problem becomes. The scale and distribution of such heterogeneity can range from inches to tens of yards, the most problematic scale for assessment and remediation is when the heterogeneity’s are on the order of feet. Smaller scales do not allow for the unexpected transport of contaminants over significant distances, and larger scales can be isolated and treated as homogenous blocks. While engineering is a component of assessment and subsequent remediation, remediation is ultimately a problem that can not be addressed by engineering. The engineering of a physical/chemical process entails the ability to take specific feedstocks, process them in a reactor under designed conditions, and produce a final product with a great degree of precision.
Unfortunately, with regards to the remediation of groundwater, that degree of knowledge or control is unobtainable. Rather, the process is one in which given existing data on the subsurface conditions of a site an interpretation is developed. That interpretation is based on the site data, and most importantly the education, experience and skill of the individual performing that interpretation, generally a geologist. To some degree this process is an art, the ability of which to exercise increases with practice, but as with all arts, a skill that can not be obtained by all who desire it. An important point must be made with regards to the interpretation process, a goal should always be in mind. Successful petroleum exploration geologists will interpret data in a fashion to maximize the potential for the existence of a petroleum trap. Conversely, geologists interpreting data with environmental control in mind will be best served to interpret that data to maximize potential contaminant transport away from the source zone and minimize the ability for subsequent recovery.
It should be understood that it is most likely that a remediation design from that process will ultimately be modified as more data is gathered during the installation of the remedial system and as the remedial system is operated placing the site under dynamic stress. Dynamic stress is usually induced by pumping groundwater. The pumping rate, pumping location, and pumping duration are all parameters that can be controlled. The analysis of the behavior of the groundwater table in the pumping well and surrounding monitor wells allows for the interpretation of the configuration of the flow regime between installed wells. That in turn gives some insight into the configuration of the geologic matrix through which that groundwater is flowing. This information is then used to modify the operation of the remedial system, after which observation and modification continues.
The other issue is to realize that subsurface conditions are not steady state during the remediation process. Operating conditions optimal for remediation during the first three months are unlikely to be optimal two years later. Contamination concentrations change, subsurface geochemistry is modified by remedial processes, and dominant mechanisms affecting mass transport efficiencies change.
The process is one of continuous analysis and modification of the operating conditions. The ideal is to have a system that one can turn on, leave on for the requisite period of time, and then decommission with the clean up goal achieved. The ideal rarely exists, understanding that and even exploiting it is the job of those responsible for the management of groundwater remediation projects.
Following are useful broad based assumptions that are the consequence of the above discussion:
- The first thought should be towards obtaining hydraulic control of the site. To minimize future liability and costs a best effort should be made to rapidly prevent the further migration of contaminants.
- The concentration of a dissolved plume will roughly logarithmically decrease with distance form the source area. Failure for this to occur is indicative of transport along preferential flow channels that may be due to heterogeneity of the granular media or the presence of fracture systems.
- Contaminant transport retardation increases as grain size of the matrix decreases causing transport rates to be significantly less than the groundwater flow rate. Retardation will be almost nonexistent in fractured media where contamination will be transported at a rate only slightly less than the groundwater flow rate.
- Removal of contaminants from some portions of the exposed soil matrix through pump and treat methods will take at least as long as the duration of the prior exposure to that contaminant, rapid response to new releases is therefore critical.
- Some portion of the contamination is irreversibly adsorbed.
- Bioremediation is primarily a mass transport issue which should be resolved by geologists and there are also limits to the effectiveness of bioremediation.
It is critically important that the regulatory, legal, and business communities understand the nature and limitations of the above process. Our focus must be on what is possible rather than arbitrary standards. Current thought, political debate, and regulatory action is beginning to reflect this. When applied on a site specific basis risk assessment is an integral and necessary way of integrating the possible with the assessment and remediation of environmental degraded groundwater.
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