GeoInsight, Inc. - Let's Get Physical

GeoInsighter Summer/Fall 2001 Newsletter

Let's Get Physical

As the regulatory environment continues to move toward a “cookbook” and “guideline” driven approach, environmental professionals rush to “compare” soil and ground water quality data to potentially applicable “reporting,” “risk,” and “clean up” standards. While guidelines and standards have a place in evaluating environmental risk, media-specific data are generally of limited value unless they are viewed in a larger context (i.e., with a comprehensive understanding of the magnitude and extent of impacts and associated receptors). In the onslaught of laboratory testing and analytical data, I’ve noticed that folks have been neglecting some simple tests that often provide valuable information for those instances where “standards” are not met. Yes, that’s right, I’m talking about every geologists dream……..grain size analyses!

Grain size analyses provide a summary of the distribution of soil particles in a given soil or sediment sample based upon particle diameter. The analyses involve shaking a soil sample through a series of progressively smaller diameter mesh screens called sieves, and measuring the quantity of soil retained on each screen. Certain sieve sizes correspond to certain grain size diameters, such as gravel (larger spaced mesh screens) and fine sand (smaller spaced mesh screens). The finest grained material is collected on an underlying pan and generally represents the silt and clay present in the sample. The distribution of different sized soil particles in a sample can be used to characterize the physical behavior of the sediment or soil sample. For instance, the distribution of particle sizes can be used to estimate how easily water or air will move through a soil layer. These characteristics are critical components in any evaluation of how a contaminant might migrate from a source area to a potential receptor or how effective certain remedial technologies might be at addressing a documented release.

Grain size analyses can also be used to select the type of gravel pack and well screen for monitoring and recovery wells. For example, if there is a large percentage of silt in the soil matrix you are setting your well in, you want a smaller finer grain gravel pack and screen slot size. Proper gravel packs and screen slot size will reduce well siltation, improve well efficiency, and minimize well sample turbidity.

I cannot tell you how many times I have reviewed an evaluation of possible remedial options for a site, and because basic data were not collected regarding the physical characteristics of the site soil, the consultant is left recommending several options (with a commensurately wide range of costs) and suggesting the need to perform additional testing. More often than not, the feasibility of implementing a remedial technology at a site comes down to the behavior of site soils.

The beauty of grain size analyses is that they are inexpensive ($35 to $80 per sample) and they provide information that can be used to address several issues that must be evaluated at most sites (such as hydrogeology, contaminant fate and transport, risk, and remedial options). As far as laboratory analyses go, they give you great bang for the buck! However, the time to be thinking about them is early in the project. It is costly to go back and collect soil samples once the soil borings and wells have already been installed. But unlike samples for environmental testing, there is no holding time for physical tests. Grain size analyses can be (and are routinely) performed on samples that are several months to several years old (as long as the environmental consultant had the foresight to save boring soil samples).

Another test that is simple and inexpensive (approximately $70 per sample), yet often overlooked, is Total Organic Carbon, or “TOC” for short. This test measures the amount of organic material (roots, leaves, etc.) in a sample. Because many environmental contaminants have an affinity for organic carbon (that’s right, this is the same stuff used in the “drums of carbon” commonly used in ground water pump and treat systems), knowing the quantity of organic carbon present in a soil can help evaluate how fast a contaminant may move through an aquifer, how much might be adsorbed (and thus, act as a passive long-term source to air or water), and how persistent a release might be. TOC is commonly used when modeling the behavior of contaminants in soil or ground water, and it is a critical parameter when evaluating the leaching potential of compounds in certain soils. Although there are several general rules of thumb regarding the typical range of TOC in certain geologic materials, having site-specific information limits the number of variables in these modeling efforts and reduces uncertainty in modeling results.

So, the next time you are reviewing a proposal or a scope of work to perform environmental characterization activities, check and see whether a limited number of grain size and TOC analyses are included. If not, you may want to impress the proposal’s author with your “geologic” knowledge and request that he/she “get physical” and include these analyses as part of the scope of work. I assure you, the additional strength of your site data as supported by these additional tests will provide a valuable pay back during subsequent review by regulators or third parties.

Michael J. Webster, P.G., L.S.P.
mjwebster@geoinc.com

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