The Process of Effective Soil and Sediment Investigation

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Underground dry cleaning solvent tank encountered during indoor $2,000,000 remedial excavation.

Reidar Zapf-Gilje & Guy Patrick

Many contaminated site remediation projects have failed to meet their technical goals and objectives because of an inadequate or incomplete understanding of site conditions. Particularly in urban environments, many if not most sites encounter costly surprises during remediation as a consequence of a long history of past industrial use. On land, oil-filled wood stave culverts, abandoned underground solvent tanks, and buried rock pits are some of the more common examples we have encountered. In marine sediments, we often encounter issues such as sunken vessels, abandoned gear and debris, and wood waste.

Figure 1. Sunken vessel identified by multi-beam sonar survey during marine sediment characterization study.

The Conceptual Site Model

These issues become surprises when they are not identified a priori or are not foreseen as possibilities by adequate environmental site investigation. Good environmental site characterization is a scientific process that begins with the compilation of historical and current site information, and re-configuration of that information within the framework of a conceptual site model (CSM; Figure 2). The CSM is a visual representation and narrative description of the physical, chemical, and biological characteristics of the site, as well as the anthropogenic and natural activities, events and processes that occurred historically at the site over time. An effective CSM should be able to tell the story of how the site became contaminated, how the contamination was (and is currently) being transported, where the contamination will ultimately end up, and whom it may affect (i.e., receptors). As illustrated by the iterative circle in Figure 2, the CSM is continually updated and modified as new information is obtained and forms the basis for investigation work plans until the CSM is considered sufficiently robust for remedial design.

Figure 2. The iterative site investigation process.

Investigation Approaches

The conventional approach to investigation typically involves the use of standard investigation tools (e.g., test pits, boreholes, monitoring wells) and collection of media samples for analysis by a fixed laboratory. The process may be expedited using the “Triad Approach”, where field analytical methods are used and information is interpreted in the field by experienced professionals. Real-time decisions are made based on an increased amount of useful information collected, often resulting in lower overall costs and reduced timelines. Through the “Triad Approach”, larger quantities of less precise data are often acquired rapidly to more fully develop the CSM, compared to the much slower conventional approach where a smaller number of more analytically precise data is acquired.

Soil Characterization

Site investigations usually involve collection of representative samples of one or more environmental media including soil, rock, groundwater, surface water, soil vapour, air and sediment. Soil investigations involve identifying and characterizing the contamination at the appropriate scale in terms of spatial extent (laterally and vertically), the potential to serve as a source of contamination for other media, and intrinsic soil properties that may serve to attenuate the contamination or otherwise assist in determining optimal remedial approaches. Contaminated soil has some important characteristics that are different from that of contaminated groundwater and soil vapour. These differences must be considered by the CSM and investigation strategy. Contamination in soil is often highly variable over relatively small distances. Whereas organic chemicals in groundwater tend to form plumes in a relatively predictable manner, soil contamination may be discontinuous and dispersed, depending on the contamination source. Another key difference relates to the changes in chemistry that may take place over time which, in soil, tend to be slow and generally inconsequential.

Good sampling design for soil characterization depends on the nature of the contamination and the size of the contaminated area. The three most prevalent sampling designs are judgemental, systematic grid sampling and transect sampling. Systematic grid sampling (see Figure 3) is often used to provide broad coverage of an area of suspect contamination such as imported fill material. Where relatively small hot-spots are suspected, several step-out samples evenly spaced in a radial pattern (i.e., spokes of a wheel) are typically used for delineation purposes. Transect sampling is sometimes used where concentrations are suspected to decrease with distance from a source zone, following a somewhat predictable pattern. During the field program, adherence to a rigid sampling program is sometimes not warranted, as sampling may be better guided using the results of real-time field measurement technologies and observations.

There are two common and basic methods for obtaining representative soil samples below ground surface: 1) excavation of test pits, and 2) drilling of boreholes. Test pits can be excavated by hand or with hand tools to shallow depths (about 1 m or less) or by machine (e.g., excavator) to greater depths (typically to depths of 3 m to 5 m). An advantage of test pits over drilled boreholes is that more soil is exposed, enabling better visual inspection of soil horizons and possible contamination zones, and collection of a larger volume of soil sample. The disadvantages of test pits include greater disturbance to the site surface, the limited sampling depth, and an inappropriate means to install groundwater or soil vapour monitoring wells. The drilling of boreholes allows for soil sampling at greater depths and for the installation of monitoring wells for groundwater sampling.

Figure 3. Systematic Grid Sampling.

Sediment Characterization

Unlike soil sampling, sediment sampling presents unique challenges. Aquatic systems are less well understood than soil systems, mainly because sediments have limited accessibility. In addition, sediments are inherently dynamic, requiring assessment over time to identify and characterize changes in physical and chemical composition and biota. These changes are caused by natural (e.g., spring freshet in rivers; tidal forcing) or anthropogenic (e.g., propeller wash) forces, which can affect sediment contaminant transport and increase the difficulty of defining the spatial extent of contamination. The dynamic nature and unique physicochemical properties of sediment requires the use of specific sampling equipment and defined sample acquisition and handling procedures.
As with soil investigations, the sediment investigation process starts with CSM, and is iterative. Because the samples are costly to acquire, a common pitfall is the collection of too few sediment samples. Regardless of the number of sediment samples collected, it is essential that the samples are representative and that data obtained from these samples are of high quality. The sediment sampling design process typically consists of a site reconnaissance, the delineation of the study area, the identification of reference area (usually required as part of an ecological risk assessment), selection of sediment sampling approaches, selection of sampling stations, and the determination of the number of samples. A typical sampling design to characterize an area where dredged sediment has been deposited is shown in Figure 4.

Sediment sampling devices suitable for sediment characterization studies fall into two types: 1) discrete surface samplers, which are typically used to collect surface sediment (and infauna benthos) for the areal evaluation of sediment characteristics and contaminant distributions; and 2) core samplers, which are typically used to sample sediments where the vertical distribution of sediment type and contaminants is important. The challenge for both sampling methods is access, and often involves deployment from dedicated watercraft.

Figure 4. Possible scenario to stratify sampling stations at a disposal site receiving 45,000m3.

Concluding Remarks

Ultimately, the succvessful investigation of contaminated sites requires a full dataset and the collaborative problem-solving skills of experienced practitioners in order to develop a robust CSM that will guide the remedial selection process and implementation.

These practitioners may have some formal academic training in environmental engineering or applied science, but probably gained most of their insight from “on-the-job” hard-earned experience. Education in science and engineering frequently neglects practical approaches to solving real-world issues. To fill this need, experiential technical courses and on-line training webinars are emerging that have been designed to engage the practitioner as a team player in solving simple to complex problems using real and synthetic data sets to explain and underpin key concepts and processes. Professional development training can help achieve success in remediation by providing practical insights from experienced practitioners.