Source control (i.e., the reduction of contamination from an upstream point or diffuse sources) is a critical element in any management plan for contaminated waterways. In order to understand the issues surrounding source control, it is essential to have some understanding of the sources of contaminated particles, their transport pathways, and their sinks. Particle tracking offers a practical means to investigate source-sink relationships and map the transport pathways of contaminated sediments both at the point of and following delivery into waterways, through time and across space. It is a relatively straightforward methodology which involves the introduction of particulate tracers into a water body. These particulate tracers are labeled with one or more signatures so that they may be unequivocally identified following release. Particle tracking studies are often done as part of a larger sediment transport modeling effort in order to provide actual field data for model validation. Particle tracking is not a panacea, but when applied correctly, it can provide an excellent ‘tool in the box’ to assist in the validation of sediment transport models. These models can then be used to investigate sediment transport dynamics over greater spatial and temporal scales, nominally with a greater degree of confidence in the model outputs.
The objective of this project was to demonstrate a particle tracking technology to quantitatively map the spatiotemporal distribution and depositional footprint of particles released from typical Department of Defense (DoD) contaminant sources into adjacent aquatic environments. Fluorescent ferrimagnetic particles were released from specific sources and tracked through the water column. The particles were collected at the sediment surface to determine their spatial distribution and depositional pattern and quantitatively demonstrate the linkage between sources and receiving water areas where these particle sources are most likely to impact the sediments.
The demonstrated technology used proprietary tracers called dual signature tracers (Figure 1), which means that each particle (grain) of tracer had two signatures that were used to unequivocally identify the particle following introduction into the environment. The use of two signatures was an advancement and improvement on previously used mono-signature tracers. The two signatures were fluorescent color and ferrimagnetic character.
Two types of dual signature tracers were available: coated particles and entirely artificial particles. Coated particles, which were samples of natural sands or silts directly coated with a fluorescent-magnetic mono-layer, possess a fixed grain density of 2.6 grams per cubic centimeter (g/cm3; i.e., mineral density), whereas fixed grain density for artificial particles can be adjusted through a range of 1.01 to 3.75 g/cm3. Coated particle grain sizes range from 20 to 5000 micrometers (µm) and were commonly used in sediment transport/particulate contamination studies. Artificial particles are commonly used to mimic low settling velocity particulates, such as biological larvae and activated carbon, and for engineering scale model studies.
While compositional data for each tracer type is commercially confidential, coated particles (used most frequently in tracking studies) were made from entirely natural materials plus a geochemically inert fluorescent pigment. A coated particle with a density of ~2.6 g/cm3 was used at each of the demonstration sites. A second artificial particle tracer was used at the second demonstration site to simulate the activated carbon amendment at the cap site.
Four spectrally distinct fluorescent colors were available with which to label tracers. The colors were commercially available fluorescent pigments, which themselves comprised polymer nano-spheres embedded with a water insoluble dye. This means that, aside from a very minor dust fraction produced by the tracer manufacturing process, no free dye was released into the aquatic system (the dust fraction can be removed/minimized by prior screening/washing).
Each pigment was characterized by specific excitation and emission wavelengths, which facilitated a targeted sample analysis procedure. The emission and excitation spectra of the pink and chartreuse (visually green and referred to as ‘green’ hereafter) tracers are presented in Figure 2. The peak emission wavelength (λ) for each dye is λpink = 625 nanometers (nm) and λgreen= 530 nm. Use of multiple colors means that the technology can be used to label multiple sources in the same general area, or to perform consecutive studies in the same area under differing hydrodynamic conditions (e.g., high discharge, low discharge).
Every tracer particle was also ferrimagnetic. Magnetism is controlled by the forces created by the spin and orbital angular states of the electrons within atoms. The manner in which these motions are aligned, the number of electrons, and the type of motions determine the magnetic moment of the atoms. Ferrimagnetic materials have populations of atoms which are strongly aligned but exist as two sets of opposing forces. These materials display high susceptibility and are considered (colloquially) highly ‘magnetic’ materials insofar as tracer particles will adhere to any permanent or electro-magnet if they come in close proximity. This facilitates a simple separation of tracer within environmental (water, sediment, soil) samples, a process that can also be exploited in situ (e.g., through use of submerged magnets in a water course). The integration of tiny magnetic inclusions onto the kernel particle during tracer manufacture was a substantial innovation over mono-signature, fluorescent-only tracers, for which there was no effective means of tracer separation within samples prior to analysis. This has profoundly limited tracer enumeration in previous studies.
The efficiency of high field magnets for sampling suspended sediment was shown to be >90%. There was a slight reduction in the efficiency of the technique where higher concentrations of tracer material were in suspension, though generally the data revealed high efficiency in determining the concentration of suspended tracer material within a water sample (1 L). The efficiency of traditional water sampling techniques and analyses (i.e., via filtration and gravimetric analyses) to determine the concentration of total suspended solids, where correct procedures are followed, can be considered to be >90%. Thus, the two techniques can be considered comparable in terms of efficiency.
Within the HPS demonstration study, magnet frames specifically designed to capture sediments as they deposit on the seabed were not used. Instead, bed frames with a single vertical magnet positioned in the center of the frame were utilized to capture tracer particles moving in suspension across the site to investigate near bed sediment transport. In essence, these frames are no different in regards to sample efficiency.
Although performance objective 4 was passed (less than 30% difference between tracer collected on bedframe magnets compared to standard sediment grab), the research team still decided to use a standard grab to recover tracer from the sediment surface at both demonstration sites. This was based on the grab being able to recover > 90% of the tracer released in laboratory settings compared, to the various bedframe designs which never exceeded about 80% recovery. Additional work to better design magnet bedframes is in progress, as the magnet bedframes did not perform as well as expected based on field performance of deposited tracer recovery from the sediment surface. The grab data at both field sites provided lower mass balance values compared to magnet results.
The spectrofluorometric analytical procedure was adapted and developed to exploit the fluorescent attribute of the tracer particles to directly provide a dry tracer mass (in grams). The technique has sufficient spectral resolution to distinguish low concentrations (< 0.01 g) of two spectrally unique tracer colors. The dye concentration was proportioned to dry mass of fluorescent tracer particles through the use of color specific reference standards. Consistently high coefficients of determination (r2) were recorded throughout both demonstration studies.
Consequently, the performance of the spectrofluorometric analytical approach can be judged from the following key findings:
The evaluation of mass balance was based on the magnet data for the first demonstration at NBSD. Assuming the tracer mass collected on each magnet represents collection of tracer over 1 ft2 of the sediment surface, we interpolated the amounts of tracer that would be present on the sediment surface between the collection points. A total of 668 kg was calculated and represents 84% of the total 800 kg of released tracer, so although only a small fraction of the released tracer (<100 grams) was directly collected on the magnets, a mass balance could be calculated.
The evaluation of ease of use was based on comparisons to previous Space and Naval Warfare Systems Center Pacific (SPAWAR) experience with dye and particle tracer studies. The use of magnets for collection of particle tracers was easier and faster compared to collecting water samples with suspended tracers followed by filtration to collect solid tracer particles. The use of laboratory spectrofluorometric techniques to quantify tracer levels was a distinct advantage over standard analysis techniques of counting fluorescent particles under a microscope. Overall, the Partrac methodology and the dual-signature nature of these tracers proved “easier” to complete both field and laboratory aspects of a particle tracer study, in comparison to standard (mono-signature) tracer studies.
This effort provided an innovative and cost-effective methodology for tracking sediment particles (and associated sediment contaminants) to specific sources and dischargers in water bodies affected by DoD activities. This methodology is based on demonstrated scientific procedures and supports the correct assignation of regulatory enforcement to DoD and other users, resulting in appropriate cost distributions for compliance and restoration. This project has potentially far reaching benefits for DoD by directly linking sediment contamination to particular sources, thereby allowing any upstream corrective actions and downstream remedial costs to be applied to the correct potential responsible party.
Collins, A.L., Y.S. Zhang, D. Duethmann, D.E. Walling, and K.S. Black. 2013. Using a Novel Tracing-tracking Framework to Source Fine-grained Sediment Loss to Watercourses at Sub-catchment Scale. Hydrological Processes, 27:959-974.