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INVESTIGATING SEDIMENT COLOR CHANGE DYNAMICS TO PROMOTE BENEFICIAL USE APPLICATIONS J.F. Berkowitz 1, C.M. VanZomeren2 and A.M. Priestas3 ABSTRACT Sediment color is important in determining aesthetic and habitat suitability for beneficial use projects, including the placement of dredged materials during beach nourishment efforts. Generally, dark colored sediments 1) are considered less attractive to the public and 2) can impact habitat for a number of species including sea turtles. As a result, regulations preclude the beneficial use of many sediments based on in situ sediment color characteristics, thus removing sediment from the coastal system while increasing costs. However, sediment color change has been reported following beneficial use placement, potentially expanding the pool of available sediments for project application. Sediment color concerns remain poorly defined and no studies systematically evaluate sediment color change potential in a beneficial use context. In response, the current work presents regulations and environmental considerations related to sediment color across the United States, and evaluates factors driving sediment color change with an emphasis on chemical processes. Results from a sequential laboratory procedure examining potential sediment color change suggest that significant sediment color shifts may occur following beneficial use application. Preliminary findings of sediment mixing and photochemical bleaching experiments also indicate the potential for sediment color changes following sediment placement. Based upon available data, a conceptual model incorporating chemical processes, physical mixing, and photochemical bleaching is proposed. Further research will be required to link laboratory findings with real-world scenarios, promoting additional utilization of limited sediment resources. Keywords: Sediment color, beneficial use, beach nourishment, dredging, color change, bleaching. INTRODUCTION Sediment color represents an easily recognizable characteristic of materials being considered for beneficial use applications including projects that utilize dredged materials to protect infrastructure, improve/create habitat, and nourish beaches. Examining sediment color provides a mechanism for evaluating sediment properties and processes across coastal, riverine, or riverine settings (Andrews and Freedman 1996). As a result, color remains one of the most commonly reported substrate characteristics (Post et al., 1993; Soil Survey Staff, 1951) and sediment color is utilized in a number of fields including marine geology, ecology, soil science, and coastal sciences (Bigham and Ciolkosz 1993). For example, Giosan et al. (2002) used color to determine marine sediment origins and investigate sediment deposition processes. Others linked erosion and sedimentation processes through evaluating diagnostic sediment color characteristics (Martínez-Carreras et al. 2010). The following work examines sediment color change dynamics in a beneficial use context, promoting additional applications of available sediments to achieve both habitat and shoreline improvement objectives. Sediment color origin and color change dynamics Sediment color results from a combination of intrinsic properties (i.e., mineralogy, particle size) and secondary constituents including sediment coatings that mask or alter the underlying color of the naked mineral grain (Giosan
1
Research Soil Scientist, US Army Corps of Engineers, Engineer Research and Development Center, 3909 Halls Ferry Rd, Vicksburg, MS, 39180, USA, T: 601-634-5218, Email:
[email protected]
2
Research Ecologist, US Army Corps of Engineers, Engineer Research and Development Center, 3909 Halls Ferry Rd, Vicksburg, MS, 39180, USA, T: 601-634-3702, Email:
[email protected]
3
Research Physical Scientist, US Army Corps of Engineers, Engineer Research and Development Center, 3909 Halls Ferry Rd, Vicksburg, MS, 39180, USA, T: 601-634-2978, Email:
[email protected]
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et al. 2002). Sediment mineralogy has been linked with color characteristics, including white/light colors (e.g., quartz, calcium carbonate), black (e.g., augite, magnetite), deep red (e.g., hematite), and even green (e.g., glauconite, olivine) sediments that occur along beaches and other coastal settings (Balsam and Deaton 1991; Potter et al. 1980). While changes in structure and composition occur, coloration resulting from a sediment grains geologic origin remains constant within project relevant timescales (typically 50 years or less). Secondary constituents, on the other hand, encompass a number of factors impacting sediment color across short-, mid-, and long-term temporal scales. For example, water content can change within a period of minutes, having a marked effect on color perception as sediments typically exhibit lighter colors as drying occurs. As a result, sediment colors are often reported under both dry and moist conditions (Soil Survey Staff 1951). Secondary constituents coating sediment particles are some of the most common sources of perceived particle color. For example, organic matter coatings mask the inherent sediment color making particles appear darker (Mayer 1999), with as little as 5% coatings imparting a black color on underlying minerals (Vasilas and Berkowitz 2016). Organic coatings can oxidize and separate rapidly under optimal conditions, revealing the underlying naked sediment grain color (Canfield 1993). Iron oxides also coat many sediment particles derived from a variety of sources, resulting in the brown, red, and yellow colors observed across terrestrial, near-shore, and marine environments. Iron-oxide coatings also respond to environmental conditions, with changes in sediment oxidation-reduction potential resulting in a rapid sediment color shift due to dissolution or precipitation reactions (Vepraskas et al. 2016). As a result, sediment color represents an important, dynamic, yet often overlooked component of the sediment system and recent interest has focused on sediment color related to beneficial use applications (Figure 1).
Figure 1. Example of dynamic sediment color change following a pilot beach nourishment project near Mobile, AL (reproduced from USACE 2013). From left to right photos represent sediment color following sediment placement, 3.5 weeks post application, and 6.5 weeks post application. The sediment color was measured over time, indicating that redder colors transitioned to lighter colors during the monitoring period. However, color changes could not be attributed to any particular mechanism and likely resulted from a combination of mixing with native sand, sediment transport, photochemical bleaching, oxidation, and/or alteration of iron oxide or other sediment coatings. Measuring sediment color Field determinations typically rely on visual assessment via the Munsell Soil Color system that applies three parameters (hue, value, and chroma) to characterize sediment color (Gretag/Macbeth 2000). Hue is the spectral color a sediment exhibits, designated the distribution of red (R), yellow (Y), neutral (N), green (G), blue (B), and purple (P) wavelengths observed. For example, the designation YR describes the yellow-red color range. The Munsell value notation represents a sediment’s lightness or darkness, and is the most commonly applied measurement in the beneficial use context. Munsell values range from zero to 10, with lower values assigned to dark sediment colors and higher values denoting light or white sediment colors. Munsell chroma represents the saturation or intensity of color, with higher chroma corresponding to brighter, bolder color characteristics. Vasilas and Berkowitz (2016) describe how sediment colors are determined and provide guidance on using the Munsell color notation. Other color designation systems exist including the Commission Internationale de l'Eclairages’ (CIE) L*a*b* color space;
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however, the current report utilizes the Munsell Color notation throughout due to its common application within the United States. Appropriately managing limited sediment resources requires accurately determining sediment color. Sediment color measurements remain challenging especially under field conditions (Post et al. 1993), with some practitioners suggesting visual sediment color observation is unreliable or semi-quantitative. However, available scientific studies report positive relationships between visual color estimates and spectrographic methods (Kamrunnahar et al. 2004; Rabenhorst et al. 2015). For example, experienced practitioners accurately described Munsell values, with minimal variability among test subjects, demonstrating that trained professionals can accurately determine sediment color (Shields et al. 1966). Recent advances in spectroscopy allow for more accurate determination of sediment color in laboratory and field settings using hand held, portable digital colorimeters providing mechanisms to further improve color measurements in the future. Color compatibility requirements, aesthetics and habitat considerations Sediment color has been recognized as an important consideration with regards to beneficial use projects (Pranzini et al. 2010; 2016; Gravens, et al. 2008). As a result, a number of entities established color compatibility requirements to ensure beneficial use applications to maintain desired physical sediment properties such as grain size, composition, and color (Table 1; Berkowitz et al. In Press); many of these are qualitative and lack practical guidance for application (Williams 2005). Stauble et al. (1991) reported that grain size and sediment color should be considered when planning and implementing beneficial use projects across the United States. Other state and local groups established specific sediment requirements including color component thresholds (i.e., Munsell value ≤5.0). Factors indirectly linked with sediment color such as the allowable amount of shell fragments are also considered. Note that requirements often differ within a given state or regional boundary (Table 1) and few regulations provide technical justification for sediment color constraints. Despite the observed variability, beneficial use projects generally require light colored sediments, corresponding to Munsell values of ≥5.0. Additionally, some requirements also specify that sediments must have low chroma (e.g., ≤2). Sediment hue is less frequently addressed, but when applied hues are typically specified in the yellow-red or yellow spectral ranges. As a result of these regulations, potential sediment sources lacking the specified color characteristics in situ are precluded from consideration for beneficial use applications. However, practitioners report that substantial color change occurs following sediment placement, potentially surpassing sediment color thresholds. For example, if a sediment exhibiting a Munsell value of 4.5 (below the threshold) prior to placement, but transitions to a Munsell value of 6.0 during/after sediment application (thus surpassing the threshold), should the source material be disbarred from use? The authors would suggest that the propensity for sediment color change to occur following beneficial use applications should be considered when evaluating source materials as this potentially expands the application of limited sediment resources, maintains sediments with the coastal zone, and helps support both habitat and shoreline improvement objectives (Torres et al. 2017; Rosati et al. 2004). Table 1. Examples of sediment characteristics related to color required for beach nourishment projects. Note, "X" denotes citations that reference sediment compatibility but lack specific numerical values. See Berkowitz et al. (In press) for an expanded list of requirements. Size Silt Shell CaCO3 Region (mm) (%) (%) (%) Color Hue Value Chroma Citation United States X X USACE (1995) United States X X Stauble (1991) Orange FL 0.35-0.65
