Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 21.0

21.0 Ecological Risk Assessment Guidance for Coastal Marine Environments in Hawaiʻi

An investigation of contaminants in coastal marine and estuarine sediments in Hawaiʻi is necessarily influenced by the geophysical realities of the islands themselves and the dynamic Pacific Ocean. A brief introduction to the processes that create and redistribute sediments in Hawaiʻi provides a context for the specific guidance on conducting ecological risk assessments (ERAs) in Hawaiʻi.

The shield volcanoes that make up the main Hawaiian Islands are composed mainly of basaltic lavas. Erosion by wind and water break down these basaltic rocks into smaller particles that are transported into streams and ultimately deposited along the coast. At the same time, carbonate sediments derived from marine organisms in the surrounding waters are carried shoreward and deposited along the coast to form beaches (Fletcher et al. 2012). The processes of erosion and deposition of these two major sediment types creates a patchwork of unconsolidated substrates throughout coastal Hawaiʻi. Physical characteristics of the sediment particles, such as grain size and associated organic carbon, play a substantial role in the fate and transport, bioavailability, and toxicity of contaminants in the marine environment. These topics are introduced briefly below.

Grain size is a primary characteristic of sediment that influences the fate and transport of chemicals within the marine or aquatic environment.  Geologists identify sediments by size fractions (gravel, sand, silt, and clay) and classify sediments based on the ratio of size fractions using the Wentworth grade scale (USGS 2006):

gravel 2 mm
sand < 2 mm to > 62.5 µm
silt < 62.5 µm to > 4 µm
clay < 4 µm

Geological reports typically define the top 2 cm below the sediment/water interface as surficial sediment (USGS 2006).  However, standard practice in ERAs is to focus on the top 10 to 15 cm (about 4 to 6 inches), the biotic zone, where exposure of ecological receptors is greatest.

Many chemicals that cause ecological effects (such as metals, pesticides, PCBs) are known to be associated most strongly with finer-grained sediment, especially silts and clays (also called “muds”) (Morrison et al. 2011).  Fine-grained sediments generally accumulate in coastal bays and other sites where wave energy is low or absent. Contaminant concentrations are expected to be highest in such depositional areas where particles smaller than 62.5 µm accumulate (NRC 1989, Grabe and Barron 2004). In contrast, sites with predominantly sand or gravel are less likely to contain toxic levels of contaminants (Morrison et al. 2011).

One of the first studies to demonstrate the importance of grain size in sediment toxicity and bioavailability evaluations focused on PCBs in coastal marine sediments on the Mediterranean coast of France. The survey documented accumulation of low chlorinated PCB congeners with the sand-size fractions (> 63 µm) and of high chlorinated congeners with the silt-size fractions (< 63 µm). Greater bioavailability and toxicity were associated with the congeners in the fine-grained sediments (Pierard et al. 1996). Later studies in coastal marine harbors in the mainland United States corroborated these findings (Ghosh et al. 2003). Concentrations of dioxins and furans (PCDD/Fs) are also known to increase as grain size decreases in marine sediments (Lee et al. 2006). However, higher chemical concentrations may not accurately represent bioavailable fractions when chemicals are bound to finer-grained sediment.

The association of PCBs with fine-grained sediments has been demonstrated in tropical habitats, as well. In a highly-contaminated marine bay in Puerto Rico, PCB concentrations were shown to be influenced not only by grain size, but also by organic content. Moreover, microbiological characteristics (biofilm, bacteria levels, and microbial community composition) acted on the PCBs to reduce chlorination levels both in deeper anoxic sediment and shallow well-oxygenated sediments (Klaus et al. 2016). Toxic levels of lead are reported to be associated with fine-grained particulates carried by certain urban streams on Oʻahu, Hawaiʻi (Hotton and Sutherland 2016).

Coastal habitats in Hawaiʻi may contain a mixture of sediment grain sizes from various sources, creating complex sediment profiles and challenging risk assessment scenarios. For example, Hanalei Bay on the north side of Kauaʻi receives fine-grained terrestrial basaltic sediment from taro fields delivered by the Hanalei River.  Sand-sized sediment particles composed of calcium carbonate from nearshore coral reefs are transported into the bay by wave action. The Hanalei River carries so much suspended sediment that it often exceeds federal water quality standards for turbidity (Takesue et al. 2009). Despite the dominance of fine-grain sediments near the river mouth, organochlorine pesticides, PAHs, and metals were detected in sediment at very low levels. Concentrations of organic chemicals in Asian clam (Corbicula fluminea), giant mud crab (Scylla serrata), and Akupa sleeper fish (Eleotris sandwicensis) were also below ecological effect levels (Orazio et al. 2010). In contrast, sediment pore water was toxic to sea urchin fertilization (but not development) in clay and mud samples near the river mouth (Carr and Nipper 2007; Cochran et al. 2007). Further complicating the interpretation of ecological risk at this site is the seasonal influence of waves, which can flush out the finer-grained sediment from the bay during winter storms (Takesue et al. 2009).

The studies in Hanalei Bay illustrate the difficulty of drawing conclusions about ecological risk from a single line of evidence. Concentrations of chemicals in sediment of different grain sizes, surface water, pore water, and biota may all contribute to risk, but no single measure can adequately characterize the site. Actual exposure of ecological receptors to contaminant in sediment is influenced by both the presence and bioavailability of contaminated sediment and the absence of wave energy that removes sediment from the site. Although substantial deposition of fine-grained terrestrial sediment containing contaminants could indicate potential ecological risk, the regular winter flushing at this site reduced the risk to acceptable levels (Orazio et al. 2007, Takesue et al. 2009).

Beaches are eroding across Hawaiian Islands that have been evaluated (Kauaʻi, Oʻahu, Maui) more than accreting (Fletcher et al. 2012) and coastal erosion is expected to nearly double over the next few decades across areas studied, except Kailua Beach on Oʻahu. Nevertheless, sediment dynamics are spatially variable, and areas of erosion and accretion may be separated by only a few hundred meters.  Each small embayment created by rocky headlands is influenced by local wave energy and terrestrial processes, creating a patchwork of erosion and accretion along the shore. The most recent data on coastal erosion and accretion of shorelines on Kauaʻi, Oʻahu, and Maui are available at (Fletcher et al. 2012). This USGS information should be consulted during the site characterization phase of the SLERA (See Subsection 21.3.3).

Data on grain sizes are site-specific; there is no comprehensive assessment for the state, as grain size on beaches changes seasonally due to wave energy. Most beaches are sand and thus less conducive to adsorbing contaminants compared with finer-grained silt and clay fractions (Storlazzi 2016, personal communication). The risk assessor should review available data on grain size at the site. If grain size has not been adequately characterized at the site (considering season and specific location), data collection should be considered prior to initiating an ERA.  If the site is predominantly sand, the need for conducting additional chemical characterization in the area should be evaluated. Based on the CSM, additional chemical characterization may or may not be necessary.  If the site has a patchy distribution of grain sizes, chemical characterization should focus on areas where silts and clays are dominant.

The HEER Office ERA program for marine coastal environments provides guidance for conducting screening level ERAs (SLERAs) and Baseline ERAs (BERAs) in these coastal habitats. Alternative approaches or methods to the guidance provided in this section may be acceptable but should be discussed with the HEER Office for approval. The ERA program is process-oriented in that a site progresses only as far as required by the site-specific characteristics. The level of effort devoted to preparing and submitting information to the HEER Office is determined by the level of risk posed by the site. A site may exit the process at any of several points marked by management decisions and supported by technical analysis.

An ERA at a marine sediment site typically begins as a SLERA, and then may proceed to a more site-specific and in-depth BERA, if necessary. In many cases, the ERA will be conducted as part of a larger site investigation, although some sites may be addressed as strictly ERA sites. In both instances, the overall approach to conducting an ecological site investigation should generally be consistent with guidance elsewhere in the TGM, particularly in the following sections:

This ERA guidance is specific to the tropical marine environment of Hawaiʻi, but draws on decades of technical development of ERA methods by federal agencies in the U.S. and their counterparts in Australia and New Zealand, individual state agencies, independent researchers, and universities. This guidance combines the widely-used U.S. EPA framework, which provides a logical step-wise approach to conducting ERAs, with a more regionally focused approach suitable for tropical marine ecosystems. The HEER Office has developed this regionally-focused guidance to efficiently evaluate exposure and effects using Hawaiʻi-specific receptor and toxicity data wherever it is available. Readily available ecological exposure and effects data for 22 marine species in Hawaiʻi are compiled in this guidance (see Appendix 21-A). As additional ERAs are prepared and more Hawaiʻi-specific data become available, the on-line ERA TGM guidance will be updated to fill data gaps and refine exposure and effect default values and assumptions.

The HEER Office assumes that consultants and risk assessors using this guidance are familiar with the concepts and terminology of ERAs. Complete citations for references cited in this ERA guidance are provided in Subsection 21.7. Appendices to this guidance contain additional information, as follows:

APPENDIX 21-A Species Profiles and Exposure/Effects Data

The risk assessor is responsible for providing technical justification for the methods and assumptions that underlie the ERA. All references cited in the ERA must be made available for review by the HEER Office upon request. The HEER Office maintains a large library of peer-reviewed literature and government reports that may be useful to the risk assessor. Close coordination with the HEER Office will provide opportunities to share references and ensure that the most current useful information is available throughout the ERA process.