Sediment Quality

The percentage of mud (silt/clay) in estuarine sediments can SCDNR scientist deploying benthic grab

impact both the structure of the biotic assemblage as well as the bioavailability of certain contaminants to local biota. The average percentage of mud in both open water and tidal creek sites was less than 50% (Appendix 3.1), with open water sites having a mean of 19% silt/clay compared to a mean of 32% silt/clay in tidal creeks. This difference was statistically significant (p < 0.001). However, there was considerable variability in the percent of silt/clay observed among the stations sampled in both habitats (< 2% to > 95% in both habitats; Appendix 3.1).

Approximately 72% of the open water habitat sampled in 1999 - 2000 was composed predominantly of sand (< 20% silt/clay) while only 49% of tidal creek habitats contained predominantly sandy sediments (Appendix 2.5, 3.1). Less than 10% of both habitats had primarily muddy sediments (> 80% silt/clay).

Average percent of sand versus mud (silt/clay) at open water and tidal creek sites sampled in 1999 - 2000 and estimates of the proportion of the state's coastal habitat that has predominantly sandy (> 80% sand), mixed (20-80% sand), or muddy (< 20% sand) sediments.

The following sediment characteristics, contaminants, and toxicity were evaluated in the SCECAP Program:

TOC:

Total Organic Carbon (TOC) provides a measure of how much organic matter occurs in sediments. The TOC of sediments in tidal creeks ranged from 0.0 to 5.4% with a mean of 1.2% (Appendix 3.1). Sediments in open water habitats contained less TOC with a mean of 0.8% and a range of 0.04 to 6.6% . The difference between total organic carbon content in tidal creeks and open water sites was statistically significant (p < 0.001). The proximity to decomposing salt marsh plants and upland runoff probably explains the higher organic content in tidal creeks compared to the more distant open water sites. Total organic carbon was significantly correlated with the amount of silt/clay in the sediments (r² = 0.86, p < 0.001). As the percentage of silt/clay increased in sediments the total organic content increased.

Hyland et al. (2000) found that extreme concentrations of TOC can have adverse effects on benthic communities. TOC levels below 0.5 mg/g (0.05%) and above 30 mg/g (3.0%) were related to decreased benthic abundance and biomass. Approximately 15% of the tidal creek habitats in the SCECAP study had TOC levels that were either less than 0.05% or greater than 3%, which may be indicative of a stressful environment for the benthos. Approximately 13% of the open water habitats had TOC levels that were indicative of possible stress (Appendix 2.5).

Average percent total organic carbon (TOC) concentration in sediments at open water and tidal creek sites sampled in 1999 - 2000 and estimates of the proportion of the state's coastal habitat having TOC levels (< 0.05 or > 3%), which may cause stress in benthic communities.

TAN:

Total ammonia as nitrogen (TAN) in sediment porewater is a measure of another source of potential toxicity in sediments. The effects of TAN on marine biota are highly variable depending on the species considered (Sims and Moore, 1995; Moore et al., 1997). The No Observable Effects Concentration (NOEC) of TAN in porewater reported for the 7-day seed clam (M. mercenaria) assay used in this study program was 14-16 mg/L (Ringwood and Keppler, 1998). The NOEC TAN concentrations for four species of amphipods in 10-day sediment exposures ranged from < 30 to < 60 mg/L dependent on the species used (< 30 mg/L for A. abdita used in this study).

In the 1999-2000 survey, TAN levels were similar between open water sites (mean = 2.82 mg/l) and tidal creek sites (mean = 2.93 mg/L), and generally well below levels considered to be toxic (Appendix 3.1). There was no statistically significant difference between TAN levels in open water and tidal creek habitats. Only 1% of open water or tidal creek habitats had TAN concentrations > 14 mg/L and none of the sites sampled in 1999-2000 had porewater TAN concentrations > 30 mg/L (Appendix 2.5).

Average percent total ammonia nitrogen (TAN) concentration in sediment porewater at open water and tidal creek sites sampled in 1999 - 2000.

Contaminants:

    The 24 contaminants measured include both trace metals and organic compounds for which there are published bioeffects guidelines based on laboratory and field studies of estuarine and marine organisms (Long et al., 1995). None of the sites sampled in 1999-2000 had contaminant concentrations that exceeded values considered to be high (i.e. cause adverse effects in at least 50% of the studies evaluated by Long et al., 1995). However, several sites had moderately high concentrations (i.e. cause adverse bioeffects in at least 10% of the studies evaluated by Long et al., 1995). More tidal creek sites had elevated contaminants compared to the sites in larger water bodies (15 vs 9 sites). The elevated contaminants included arsenic, cadmium, copper, chromium, and several polycyclic aromatic hydrocarbons (PAHs) commonly associated with fuel combustion, petrochemical industrial effluents, etc. Arsenic is naturally elevated in South Carolina estuarine sediments (Scott et al., 1994; 2000; Sanger et al., 1999a) and the values observed cannot be attributed solely to anthropogenic stress. Only one site had many contaminants at levels known to cause adverse bioeffects. This station was located in the turning basin of Shipyard Creek, and industrialized drainage basin in Charleston Harbor.

    While individual contaminants were elevated at some sites, a better assessment of overall pollution exposure can be derived from the combined concentrations of all 24 contaminants present at a site relative to the bioeffects guidelines for each of those contaminants. Hyland et al. (1999) published sediment quality guidelines that have been shown to be predictive of a high, moderate, or low risk of observing adverse effects in bottom dwelling invertebrate communities (benthos) inhabiting southeastern estuaries. Using these guidelines, approximately 21% of the tidal creek habitat assessed in 1999 and 2000 had contaminant concentrations indicative of a moderate risk to bottom-dwelling assemblages. In comparison, only 11% of the open water habitat had similar contaminant concentrations. In both habitats, many of these sites were in developed watersheds. The remaining creek and open water stations sampled in 1999 and 2000 had low contaminant concentrations that would support a healthy bottom community, except for Shipyard Creek, which had a combined chemical concentration that represented a high risk to benthic communities.

    Although there are several locations in South Carolina's estuarine waters known to be polluted, the areal extent of these polluted areas is limited and not likely to be routinely represented in the 60 sites randomly selected for sampling each year. The lack of widespread contamination in South Carolina's estuaries is a positive indication that our estuaries are not experiencing extensive chemical degradation. More importantly, the SCECAP database provides valuable information for determining whether conditions at sites where potential human impacts are occurring are different than typical conditions found in unimpacted tidal creek and open water habitats.

Average cumulative sediment contaminant concentrations at open water and tidal creek sites sampled in 1999 - 2000 and the proportion of the state's coastal habitat having concentrations representing a low, moderate, or high risk of observing stress in benthic communities.

Toxicity:

Even if estuarine sediments have high contaminant levels, the contaminants may not be available to biota living in the sediments. Laboratory bioassays are used as indicators of both contaminant bioavailability and potential for toxicity. For SCECAP, two (1999) to three (2000) bioassays were conducted to test for toxicity using marine bacteria, juvenile hard clams, and a small sediment-dwelling crustaceans (amphipods). When two or more of the tests showed positive toxicity, the sediment was considered to be poor (high probability of toxicity); one positive test indicated marginal sediment quality and no positive test results indicated good sediment quality. Approximately 7% of the state's creek habitat and 14% of the open water habitat had poor sediment considered to be toxic, with an additional 46% and 30%, respectively, showing some evidence of toxicity.

Summary of sediment bioassay results using multiple assays. Two toxic assays represent poor sediment quality, one toxic assay represents marginal sediment quality, and no toxic assays represent good sediment quality.

Integrated Assessment of Sediment Quality:

The best estimate of overall sediment quality incorporates the combined measures of sediment contaminant concentrations and the sediment bioassay test results. An overall sediment quality score computed using these measures indicated that none of the state's tidal creek habitat sampled in 1999 - 2000 had poor sediment quality and only about 3% of the state's open water habitat had poor sediment quality. A slightly higher percentage of the state's creek habitat had marginal sediment quality compared to open water areas, but this difference was not significant (Appendix 2.5).

Proportion of the South Carolina's estuarine habitat that ranks as good (green), marginal (yellow) or poor (red) using the integrated sediment quality score developed for SCECAP. The left portion of the figure shows examples of how individual stations coded for each sediment quality parameter and the average integrated score for each station based on numerical ratings described by Van Dolah et al. (2002). The right portion of the figure shows the estimated proportion of sediment quality conditions for the entire coastal zone of the state.