Report Number: 93
Year: 2001

Contaminant Assessment of Surface Sediments from Tanapag Lagoon, Saipan

In June 1999, a pilot study was undertaken to determine levels of heavy metals, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) in surface sediment cores (5 x 15 cm) from the southern end of Tanapag Lagoon off the western shore of central Saipan. Samples were collected from 32 nearshore sites (<500 m offshore) and nine outer lagoonal sites (>500 m offshore) between Flores Point to the north and Muchot Point to the south. Sets of three sediment cores were collected within an approximate 3m-diameter circle at each site. Each core was homogenized and analyzed separately, following petrographic and organic carbon determinations. Site mean values for each contaminant were subsequently derived from each core set. All data presented here are expressed on a dry weight basis.

The range of mean heavy metal concentrations (μg/g) for all nearshore sites were as follows. silver (Ag): <0.1-0.28; arsenic (As): 1.33-10.0; cadmium (Cd): <0.1-0.58; chromium (Cr): 1.40-9.67; copper (Cu): 0.22-27.8; mercury (Hg): 0.012-0.347; nickel (Ni): <0.2-5.06; lead (Pb): <0.4-40.6; tin (Sn): 0.08-36.1, and zinc (Zn): 1.63-127. Levels determined in the offshore sites were generally lower with maximum mean values of <0.1, 1.87, <0.1, 2.43, <0.1, 0.016, <0.2, 1.2, 0.24 and 2.09, determined for Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Sn and Zn, respectively. Highest overall values for all metals, except Ag and Sn, were found at site 14, in intertidal sediments at the southernmost end of Puerto Rico Dump.

Total PCB levels (Σ20 congeners) were generally low throughout the study area and ranged from a mean of <1 ng/g at all outer lagoonal sites to 16.6 ng/g near the dump (site 14). A relatively high mean level of 11.1 ng/g was also found in sediments near Saipan Harbor (site 9). PCB profiles in sediments from this site closely resembled Aroclor 1260, whereas those from site 14 appeared more like a combination of Aroclor 1260 and one of the lower chlorinated commercial mixtures, possibly Aroclor 1254. Almost all samples with detectable quantities of PCB were dominated by Cl5-Cl7 homologues. The most frequently encountered congeners were PCBs 101 and 153, detected in 57% and 59% of all sediment cores, respectively. The highly toxic planer/near planer congeners (PCBs 77, 105 and 126) were not detected in any sample analyzed.

PAHs were only detected in 33% of all offshore sediment cores compared with 81% from nearshore sites. Detectable PAH levels in sediments from the outer lagoonal sites were barely measurable in the majority of cases. Closer to shore, levels were often appreciably higher with mean values (Σ16 congeners) rising to 1.18 μg/g off the west edge of the dump (site 30), 1.22 μg/g in Outer Cove Marina (site 4) and 1.39 μg/g by the new docks (site 7). PAH profiles were highly variable throughout the study area and were dominated by the higher molecular weight compounds. The most frequently encountered and most abundant PAHs were the 4- ringed members, chrysene, benzo(a)anthracene, and pyrene. These congeners were detected in 71%, 61% and 60% of all sediment cores respectively. The high prevalence of pyrene indicated that combustion sources were a significant source of PAHs in the area. Sedimentary fluoranthene/pyrene and pyrene/benzo(a)pyrene ratios also suggested that hydrocarbon fuel spills contributed to the PAH contamination of nearshore waters.

The data for all three chemical groups are measured against previously proposed numerical guidelines for Guam sediments in order to determine degrees of relative abundance. They are also discussed in relation to possible sources of localized input, and are compared and contrasted with levels reported by other researchers elsewhere in the world. Discrepancies are addressed between our data and the earlier DEQ findings for sediments taken near the dump. Baseline contaminant levels for clean, nearshore and offshore tropical reef sediments are proposed. These are intended to assist environmental managers in preserving the integrity of pristine sites, identifying impacted and potentially impacted sites, and prioritizing management decisions regarding sediment disposal and remediation strategies where necessary. Finally, the data are compared with numerical sediment quality guidelines developed from an extensive biological effects database by NOAA scientists. Future directions for continued research are recommended in the closing comments.

Gary R. W. Denton
Brian G. Bearden
Lucrina P. Concepcion
Galt H. Siegrist Jr.
David T. Vann
Rick H. Wood