Reports
Poplar Island Sediment Quality Monitoring: March 2016
2019, Gillmor, A., Sylvia E.R., and Van Ryswick, S.
File Reports, Coastal and Estuarine Geology, File Report 17-14
Executive Summary
File Reports, Coastal and Estuarine Geology, File Report 17-14
Executive Summary
The Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (Poplar Island) is an innovative approach to island habitat restoration through dredged material placement. The facility was constructed to produce a combination of upland and tidal wetland habitat. This facility is establishing new habitats for a wide range of aquatic and terrestrial wildlife. Depending on the success of the operation of this facility, similar facilities may be constructed based on this design. As a prototype facility in the Chesapeake Bay, it is crucial to ensure the integrity of the Chesapeake Bay environment is not diminished by the placement of dredged material for beneficial use.
In order to assess the impact of Poplar Island on the surrounding sedimentary environment, a monitoring program was initiated, beginning with a baseline survey prior to construction of the facility. The baseline survey established the regional (i.e., Chesapeake Bay segment encompassing the Poplar Island vicinity) sedimentary background on sediment samples collected at 103 stations in October 1995 and July 1996. The samples were analyzed for physical properties (water content, bulk density, and grain size); carbon (C), nitrogen (N), and sulfur (S); and eight target metals: cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn). Phosphorus (P), calcium (Ca) and mercury (Hg) were added in subsequent sampling events. The next monitoring samplings were conducted in 2001 and 2002, during which sediments were collected at the 11 water quality (WQ) stations around the facility when the dikes were completed. In the fourth and fifth monitoring events a denser spatial sampling grid, which included 16 sediment quality (SQ) stations in addition to 12 WQ stations, was used to document the sedimentary environment before and after Tropical Storm Isabel (September 2003). The same 28 sampling stations have been used each spring since 2003. This report documents the state of the sedimentary environment adjacent to Poplar Island based on samples collected in March 2016.
With regard to the grain size over time, there was alteration of the sediment distribution as a result of the construction of Poplar Island, as observed in the early post-construction sampling events. The variations in the grain size were attributed to the change in the hydrodynamic regime of the area resulting from the construction of Poplar Island. Most notable was an accumulation of fines in the sheltered area east of Poplar Island, adjacent to Coaches Island. In 2002, increased concentrations of the target metals were observed at those stations where sediments were finer grained compared to previous years, likely due to the change in hydrodynamic conditions. Storm action from Tropical Storm Isabel in 2003 then scoured the newly deposited fines from the southeastern portion of Poplar Harbor. Storm action from Tropical Storm Isabel was also responsible for a limited breach of the northeastern dike, and a localized, limited thin veneer of finer sediments was observed there during the Storm response sampling. The samples collected in June 2004 exhibited a sediment distribution that was in transition, returning to the pre-Isabel state. The May 2006 sediment pattern was similar to the patterns observed previously, during the post-construction, pre-Isabel sampling period (Year 2000 – April 2003). The distribution of sand in April 2007 was similar to the May 2006 sampling; however, it was much coarser. The coarsening trend continued until the April 2011 monitoring event, when average mud content increased around the facility. The following year (April 2012) samples were coarser overall compared to the 2011 samples. The 2012 clay content represented a historical low, both in average and range of values. Although the 2013 samples showed a slight decrease in sand similar to that of 2011 sampling, the 2014 sampling saw a slight increase in sand, amounts of which are similar to 2012 conditions.
In 2016, the average concentrations of P, Ca and S were higher or slightly higher than the preceding year, whereas the average concentrations of C and N were slightly lower. For most nutrients and S in sediment, the range observed in 2016 was small and tracked proportionately with the average, with the exception of the S maximum. The summary statistics for C and P closely resemble those for Ca, which suggests that shell material contributes to the concentrations of these components. Shell is estimated to contribute approximately a third of the total C measured in Poplar Island sediments.
Compared relative to the Effects Range Low (ERL) for metals, no single sample exceeded this evaluation threshold in 2016. ERLs are established for Cd, Cr, Cu, Hg, Ni, Pb and Zn, and not for Fe or Mn. In aggregate, the average concentrations of metals in 2016 were between 6% (Hg) and 40% (Ni) of their respective ERLs.
Compared relative to the trends in absolute metals concentrations (mg/kg or %) over time, average Cr, Cu, Fe, Pb and Zn (Type I metals) exhibited slight decreases in 2016 relative to the preceding year, average Ni exhibited a slight increase and the overall trend-to-date for Type I metals remains stable to slightly decreasing. The absolute concentrations averages of Mn, Cu and Cr (Type II metals), exhibited slight decreases in 2016 relative to the preceding year, and the overall trend-to-date for Type II metals remains stable to slightly increasing. The maximum Mn concentration has been trending upwards since 2012. The absolute concentrations of the metals continued to be low with respect to the main Bay.
The concentrations of target metals from the 2016 samples were directly correlated to grain size distribution, a pattern which has been observed consistently since the 1995-1996 baseline study. Metal concentrations continued to be low with respect to the main Bay, primarily due to the coarse nature of the sandy sediment around Poplar Island.
These observations continue to support the working hypothesis for the study: “Project conditions will not change the metals behavior in Poplar Harbor when compared to regional background sediments.”
