Reports
Poplar Island Sediment Quality Monitoring: May 2015
2017, Gillmor, A., Sylvia, E.R., and Van Ryswick, S.
File Reports, Coastal and Estuarine Geology, File Report 16-06
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 May 2015.
With regard to the grain size, there was alteration of the sediment distribution as a result of the construction of Poplar Island, as observed in the previous post-construction sampling events. The variations in the grain size are attributable to the change in the hydrodynamic regime of the area resulting from the construction of Poplar Island. Most notable is the 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, potentially due to the change in hydrodynamic conditions. However, the increases in metal concentrations were expected given the grain size change. Storm action, from Tropical Storm Isabel in 2003, altered the grain size distribution by scouring out newly deposited fines in the southeastern portion of Poplar Harbor, and depositing a localized thin veneer of finer sediments from the breach caused by the storm in the northeastern dike of Poplar Island. 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 the most consistent with the post-construction, pre-Isabel sampling period (Year 2000 – April 2003) to date. 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.
The 2015 average concentrations of C, N, S and Ca were slightly higher than the preceding year (2014), whereas P was essentially the same. 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.
The concentrations of target metals from the 2015 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.”
