Reports
Bathymetric and sediment assessment in the Bishopville Prong of St. Martin River
2012, Wells, D.V., Ortt, R.A., Jr., Van Ryswick, S., and Sylvia, E.R.
File Reports, Coastal and Estuarine Geology, File Report 2012-02
Executive Summary
The Maryland Geological Survey (MGS) conducted hydrographic and sediment surveys of the Bishopville Prong. The purpose of this study, funded through Maryland Coastal Bays Program Implementation Grant, was to document existing environmental conditions of the Bishopville Prong. This information can be used to assessment of the environmental impact, particularly with sediment quality issues, from the proposed Bishopville Dam Removal project and provide a baseline for future monitoring efforts.
MGS conducted the hydrographic surveys in June, 2011, using differential global positioning service (DGPS) techniques and digital echosounding equipment. MGS also collected sub-bottom seismic profiles along with the sounding data. Over thirty thousand discrete soundings were collected and used to generate a current bathymetric model of the Prong.
Concurrent with hydrographic surveys, MGS collected surficial sediment samples in Bishopville Prong and Pond. The sediments were analyzed for texture (sand, silt, clay contents), and thirty-seven element constituents, including total carbon (C), nitrogen (N), phosphorus (P), sulfur (S), arsenic (As), silver (Ag), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), lead (Pb), nickel (Ni), and zinc (Zn).
Results of chemical analyses of the Pond sediments are similar to those reported by in an earlier feasibility study of the Pond (KCI, 2002). Any discrepancies in results from the two studies are attributed to differences in sampling techniques and analytical methods, and variation in sample textures, which range from sand collected in the upper end of the pond to fine grained silty clay collected near the dam. Results from the MGS study confirm that the Pond contain high nutrient levels. C, N, and P contents were some of the highest observed in the coastal bays watershed. Pond sediments also contain S at levels higher than what would be expected in a fresh water environment. The amount of S is fairly consistent in sediments (~0.5 %) regardless of clay content, suggesting that S is in a stable mineral form. However, sediment samples collected in this study represent the top 5 cm of the sediment profile. Analyses of a sediment core collected in 2013 in the middle of the pond indicate that in some areas of the pond sediment deeper than 10 cm contain higher S levels. The source of S is thought to be from historic spills from a poultry processing plant within the Pond’s watershed.
The Pond sediments also contain higher concentrations of several metals: Cd, Co, Ni, Pb and Zn, compared to sediment in Bishopville Prong and other Coastal Bay tributaries. All Bishopville Pond samples contained concentrations of As and Cd exceeding the Threshold Effects Level (TEL) value and over half the samples had concentrations of Cr, Cu, Ni, Pb, and Zn exceeding the TEL. Several samples (closer to the dam) contain Ni and Zn concentrations exceeding the Probable Effects Level (PEL).
Bishopville Prong can be divided into three segments, defined, in part, by geomorphology and hydrodynamic regime. These segments are useful in explaining the bathymetry and sediment data from this study. The upper 1500 m segment, immediately downstream of the Bishopville dam, represents the narrowest portion of the Prong and encompasses the deepest portions of the Prong, with depths greater than 4 meters. The deep areas are probably from scour during high stream flow events. Sub-bottom data indicate that very little sediment is accumulating in this segment. Sediments collected in this segment ranged from coarser sand to sand-silt-clay to silty clays. However, the sediments collected in the segment contained the highest C, N, P and S contents measured for this project. N:C:P ratios suggest that the organic matter is from algae, mostly likely transported from the Pond.
The second segment of the prong, between N4253500 and N4251750, is generally wider and characterized by shallower depths (> 3 m). Sub-bottom data show accumulation of sediments along outside edge of stream meanders. Sediments in this segment range from sand in the upstream portion to silty clays downstream. C, N, P and S contents in sediments in this segment are lower compared to upstream areas. When normalize to clay content, C, N, P and S gradually decrease in the downstream direction within this segment.
The mouth of the Prong represents the third segment. Here, the Prong broadens to 300 to 500 m wide before the confluence with the St. Martin River. Water depths increase slightly along this segment of the Prong. However, there is no distinct central channel. Sub-bottom data show what is interpreted to be thick layer (1 meter thick) of recently deposited sediments, the texture of which is predominately silty clay. C:N:P ratios approach those ratios reported in main stem of the coastal bays (Wells et al., 1994).
Data from this study provide a general characterization of the Bishopville Prong study area. This assessment may be use as a baseline for future assessments after the removal of Bishopville Dam, particularly if the removal results in major alteration of stream hydraulics.
