Reports
Sediment Quality Assessment of St. Mary's Lake, St. Mary's County, Maryland
2022, Gillmor, A., Van Ryswick, S., Sylvia, E.R., Connallon, C., and Nicholson, B.
File Reports, Coastal and Estuarine Geology, File Report 20-06
Executive Summary
Maryland Geological Survey, in conjunction with other Maryland Department of Natural Resources programs, including Tidewater Ecosystem Assessment (TEA) and Fishing and Boating Services, was asked to perform sediment quality assessments of state-owned lakes in Maryland. The goal is to provide surficial sediment data for each lake to determine general sediment physical and chemical characteristics as a pre-assessment for potential future lake sediment dredging. Physical and chemical analysis of the sediments from each lake will pose as a baseline for the lake’s sediment data. The sediment data will be incorporated into other study aspects, including fish populations, water quality, and submerged aquatic vegetation (SAV) abundance and distribution in order to generate an overall lake health and status.
This report focuses on St. Mary’s Lake in St. Mary’s County, Maryland. The lake is 250 acres and is located within St. Mary’s River State Park. In September 2019, 35 surficial sediment samples were collected throughout the lake, and coordinates were stored using Real-Time Kinematic Global Positioning System (RTK GPS). Sediment at each site was described and sampled for analysis. The analysis includes physical properties, including bulk density and grain size, and elemental analysis.
Total carbon (C) content measured in St. Mary’s Lake sediments had an average of 2.5% by dry weight. Total nitrogen (N) averaged, 0.15%, total phosphorus (P) averaged 0.016% and total sulfur (S) averaged 0.07%. Concentrations of C, N, P and S were similar to that found in other reservoirs within the state. Concentrations of N and P were the lowest of any of the freshwater lakes and reservoirs studied by MGS. C, N and S concentrations were well inter-correlated at St. Mary’s Lake, which is suspected to be due to the high amount of fresh organic material in the sediments.
The elemental concentrations of 35 surficial sediments followed similar spatial patterns to each other when plotted by station, and correlated well with clay content. This was true for major rock-forming elements aluminum (Al), calcium (Ca), iron (Fe), potassium (K), sodium (Na) and magnesium (Mg). Most trace elements had quite similar profiles to each other, and they also are well correlated with clay content. These include arsenic (As), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn), whereas manganese (Mn) concentrations were only partially paired with clay content. When compared relative to two eco-toxicological screening levels for freshwater sediments, only a few of the metals of concern (As, Cr, Fe and Ni) are above the more conservative lowest effect level (LEL) in some or most sediments, while no concentrations of these metals are higher than the severe effect level (SEL) value. Since these concentrations represent more than that which is environmentally available, no eco-toxicological harm is anticipated. When compared to relative crustal abundance via the use of Fe-normalized enrichment factors, the elements. As, Cs, Eu, Pb, Hf, Sb, Th, U and Y were elevated. However, these elements are often encountered in high concentrations in clay-rich sediments, clay-rich sedimentary rocks and sandstones, as well as in minerals where they may substitute for K.
No abnormalities were found in regard to the physical or chemical properties of the bottom sediments of St. Mary’s Lake.
