Reports
Deep Creek Lake Sediment Study: Physical and Chemical Characteristics of Lake Sediments
2011, Wells, D.V., and Ortt, R.A.
File Reports, Coastal and Estuarine Geology, File Report 2011-05
Abstract
The State of Maryland, Department of Natural Resources (DNR) recently added Deep Creek Lake to its public land holdings. Increased development of the surrounding land and a growing public concern over lake sedimentation has prompted a detailed examination of this resource. While Deep Creek Lake appears generally healthy based on existing water quality data, there are gaps in data, particularly with regard to sediments. Additional information is needed on the bottom sediments contained within this system, capacity of the lake itself, and identification, where possible, of the impacts of changing land use patterns on sedimentation and sediment character within the lake.
In order to characterize the bottom sediments in Deep Creek Lake, the Maryland Geological Survey (MGS), a program within DNR’s Resource Assessment Service, collected surficial sediments at 50 locations throughout the lake. The sediments were analyzed for textural properties, total nitrogen (N), total carbon (C), reactive carbon (CR), total phosphorus (P), and 48 additional elements including arsenic (As), antimony (Sb), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), nickel (Ni), lead (Pb), sulfur (S), zinc (Zn). These data provide baseline information on the bottom sediments.
Based on the textural analyses of the 50 surficial sediment samples (representing the top 5 cm of the sediment column), the majority of the samples collected are fine-grained sediments, with an average textural content of 18% sand, 39% silt and 43% clay. Clay represents a major component of the collected sediments, which is not unexpected given the abundance of shale in the underlying formations within the watershed. Sediments with the highest clay content were collected in the deepest part of the lake. Sand is a relatively minor component with only four samples classified as sand and silty-sand. The sand sample is the only sample to contain appreciable amounts of gravel (i.e., particles with diameter> 2mm). This sample was collected in the upstream reach of Cherry Branch.
Average N, C, and P in Deep Creek Lake sediments fall within the range of those measured in other Maryland freshwater lakes. On average, 70% of the total C contained in Deep Creek Lake sediment is reactive, readily available to the biological community. Coarser sediments (i.e., low clay content) tend to contain a lower portion of reactive C. Sediments in the northern portion of the lake contain overall higher C content (both total and reactive C) compared to the southern end. There is less variation in the non-reactive C content with regard to distribution.
Total C has little correlation with grain size; the poor correlation due to inclusion of non-reactive C which has no association with any particular sediment type. However, reactive C has higher correlation with clay as well as with N and P (compared to total C), indicating that a significant portion of the reactive carbon in the sediment comes from primary productivity (plankton and algae blooms). Total N has the highest correlation with reactive C indicating that most N is associated with organic material, most likely from primary productivity (algae). Total P is associated with the clay content of the sediment as well as many of the metals and S. When comparing the relative amounts of C, N and P in the Deep Creek Lake sediments to those of dried algae, P appears to be the limiting nutrient. In other words, mean C:P and N:P ratios are greater than those ratios of dried algae.
Compared to other freshwater lakes, S is significantly higher in most of the Deep Creek Lake sediments, particularly those collected in the deepest area at the downstream end (north end). The very dark color (black and dark grey) of some sediments collected indicated the presence of S, in the form of mono-sulfides (as FeS). Sources of S include sulfates from acid mine drainage, and atmospheric deposition in the watershed. Reduced S and sulfate (SO4-2) concentration is an extremely important variable controlling P release from sediments. The increased P release from sediments at higher sulfate concentrations may help explain why primary production in freshwater systems (with relatively low S concentrations) tends to be P limited, whereas in many saline systems (with high sulfate concentrations) production is often P sufficient. Sulfur also plays an important role in arsenic cycling, which explains the high correlations between S, As, and Fe seen in this study.
Compared to other freshwater lakes, S is significantly higher in most of the Deep Creek Lake sediments, particularly those collected in the deepest area at the downstream end (north end). The very dark color (black and dark grey) of some sediments collected indicated the presence of S, in the form of mono-sulfides (as FeS). Sources of S include sulfates from acid mine drainage, and atmospheric deposition in the watershed. Reduced S and sulfate (SO4-2) concentration is an extremely important variable controlling P release from sediments. The increased P release from sediments at higher sulfate concentrations may help explain why primary production in freshwater systems (with relatively low S concentrations) tends to be P limited, whereas in many saline systems (with high sulfate concentrations) production is often P sufficient. Sulfur also plays an important role in arsenic cycling, which explains the high correlations between S, As, and Fe seen in this study.
