Reports
Sediment mapping and sediment oxygen demand of Triadelphia and Rocky Gorge Reservoirs
2007, Wells, D.V.; Hill, J.M.; Ortt, R.A., Jr.; Van Ryswick, S.
File Reports, Coastal and Estuarine Geology, File Report 2007-02
Executive Summary
Triadelphia and Rocky Gorge Reservoirs, located in the Patuxent River watershed, are sources of drinking water for the Washington, D.C. area. The Interstate Commission on the Potomac River Basin is developing a model to determine the Total Maximum Daily Loads (TMDLs) for nutrients that would be permissible in the reservoirs. In order to provide additional chemical sediment data for TMDL modeling of Rocky Gorge and Triadelphia Reservoirs, the Maryland Geological Survey (MGS) conducted a study to (a) characterize the physical and chemical properties of recently deposited reservoir sediments and (b) provide estimates of the sediment oxygen demand (SOD) for different sediment types. In 2005, MGS collected and analyzed 40 sediment samples in Triadelphia Reservoir and, in 2006, 42 sediment samples in Rocky Gorge Reservoir. The samples were analyzed for textural properties (bulk density and gravel-, sand-, silt- and clay-size components), nutrients (total carbon, reactive organic carbon, inorganic carbon, total nitrogen and total phosphorus), and selected metals (including cadmium, chromium, copper, iron, manganese, magnesium, nickel, lead and zinc). In mid-fall of 2006, MGS conducted in situ SOD measurements at four sites in Rocky Gorge Reservoir.
Results of the sediment analyses show that Rocky Gorge Reservoir contains a broader range of sediments, in terms of grain size, compared to Triadelphia Reservoir. Water depth appears to have less control over grain size distribution in Rocky Gorge than it does in Triadelphia Reservoir. Spatial patterns of sediment accumulation in both reservoirs reflect the morphology of their original stream valleys.
The surficial sediments do not contain extreme amounts of nitrogen, carbon or phosphorus in either reservoir. Overall, the total nutrient content of Rocky Gorge Reservoir sediments is slightly less than that of Triadelphia Reservoir sediments. Approximately two-thirds of the total organic carbon in Rocky Gorge Reservoir sediments is reactive, that is, readily available to the biological community. The ratio of reactive organic carbon to total carbon in Rocky Gorge Reservoir is twice that of Triadelphia Reservoir. Carbon is derived from two sources; terrestrial carbon and algal carbon. Terrestrial carbon is derived from detritus washed in from the drainage basin. These carbon compounds generally contains lower amounts of reactive organic carbon compared to algal detritus which is carbon generated within the reservoir. The lower reactive carbon component in Triadelphia Reservoir is attributed to the higher terrestrial carbon input.
Reactive organic carbon content and textural parameters were used to calculate the total potential sediment oxygen demand (TPS) for the sediment in both reservoirs.
In-situ SOD measurements were conducted in late October and early November 2006. The timing of the measurements was driven by the maintenance activities related to water intake structure at Rocky Gorge Dam which impacted the contract schedule. During this period, the mean pool elevation was approximately 5 meters below normal in Rocky Gorge Reservoir. The bottom dissolved oxygen (DO) levels were high ( >8 mg/l) at the time of the measurements, suggesting minimum stratification (i.e., absence of a hypolimnion) and a potential for aerobic bacterial activity at the reservoir bottom. However, data to confirm the conditions of reservoir stratification, chlorophyll, and light penetration were not available at time of the SOD measurements.
Results of the In-situ SOD measurements yielded ambient temperature SOD values (SODT) ranging from-0.831 g O2/m2/day to -2.897 g O2/m2/day. Water column oxygen demand (WOD) at all sites were positive, indicating that active photosynthesis in the lower water column was taking place at the time of measurement, even at the deeper sites. To determine the decay rate of reactive organic carbon at each SOD site, in situ SOD values were related to total potential SOD (TPS) calculated for the sediments at each site and a decay rate constant, k, was calculated. The decay rate constant reflects the reactivity of the reactive organic carbon; the higher the k value the more reactive the organic matter. Calculated decay rate constants range from 0.362 yr -1 to 1.312 yr -1. An average decay rate constant value was used to calculate SOD rates for the sediments at other sites in Rocky Gorge Reservoir. Calculated SOD values (adjusted to 20℃) range from -0.455 g O2/m2/day to -4.657 g O2/m2/day.
The In-situ SOD measurements conducted for this study represent a snap shot of the bottom chemical environment. The results of this study are directly applicable to water column modeling during times when DO is present. In order to gain insight to seasonal changes, additional In-situ SOD measurements should be made during the spring and in the summer when the hypolimnion is present and bottom dissolve oxygen levels are lower (hypoxic). Future measurements should also include chlorophyll level and light readings to address the possible contribution of photosynthesis to water column dissolved oxygen levels.
