Investigation of Flow Patterns, Water-Quantity and Water-Quality Conditions in the Bushy Park Reservoir, Charleston, South Carolina
Science Center Objects
The U.S. Geological Survey South Atlantic Water Science Center, South Carolina office, in cooperation with Charleston Water System (CWS), is evaluating the hydrodynamics of the Bushy Park Reservoir to determine the effects on water-quality conditions.
The U.S. Geological Survey South Atlantic Water Science Center (SAWSC), in cooperation with Charleston Water System (CWS), is evaluating the hydrodynamics of the Bushy Park Reservoir to determine the effects on water-quality conditions. Specifically, the CWS has three areas of concern that this investigation will address:
(1) hydrologic monitoring of the reservoir to establish a water budget,
(2) water-quality sampling, profiling, and continuous monitoring to evaluate the causes of taste-and-odor occurrence, and
(3) technical evaluation of appropriate hydrodynamic and water-quality simulation models for the reservoir.
Data collection will include real-time continuous monitoring, discrete water-quality sampling, and vertical and longitudinal profiling by using an Autonomous Underwater Vehicle (AUV). The data-collection effort will help to identify probable “hot spots” of elevated chlorophyll and blue-green algae (BGA) concentrations within the reservoir and give insight to the principal causes of the Bushy Park Reservoir taste-and-odor episodes.
Autonomous Underwater Vehicle.
Deployment of the autonomous underwater vehicle
Periods of elevated trans-1, 10-dimethyl-trans-decalol (geosmin) and 2-methylisoborneol (MIB) concentrations have occurred in the Bushy Park Reservoir including an extreme case during the spring of 2012. Occurrence of geosmin and MIB, which produce musty, earthy tastes and odors, are one of the primary causes of taste-and-odor episodes in drinking water. Although not a human health problem, geosmin and MIB are problematic because the human taste-and-odor detection threshold for these compounds is extremely low and conventional water-treatment procedures such as particle separation, oxidation, and adsorption typically do not reduce concentrations below the threshold level. Geosmin- and MIB-producing BGA blooms are attributed to a range of environmental factors, including nutrient concentrations and ratios, light availability, water temperatures, water-column stability, and reservoir flushing rates. Remediation efforts of reservoir conditions where BGA dominance occurred has hinged upon a strong scientific understanding of the mechanisms controlling the algal community.
Evaluating the physical and chemical conditions within the Bushy Park Reservoir with respect to changes in BGA concentration may provide CWS a basis to further modify management strategies to reduce taste-and-odor occurrence. Management strategies can be implemented long-term to prevent conditions under which BGA dominate and additional treatment technologies, like the powdered activated carbon that CWS currently (2014) utilizes, can be implemented short-term to reduce or remove the taste-and-odor compounds.
Hydrologic monitoring of the reservoir to establish a water budget and document reservoir circulation dynamics
The data to determine the water budget and reservoir circulation dynamics of Bushy Park Reservoir will include measurements of the volume of the reservoir and the inflows and outflows to the reservoir; bathymetric data collected using an AUV; withdrawal information collected from area industries and CWS; water-level data from the new installation of a continuous real-time water-level gage located near the Bushy Park Reservoir dam; and collection of index velocity measurements at the Durham Canal gage and at temporary locations throughout the reservoir. To measure the flow in the water tunnels, real-time monitoring stations will be established and acoustic Doppler velocity meters will be located in the tunnels.
Water-quality sampling, profiling, and continuous monitoring to understand the causes of taste-and-odor issues
To analyze the possible convergence of conditions that cause taste-and-odor problems, data will be collected over a two-year period. A combination of continuous monitoring of water-quality and meteorological parameters, longitudinal water-quality profiling of the reservoir and tributaries, and discrete water-quality sampling will be used to capture the temporal and spatial water-quality dynamics of Bushy Park Reservoir.
Meteorological parameters that will be collected continuously at the real-time gage located near Bushy Park Reservoir dam are temperature, precipitation, wind speed and direction, and photosynthetically active radiation (PAR). A series of bimonthly longitudinal water-quality profiles will be collected with an AUV to identify location and conditions for elevated chlorophyll and BGA concentrations. The information collected from the spatial water-quality surveys will be used to produce a longer-term monitoring plan to assess potential environmental factors related to increases in geosmin production and identification of easily and continuously measured surrogates to indicate its presence in the source water.
Concurrent with three of the bimonthly surveys and during taste-and-odor response surveys, a manned vehicle will be used to collect water-column samples and other depth-specific locations within Bushy Park Reservoir that represent a range in the chlorophyll and BGA concentrations measured by the AUV. Samples will be analyzed for geosmin, MIB, chlorophyll a, pheophytin a, nutrients, actinomycetes, and suspended sediment concentrations, and for BGA biovolumes. Field measurements of PAR, light absorption at 1-meter depth intervals and transparency as Secchi disk depth will be measured at each location at the time of sampling. Field measurements of water temperature, dissolved oxygen, pH, specific conductance, turbidity, chlorophyll (estimate of algal biomass), and phycocyanin (estimate of BGA biomass) also will be collected with a calibrated sonde at the sampling locations for the specified depth.
Technical evaluation of the hydrodynamic and water-quality simulation model for the reservoir
The previously documented three dimensional Environmental Fluid Dynamics Code (EFDC) model for both hydrodynamics and water quality (Tetra Tech and Jordan Jones and Goulding, 2008; Hamrick, 1992; Park and others, 1995; Cerco and Cole, 1993) will be evaluated to 1) determine if the calibration is adequate to use as a planning tool for the Bushy Park Reservoir and 2) if the model calibration is not adequate, determine the modifications to the model application that would be necessary for recalibration and application as a planning tool. The EFDC model will be evaluated with existing data and the data collected during this investigation. If necessary, a list of recommended changes to the model and the level of effort required will be generated. A thorough reconfiguration and recalibration of the model is not part of this investigation. Making a limited number of changes to the model will help to inform the level of effort needed to develop a good reservoir management models. In addition, preliminary reservoir management scenario simulations will provide information on the feasibility of alternative management options. The following modifications to the model will be made:
- Update the bathymetry data for the reservoir,
- Generate simulated boundary conditions for Durham Canal to establish a small, faster run-time model,
- Refine the computational model grid, and
- Evaluate the water-quality kinetics to reflect the new data collection.
RELEVENCE AND BENEFITS
Evaluating the physical and chemical conditions within the Bushy Park Reservoir with respect to changes in BGA concentration may provide CWS a basis to further modify management strategies to reduce taste-and-odor occurrence. Management strategies can be implemented long-term to reduce the frequency of conditions under which BGA dominate and additional treatment technologies to those currently (2014) being used can be implemented short-term to reduce or remove the taste-and-odor compounds.
In support of the USGS Water Mission Area (http://water.usgs.gov/mission.html), the proposed study will provide data and information to “protect and enhance water resources for human health, aquatic health, and environmental quality.” Benefits of this investigation to the CWS and others include:
- Accurate data and analysis on the water-quantity and water-quality of Bushy Park Reservoir that will provide reference conditions and understanding of the available quantity of freshwater for the reservoir and will provide baseline conditions and understanding of the causes of taste-and-odor issues.
- Accurate data on the flow rates in the supply tunnels from the Edisto River to the CWS water-treatment plant and accurate measurement of the flow volume of diversions in the tunnels to large water users.
- Technical evaluation of available computer model for Bushy Park Reservoir.
- Preliminary model simulation to determine the magnitude of a model development effort for the reservoir and to determine the feasibility of alternative reservoir management scenarios.
The results of this work will be documented in two USGS Scientific Investigations Reports and made available to the general public at the USGS publication website https://water.usgs.gov/pubs. The reports will describe the data-collection effort, interpretation of the data, and technical evaluation of the hydrodynamic and water-quality simulation models for the reservoir.
Cerco, C. F., and T. Cole, 1993, Three-dimensional eutrophication model of Chesapeake Bay, J. Environ. Engnr., 119, 1006-1025.
Hamrick, J. M., 1992, A Three-Dimensional Environmental Fluid Dynamics Computer Code: Theoretical and Computational Aspects. The College of William and Mary, Virginia Institute of Marine Science, Special Report 317.
Park, K., Kuo, A.Y., Shen, J., and Hamrick, J.M., 1995, A Three-Dimensional Hydrodynamic-Eutrophication Model (HEM-3D): Description of Water Quality and Sediment Process Submodels (EFDC Water Quality Model). The College of William and Mary, Virginia Institute of Marine Science, Special Report 327.
Tetra Tech and Jordan, Jones & Goulding, 2008, 3-D Modeling Report for the Charleston Harbor System, prepared for Berkeley-Charleston-Dorchester Council of Governments, 455 pp.