Stratification and Water Quality of Jordan Lake, North Carolina

Science Center Objects

The New Hope arm of Jordan Lake, located in Chatham County, North Carolina, serves as a drinking-water supply for the Towns of Cary, Apex, and Morrisville. The reservoir is listed as impaired due to nutrient over-enrichment and occasionally experiences algal blooms and fish kills.

In order to evaluate effects of installing a vertical-mixing system on the New Hope arm of Jordan Lake, the U.S. Geological Survey (USGS) is documenting water-column stratification, chemical quality, taste and odor compounds, and phytoplankton characteristics before and after installation. 

Summary:

Sunset at Jordan Lake

(Credit: Doug Harned, USGS. Public domain.)

The New Hope arm of Jordan Lake, located in Chatham County, North Carolina, serves as a drinking-water supply for the Towns of Cary, Apex, and Morrisville. The reservoir is listed as impaired due to nutrient over-enrichment and occasionally experiences algal blooms and fish kills.

Water quality in Jordan Lake varies with depth, particularly during warm months when the lake stratifies.

  • Water near the surface typically is well oxygenated and supports abundant algae.
  • Bottom waters are oxygen-poor and contain elevated levels of dissolved iron, manganese, and nutrients.

Both scenarios can lead to high water-treatment costs and taste and odor issues.

The Town of Cary installed an innovative, high-capacity, water-column mixing system in the New Hope arm of Jordan Lake during spring 2017.

The vertical-mixing system is expected to enhance drinking-water treatment, alleviate algal blooms, and enhance recreational use by:

  • Keeping the entire water column well oxygenated
  • Lowering levels of dissolved iron and manganese
  • Altering phytoplankton dynamics and the distribution of nutrients

However, if the system does not completely mix the water column as expected, nutrients transported upward from bottom waters could fuel algal blooms near the surface.

In order to evaluate effects of installing a vertical-mixing system on the New Hope arm of Jordan Lake, the U.S. Geological Survey (USGS) is documenting water-column stratification, chemical quality, taste and odor compounds, and phytoplankton characteristics before and after installation. This project is being conducted in cooperation with the Town of Cary, NC, and with support from the USGS Cooperative Water Program.

Objectives:

  1. Evaluate the spatial and temporal scope of the vertical-mixing system's effects on lake stratification
  2. Document concentrations of iron, manganese, nutrients, and taste and odor compounds before and after the installation of the mixing system

Approach:

To achieve the study objectives, the USGS collected data:

  • At 4 locations in Jordan Lake
  • Biweekly during the months of April through October
  • April 2012 - October 2013 and April 2017 - October 2018

Water Column Stratification

Vertical profiles of water temperature, dissolved oxygen, pH, and specific conductance were measured through the water column.

The vertical profile data will be used to evaluate the spatial and seasonal extent of the mixing system's effects on water-column stratification.

The USGS also will compare local circulation patterns in the lake induced by the mixing system using hydroacoustic techniques.

Water-Quality Characteristics

Secchi depth, the depth to 1 percent incident light, and turbidity were measured near the water surface. Iron and manganese were sampled from 2 depths that represent surface and bottom waters.

Nutrients and chlorophyll a were analyzed from vertical-composite samples of the photic zone at all sites; nutrients also were collected from bottom waters. At site 4, phytoplankton and taste and odor compounds (geosmin and 2-methylisoborneol) were collected from photic-zone and bottom waters.

The USGS will document water-column and water-quality conditions in the study area before and after the vertical-mixing system was installed in a final report. Evaluation will focus on the spatial and temporal scope of mixing system's effects on lake stratification and concentrations of iron, manganese, nutrients, and taste and odor compounds.

Table 1. Water-quality properties and constituents measured for the Jordan Lake study

Property or Constituent                        Biweekly sampling at 4 sites, April-October, 2012-2016           
                                                            Vertical        1 meter below        Photic-zone       1 meter above
                                                            profile              surface                composite           lake bed      
                                                         Field Properties                                                                            
Water temperature                                     X
Dissolved oxygen                                       X
Specific conductance                                 X
pH                                                              X
Turbidity                                                                           X
Secchi depth                                                                 At surface
Depth to 1% incident light                                             At surface                                                         
                                                      Iron and Manganese                                                                        
Iron, total                                                                          X
Iron, dissolved                                                                  X
Manganese, total                                                              X
Manganese, dissolved                                                      X                                                                  
                                                           Nutrients                                                                                     

Nitrogen, ammonia                                                                                     X
Nitrogen, total ammonia + organic                                                              X
Nitrogen, nitrite + nitrate                                                                             X
Phosphorus, orthophosphate                                                                     X
Phosphorus, total                                                                                       X                                        
                                  Phytoplankton and Taste and Odor Compounds                                                   

Chlorophyll a                                                                                               X
Phytoplankton id and enumeration                                                             X(1)                        X(1)
Geosmin                                                                                                     X(1)                        X(1)
2-Methylisoborneol (MIB)                                                                            X(1)                        X(1)       
                                                          (1) Collected at site #0209799150

References

Measuring Discharge with Acoustic Doppler Current Profiles from a Moving Boat
By David S. Mueller and Chad R. Wagner
Techniques and Methods 3-A22

Links

Town of Cary, North Carolina

Town of Cary, NC, Jordan Lake Aeration System

USGS Cooperative Water Program