Sediment Dynamics in Jordan LakeBrent McKee and Sherif GhobrialDepartment of Marine Sciences
UNC Chapel Hill
Special Thanks to UNC undergraduate students:Naomi Becker
Michaela MonahanAnnie WilliamsIvonne GuzmanSarah Brooker
1) What question are you asking?
QuestionWhat is the fate of sediments entering Jordan Lake
Where do they go spatially (throughout the lake)?
How do they move temporally (throughout the year)?
Why do we care?An important fraction of nutrients and contaminants are associated with particles and follow particle pathways
Once particles settle to the lake bottom they experience a different geochemical environment (e.g. pH, O2) and
transformations can occur (release from particles)
Particulates themselves can cause water quality issues resulting in light limitation
1) What question are you asking?
Water Discharge: 2.4 m3s-1
Total Suspended Matter = 7 mg l-1Water Discharge: 1213.5 m3s-1
Total Suspended Matter = 335 mg l-1
Haw River at Bynum
Sediment inputs to the lake from rivers and streams are episodic
Where do Haw River sediments go as lake conditions and river inputschange ?
2) What have we found out?
Our Experimental Design
Collect cores from 5 sites from the Haw River arm through the “Narrows” to middle lake
Reoccupy sites every 4-6 weeks
Dissect each core at 1 cm intervals and analyze for radiotracer Be-7
(atmospheric source; 53 day half life)
Sediment Coring and Dissection
7Be Half life = 53 d
Constant Supply
Naturally Occurring
Locks onto particles and tracks particle movement.
2) What have we found out?
0
200
400
600
800
1000
1200
1400
1600
4/10/2018 5/30/2018 7/19/2018 9/7/2018 10/27/2018 12/16/2018 2/4/2019 3/26/2019 5/15/2019
Disc
harg
e (c
ubic
met
ers p
er se
cond
)9/17/2018 11/13/2018
2/24/2019
Haw River Discharge and Narrows Sampling Dates
0
2
4
6
8
10
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Be-7 (dpm/g)
Station 2Sampling D
00.5
11.5
22.5
33.5
4
0.00 0.50 1.00 1.50 2.00 2.50
Be-7 (dpm/g)
Station 5Sampling F
Depth of Be-7 tracer
This does not give us enough resolution
we want to know what happened each month
Deepest Be-7 in the core corresponds to particles that locked on Be-7 less than 8
months before
(1) Decay correct to determine remaining Be-7 at site (Residual) from Time 1
(2) Subtract Residual from total Be-7 measured at Time 2
This determines “NEW” Be-7
(3) Using mean Be-7 concentration at core surface then calculate mass flux of sediment during interval from Time 1 to Time 2
Time 1 Time 2
from Canuel et al 1990
We need to re-sample each location ~ every few weeks
2
51
3
4
Our Experimental Design
2) What have we found out?
0
200
400
600
800
1000
1200
1400
1600
4/10/2018 5/30/2018 7/19/2018 9/7/2018 10/27/2018 12/16/2018 2/4/2019 3/26/2019 5/15/2019
Disc
harg
e (c
ubic
met
ers p
er se
cond
)9/17/2018 11/13/2018
2/24/2019
Haw River Discharge and Narrows Sampling Dates
Rates of Deposition
Rates of Erosion
What are policy implications from these findings?
Better understand the extent to which Haw River sediment get to the middle lake and when that happens
Analyze sediment flux data with information about Haw River discharge and circulation insights from physical group to determine what drives sediment flux to middle lake
Possible to predict when and how turbidity plumes get to middle lake where municipal water intakes are located.
Help understand dispersal of sediment associated nutrients and contaminants
