Nov 16, 2017 - 4. 2- concentration to NE MN. ⢠< 10 mg/L SO. 4. 2- very sensitive to ... methylmercury accumulation by liquid calcium nitrate amendment: redox.
Eutrophication effects of sulfate reduction in lakes and reservoirs: Case studies and foundations
David Austin, P.E., Senior Ecologist ESA SETAC Minneapolis, Minnesota November 16, 2017
Overview
• Regional sulfate concentrations • Sulfate reduction: mechanism of impacts • Sulfate reduction remediation: case studies • Synopsis and prognosis
MN sulfate in surface waters • Most of Lake Superior Laurentian Shield drainage similar SO42concentration to NE MN • < 10 mg/L SO42- very sensitive to sulfate enrichment
Effect of SO42- reduction on WQ Anoxic positive feedback:
Anoxic negative feedback:
Fe, Mn, PO4, NH4, Hg-CH3
SO4
Mercury methylation efflux to water column starts here Lake Ann, Chanhassen, MN. 8oC, 14 m depth Austin et al. 2016. Suppression of hypolimnetic methylmercury accumulation by liquid calcium nitrate amendment: redox dynamics and fate of nitrate. Lake and Reservoir Management. 32(1):61-73.
6 900 Non-linear sediment efflux induced Lake Ann Data 800 by anoxia Fe-dissolved, mg/L 5
700
NH3-N, mg/L
4
3
Fe-dissolved, mg/L
NH3-N, mg/L
600
• Exponential time scale of nutrient release at 8oC • ~4x faster at 20oC • ~5x at 25oC • ~6x at 30oC
500 400 300
2
200
29-Sep
1-Sep
4-Aug
9-Jul
16-Jun
1
100 5
Temperature dependence of sulfate reduction O2 H2S
COOL/COLD SEDIMENTS
• Deep, cold sediments: O2 depletion rate delays onset of nutrient release • Shallow, cool sediments: o O2 regularly replenished o Nutrient release quenched
O2
H2S WARM SEDIMENTS Ingvorsen et al, 1982
• H2S production at 25oC is 5 times over 10oC • Shallow, warm sediments o H2S flux out of sediments > O2 flux in o Rapid, positive feedback release of nutrients
Sulfate reduction “bottom up” biogeochemical cascade effect on water quality
-
HgCH3+
ALGAE BLOOMS -
CYANOTOXINS
TASTE & ODOR
+
-
SULFATE REDUCTION
+
H2S, Mn, Fe, NH4+
+
Reservoir water quality 101 – The big problems
Poor WQ from any level
High TP, cyanobacteria, Temp, and pH Thermocline Intake Pipe
Anoxia
High Fe, Mn, NH4+ and H2S
Reservoir management 101 – Big solutions
Good WQ from any level
Low TP, lower algae, lower pH, high Temp. Thermocline Intake Pipe
High D.O.
Very low Fe, Mn, NH4+ and H2S, neutral pH
Hypolimnetic oxygenation
Case Study: Saint Paul Regional Water Services, Minnesota Average 50 MGD (189 MLD) PLEASANT LAKE
Side stream reservoirs, historic poor water quality requiring remediation – Hypolimnetic aeration (HA) + FeCl3:
• Pleasant: 1994 - 2006 • Vadnais: 1987 - 2010 – Hypolimnetic oxygenation (HO) + FeCl3 :
• Pleasant: 2013 - Present • Vadnais: 2011 - Present
Pre-oxygenation
Pre-oxygenation
Hypolimnion Mn, Fe 4
5
3.5
3
2
1
3 Fe, mg/L
Mn, mg/L
4
2.5 2 1.5 1 0.5
0
0
Total phosphorus Hypolimnion 3 2.5
TP, mg/L
2 1.5 1 0.5 0
• TP release from sediments integral part of redox system • Quenching SO42- reduction sharply reduces internal TP loading
Shamrock Reservoir, Georgia
Bottom TP, mg/L
5 4
• Hypereutrophic
3
• Internal TP loading
2
almost entirely a
1
function of SO42-
0 30
reduction 25 20 15 10 5 Bottom sulfate, mg/L
0
Shamrock Reservoir cyanobacteria
SO42- reduction is ecosystem forcing function
8
Cyanobacteria, cells/ml
3.5 10
3 108
• No SO42- reduction →
2.5 108
no blooms
8
2 10
• High SO42- reduction
1.5 108
→ bloom trigger
8
1 10
• Why? Cyanobacteria
7
5 10
0 30
sink into nutrient-rich 25 20 15 10 5 Bottom sulfate, mg/L
0
anoxic water and then float to surface.
Synopsis • Sulfate reduction is a fundamental ecosystem forcing function o Eutrophication o Mercury methylation o Harmful algae blooms / cyanotoxin potential
• Quenching sulfate reduction: engineering design basis for WQ remediation if sediment surface is anoxic o Reversal of eutrophication o Sharp reduction of potential for harmful algae blooms
Outlook for upper Great Lakes • Lake Superior watershed aqueous biogeochemistry is sulfate-limited • Sulfate enrichment consequences: o Non-linear amplification of internal nutrient loading o Accelerated eutrophication o Deterioration of H2S sensitive habitat
• Global warming will amplify effects of sulfate enrichment • Prognosis:
o Unequivocal scientific grounds for concern over sulfate enrichment o Site-specific remediation of sulfate-impacted lakes feasible o Regional impacts require regulatory intervention
Questions?
