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The Contribution of Sacramento Valley Rice Systems to Methylmercury in the
Sacramento River
Christy Tanner1, Lisamarie Windham-Myers2, Jacob Fleck2, Kenneth Tate1, Stephen McCord3 and Bruce Linquist1
1. UC Davis, 2. US Geological Survey, 3. McCord Environmental
8th Biennial Bay-Delta Science Conference
October 29th 2014
Introduction
• Methylmercury (MeHg) is toxic and bioaccumulative
• Total Maximum Daily Load (TMDL) • 0.06 ng/L goal
• Produced in flooded soils
• 200,000 Ha of paddy rice in the Sacramento Valley
Objectives and Hypothesis
• Gather data from previous studies of MeHg concentrations
• Assess spatial and temporal patterns in MeHg concentration • Seasonal patterns?
• Differences between the Sacramento and Feather Rivers?
• Hypothesis: Rice drainage waters are an important source of MeHg to the Sacramento River • Are MeHg concentrations higher in ag drainage canals?
• What is the MeHg load from agricultural drainages?
Outline
• Methods and overview of the dataset
• Results
• Temporal patterns
• Spatial patterns
• Loads from agricultural drainages
• Conclusions
Ten years of Data Available
Program
Sampling period 1996-2007
# of sites
# of samples/site
USGS National Water Quality Assessment
5 23-29
Sacramento River Watershed Program
10 14-18
8 4
12 17-18
CALFED Bay-Delta Program 16 23-31
2.5 years
3.5 years
1.5 years
<1 year
3.5 years
Selected most relevant sites • Location with
respect to rice • Data
availability
Only three sites sampled in all studies Confluence sites sampled in one study each
Upstream Sacramento
Upstream Feather
Confluence
Agricultural Drains
Rice growing area
25 mi
Methods
• Un-balanced data set • Not all sites sampled in all years
• Mixed effects model • Random effects: site and year • Fixed effects: location and season
Outline
• Methods and overview of the dataset
• Results
• Temporal patterns • Spatial patterns
• Loads from agricultural drainages
• Conclusions
Long term trends
• 566 samples at selected sites
• Concentrations ranged from method detection limits to 1.98 ng/L
• 75% of samples >0.06 ng/L (TMDL goal)
0
0.5
1
1.5
2
2.5
1/96 6/01 12/06
Me
Hg
(ng
/L)
Date (month/year)
TMDL goal
Upstream Sacramento
Ag Drains
Upstream Feather
Confluence
‘97 Flood
Seasonal pattern
Seasonally Low MeHg
Concentrations
0
0.5
1
1.5
2
2.5
1/1 3/2 5/2 7/2 9/1 11/1
Me
Hg
(ng
/L)
Julian day
Upstream Sacramento
Ag Drains
Upstream Feather
Confluence
TMDL goal
Rice Growing Season
Outline
• Methods and overview of the dataset
• Results
• Temporal patterns
• Spatial patterns • Loads from agricultural drainages
• Conclusions
No difference between Sacramento and Feather Rivers
0.001
0.01
0.1
1
10
UpstreamSacramento
UpstreamFeather
UpstreamSacramento
UpstreamFeather
Me
Hg
(n
g/L
)
Maximum 75th percentile Median 25th percentile Minimum
Summer / Fall Winter / Spring
0.060 0.075 0.066 0.076
Winter/Spring MeHg concentrations are higher in Ag Drains
0.001
0.01
0.1
1
10
Ag Drains Sacramento R. Feather R. Confluence
Me
Hg
(ng
/L)
a b b
ab
Maximum 75th percentile Median 25th percentile Minimum
0.18
0.075 0.076 0.12
Summer/Fall: Same pattern but not significant
0.001
0.01
0.1
1
Ag Drains Sacramento R. Feather R. Confluence
Me
Hg
(ng
/L)
a a a a
Maximum 75th percentile Median 25th percentile Minimum
0.12 0.079
0.066 0.060
Location - Season Interaction
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
Summer Winter
Me
dia
n M
eH
g co
nce
ntr
atio
n
(ng
/L) Sacramento R.
Feather R.
Ag Drains
Confluence
Ag Drain Concentrations lower than Yolo Bypass rice study
Alpers CN, Fleck JA, Marvin-DiPasqualebe M, Stricker CA, Stephenson M, Taylor HE. Mercury cycling in agricultural and managed wetlands, Yolo Bypass, California: spatial and seasonal variations in water quality. Sci Total Environ 2014e;484:276–87.
0.001
0.01
0.1
1
10
100
winter summer
Me
Hg
(ng
/L)
TMDL goal (0.06 ng/L)
Outline
• Methods and overview of the dataset
• Results
• Temporal patterns
• Spatial patterns
• Loads from agricultural drainages • Conclusions
Load Calculations
• Load = Concentration x Flow
• Used Ratio estimator method:
Over all load = 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑙𝑜𝑎𝑑𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑓𝑙𝑜𝑤 * Over all flow
• Gaps in data: • No flow data for eastern Ag Drain
• Western Ag Drain water partially diverted to Yolo Bypass
• Limited data for confluence
? ?
X
MeHg load from Ag is small compared to loads in the Rivers
0
1
2
3
4
5
6
7
UpstreamSacramento
Upstream Feather Ag Drain (west)
MeH
g lo
ad (
g/d
ay)
Upstream Loads, all avaliable data
Error bars = RMSE
(Not sampled in earliest dataset)
Sum of upstream loads is similar to confluence load
0
10
20
30
40
upstream confluence upstream confluence
MeH
g Lo
ad (
g/d
ay) First Dataset Last Dataset
Upstream Sacramento Upstream Feather
Ag Drain (west) Confluence Error bars = RMSE
Load Estimates are consistent across studies
*Log scale
Conclusions
• Future studies should account for seasonal variation
• MeHg concentrations are elevated in agricultural drains • particularly in the winter
• MeHg concentrations were low compared to Yolo Bypass
• Measurable MeHg loads from agricultural drains were small compared to Loads in the Sacramento and Feather Rivers • Due to differences in flow
Acknowledgments
• Funding • Rice Research Board
• UC Davis Plant Sciences
• Data • Sacramento River Watershed Program
• J. Domagalski, National Water Quality Assessment
• C. Foe, CALFED Bay-Delta Program
• Coauthors
• Agroecosystems Lab
Precipitation
-5
-4
-3
-2
-1
0
1
0 0.2 0.4 0.6 0.8 1
Ln(M
eH
g (n
g/L
))
Valley Average Precipitation (in)
agConfluenceFeatherSacramento