Post on 01-Jan-2016
Almost Everywhere:Naturally Occurring Arsenic in
Wisconsin’s Aquifers
Madeline Gotkowitz
Wisconsin Geological and
Natural History Survey
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Arsenic concentrations in Wisconsin
> 10 %
% samples > 5 ppb
2 to 10%
<2 %1 sample > 5ppb
insufficient samples
Study area: eastern
Wisconsin> 10 ppb contour
4 ppm As
133 ppm As
<2 ppm As
Eastern Wisconsin Stratigraphy
dolomite
sandstone
Sulfide-cement horizon
Quarry on Leonard Pt Road
Pre-development
Clay till
St. Peter sandstone
Cambrian Aquifer
Sinnipee dolomite
250
200
150
100
50
0
700
650
Dep
th, f
eet
Prairie du Chein dolomite
PreCambrian granite
Sulfide horizon
confinedClay till
St. Peter sandstone
Cambrian Aquifer
Sinnipee dolomite
250
200
150
100
50
0
700
650
De
pth,
feet
Prairie du Chein dolomite
PreCambrian granite
Sulfide horizon
O2 Pumped system
Clay till
St. Peter sandstone
Cambrian Aquifer
Sinnipee dolomite
250
200
150
100
50
0
700
650
De
pth,
feet
Prairie du Chein dolomite
PreCambrian granite
Sulfide horizon
O2 Pumped system Managed system
Clay till
St. Peter sandstone
Cambrian Aquifer
Sinnipee dolomite
250
200
150
100
50
0
700
650
Dep
th, f
eet
Prairie du Chein dolomite
PreCambrian granite
Sulfide horizon
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
Year
0
50
100
150
200
250
300
Dep
th, f
eet
0
50
100
150
200
250
300
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
0
200
400
600
800
Num
ber
of W
ells
Mandatory well casing depth based on regional hydrogeology
A = < 2 g/L
B = 2-10 g/L
C = 10-100 g/L
D = >100 g/L
Fe SO4
pH
Schreiber et al. 2003
High Fe
High SO4
Low pH
Geochemistry indicates sulfide oxidation at high-arsenic wells, FeOH reduction at others
Without pumping, the well becomes strongly reducing
ORP
DO
Gotkowitz et al. 2004
Arsenic, iron and sulfate within a well, non-pumping conditions
Effect of pumping rate and volume on redox
100 gallons every 8 hours
200 gallons every hour
Gotkowitz et al. 2004
2004 2005 2006 2007 2008 2009 2010Year
363 381
583670
732 764 801
Residences transferred to public
water supply
In situ disinfection
In situ disinfection treatments
0 5 10 15 20Elapsed Time (days)
0
10
20
30
40
50
60
Tot
al A
rsen
ic, u
g/L
Gotkowitz et al. 2008
6.5
7.5
8.5
pH
0
8
16
DO
(m
g/L
)
LegendP um ping on ly
Low dose
A cid surfactant/Low dose
H igh dose
0 1 2 3Time (d a ys s in ce tre a tme n t)
-500
0
500
1000
OR
P (
mV
)
6.5
7.5
8.5
pH
0
8
16
DO
(m
g/L
)
LegendP um ping on ly
Low dose
A cid surfactant/Low dose
H igh dose
0 1 2 3Time (d a ys s in ce tre a tme n t)
-500
0
500
1000
OR
P (
mV
)
Desorption of As (pH) orreductive dissolution of HFOs (microbially mediated)
As-bearing Pyrite
Primary reservoir
Release from
primary reservoir
Arsenic cycling following sulfide exposure to DO, chlorine
Cl2
O2 (minor)
Release from secondary reservoir
Secondary reservoir
HFOs w/adsorbe
d As
Fe(II)SO4
As species
Fe(III)As(V)
Cl2
O2 (minor)
West et al. In Review
Arsenic in bedrock aquifers• High concentrations of solid-phase arsenic are associated with sulfide minerals
• Arsenic mobilized under oxiding conditions but can become sequestered on iron-oxide minerals
• Iron-oxide minerals become a secondary source of arsenic to groundwater under reducing conditions
• Complex cycling of arsenic is affected by the water table, pumping rates and volumes, and well disinfection
Arsenic > 10 µg/l in 10% of wells
Southeastern Wisconsin
Arsenic > 10 µg/l in 20% of wells
Discontinuous sand and gravel lenses provide groundwater; these may be overlain by low-conductivity glacial tills
Low As
Moderate AsHigh As
aquifer
Root et al. 2009
Organic carbon triggers arsenic-iron-oxide dissolution
Low As
Moderate AsHigh As
Root et al. 2009
Arsenic, mg/kg
Organic matter, %
Arsenic in glacial deposits
• Low concentrations of solid-phase arsenic is associated with iron and manganese oxide minerals , within all stratigraphic units
• Solubility of the solid-phase arsenic results from reducing conditions along deep groundwater flow paths; driven in part by organic carbon leading to reductive dissolution of Fe- and Mn- oxides
Implications of arsenic cycling for managing exposure to arsenic…
1. Groundwater extraction affects arsenic mobilization
• Alters flowpaths• Alters redox conditions
2. Rate and frequency of pumping impacts biogeochemistry
3. Strategies must control redox conditions• Well construction: prevent
introduction of oxygen, controls flowpath
• Extend community water supplies (avoids well disinfection, provides routine testing and /or treatment)
• Q&A
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