Transcript Development of a Highly-Resolved Three

Arsenic in Iowa’s Groundwater—The Unknown
Threat: A Pilot Study in Cerro Gordo County
Douglas J. Schnoebelen, PhD: University of Iowa
Acknowledgements
Peter Weyer, CHEEC, (Center for Health Effects Env. Con.)
Brian Hanft, Cerro Gordo Health Department
Daniel Ries, Cerro Gordo Health Department
Sophia Walsh, Cerro Gordo Health Department
Lorelei Kurimski, UHL
Sherri Marine, UHL
Pam Mollenhauer, UHL
Mike Wichman, UHL
Paul Van Dorpe, DNR
Gary Shaver, Shaver Well Drilling
CDC--Funding
Overview
• Is arsenic in groundwater a new problem?
• Why Cerro Gordo County? SWRL2 study
• Background on:
• Geology and rock types
• Geochemistry and Redox
• Geochemical scenarios
• Using the geology and geochemistry to design the
sampling plan
• Database and communication
Map of U.S. showing arsenic concentrations in
groundwater
Does not tell the whole story
Arsenic concentrations found in at least 25%
of GW samples within a moving 50km radius
(Ryker, 2001)
Arsenic in Groundwater
• Arsenic concentrations may be high groundwater in
many areas across the United States & the world
• The old drinking water standard of 0.05 mg/L (50
ppb) was in the U.S. from 1942 - 2005
• New standard for drinking water of 0.01 mg/L (10
ppb) was set in the U.S. in 2006
• Arsenic is a carcinogen (skin, lung, bladder
cancers) and linked to cardiovascular disease
• Private wells are unregulated and may be at risk
High As levels near Clear
Lake/Mason City
Map from IDNR,
Arsenic in Cerro Gordo County
Arsenic in groundwater can come from a variety of
sources:
Ruled out anthropogenic sources
• Arsenic in the Soil and Rock
• Geochemical and Biogeochemical Reactions
• Potential redox zones and solubility/mobility
• Bacterial activity
• Both are important!
Bedrock Units--Lime Creek and Cedar Valley
• Cedar Valley Group-- (Middle and Upper Devonian in age).
Records T-R cycles of the Devonian sea. Carbonates (limestones
and dolostones) principle rock types. Wide variety of fossils (ie
Devonian Fossil Gorge in Coralville).
• Lime Creek Formation--(Upper Devonian in age). Other T-R
cycles in the Devonian sea. Generally shallower water deposition.
Shale, calcareous shale, and argillaceous limestone.
• Pyrite (FeS) certainly common in the shales of the Lime Creek,
but also found in the Cedar Valley as well. Other iron minerals
too!
Some minerals may contain small amounts of
arsenic: iron hydroxides and sulfides
Iron hydroxides: are not crystalline (no structured crystal
lattice)—also known as hydrous ferric oxides (HFOs) or
Hydrous iron oxides (HIOs) or oxhydroxides. Often as
grain coatings or weathering deposits.
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Numerous iron oxides(16 different ones)
Iron II hydroxides: Fe(OH)
2
Iron III Hydroxides: Fe(OH)3
Examples: Hematite (Fe2O3)---common coating on grains
Also Goethite: FeO(OH)
Sulides: (Pryrite, FeS2), arsenopyrite (FeAsS),
realgar(AsS), marcasite (FeS2)
Arsenic and Redox
• Oxidation and Reduction (Redox) reactions are
important in understanding arsenic concentrations and
speciation in the groundwater
• Redox reactions are simply a family of reactions that are
concerned with the transfer of electrons between species.
• Oxidation refers to reactions of an element & oxygen
where electrons are lost (ie rust)
• Reduction refers to reactions to the element or
compound where electrons are gained
• Oxidation—involves an increase in the oxidation number of
an atom while Reduction—occurs when the oxidation
number of an atom decreases
Arsenic Species and Oxygen
Arsenic in groundwater can exist with formal oxidation
states of III (arsenite) and V (arsenate). Arsenate (As V)
would be more typical in oxygen rich waters (DO over 1 mg/L)
• EPA regulations are for TOTAL arsenic (III and V
combined). Most lab analyses report TOTAL arsenic
• Arsenic III is more toxic than Arsenic V
• Arsenic III is more difficult to remove (with conventional
treatment) than Arsenic V
Sulfide Minerals and Oxygen
(one of several geochemical scenarios)
• Fluctuating water levels in the well bore introduce
oxygen to the sulfide minerals– resulting in high As
concentrations in the groundwater (perhaps Wisc. example
Galena/St. Peter Ss)
• Arsenic (As) concentrations will be highest in wells
with flucuating water levels and sulfides at the same
level (O2 is the source)
• However, if sulfides are above OR some distance
below the water table then As concentrations are
typically lower in the groundwater (but may still be a
problem).
Iron oxides and Reducing conditions
(another geochemical scenario of many)
• The reducing conditions in the aquifer drives a
complex series of Redox reactions with the sequential
loss of dissolved oxygenreduction of nitrateMn
(IV) and Fe(III)SO4methane produced
• Microbial activity important in catalyzing reactions and
arsenic from iron oxides released (India example)
• Also can have a change under reducing conditions of
Fe(III) to mixed form Fe(II)/Fe(III) oxides that can affect
the affinity of the minerals binding the ARSENIC
therefore releasing more As to the water
Groundwater Sampling Plan—How to design?
No plan is perfect---funds are not unlimited (best bang
for the buck to achieve our objectives
• Better understand the arsenic problem: source,
geochemistry, causes
• Improve data to homeowners and drillers
• Enlarge the data set over a larger geographic area
• Select wells with complete geology (new wells)
• Bedrock wells
• Analysis of rock cuttings from the wells
• 50 wells per twice per year (wet/dry seasons) for 3
years
Groundwater Sampling—What to sample?
Dissolved Oxygen and Eh: Redox conditions
pH: Sorption and Desorption Temp and SpC: Field
parameters
Hydrogen sulfide: Redox conditions
Major Cations and Anions: Geochemistry
Organic Carbon: Geochemistry
Fe, Mn and other metals: Reductive vs Desorption
Arsenic Species: As III and As V
These can test:
Aresenic sources, solubility, mobility, potential redox
zones, potential bacterial activity
Groundwater sampling equipment used
Groundwater Sampling
• Flow through Cell and Sonde (field readings critical)
• Hach Spectrophotometer (sulfide in the field)
• Sample Chamber (reduces contamination)
• Replace tubing each time (no carryover)
• Series of field readings before final measurement
Groundwater Sampling
• In-house training
• Data Sheets and good communication with the Lab
• Field test site with actual sample to track data
handling
• 10% QA/QC samples
• Lab is measuring Arsenic as total, III and V
Database—Goals and Objectives
Geochemical modeling of the data
• Spatial and temporal patterns
• Any correlation with simple field measurements
• Trends, patterns, correlation with rock samples
• Would like to follow-up with lab studies & more
targeted samples
Thank You!!!!
Doug Schnoebelen, The University of Iowa,
IIHR—Hydroscience and Engineering:
[email protected]