Douglas J. Schnoebelen, PhD: University of Iowa

Arsenic in Iowa’s Groundwater—The Unknown Threat: A Pilot Study in Cerro Gordo County

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

Acknowledgements

• 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

Overview

Map of U.S. showing arsenic concentrations in groundwater

Arsenic concentrations found in at least 25% of GW samples within a moving 50km radius (Ryker, 2001)

Does not tell the whole story

• 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

Arsenic in Groundwater

Map from IDNR, VanDorpe, 2010

High As levels near Clear Lake/Mason City

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!

Arsenic in Cerro Gordo County

• 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!

Bedrock Units--Lime Creek and Cedar Valley

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.•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)

Some minerals may contain small amounts of arsenic: iron hydroxides and sulfides

• 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 and Redox

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

Arsenic Species and Oxygen

• 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).

Sulfide Minerals and Oxygen(one of several geochemical scenarios)

• The reducing conditions in the aquifer drives a complex series of Redox reactions with the sequential loss of dissolved oxygenreduction of nitrateMn (IV) and Fe(III)SO4methane 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

Iron oxides and Reducing conditions(another geochemical scenario of many)

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 Plan—How to design?

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—What to sample?

Groundwater sampling equipment used

• 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

Groundwater Sampling

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

Database—Goals and Objectives

Thank You!!!!

Doug Schnoebelen, The University of Iowa, IIHR—Hydroscience and Engineering:

douglas-schnoebelen@uiowa.edu