Almost Everywhere: Naturally Occurring Arsenic in Wisconsin’s Aquifers Madeline Gotkowitz Wisconsin Geological and Natural History Survey

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 confined Pumped system

Managed system

Mandatory well casing depth based on regional hydrogeology

A = < 2  g/L B = 2-10  g/L C =  g/L D = >100  g/L Fe SO 4 pH Schreiber et al High Fe High SO 4 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

Residences transferred to public water supply

In situ disinfection

In situ disinfection treatments Gotkowitz et al. 2008

Desorption of As (pH) or reductive dissolution of HFOs (microbially mediated) As-bearing Pyrite Primary reservoir Release from primary reservoir Arsenic cycling following sulfide exposure to DO, chlorine Cl 2 O 2 (minor) Release from secondary reservoir Secondary reservoir HFOs w/adsorbed As Fe(II) SO 4 As species Fe(III) As(V) Cl 2 O 2 (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 As High As aquifer Root et al. 2009

Organic carbon triggers arsenic-iron-oxide dissolution Low As Moderate As High As Root et al 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)

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