arsenic in minnesota groundwater: occurrence and geochemical mobilization mechanisms

Arsenic in Minnesota Arsenic in Minnesota Groundwater: Occurrence Groundwater: Occurrence

and Geochemical and Geochemical Mobilization MechanismsMobilization Mechanisms

Arsenic in Minnesota Arsenic in Minnesota Groundwater: Occurrence Groundwater: Occurrence

and Geochemical and Geochemical Mobilization MechanismsMobilization Mechanisms

Mindy L. Erickson, Ph.D, P.E.US Geological Survey

November 9, 2011

Funding provided by U of MN (CURA and WRC), MDH, and USGS

Why Arsenic? Why Now?Why Arsenic? Why Now?

At 50 ug/l in drinking water, lifetime cancer risk is about 2 in 100 (similar to second-hand smoke)

MCL change to 10 ug/l affected bout 250 upper Midwest Public Water Systems

Treatment plants for arsenic are expensiveBeginning in 2008, all new MN wells tested for As

GoalsGoalsCharacterize arsenic occurrenceCharacterize arsenic temporal variabilityTest potential geochemical mechanisms and

geological controlsProvide interested parties with results

Completed WorkCompleted WorkDatabase compilationSediment

Geochemistry: metals, organic carbonSequential extraction Scanning electron microscopy

Ground water Geochemistry: trace elements, arsenic species organic carbon, ammoniumField parameters: pH, ORP, conductivity, DO, temp

Literature reviewGeochemical modeling, statistical analysis

Arsenic OccurrenceArsenic Occurrence

Arsenic in rock and sediment at 1 to 100s mg/kgCrustal average is 1.8 mg/kgAt 1.8 mg/kg, solubilization of <0.1% yields 10 ug/L

arsenic in water Certain geochemical conditions leach arsenic into

ground waterPrevious study proposed link between Des Moines

lobe till and elevated arsenic

Database CompilationDatabase Compilation

State drinking water agencies/departmentsUSGSState geological surveys

Understanding the Geology

Understanding the Geology

Northwest provenance late Wisconsin-aged tillLarge fraction of fine-grained materialEntrained organicsActive anaerobic biological activityReduced conditions

Understanding the Geochemistry


Arsenate (As+5 H2AsO4-, HAsO4

-2)Oxidized formAdsorbs to metal oxides

Arsenite (As+3 H3AsO3)Reduced form; more toxic inorganic formAdsorbs to iron oxides

Organic Arsenic (many forms)In foods; highest in seafoodUncommon in groundwater



Arsenic Release MechanismsReductive DesorptionReductive DissolutionAnion CompetitionMineral Oxidation (often pyrite)

Understanding the GeochemistryUnderstanding the Geochemistry

Northwest provenance sediment has 2 to 26 mg/kg arsenic

Northeast provenance sediment has 1 to 17 mg/kg arsenic

In 9 domestic and monitoring wells in Minnesota, As in sediment is not correlated to As in water


0

2

4

6

8

10

12

0 10 20 30

Water As Concentration, average (μg/L)

Se

dim

en

t A

s C

on

ce

ntr

ati

on

(m

g/k

g) Sand

Till



y = 0.9915xR2 = 0.9987

0

10

20

30

40

50

0 10 20 30 40 50

Total Arsenic Concentration (ug/l)

Aq

ue

ou

s A

rse

nic

Co

nc

en

tra

tio

n (

ug

/l)



2

4

6

8

Fe

Con

cent

ratio

n (m

g/l)

0 5 10 15 20 25 30 35As Concentration (ug/l)

0

250

500

750

1000

0 10 20 30 40

As (ug/l)

An

ion

Co

nc

en

tra

tio

n (

mg

/l)

Bicarbonate

Sulfate




At 1.8 mg/kg arsenic, solubilization of <0.1% yields 10 ug/L arsenic in water

Total sediment arsenic concentration less important than the availability of arsenic

Measured 0.4 - 0.8 mg/kg arsenic adsorbed to/ coprecipitated with metal oxides

Adsorbed/coprecipitated arsenic labile

Inside Footprint

Aquifer Type

All Wells PWS Wells Other WellsExcluding

MARS

Count % > 10 μg/l As Count

% > 10 μg/l As Count


% > 10 μg/l As

Glacial 4,275 30.9% 551 16.3% 3,724 33.1% 2898 27.7%

Bedrock 1,174 7.0% 583 7.2% 591 6.8% 591 6.8%

Unknown 1,652 17.1% 630 12.7% 1,022 19.9% 1022 19.9%

All 7,101 23.8% 1,764 12.0% 5,337 27.7% 4511 23.2%

Outside Footprint

Aquifer Type

All Wells PWS Wells Other Wells

Count % > 10 μg/l As Count


% > 10 μg/l As

Glacial 1,061 7.0% 219 0.5% 842 8.7%

Bedrock 1,528 3.5% 642 2.2% 886 4.4%

Unknown 1,744 3.5% 1,321 2.8% 423 5.7%

All 4,333 4.3% 2,182 2.4% 2,151 6.3%

Statistics ResultsStatistics Results

Well Type CountDepth Range

(m)% Wells >10 μg/l

As

Bedrock

132 800 - 186 1.5%

263 185 - 92 3.8%

131 91 – 4a 22.1%

Glacial

120 157 - 65 8.5%

236 64 - 28 27.0%

118 28 - 7 7.4%

a All but 4 of these wells are more than 30 m deep.

SummarySummary Inside the footprint:

Glacial wells are deeperA higher proportion of wells exceed 10 ug/l AsGlacial and bedrock wells 30 – 90 m deep most affected

Arsenic weakly correlated to ironArsenic not correlated to competing anionsArsenic in ground water is dissolvedArsenic in ground water is predominantly As(III)

SummarySummary

Elevated arsenic concentrations spatially correlatedTotal northwest provenance sediment arsenic not

particularly high compared with other regional sediment

Sediment and ground water arsenic not correlated 0.4 – 0.8 mg/kg labile arsenic present in aquifer

sedimentAquifers are moderately reduced

ConclusionsConclusionsLate Wisconsin-aged till causes the upper

Midwest’s widespread area of elevated arsenic in ground water

Reductive dissolutionReductive desorption

High-arsenic sediment is not necessary to cause arsenic-impacted ground water

Large but imperfect data sets allow inexpensive observation and characterization of regional environmental problems

Aquifer Type

PWS Wells Other WellsExcluding

MARS

Count % > 10 μg/l As

Count % > 10 μg/l As

Count% > 10 μg/l As

Glacial 551 16.3% 3,724 33.1% 2898 27.7%

Bedrock 583 7.2% 591 6.8% 591 6.8%

Unknown 630 12.7% 1,022 19.9% 1022 19.9%

All 1,764 12.0% 5,337 27.7% 4511 23.2%

Well CharacteristicsWell Characteristics

Well characteristicsWell characteristics

Till

Sand


Aquifer Type

PWS Wells Other Wells

Count Median

Depth (m) Count

Median Depth (m)

Glacial 474 44 3,589 27

Bedrock 526 130 583 85

Unknown 57 49 466 15


DescriptionAvg As (ug/l)

% As

> 10Avg Fe (ug/l)

Count

Screen ≤ 8 feetClay ≤ 4 feet

20 60 2,484 224

Screen > 8 feetClay > 4 feet

12 40 1,660 71

Screen sand or clay 18 53 2,117 754

Screen gravel 13 41 1,836 56

Summary and ConclusionsSummary and ConclusionsPWS and domestic wells have distinctly different

well construction characteristicsPWS well construction coincidentally yields

lower arsenicCoarser aquifersLarger aquifersLonger screens

Reductive arsenic mobilization mechanisms active at the till-aquifer interface

Changing routine domestic well drilling practices may yield fewer high-arsenic domestic wells

Goals and ResultsGoals and ResultsCharacterize Upper Midwest arsenic occurrence

Northwest provenance late Wisconsin-aged sedimentBedrock and glacial wells 30-90 m deep

Test potential geochemical mechanisms and geological controls

Reductive mobilization mechanisms– Labile arsenic is present– As(III) predominates– Fe correlation, no competing anion correlation

Well characteristics

Next stepsNext steps

Micro-to-macro scale look at arsenic occurrence in conjunction with other metals

Assess new well As resultsBuild and test As prediction model

Questions?Questions?

Mindy L. EricksonUS Geological [email protected]

arsenic in minnesota groundwater: occurrence and geochemical mobilization mechanisms

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