Using GIS to Predict Arsenic Over 10 ppb in Drinking Water

OrHow to lie with statistics and maps

Rich SouleMDH

All Blue

All Red

Michael Gastner, Cosma Shalizi, and Mark Newman University of Michigan

Michael Gastner, Cosma Shalizi, and Mark Newman University of Michigan

Make a Statewide Map of Arsenic in Groundwater in Less Than 3 Hours

1) Come up with a single physical process.It must be mapped at state scale.

2) Find the data sets representing this process and arsenic.

Evaluate data set bias.3) Generate a Probability Map for Arsenic

over 10 ppb.

Clay

Sand

Sand withIron

ReducedBlack, green and grey

Rotten eggs odor.

Red, brown and, tanLittle to no odor.

Deep

Shallow Oxygen+

Carbon+

MicrobesHAsO4HAsO4--22

HAsO4HAsO4--22

As+5

HAsO4-2

As+3

H2AsO3 Ars

enic

Confining Thickness

HydrogeologicModel

Ars

enic

Time

BiologicalModel

GeochemicalModel

Oxidized

Conceptual Model• Pr(As) > 10 ppb ≈ Clay Thickness.

• Groundwater is reduced by microbes at a constant rate and is limited by electron acceptors (O2, FeNO3, Fe ).

• The flow of electron acceptors is determined by vertical groundwater recharge. – Q=KiA, Assume average “K” and “i” .

• Assume Steady State.

Why?: Spatial RelationshipsArsenic & DM Lobe Wood & DM Lobe

Arsenic >10 ppbArsenic <10 ppb

Data Sets: Geology (CWI)VerifiedLocations

RawLocations

170,000Wells

200,000Wells

Use it All.

VerifiedLocations

RawLocations

Bias: Well Density

Summarizing Well Logs

37 ft

56 ft

21 ft

15 ft

Clay Thickness

+Total 138 ft

Clay

Clay

Clay

Clay

Do this370,000 Times.Takes 20 min.

5 kmGrid

10 kmGrid

Grid Clay Thickness

> 100 Ft.

None

Clay Thickness

Grid Clay Thickness

1 kmGrid

1 kmGrid with Brakes

> 100 Ft.

None

Clay Thickness

No Data

MARSAreaLooks Really Bad….Population Declining….Co-incidence?

Data Sets: Arsenic

Yes

No

Yes

Location

High As?Deeper?

YesWest Central

MARS

State

State

Area

No

Yes

Geology Bias Data

No LocationShallower?

Construction?

CDC/MDH

Deeper?Construction?

GWMAPS

Use MARS Data?

Over 10 ppbNumber in AreaData Set

49%47CDC48%50GWMAPS52%893MARS

Yes, but cautiously.

Get Clay Grid Values

10

%

%

0 20 40 60 80 100Avg. Feet Clay Above Screen

0

20

40

60

80

Pr (

As>

10)

Probability As >10 ppb All Arsenic Data

All Data

MARS Data

GWMAPS

CDC Data

Low

Medium

High

Medium

0 20 40 60 80 100Avg. Feet Clay Above Screen

0

20

40

60

80

Pr(

As>

10)

Mapping Arsenic Using “Blocks”

All Data

MARS Data

GWMAPS

CDC Data

Results

10 ppb

Results

10 ppb

Low

Medium

High

Medium

Score Card

0 20 40 60 80 100Avg. Feet Clay Above Screen

0

20

40

60

80

Pr(

As>

10)

MARS

All

MARS

MARS

MARS

All

AllAll

GWMAPS

GWMAPS

GWMAPS

GWMAPS

CDC

CDC

CDC CDC

Implications• State Scale: the proposed processes may impact

the arsenic concentration in groundwater.• County Scale: These processes are probably

variable (i.e. K, infiltration…).• Section Scale: The change of the redox state of

groundwater through time may be the most important factor.

MPCA & others

Changing the Redox StateBorehole Chemistry & Microbial Ecosystems

“Reducing Factors” Increase Arsenic:Decreasing Infiltration (Tile Drains)Increasing Carbon Load (Monoculture)

“Oxidizing Factors” Decrease Arsenic:Increased Nitrate Loading

Acknowledgements• Arsenic Lunch Group:

Mindy Erickson Randal BarnesRoman Kanivetsky Karla PetersonDavid Rindal Mike Berndt

• MDHBruce Olsen Sheila GrowBrian Johnson Justin BlumAl Epp Mike Baker