[PPT] - Occurrence and Distribution of Ground Water Arsenic in Public Water PowerPoint Presentation

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Occurrence and Distribution of Ground Water Arsenic in Public Water Supply Wells in Ohio

Michael W. Slattery Christopher Kenah

Division of Drinking and Ground Waters Program Development Unit

SLIDE 2

Acknowledgments

Data: OEPA District Ambient Network Field Personnel OEPA District Drinking Water Field Personnel GIS Support: Brian Gara and Dick McClish of the DDAGW GIS Unit

SLIDE 3

Arsenic in Ohio’s PWS

Arsenic Rule driven by health concerns
State-wide As appears widespread

Reduction of MCL to 10 ug/L from 50 ug/L Analysis suggests that redox controls are more important than lithologic or stratigraphic controls Regional patterns do not support anthropogenic source Understanding As provenance can help optimize reduction strategies

SLIDE 4

Data Sources

Ambient Monitoring Network

214 Wells, Untreated Water Good (electronic) locational, geologic control

Public Water Supply Database

2922 PWSs, Treated Water Poor (electronic) locational, geologic control Single modern MDL = 2.0 ug/L Multiple MDLs = 2, 3, 5, 10 and 20 ug/L

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N

Div ision of Drinking and Ground Waters Program Dev elopment Unit

Ohio Environm ental Protection Agency

Car tography by: Michael Slattery

Carbonate Aquifers Sandstone Aquifers Interbedded Shale and Carbonates Sand and Gravel Aquifers

Aquifer Type/Lithology

# S

0 - 3

# S

3.1 - 5

# S

5.1 - 10

# S

10.1 - 20

# Y

20.1 - 91

Mean Ambient and PWS As Concentrations (ug/L)

20 20 40 Miles

Mean Ambient and PWS As Concentrations

SLIDE 6

8 16 24 32 40 48 56 64 72 Average As concentration ( g/L 100.0 101.0 102.0

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2

Counts

N = 214 sites

Histogram of Mean Ambient Arsenic Concentrations

Proposed MCL = 5 g/L

SLIDE 7

100 101

2 3 4 5 6 7 8 2 3 4 5 6 7 8 9

Arsenic Concentration ( g/L)

(43)

Sandstones

Mean As Concentration by Lithology

Ambient Monitoring Network

median = 2.00

(31) (114)

Carbonates Sand and Gravel Current MCL = 50 g/L New MCL = 10 g/L

median = 2.95 median = 2.16

SLIDE 8

SLIDE 9

4 8 12 16 20 24 28 32 Average As concentration ( g/L 100.0 101.0 102.0

2 3 4 5 6 7 9 2 3 4 5 6 7 9 2 3 4 5 6 7

Counts

N = 2922 PWSs

Historical Detection Limits

Histogram of Mean PWS Arsenic Concentrations, 1980-1999

10 g/L 20 g/L 5 g/L 3 g/L

SLIDE 10

1 10 100

4 5 6 7 8 2 3 4 5 6 7 8 2 3 4 5 6 7 8

As (µg/L)

Mean PWS As concentrations, 1980-1999

Proposed MCL = 10 ug/L current detection limit = 2 ug/L Current MCL = 50 ug/L

Mean: 7.23 Median: 6.09 Max: 90.11 Std Dev.: 5.1

(2922)

SLIDE 11

4 8 12 16 20 24 28 32 Average As concentration ( g/L 100.0 101.0 102.0 103.0

2 3 4 5 6 8 2 3 4 5 6 8 2 3 4 5 6 8 2

Counts

N = 2175 PWSs

Historical Detection Limits

Histogram of Mean PWS Arsenic Concentrations, 1996-1999

5 g/L 3 g/L

SLIDE 12

1 10 100

2 3 4 5 6 7 8 2 3 4 5 6 7 8 2 3 4 5 6 7 8

As (µg/L)

Mean PWS As concentrations, 1996-1999

Mean: 5.59 Median: 5.00 Max: 90.34 Std Dev.: 4.56

(2175)

Proposed MCL = 10 ug/L current detection limit = 2 ug/L Current MCL = 50 ug/L

SLIDE 13

100 101 102 103 104

2 3 4 5 67 2 3 4 5 67 2 3 4 5 67 2 3 4 5 67 2 3 4 5 67

Number of Service Connections

100 101 102

4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7

Mean Total Arsenic Concentration ( g/L)

Community PWSs Transient PWSs Non-Community NT PWSs

Mean Arsenic in Ohio PWSs

as a function of PWS Type and Number of Service Connections

proposed As MCL = 10 g/L

SLIDE 14

500 1000 1500 2000 2500

Pumping Rate (gal/min)

20 40 60 80

Mean Total Arsenic Concentration ( g/L)

Community PWSs Transient PWSs Non-Community NT PWSs

Mean Arsenic Concentrations in Ohio PWSs

as a function of Pumping Rate and System Type

proposed As MCL = 10 g/L

SLIDE 15

Arsenic Mobilization

As derives from dissolution of Fe oxyhydroxides

Arsenic is strongly sorbed onto Mn, Fe oxide coatings
Microbially mediated reduction of Fe oxides,
xidation of C releases adsorbed As through reaction

sequence similar to natural attenuation

Fe oxides exist as dispersed phases
Arsenopyrite is a possible As source from near-

surface oxidized environments to FeOx

SLIDE 16

Arsenic appears ubiquitous in all aquifer types

Evidence for Fe oxide source

Increase in As with depth (reducing conditions)
Lack of elevated As in known oxidized conditions
Pyrites common locally, not regionally, in carbonates
Increase in alkalinity supports reductive scheme
In-phase changes of Fe and As suggests co-variation

SLIDE 17

102 103 104

2 3 4 5 6 7 8 2 3 4 5 6 7 8 2 3 4 5 6 7 8 2

Total Fe ( g/L)

100 101

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9

Arsenc concentration ( g/L)

Mean Arsenic - Total Fe Relations in Ambient Wells

sand and gravel aquifers sandstone aquifers carbonate aquifers

10 g/L As 700 g/L Fe

SLIDE 18

102

4 5 6 7 8 9 2 3 4

Alkalinity as CaCO3 (mg/L)

100 101

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9

Arsenic Concentration ( g/L)

Mean Arsenic - Alkalinity Relations in Ambient Wells

sand and gravel aquifers sandstone aquifers carbonate aquifers

10 g/L As

Baseline Alkalinity 200 mg/L

SLIDE 19

20 40 60 80 Arsenic Concentration ( g/L) 400 300 200 100 Well Depth below Land Surface (feet)

As - Depth Relations in Ambient Wells

sand and gravel aquifers sandstone aquifers carbonate aquifers

Robust regression slope yields 2.5 g/L increase/10 feet up to about 200 feet (25 g/L)

SLIDE 20

102

4 5 6 7 8 9 2 3 4

Alkalinity as CaCO3 (mg/L)

102 103 104

2 3 4 5 6 7 9 2 3 4 5 6 7 9 2 3 4 5 6 7 9 2

Total Fe ( g/L)

Mean Iron - Alkalinity Relations in Ambient Wells

sand and gravel aquifers sandstone aquifers carbonate aquifers

Baseline Alkalinity 200 mg/L

Elevated HCO3- suggesting Fe oxyhydroxide reduction, i.e., 4FeOOH + 8CH20 7H2CO3

--> 4Fe2

+ + 8HCO3

+ 6H2O

SLIDE 21

5 10 15 20 25 Arsenic (ug/L) 4 8 12 Nitrate/Nitrite (mg/L)

Mean Arsenic-Nitrate/Nitrate Relations in Ambient Wells

sand and gravel aquifers sandstone aquifers carbonate aquifers

SLIDE 22

Temporal As Variation

As appears to have two end-member modes with respect to N

1. Positive co-variation between As and Fe, NOX (N) excluded 2. As and Fe non-detect, NOX (N) detected

Pattern appears to follow “natural attenuation” reduction

sequence : O2 > NO3 > Mn > Fe > SO4

Variability has recharge, pumping, and seasonal components

SLIDE 23

1/1/1987 1/1/1989 1/1/1991 1/1/1993 1/1/1995 1/1/1997 1/1/1999 1/1/2001 16 32 48 64 80 96

Total Arsenic, ug/L

asfeplots298 Feb. 16, 2001 2:06:01 PM

2500 3000 3500 4000 4500 5000 5500

Total Iron, ug/L

Total Arsenic, (ug/L) Total Iron, (ug/L) Ammonia, (mg/L) Nitrogen, (mg/L)

Arsenic-Iron Relations

AMBIENT MONITORING NETWORK, OEPA

PLEASANT CITY WELLFIELD,Well Number 2, DB File=Plant298, GeoAge:QUAT, Total Depth:50ft,Lithology:USG (Sand/Gravel), StationID:39GUE00093, Status:(AS) ACTIVE STANDARD

--- mean As = 75.4

..... mean Fe = 3700

SLIDE 24

1/1/1995 1/1/1997 1/1/1999 1/1/2001 5 10 15 20 25 30

Total Arsenic, ug/L

ASFEPLOTS153 Feb. 16, 2001 1:59:20 PM

1100 1500 1900 2300 2700 3100 3500

Total Iron, ug/L

Total Arsenic, (ug/L) Total Iron, (ug/L) Ammonia, (mg/L) Nitrogen, (mg/L)

Arsenic-Iron Relations

AMBIENT MONITORING NETWORK, OEPA

HEBRON WELLFIELD,Well Number 1, DB File=Plant153, GeoAge:QUAT, Total Depth:155ft,Lithology:USG (Sand/Gravel), StationID:39LIC00221, Status:(AS) ACTIVE STANDARD

--- mean As = 15.7

..... mean Fe = 2100

SLIDE 25

1/1/85 1/1/87 1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 3 6 9 12 15 18

Total Arsenic, ug/L

GS12 Feb. 2, 2000 12:54:45 PM

700 1400 2100 2800 3500 4200

Total Iron, ug/L

Total Arsenic, (ug/L) Total Iron, (ug/L) Ammonia, (mg/L) Nitrogen, (mg/L)

Arsenic-Iron Relations

AMBIENT MONITORING NETWORK, OEPA

MIDDLETOWN WELLFIELD,Well Number 17, DB File=Plant229, GeoAge:ORD, Total Depth:146ft,Lithology:USG (Sand/Gravel), StationID:39BUT00155, Status:(AS) ACTIVE STANDARD

--- mean As// = 6.77

..... mean Fe// = 1600

SLIDE 26

1/1/94 1/1/96 1/1/98 1/1/00 0.0 1.6 3.2 4.8 6.4 8.0 9.6

Total Arsenic, ug/L

GS18 Feb. 2, 2000 12:54:54 PM

1100 1300 1500 1700 1900 2100 2300

Total Iron, ug/L

Total Arsenic, (ug/L) Total Iron, (ug/L) Ammonia, (mg/L) Nitrogen, (mg/L)

Arsenic-Iron Relations

AMBIENT MONITORING NETWORK, OEPA

MT. STERLING WELLFIELD,Well Number 1, DB File=Plant247, GeoAge:SIL-DEV,

Total Depth:180ft,Lithology:CLS (Limestone), StationID:39MAD00276, Status:(AS) ACTIVE STANDARD

--- mean As// = 6.27

..... mean Fe// = 1600

SLIDE 27

1/1/86 1/1/88 1/1/90 1/1/92 1/1/94 1/1/96 1/1/98 1/1/00 0.0 1.4 2.8 4.2 5.6 7.0 8.4

Total Arsenic, ug/L

GS54 Feb. 2, 2000 12:56:05 PM

20 30 40 50 60 70 80

Total Iron, ug/L

Total Arsenic, (ug/L) Total Iron, (ug/L) Ammonia, (mg/L) Nitrogen, (mg/L)

Arsenic-Iron Relations

AMBIENT MONITORING NETWORK, OEPA

BEVERLY WELLFIELD,Well Number 4, DB File=Plant031, GeoAge:QUAT, Total Depth:63.7ft,Lithology:USG (Sand/Gravel), StationID:39WAS00068, Status:(AS) ACTIVE STANDARD

--- mean As// = 2

..... mean Fe// = 50

SLIDE 28

Arsenic Summary

Arsenic derives from microbially mediated reduction of FeOx,
xidation of sedimentary organic C
Distribution of elevated As favors reduced carbonate and

glacial sediment aquifer settings

Pattern appears to follow “natural attenuation”

sequence of O2 > NO3 > Mn > Fe > SO4 reduction

Time series correlations indicate two end-members with

respect to nitrate/nitrite

SLIDE 29

Conclusions

Arsenic derives from microbially mediated reduction of

FeOx, oxidation of sedimentary organic C

Distribution of elevated As favors reduced

carbonate and glacial sediment aquifer settings

Pattern appears to follow “natural attenuation”

sequence of O2 > NO3 > Mn > Fe > SO4 reduction

SLIDE 30

Future Directions

Increase data sources (ODNR, USGS, ODH etc)
Aquire redox data (O2 concentrations, ORP, eH)
Speciate As(III), As(V)
Review / aquire whole rock As data
Formalize reductive sequence model
Short term sampling to evaluate As variation

SLIDE 31