Naturally Occurring Arsenic in Groundwater from Glacial Deposits in King County, Washington

Eric Ferguson / Ken JohnsonKing County Groundwater Protection Program

NGWA Naturally Occurring Contaminants ConferenceFebruary 6, 2006

February 6, 2006 King County GWPP

Talk outline King County, WashingtonData presentation– “Ambient Monitoring” Program– Sammamish River Valley GW Study

PhosphorusPeat-derived hypothesisConclusion

February 6, 2006 King County GWPP

February 6, 2006 King County GWPP

Ambient Monitoring ProgramData collected 2001-2004– Follow up of previous work (1989-95)

Sampled for Metals, Nutrients & Conventionals– Sampling events twice annual– Timing at the end of wet and dry seasons

Sampled 68 sites in 4 Groundwater Management Areas (GWMA)

February 6, 2006 King County GWPP

Ambient Monitoring WQ sites

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68 sites in 4 GWMA– 21 VMI– 16 RBC– 16 ICV– 15 EKC

– Predominantly domestic wells

– Few public water wells

February 6, 2006 King County GWPP

Ambient Monitoring DataArsenic was detected in all GWMAs– MDL for As is 0.0005 mg/L

11 sites of 68 have concentrations over the MCL (0.01 mg/L)– Range of values (0.011 to 0.075 mg/L)

Exceedances by GWMA– EKC: 8 of 15 sites– ICV: 0 of 16 sites– RBC: 1 of 16 sites– VMI: 2 of 21 sites

February 6, 2006 King County GWPP

Arsenic concentrations vs. depth

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0.0001 0.001 0.01 0.1 1Arsenic (mg/L)

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February 6, 2006 King County GWPP

Arsenic concentrations vs. time (EKC)

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EKC13 EKC14EKC17 MCL

February 6, 2006 King County GWPP

Arsenic concentrations vs. time (VMI)

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VAS_w-02a VAS_w-04VAS_w-07 VAS_w-09aVAS_w-18 VAS_w-21MCL

February 6, 2006 King County GWPP

Shaded Relief map of King County w/ GWMAs

February 6, 2006 King County GWPP

Interpretation

Initial interpretation: derived from natural leaching of arsenic from locally-derived rocks and soil minerals Results similar to other reports:– USGS Water Supply Papers 94-4082 & 92-4098– Public Health - Seattle-King County (PHSKC)

2000

February 6, 2006 King County GWPP

Sammamish River Valley Groundwater Study

21 monitoring wells

20-70’ deep

February 6, 2006 King County GWPP

Sammamish River Valley (SRV) Groundwater Study Data

Project specific work in a river valley 21 monitoring wells installed– 5 sampling events (2003-2005)

Arsenic detected in 20 of 21 wells– MDL: 0.0005 mg/L

4 sites of 20 have concentrations over the MCL (0.01 mg/L)– Range of values (0.014 to 0.169 mg/L)

February 6, 2006 King County GWPP

Arsenic concentrations vs. depth (SRV)

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February 6, 2006 King County GWPP

PhosphorusInitially not part of the Ambient Monitoring programKC Lakes Programs monitors P– wanted to obtain the GW component

Total Phosphorus added– Ortho P only done in SRV– R2 = 0.979 for Total vs. Ortho P

Noted a correlation between As and P

February 6, 2006 King County GWPP

Arsenic vs. Phosphorus

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February 6, 2006 King County GWPP

Peat-derived hypothesisP vs. As correlationReduction of FeOOH– Ravencroft et al.,2001 – Bangladesh area– Anoxic conditions (<2.5 mg/L DO) in most wells

High Arsenic concentrations in Peat Deposits in KCRecent “uncovering” of peat deposits in SRV

February 6, 2006 King County GWPP

Other Arsenic findings in KCSamples from a peat bog in western King County (Des Moines Creek) were found to have arsenic as high as 432 mg/kg, – samples in surface water systems downstream

from the peat bog • water samples: 0.003 mg/L• sediment samples: 47.6 mg/kg

buried peat bog located further inland (~30 km east) has arsenic: 3.49 mg/kg

February 6, 2006 King County GWPP

Peat deposit “uncovered”Near SRV Site: SAMM_W-6

Well concentrationsavg. As: 0.109 mg/Lavg. P: 0.12 mg/L

Probable Peat deposits nearSAMM_W-4D as well due towell concentrations ofavg. As: 0.161 mg/Lavg. P: 1.03 mg/L

February 6, 2006 King County GWPP

ConclusionsTwo probable types of Arsenic derived sources appear to be present in King County– Peat derived…for a few locations– Local rocks and soil minerals

derived…predominant sourceFuture Work ?!?