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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
0
100
200
300
400
500
600
700
0.0001 0.001 0.01 0.1 1Arsenic (mg/L)
dept
h (ft
)
EKCICVRBCVMIMCLMDL
MDL MCL
February 6, 2006 King County GWPP
Arsenic concentrations vs. time (EKC)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
1/1/01 7/2/01 1/1/02 7/2/02 1/1/03 7/2/03 1/1/04 7/1/04 12/31/04
Date
Arse
nic
Conc
entr
atio
n (m
g/L) EKC04 EKC09
EKC13 EKC14EKC17 MCL
February 6, 2006 King County GWPP
Arsenic concentrations vs. time (VMI)
0
0.01
0.02
0.03
0.04
0.05
0.06
1/1/01 7/2/01 1/1/02 7/2/02 1/1/03 7/2/03 1/1/04 7/1/04 12/31/04
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)
0
10
20
30
40
50
60
70
80
0.0001 0.001 0.01 0.1 1Arsenic (mg/L)
Dep
th (f
t)SRVMCLMDL
MDL MCL
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
0.0001
0.001
0.01
0.1
1
0.001 0.01 0.1 1 10Avg. P (mg/L)
Avg
. As
(mg/
L)
EKCICVRBCVMISRVMCLMDL
MDL
MCL
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 ?!?