SEHIC Arsenic in groundwater v1 10/27/06
DRINKING WATER INDICATOR: Geographic and Geologic Distribution of
Arsenic in Groundwater Used for Drinking Water
Measurement Unit
Arsenic in μg/l
Geographic Area
There are two USGS national scale data sets that can be used to define the geographic
and geologic distribution of arsenic in groundwater used for drinking water within a state.
First, National Water Information System (NWIS) data provides the geographic distribution
of arsenic concentrations in samples from domestic water supplies. Second, National
Water Quality Assessment (NAWQA) Program data also provides the geographic
distribution of arsenic but geologic information is also provided because samples were
collected from designated aquifers that are fully or partially within a State.
Measures Of
Frequency
Single measurements for each sampled well; however, NAWQA is an ongoing monitoring
program that will provide information on changes in concentrations with time.
Time Period
1973-2001 for NWIS, 1990-2005 for NAWQA
Significance &
Background
Exposure to arsenic in drinking water has been associated with several different types of
cancer including bladder, liver, and skin cancer. The maximum contaminant level (MCL)
has been reduced from 50 ug/l to 10 ug/l and will be effective starting in 2006. This
change will have both economic and public health consequences. Arsenic levels in ground
water are a result of natural processes and for the most part there are limited man-made
sources of arsenic.
Rationale
This indicator can be used to (1) define the geographic distribution of potential arsenic
exposure to populations served by domestic water supplies, and (2) define the
concentrations arsenic in groundwater within major aquifers used for drinking water. The
geographic distribution of potential exposure is important if these data will be compared
with human health data. Concentrations in aquifers used for drinking water can also be
used to help define exposure (if water use information is available by aquifer) but can also
be used to help define geologic sources in arsenic in groundwater.
These arsenic data, if combined with water use and health information, can be used to
determine how arsenic concentrations in drinking water affects human health and how
reducing exposure remediate these effects. Understanding the geological sources of
arsenic and geographic distribution of exposure can be used to help reduce exposure;
particularly, to elevated arsenic concentrations among the state’s population.
Limitations Of
Measure
Limitations for assessing exposures
- Don’t know actual consumption of drinking water and use of drinking water in the
home; for example, some people may use bottled water.
- Does not give a sense of smaller scale geographic variation across the state
- Not a measure at the tap; for example, some people may treat ground water
before use.
- Assumes that drinking-water measure is the only route of exposure
- One time measure
Data Resources
NWIS and NAWQA data
Limitations Of Data
Resources
-
Samples untreated and as close the sampled well as possible
To be most useful, these data need to be combined with water use information that
is not currently available at the national scale.
SEHIC Arsenic in groundwater v1 10/27/06
Related Sets Of
Indicators
Additional Data To
Collect
EPA, health people 2010, CSTE
-
General feedback
Water use by census tract
More specific demographics (Age, Race, ethnicity)
Treatment type
Consumption patterns (BRFSS state added module)
Distribution of aquifers within a state
(http://www.nationalatlas.gov/mld/aquifrp.html)
Descriptions of aquifers and hydrologic systems
(http://capp.water.usgs.gov/gwa/gwa.html)
Concentrations less than the MCL may have health implications. Need to get additional
data on county level estimates of population served by community and domestic water
supplies within a state (e.g., 1990 Census Data). Political boundaries are not geological
boundaries.
SEHIC Arsenic in groundwater v1 10/27/06
How to Guide- Geographic and Geologic Distribution of Arsenic in Groundwater
1)
2)
3)
4)
5)
NWIS data for arsenic concentrations is available at http://water.usgs.gov/nawqa/trace/arsenic/,
Click on USGS data (excel spreadsheet).
Download data
Sort on State Code and delete all data from states that are not of interest.
Sort on “WATERUSE” and delete all data from well types that are not of interest (H=Domestic Wells,
P=Public Water Supply)
6) Create a new column of concentration data so that a single column of numbers can be used to create
graphs and maps. If the measured concentration was “less than” the reporting level then assign the
concentration to a common small value (for example 0.5 μg/L). If an arsenic concentration was
detected then the concentration in new column is assigned the measured concentration. The following
command “=IF(J97="<",0.5,K97)” can be used to create this new column of concentration data.
7) Create a map of arsenic concentration data
8) Create a quantile plot of concentration data to determine how frequent concentrations exceed threshold
values. For example, the following quanitle plot of all the NWIS arsenic data shows that 10 percent of
the samples exceed 10 ug/L and about 60 percent of the samples had a detectable concentration of
arsenic.
(Note:May want to consider additional ways for graphically displaying the data, could
use similar bin like structure)
SEHIC Arsenic in groundwater v1 10/27/06
1) NAWQA data are available at:
http://discoviewer.er.usgs.gov/discoverer/viewer?&cn=tm4i_connection1&us=%7E27JQPUBL
IC%7E27&db=wiprod2&lm=discoverer&eul=NWQ_STAR_EUL_PUB&nlsl=enus&_po=100000100001001110000000&wbk=CDC_ARSENIC1&pw=jqpublic
2) Select state of interest
3) Arsenic is already selected.
4) Run the query.
5) To export data click on “export” on right side of page.
6) Select file type you would like to create.
7) Create quantile plots and maps of arsenic concentrations for each different Su Code Sum (aquifer
study). For quantile plots, a new column of concentration data will have to be created so that a single
column of numbers can be used to create the plots. If the sample had a remark of “less than” the
reporting level, then assign a concentration to a common small value (for example 0.001 μg/L used in
the plot below). If an arsenic concentration was detected then the concentration in new column is
assigned the measured concentration. The following command “=IF(M4="<",0.001,L4)” can be used
to create this new column of concentration data in column “N”. Please note, an “E” value in the remark
column means that arsenic was detected in the sample but the concentrations were estimated indicating
some uncertainty in the reported concentration.
8) Quantile plots will show which aquifers have the largest concentrations of arsenic and the distribution
of these concentrations. For example, the quantile plot below shows that concentrations of arsenic are
largest in nvbrsus2 and nvbrsus4—about 40% of samples in these two studies are equal to, or exceed,
10 μg/L. “nvbrsus2” had about 10% of the samples that exceeded 50 μg/L.
SUCODE
gaflsus1
gaflsus2
gaflsus3
grslsus1
nvbrsus1
nvbrsus2
nvbrsus3
nvbrsus4
sacrsus1
sanasus1
sanasus2
sanasus3
sanjsus1
soflsus1
wmicsus1
wmicsus2
1.0
0.9
Fraction of Data
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
01
0.0
10
0.0
00
0.1
00
1.0
10
.00
0
00
0.0
10
ARSENIC_1000
9)
10)
11)
12)
13)
14)
To download aquifer information go to: (http://www.nationalatlas.gov/mld/aquifrp.html)
On right side of page, Click on aquifers
Zoom in on your state.
Add aquifers of alluvial and glacial origin. Redraw Map.
Click at top of page to print map.
Download aquifer descriptions (http://capp.water.usgs.gov/gwa/gwa.html).