GWI Program
Groundwater Sources Under the Direct Influence
of Surface Water
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Background
The federal Surface Water Treatment Rule applies to all Group A
public water systems (community and non-community) that use:
- Surface water sources.
- Groundwater sources under the direct influence of surface
water (GWI).
Such sources are vulnerable to microbiological contamination.
For most affected systems, the rule requires both filtration and
disinfection to control this contamination.
Definitions and source classification
The federal rule defines groundwater under the direct
influence of surface water as"
"any water beneath the surface of the ground with:
- significant occurrence of insects or other macroorganisms,
algae or large-diameter pathogens such as
Giardia lamblia,
or
significant and relatively rapid shifts in water
characteristics such as turbidity, temperature, conductivity,
or pH which closely correlate to climatological or surface
water conditions."
"Under the direct influence of surface water" means the
groundwater source is located close enough to nearby surface
water, such as a river or lake, to receive direct surface water
recharge. Since a portion of the groundwater source's recharge
is from surface water, the groundwater source is considered at
risk of contamination from pathogens such as Giardia lamblia
and viruses, which are not normally found in true
groundwaters.
Sources most likely to be under the direct influence of
surface water are:
- Infiltration galleries and Ranney wells located near
surface waters.
- Poorly constructed springs.
- Shallow wells located near surface waters.
Process for designating sources as GWI
Records review. DOH conducts a records review to identify
"potential" GWI sources. Potential GWI sources are defined as
all infiltration galleries, Ranney wells, springs, and wells
less than 50 feet deep located within 200 feet of surface water.
Determination of hydraulic connection. A water system may
either conduct a hydrogeologic investigation or use the water
quality monitoring (WQM) method to determine whether the
potential GWI source is hydraulically connected to nearby
surface water. The hydrogeologic investigation requires a
licensed geologist; the water quality monitoring method does
not.
The WQM method requires one year of weekly measurements of
temperature and conductivity (or other parameters) at both the source and the surface
water. The purveyor must arrange for statistical analysis of the
data to determine if there is a correlation between source
measurements and surface water measurements. The department can
provide assistance and will check the validity of the analysis.
If either the WQM method or a hydrogeologic investigation
indicate a hydraulic connection, the source is designated as
a groundwater in hydraulic connection with surface water, requiring
disinfection and microscopic particulate analysis.
Microscopic particulate analysis. The water system
collects a sample of source water and sends the sample to a
laboratory for a microscopic particulate analysis. If certain
numbers or types of surface water organisms are found in the
groundwater samples, the source is designated to be under the
direct influence of surface water. Such sources are classified
as GWI and are subject to the Surface Water Treatment Rule.
Private labs that analyze MPA samples
- Analytical Laboratories, Inc., Boise, Idaho,
1-800-574-5773 - ($275/MPA sample, which includes the
sampling kit)
- Labcor, Inc., Seattle, Washington, (206) 781-0155
- ($400/MPA sample)
Treatment requirements for systems with GWI sources
Water systems with GWI sources must achieve at least 99.9%
removal or inactivation of Giardia lamblia cysts and at
least 99.99% removal or inactivation of viruses. To accomplish
this, systems must do all of the following:
- Filter, unless certain source quality and site-specific
conditions are met to avoid filtration.
- Disinfect.
- Be operated by qualified personnel.
Compliance options
Systems with GWI sources or sources identified by the
department as being "potential" GWI sources have several
compliance options to choose from, including:
- Modify the groundwater source to eliminate direct surface
water influence.
- Develop an alternate DOH-approved source (for example,
develop a protected groundwater source or purchase from a
nearby approved public water system).
- Attempt to meet the source quality and site-specific
criteria to remain unfiltered.
- Install filtration.
More information and assistance
Department of Health, Office of Drinking Water
Kitty Weisman, GWI Program Coordinator, (360) 236-3086.
- Regional Office GWI Program contacts:
Eastern Regional Office,
Andres Cervantes (509)
329-2120
Northwest Regional Office,
Derek Pell (253) 395-6763
Southwest Regional Office,
Regina Grimm (360) 236-3035
- Our Drinking Water home page
has publications on
Cryptosporidium and other contaminants, information on how
to hire an engineer, and many other resources. It also includes
related links to other sites.
- Toll-free number: 1-800-521-0323
US Environmental Protection Agency -
Safe Drinking Water Hotline 1-800-426-4791
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