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3.20 Groundwater Under the Direct Influence of Surface Water 109
types of groundwater sources potentially regulated under the SWTR include dug wells,
springs, infiltration galleries, shallow or improperly constructed wells, or other collectors
in subsurface aquifers near surface waters.
Each local health department (LHD) throughout the United States is responsible for
identifying which public water sources are subject to the SWTR. However, it is the re-
sponsibility of the water supplier to provide the information needed to make this determi-
nation to the LHD. The LHD is also responsible for recording and reporting the criteria
used and the results of determinations. All groundwater sources used to supply public
water systems must be evaluated for evidence of groundwater under the direct influence
of surface-water GWUDI. This evaluation will focus on the likelihood that the groundwater
source could be contaminated with large-diameter pathogens, such as Giardia lamblia
and Cryptosporidium, through a hydraulic connection with surface water. If a drinking
water source has been identified as GWUDI, the source must meet the criteria established
under the SWTR.

3.20.1 GWUDI Determination: Source Screening Phase
Information gathered during sanitary surveys is important when making GWUDI determi-
nations. In addition, information such as compliance monitoring data, topographic maps,
geologic reports, well logs, and data on potential contaminant source(s) is useful.
A two-phase methodology is being used in the United States to determine whether or
not a groundwater source is under the direct influence of surface water: The source screen-
ing phase is used first to separate those sources that are clearly not subject to surface water
influences from those sources in need of further evaluation. Then the second phase, the
detailed evaluation phase, is applied to sources identified for testing to evaluate their degree
of hydraulic connection with surface water.
The source screening phase should be used to separate those sources that are clearly
not subject to surface-water influences from those sources in need of further evaluation. A
schematic of the screening procedure is shown in Fig. 3.17, and the overall methodology
for the detailed evaluation phase is presented as a flowchart in Fig. 3.18.
Box 1 in Fig. 3.17 includes criteria that will immediately select a groundwater source
for further review. These source screening water criteria include the following: (a) A bio-
logical waterborne disease outbreak has been associated with the source; (b) a chemical
contaminant incident has occurred in the source that was thought to have originated from
the surface water; (c) the source is a spring, infiltration gallery, crib intake, dug well, or
shallow horizontal collector; (d) rapid fluctuations have been noted in the source output,
well water level, and/or chlorine demand (particularly when associated with runoff
events); (e) the source is located in a carbonate aquifer; and (f) the source is within 200 ft
(60 m) of surface water in a fractured bedrock setting with 100 ft (30 m) of casing. If
none of these criteria are met, then the screening process continues in Box 2 of Fig. 3.17.
If any of these criteria is met, then the screening process continue to the box “Source se-
lected for further review.”
The criteria in Box 2 of Fig. 3.17 ask if the source is located within 200 ft (60 m) of
surface water, and/or if the well has 50 ft (15 m) or less of casing. When both criteria are
met, the source is selected for further review under the box “Source selected for further
review.” If neither of these conditions is met, the source is designated as not being under
the direct influence of surface water, or not-GWUDI. When only one of the criteria in Box
2 of Fig. 3.17 is met, the next step in the GWUDI determination is dependent on whether
or not the water source is currently disinfected. Undisinfected wells that have met the cri-
teria for a disinfection waiver and have an adequate coliform monitoring history (typically