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3.20 Groundwater Under the Direct Influence of Surface Water 115
Conductivity, or specific conductance, is the measure of water’s ability to carry an
electric current. This ability depends on the presence of ions in the water and the water’s
temperature. Groundwater is generally higher in conductivity than surface water, because
groundwater dissolves minerals from substrates through which it moves. Generally, the
longer the contact times between groundwater and its aquifer, the higher the conductivity.
However, there are exceptions to this generalization (e.g., surface-water bodies receiving
large amounts of groundwater recharge or surface-water bodies contaminated with salts,
clays, metals, or polar organics). Conductivity data are especially important when making
determinations for springs (and other situations where large seasonal fluctuations in tem-
perature are expected). Overall, conductivity tends to be a more sensitive parameter than
temperature and more difficult to interpret.
Once water quality assessment data have been collected and analyzed, a determination
must be made regarding whether there is a significant hydraulic connection between the
surface-water body and the groundwater source. A significant hydraulic connection exists
when water movement from the surface-water body to the groundwater source allows for
the transport of Giardia lamblia cysts or Cryptosporidium oocysts. Dilution and time of
travel estimates should be considered when determining the significance of the hydraulic
connection. If the time of travel estimate for the source is less than 100 days, a significant
hydraulic connection should be assumed and the supplier should proceed to the next step,
MPAs (option 3).

3.20.5 Microscopic Particulate Analyses
When the hydrogeologic assessment (option 1) and/or water quality assessment (option 2)
results suggest that the groundwater source is probably under the direct influence of sur-
face water, then the water supplier may either accept the GWUDI determination, or choose
to conduct microscopic particulate analyses (MPAs), as shown in Fig. 3.18. Information
collected as part of the characterization of hydraulic communication and time of travel will
be very important in determining the correct timing of MPA sample collection. MPA sam-
ples must be collected at least twice, and the dates of sampling should represent worst-case
conditions, when maximum potential recharge from the nearby surface water is taking
place (usually during extremely wet or dry periods).
Basically, the use of MPA in a GWUDI determination involves the careful enumer-
ation of microscopic organisms (and other particulates) in the raw drinking water.
These data are then systematically evaluated to determine if the particles found are
more indicative of surface-water or groundwater. Some of the organisms that are con-
sidered to be characteristic of surface water include Giardia, Cryptosporidium, algae,
diatoms, and rotifers.
Note that the hydrogeologic information and the results of the water quality assess-
ment must be used in conjunction with MPAs to make the GWUDI determinations. Most
importantly, MPA samples should be collected at a time when the water quality assess-
ment and hydrogeologic data indicate the greatest probability that surface water is im-
pacting the groundwater source, as indicated by hydrogeologic and water quality data. It
is relatively safe to conclude from a “high” MPA rating that the groundwater source is
under the direct influence of surface water, particularly when considered along with cor-
roborating information collected in the earlier phases of a GWUDI evaluation. Moreover,
it is more difficult to conclude from a “low” MPA rating that the groundwater source is
not under the direct influence of surface water, because surface-water influences often
only occur intermittently under particular hydrologic conditions (usually during very wet
or dry periods).