Page 104 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 3
Water Sources: Groundwater
source is not in hydraulic connection with surface water, no
further analysis will be required. However, if the LHD deter-
for information gathering.
mines that the hydrogeologic assessment does not contain
To be definitive, a hydrogeologic assessment needs to
include an interpretation of the information collected with
enough information to establish whether there is a hydraulic
respect to the potential for a hydraulic connection between
connection between surface water and the source-water col-
a surface-water body and the aquifer. If the hydrogeologic
lection device, the water supplier should collect additional
assessment indicates a potential hydraulic connection, the
hydrogeologic information or proceed with a water quality
assessment or MPAs.
water system should be required to initiate a water quality
assessment.
The hydrogeologic assessment should be designed to
provide the following information:
3.20.4 Water Quality Assessment
1. Well construction details, which provide (a) a well examples of sanitary and field survey reports that can be used
log, well construction diagram, and well description;
Option 2, a water quality assessment, entails either (a) an
and (b) its installation methods for comparison with
evaluation of water quality parameters in terms of conduc-
current well standards
tivity and temperature daily over a 12-month period or (b)
2. Aquifer characteristics, data which include (a) an evaluation of water quality parameter in terms of monthly
aquifer geometry and texture, including the unsat- coliform levels for 12 consecutive months.
urated zone; (b) saturated thickness; (c) hydraulic Conductivity, or specific conductance, is the measure
conductivity; and (d) transmissivity of water’s ability to carry an electric current. This ability
3. Prepumping and pumping groundwater flow char- depends on the presence of ions in the water and the water’s
acteristics data, which include (a) water table/ temperature. Groundwater is generally higher in conductiv-
potentiometric surface elevations; (b) groundwater ity than surface water, because groundwater dissolves min-
flow directions; (c) groundwater flow velocity; and erals from substrates through which it moves. Generally, the
(d) zone of contribution/influence of the well longer the contact times between groundwater and its aquifer,
4. Degree of hydraulic connection between surface- the higher the conductivity. However, there are exceptions to
water source and aquifer, which is indicated by (a) this generalization (e.g., surface-water bodies receiving large
geology underlying surface-water body; (b) char- amounts of groundwater recharge or surface-water bodies
acterization of bottom sediments in surface-water contaminated with salts, clays, metals, or polar organics).
body; (c) determination of vertical hydraulic gradi- Conductivity data are especially important when making
ent in surface-water body; (d) hydraulic relationship determinations for springs (and other situations where large
between the surface-water body and the well; and (e) seasonal fluctuations in temperature are expected). Overall,
calculations of travel times between the surface-water conductivity tends to be a more sensitive parameter than tem-
body and the well perature and more difficult to interpret.
Once water quality assessment data have been col-
5. Seasonal variations in hydrogeologic characteristics,
lected and analyzed, a determination must be made regarding
such as the changes in flow patterns during seasonal
whether there is a significant hydraulic connection between
fluctuations or periods of drought
the surface-water body and the groundwater source. A signif-
icant hydraulic connection exists when water movement from
The hydrogeologic assessment should include, as a min-
the surface-water body to the groundwater source allows for
imum, geologic logs and construction details for the pumping
the transport of Giardia lamblia cysts or Cryptosporidium
well and any observation wells or piezometers; aquifer pump-
oocysts. Dilution and time of travel estimates should be con-
ing test(s); a survey of the elevations of water level monitor-
sidered when determining the significance of the hydraulic
ing measuring points; water level monitoring of groundwa-
connection. If the time of travel estimate for the source is
ter and surface sources; and preparation of detailed maps
less than 100 days, a significant hydraulic connection should
of water table/potentiometric surface and geologic cross-
be assumed and the supplier should proceed to the next step,
sections. The owner/developer may need to install observa-
MPAs (option 3).
tion/monitoring wells or piezometers if these do not already
exist.
In addition to requiring an assessment of hydrogeo-
3.20.5 Microscopic Particulate Analyses
logic factors, the hydrogeologic assessment should include
a description and review of the collection device (i.e., type, When the hydrogeologic assessment (option 1) and/or water
age) and a summary of any current or historical sanitary quality assessment (option 2) results suggest that the ground-
conditions. Any information available from previous sani- water source is probably under the direct influence of sur-
tary surveys or field investigations should be included in the face water, then the water supplier may either accept the
assessment, as appropriate. Table 3.6 and Fig. 3.19 provide GWUDI determination or choose to conduct MPAs, as shown