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<br />
<br />on the water table, Lines drawn perpendicular to water-table
<br />contours usually indicate the direction of ground-water flow
<br />along the upper surface of the ground-water system, The
<br />water table is continually adjusting to changing recharge and
<br />discharge patterns, Therefore, to construct a water-table map,
<br />water-level measurements must be made at approximately
<br />the same time, and the resulting map is representative only
<br />of that specific time,
<br />
<br />GROUND-WATER MOVEMENT
<br />
<br />The ground-water system as a whole is actually a
<br />three-dimensional flow field; therefore, it is important to under-
<br />stand how the vertical components of ground-water move-
<br />ment affect the interaction of ground water and surface water.
<br />A vertical section of a flow field indicates how potential energy
<br />is distributed beneath the water table in the ground-water
<br />system and how the energy distribution can be used to deter-
<br />mine vertical components of flow near a surface-water body,
<br />The term hydraulic head, which is the sum of elevation and
<br />water pressure divided by the weight density of water, is used
<br />to describe potential energy in ground-water flow systems, For
<br />example, Figure A-3 shows a generalized vertical section of
<br />subsurface water flow, Water that infiltrates at land surface
<br />moves vertically downward to the water table to become
<br />ground watel, The ground water then moves both vertically
<br />and latelally within the ground-water system, Movement is
<br />downward and lateral on the right side of the diagram, mostly
<br />lateral in the center, and lateral and upward on the left side of
<br />the diagram,
<br />Flow fields such as these can be mapped in a process
<br />similar to preparing water-table maps, except that vertically
<br />distributed piezometers need to be used instead of water-
<br />table wells, A piezometer is a well that has a very short screen
<br />so the water level represents hydraulic head in only a very
<br />small part of the ground-water system, A group of piezome-
<br />ters completed at different depths at the same location is
<br />referred to as a piezometer nest. Three such piezometer
<br />nests are shown in Figure A-3 (locations A, B, and C), By
<br />starting at a water-table contour, and using the water-level
<br />data from the piezometer nests, lines of equal hydraulic head
<br />can be drawn, Similar to drawing flow direction on water-table
<br />maps, flow lines can be drawn approximately perpendicular to
<br />these lines of equal hydraulic head, as shown in Figure A-3,
<br />
<br />EXPLANA liON
<br />
<br />-------- WATER TABLE
<br />
<br />PIEZOMETER
<br />t Water level
<br />
<br />-20- LINE OF EQUAL HYDRAULIC HEAD
<br />
<br />Actual flow fields generally are much more complex
<br />than that shown in Figure A-3, For example, flow systems
<br />of different sizes and depths can be present, and they can
<br />overlie one another, as indicated in Figure A-4, In a local flow
<br />system, water that recharges at a water-table high discharges
<br />to an adjacent lowland, Local flow systems are the most
<br />dynamic and the shallowest flow systems; therefore, they
<br />have the greatest interchange with surface water, Local flow
<br />systems can be underlain by intermediate and regional flow
<br />systems, Water in deeper flow systems have longer flow paths
<br />and longer contact time with subsurface materials; the ref Ole,
<br />the water generally contains more dissoived chemicals,
<br />Nevertheless, these deeper flow systems also eventually
<br />discharge to surface water, and they can have a great effect
<br />on the chemical characteristics of the receiving surface water.
<br />
<br />Local flow system
<br />
<br />Direction of flow
<br />
<br />
<br />Intermediate
<br />flow system
<br />
<br />Regional
<br />flow system
<br />
<br />Figure A-4, Ground-water flow systems can be local,
<br />intermediate, and regional in scale. Much ground-water
<br />discharge into surface-water bodies is from local flow
<br />systems, (Figure modified from Toth, J" f963, A theoretical
<br />analysis of groundwater flow in small drainage basins:
<br />p, 75-96 in Proceedings of Hydrology Symposium No, 3,
<br />Groundwater, Queen's Printer, Ottawa, Canada.)
<br />
<br />GROUND-WATER DISCHARGE
<br />
<br />c
<br />
<br />The quantity of ground-water discharge (flux) to and
<br />from surface-water bodies can be determined for a known
<br />cross section of aquifer by multiplying the hydraulic gradient,
<br />which is determined from the hydraulic-head measurements
<br />in wells and piezometers, by the perme-
<br />ability of the aquifer materials, Permeability
<br />is a quantitative measure of the ease of
<br />water movement through aquifer materials,
<br />For example, sand is more permeable than
<br />clay because the pore spaces between sand
<br />grains are larger than pore spaces between
<br />clay particles,
<br />
<br />'80
<br />
<br />'80
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<br />
<br />'40
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<br />- DIRECTION OF GROUND-WATER FLOW \'Oc0
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<br />\..&{\ oto{\ i'
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<br />
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<br />
<br />30 'l>
<br />
<br />~-------- UNSATURATED-ZONE
<br />WATER FLOW
<br />
<br />B
<br />
<br />A
<br />
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<br />
<br />120
<br />
<br />100 ~
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<br />
<br />Figure A-3, if the distribution of hydraulic
<br />head in vertical section is known from
<br />nested piezometer data, zones of down-
<br />ward, iateral, and upward components of
<br />ground-water fiow can be determined,
<br />
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<br />
<br />20
<br />40
<br />60
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