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I grasp the concept of using elevation data to delineate surface water basin boundaries quite easily as surface basins have visible features.

I'm curious about understanding the less visible aspects of aquifers - specifically, their lateral and bottom boundaries. For lateral boundaries, what strategies or indicators can simplify the identification of an aquifer's horizontal limits?

Determining the bottom boundary of an aquifer seems straightforward through geological investigations, could anyone suggest techniques, or insights that make identifying both lateral and bottom boundaries more accessible in practice?

Are there remote sensing methods to use to delineate lateral boundaries of aquifers?

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You can think of this in several ways:

  1. From a lithological perspective - e.g. what are the boundaries in space of the lithological unit that forms an aquifer. This might be as simple as the lateral extents and depth of a layer of sand and gravel or more complex if you are dealing with a multi-layer system.
  2. Refining the lithological perspective - you may need to take into account lithological variation in space or with depth. A fault may (or may not!) act as a barrier and effectively compartmentalize an aquifer, or in an aquifer where fracture flow dominates there may be fewer fractures at depth, and so the base of the aquifer is defined by some measure of fracture density.
  3. From a hydraulic perspective - which is basically analogous to surface water catchments. The 'edge' of the aquifer is defined by the highest point at which water flows into the aquifer.

Groundwater scientists (and especially modellers) tend to lump these factors together as 'no-flow boundaries' e.g. the aquifer limits are the limit of the zone where if you introduce a hypothetical 'drop' of water it flows into the aquifer. The 'downstream' limit of an aquifer is usually taken as a river, or the sea' where again water doesn't cross the boundary.

So aquifer mapping requires a mix of data;

a) geological mapping to describe the lithological boundaries - which might be assisted by geophysical techniques to determine bed thickness or lithological variation.

b) information from drillers logs, pumping and the laboratory and field measurement of aquifer properties. This can provide insight into porosity and permeability, the vertical extent of aquifers and also provide evidence for no-flow boundaries such as impermeable faults.

c) topographic mapping to define hydraulic boundaries - rivers, coasts etc.

d) water level observations from wells and boreholes to establish the height of the water table and define no-flow boundaries.

The actual definition of 'an aquifer' is likely to be influenced by utilitarian considerations. Aquifers can be defined at regional scales or characterized in detail locally. Whether water quality is a factor in the definition of an aquifer boundary might vary between defining an aquifer for drinking water abstraction or defining an aquifer for environmental management.

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