1. Geometry of the Water Table
Similar in shape to land surface, but more subdued
Geometry of the Water Table
Water table has a similar shape to land surface, but is more subdued
Groundwater flows down the slope of the water table
Shape of water table mostly independent of rock units, but rock types influence how fast groundwater flows, if at all
Groundwater flows down slope of water table

2. Slope of the Water Table Influences Groundwater Flow
Steeper slope = faster flow
Slope of the Water Table Influences Groundwater Flow
Water moves faster if there is a steeper slope to the water table compared to a more gentle slope, if we are comparing the same kind of material (same permeability)
You could present an analogy of how fast you travel on a skateboard if the ramp in the parking garage is steep
Faster
Slower
Groundwater divide

3. Types of Aquifers
Unconfined aquifer open to Earth’s surface and to infiltration
Confined aquifer overlain by less permeable materials
Types of Aquifers
We’ve been talking about unconfined (surficial) and confined aquifers
Think of the coastal plain
Layers of sediment or sedimentary rock
Grains with pores between them
Unconfined aquifer is the most common type of aquifer - statewide
A confined aquifer is separated from Earth’s surface by rocks with low permeability – coastal plain
Perched water sits above the main water table - local
Artesian does not have to rise all the way to the surface – coastal plain
Artesian has no implication about water quality or taste (some artesian wells are salty water)
Perched aquifer underlain by low-permeability unit
Artesian aquifer: water rises in pipe (maybe to surface)

4. Groundwater in Fractures
Fractures can contain groundwater
May be the only pathways for water, if interconnected
Groundwater in Fractures
In the piedmont and mountains there is some surficial aquifer in the soil/saprolite zone
we don’t really have sand aquifers, we have bedrock aquifers
Requires fractures
Nearly all rocks contain some fractures, but some rocks are more fractured than others
Fractures may be the only pathways for water in some rocks, like granite
Water can enter fractures that are interconnected (permeability)
More and wider fractures provide more space for water
More fractures = more water
17.03.a2

5. Bedrock Aquifer (Cunningham & Daniel, 2001)
Typical well in a bedrock aquifer
Figure from Cunningham and Daniel, 2001
(Cunningham & Daniel, 2001)

6. Bedrock Aquifer
Photo of Groundwater discharge in New England
GeoHealth newsletter, vol 9, #3 – 2012
“Arsenic and Other Natural Contaminants in New England Drinking Water Wells”
- Arsenic and Other Natural Contaminants in New England Drinking Water Wells

7. Limestone (Karst) Aquifers
Limestone is soluble
Groundwater is slightly acidic
Above water table, cave may be dry
Limestone aquifers
Some rocks, especially limestone, have cavities
Caves and smaller cavities can be filled or partially filled with groundwater
Groundwater in caves is NOT the main way groundwater occurs in most regions
EXPLANATION
Most caves form in soluble rocks, such as limestone
Groundwater in fractures and bedding planes dissolves material
Dissolution widens bedding planes and fractures until they become cavities and caves
Part of cave above water table may be mostly dry
Cave below water table will contain water that further dissolves material, enlarging cave
A sinkhole develops over the collapsed part of the cave on the right side
A cave may not have any expression at the surface, as on the left side
Groundwater tends to be acidic
Below water table water further dissolves material
NOT where most groundwater occurs

8. Generalized geology of NC
Aquifers of eastern NC
Generalized geologic map of NC
Aquifer map of eastern NC
From USGS - North Carolina District Science Plan
Science Goals for

9. How Does Water Move Between the Surface and Subsurface?
Precipitation, snowmelt, and surface water can soak into subsurface and become groundwater
Water table is generally below surface, providing unsaturated zone into which water can seep
Where water table intersects surface, water can flow out into spring, lake, or river
Surface water is groundwater you can see!
Water table intersects surface
Why is there water in streams when it’s not raining?

10. How Are Lakes and Wetlands Related to Groundwater?
Perched above water table
Wetlands at water table or perched
Water table intersects surface
How Are Lakes and Wetlands Related to Groundwater?
Perched above water table with outflow to subsurface (transient or ephemeral)
Where water table intersects the surface (normal setting for lakes)
Lake on bench (flat area) in topography
Lake on bottom of valley, where water table is flat
Wetlands can be at water table or perched above an impermeable zone

11. How Do Streams Interact with Water Table?
Stream lower than water table
Gaining stream
How Do Streams Interact with Water Table?
River lower than water table can gain water from inflow of groundwater
River higher than water table can lose water to groundwater
Mound of groundwater can form below river due to outflow of river water into groundwater
Stream higher than water table
Losing stream

12. Topography of water table
Flow direction perpendicular to topography
Gaining stream – flow line bends toward stream
Losing stream – flow line bends away from stream

13. Stream level and groundwater table are dynamic
Depends on precipitation regime over time
Bank storage