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

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

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)

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

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

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” http://www.usgs.gov/envirohealth/geohealth/v09_n03/ - Arsenic and Other Natural Contaminants in New England Drinking Water Wells

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

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 2003-2008 http://nc.water.usgs.gov/reports/ofr041025/report.html

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?

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

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

Topography of water table Flow direction perpendicular to topography Gaining stream – flow line bends toward stream Losing stream – flow line bends away from stream http://pubs.usgs.gov/circ/circ1139/htdocs/natural_processes_of_ground.htm

Stream level and groundwater table are dynamic Depends on precipitation regime over time Bank storage http://pubs.usgs.gov/circ/circ1139/htdocs/natural_processes_of_ground.htm