ch 11 reading guide
How has the FL population changed in the past several decades?
Growing especially along the coasts.
How have the amount of water withdrawals in the US changed with time?
They have increased over time.
Name two other aquifers in the state. Where are they utilized?
Biscayne Aquifer, southeast florida. Intermediate aquifer, southwest florida
How many first-magnitude springs are in Florida? How does this compare to the US as a whole?
33 of the nation's 78 first magnitude springs
How many water management districts are in Florida? What are they? To which water management district does Gainesville belong? What about Miami?
5: Northwest Fl WMD, Suwannee WMD, St. Johns River WMD, Southwest Fl WMD, and South Fl WMD . Gainesville is part of the Suwannee WMD and the St. Johns WMD. Miami belongs to the South Fl WMD.
How much of Florida's water supply comes from the Floridan Aquifer?
60%+
What is an aquifer? Why do good aquifer strata (rock or sediment materials) need to be both porous and permeable? Give two examples of geologic materials that make good aquifers.
An aquifer is a porous, permeable rock or sediment through which groundwater flows (and can be extracted). Sand, gravel, sandstone and limestone.
What is an aquitard (or aquiclude)? Give one example of a geologic material that makes a good aquitard.
An aquitard is an impermeable rock or strata that is a barrier to groundwater flow. Clay, shale and intrusive igneous rocks.
Why can the flow of water into a well be more rapid than the flow of water elsewhere in the aquifer away from the well? (i.e. how is it that you can pump water out of an aquifer faster than it can be recharged?)
By locally steepening the slope of the potentiometric surface you steepen the hydraulic gradient which allows one to pump water from the well faster than the water can come in from elsewhere to replace it. This leads to groundwater depletion.
What is the difference between a confined and an unconfined aquifer?
Confined: the upper bound of the aquifer is defined by a confining (impermeable) strata (aquitard). Water can only enter the aquifer at a point where it is not confined. Unconfined: all materials above them are permeable strata, allowing water to percolate from the surface and enter the aquifer. Top of the aquifer is established by the water table.
Define Darcy's Law and each of the terms included in it. How does the hydraulic gradient differ in confined vs. unconfined aquifers?
Darcy's Law: Q=kiA Q= flux of groundwater (discharge) k= hydraulic conductivity (permeability specific to water) i= hydraulic gradient (gradient of the potentiometric surface) 'ki' is what is defining the velocity of the water in terms of direction and rate A= cross-sectional area of aquifer In an unconfined aquifer the 'i' can be simplified to just look at the slope of the water table but in a confined aquifer the water table is not defining the top of the aquifer so it can't be used to find the hydraulic gradient. In an unconfined aquifer the water table is the top of the aquifer and therefore establishes the potentiometric surface because water flow would just run straight down the slope. The geometry of the confining unit is what is defining the slope of the potentiometric surface in a confined aquifer.
What information would one need to know to predict the direction and rate at which contaminated groundwater might spread from the source of the contamination in an unconfined aquifer?
Darcy's law, type of contaminant, geology of the area.
How does a karstified terrain change in its appearance from the early stages of karstification to the most advanced, mature karstified terrains?
Dissolution is focused around the water table but the water table and land can move up and down. This leads to dissolution following along up and down. Eventually the dissolution cavities get large enough to cause collapse that leads to landscapes dotted with sinkholes. Early stages: Various sinkholes and disappearing streams Through time: continued subsurface dissolution causes sinkholes to coalesce and cavern, broader solution valleys form, causes subsurface subsidence. Most mature: Eventually most of the limestone is removed completely leaving a landscape close to the water table with various rock towers of leftover undissolved rock.
What is the most heavily drawn upon aquifer in Florida?
Floridan aquifer except for South Florida
Given an appropriate diagram, can you predict the direction of groundwater flow?
Groundwater flows perpendicular to the contours.
What is the chemical reaction that leads to karstification? What solid products are left behind from this reaction?
H2O(l) + CO2(aq) <-> H2CO3(aq) carbonic acid H2CO2(aq) +CaCO3(s) <-> Ca2+(aq) + 2HCO3- (aq) No solid products are left over (you get a hole)
What is the Cody escarpment? How does this feature relate to sinkhole distribution in this region?
In areas where the aquifer is deeply buried, subsidence is going to drop off. Areas west of the Cody escarpment have limestone that is very close to the surface which makes subsidence more likely.
Where is the Ogallala aquifer? What are some issues/concerns with the current utilization of this resource?
In the Midwest. It's being severely depleted (harvesting water about 10x the rate that it's being replenished).
What is the Cody Escarpment and what is its significance to the hydrogeology of North Central Florida?
It is an old coastline from when we had a different sea level. The geology of north florida has 3 main layers: underlying eocene aged limestone, miocene aged clay units that tend to form the confining layers, and more recent sands. To the east of the scarp, the layers on top of the limestone were wiped away leaving the limestone exposed (unconfined aquifers). To the west of the scarp the aquifers are confined and have all 3 layers.
Why is understanding these characteristics important when considering matters of groundwater pollution?
Its recharge zones are areas where the aquifer is unconfined and it is close to or at the surface. There are then the areas that there is the greatest potential for contaminants to enter the aquifer.
What is Karst topography? What conditions are necessary for its formation?
Karst topography is areas where the shape of the land surfaces have been strongly influenced by dissolving away (particularly by groundwater) of soluble rocks (most commonly limestone).
What are some methods for remediating groundwater contamination?
Manipulating the potentiometric surface by things like pumping. You put boundary containment wells around the contaminated areas and pump very aggressively to create drawdown to trap contamination before it spreads. You can also use vapor extractor wells (force vapors in to volatile organic compounds and extract them), bioremediation, physical stabilization, chemical stabilization (render things immobile or insoluble chemically) etc. depending on the situation.
What are the characteristics of the Floridan aquifer?
Mostly Eocene limestone with miocene clay confining unit, covers the whole state and provides 60%+ of domestic supply, state's most important aquifer, recharge zones run through the north central portion of the state (big bend area).
How can groundwater become polluted?
Nutrient pollution from runoff, gas stations with leaking storage tanks, leaks from waste-disposal sites (landfills), septic system seepage, saltwater intrusion etc.
Define porosity and permeability. How do the two differ? Are all high porosity rocks permeable and low porosity rocks impermeable? Give examples to support your answer
Porosity is the volume of open spaces in a material (rock or soil). Permeability is a measure of a material's ability to transmit a fluid (water). Permeability relates to the size and 'connectedness' of the pore spaces. Many porous materials are permeable but not always. Clay is very porous but the material size and pore sizes are very small and pores are not very connected which makes it rather impermeable. Granite is very low porosity but is fairly permeable especially via secondary porosity.
Is groundwater a renewable or nonrenewable resource? What are some possible ramifications of over utilization of groundwater resources?
Renewable but not at the rate that we are harvesting it.
What are some potential consequences with overuse of groundwater and water table drawdown. Site specific examples.
Saltwater intrusion (Biscayne aquifer), subsidence (San Joaquin).
What is saltwater intrusion? Why do we care (if we do)?
Saltwater intrusion involves saltwater coming into our aquifers and polluting them. It makes it much more expensive to harvest groundwater for drinking.
In each of the aquifer case studies presented, what was the concern in each case?
San Joaquin, CA is suffering from subsidence resulting from groundwater extraction for irrigation which lowered the surface by 10meters in 52 years. Biscayne Aquifer: overdrawing of groundwater has led to saltwater intrusion
In Florida, what conditions (relating to the thickness and type of materials overlying limestone) favor the development of cover-collapse sinkholes?
Sediment atop of limestone is moderately thick, clay-rich (impermeable), and cohesive.
What are some landforms and geologic features associated with karstified terrains?
Sinkholes, solution cavities (caves, caverns), speleothems (cave deposits like stactits, stalagmites). poorly developed surface drainage systems/patterns (few surface rivers/streams, disappearing streams). Solution valleys. Very mature terrains dominated by towering erosional remnants.
Why can't one use the local slope of the water table to determine rates of water movement in a confined aquifer system?
The water table is not defining the top of the aquifer. The geometry of the confining unit and elevation of the recharge zone is what is defining the slope of the potentiometric surface in a confined aquifer.
How are the recharge areas different between unconfined aquifers and confined aquifers?
Unconfined have much broader areas of recharge because they aren't bounded like a confined aquifer is.
How does pumping water from a well affect the water table?
Withdrawing water from a well forms a one of depression in the potentiometric surface (which in an unconfined aquifer is the water table). By locally steepening the slope of the potentiometric surface you steepen the hydraulic gradient which allows one to pump water from the well faster than the water can come in from elsewhere to replace it. This leads to groundwater depletion.
What is the difference between the zone of saturation and the zone of aeration?
Zone of saturation: area of the subsurface where pores spaces are completely filled with water. Zone of aeration: region of subsurface where pore spaces are not completely filled with water.