Chapter 3 Flashcards
Groundwater is
is an important and significant part of the hydrologic cycle. Yet, it remains a mystery to many people
Groundwater is an important and significant part of the hydrologic cycle. Yet, it remains a mystery to many people. Because we
cannot see it directly, groundwater defies our experience of the land surface as a solid, rigid boundary marking the top of the earth
In the vast majority of intermontane basins - ……………………-
the areas where groundwater exists in greatest quantity
. In the vast majority of intermontane basins—the areas where groundwater exists in greatest quantity—groundwater
fills small, often microscopic pores between the grains of sediment
Groundwater may occur in
tiny fractures and fissures of rocks hidden below the loose, or unconsolidated, surface sediments
Groundwater may occur in tiny fractures and fissures of rocks hidden below the loose, or unconsolidated, surface sediments. To the naked eye
soils and rocks may appear next to impenetrable
To the naked eye, soils and rocks may appear next to impenetrable. But most such materials are, in fact, a
microscopic universe full of empty spaces that can be occupied by water
. But most such materials are, in fact, a microscopic universe full of empty spaces that can be occupied by water. When such water completely fills the void space of sediment pores or rock fractures, the pores or fractures are said to be
saturated
When such water completely fills the void space of sediment pores or rock fractures, the pores or fractures are said to be saturated. In all but truly ……………………areas, the ground just beneath the land surface is ……………………..: water occupies only part of the pore space
swampy
aerated
The boundary between ………………………….. and ………………………… is referred to as the water table.
vadose zone water (unsaturated)
groundwater
The zone above the water table is referred to as the
“unsaturated zone” or “vadose zone
The term “soil” describes
only the first three to six feet below the land surface that is exposed to weathering plant root growth, etc.
In cold to moderate, humid climates, the vadose zone is commonly
identical to the soil layer
In cold to moderate, humid climates, the vadose zone is commonly identical to the soil layer—that is, the water table is
less than 6 feet from the land surface
In Mediterranean and semi-arid climates, it is not uncommon to have a …………… that is ……………………….
vadose zone that is several tens of feet thick.
In Mediterranean and semi-arid climates, it is not uncommon to have a vadose zone that is several tens of feet thick. In some areas, this zone may even be
be several hundred feet thick.
While the vadose zone is not a resource from which we can obtain water, it is an important
storage area, pathway, and potential barrier for pathogens, nutrients, or contaminants traveling within the water.
To understand the occurrence of groundwater at a given site, one must understand the
local geology
Geologists usually investigate local geology by studying
the type and age of the various sediment and rock layers
Geologists usually investigate local geology by studying the type and age of the various sediment and rock layers. That information also provides
important clues about the characteristics of the groundwater system in an area.
A geologic formation from which significant amounts of groundwater can be pumped for
domestic, municipal, or agricultural uses is known as an aquifer
A geologic formation from which significant amounts of groundwater can be pumped for domestic, municipal, or agricultural uses is known as an aquifer. The term is relative: it means that a geologic unit
yields water relative to surrounding materials
A geologic formation from which significant amounts of groundwater can be pumped for domestic, municipal, or agricultural uses is known as an aquifer. The term is relative: it means that a geologic unit yields water relative to surrounding materials, but
does not indicate that a specific amount of groundwater can be pumped.
A small intermontane-valley aquifer yields
significantly more water than its surrounding hard rock (bedrock) formations.
Aquifers sometimes are …………………. separated by……………………. that permit……………………………..
vertically
geologic formations
little or no water to flow.
The formation that acts as water barrier is called
aquitard
The formation that acts as water barrier is called aquitard if it is
much less permeable than a nearby aquifer but still permits flow (e.g., sandy clay).
aquiclude
If the water barrier is almost impermeable (e.g., clay) and forms a more or less formidable flow barrier between multiple levels of aquifers
Aquifers can be of two major types
unconfined or confined
In an unconfined aquifer, there is
no overlying aquitard or aquiclude
Where multiple levels of aquifers exist,
the uppermost aquifer is typically unconfined
Where multiple levels of aquifers exist, the uppermost aquifer is typically unconfined. Vertical recharge by
infiltration of rainwater or irrigation water downward to the unconfined aquifer is therefore not restricted
The water table at the top of the unconfined aquifer can
migrate freely up and down depending on how much water is stored in the aquifer
A confined aquifer, on the other hand, is “…………………….” between ………………………………………………….
sandwiched
an aquiclude above and an aquiclude or aquitard (e.g., bedrock) beneath
As a result of “backpressure”, water in the confined aquifer is
pressurized
As a result of “backpressure”, water in the confined aquifer is pressurized. Due to the pressure, the water level in a borehole drilled into a confined aquifer will
rise significantly above the top of the aquifer
An artesian well occurs where the
pressure is so large that the water level in a well drilled into the confined aquifer rises above the land surface
. An artesian well occurs where the pressure is so large that the water level in a well drilled into the confined aquifer rises above the land surface—in other words,
the well flows freely (if opened) without pumping.
. An artesian well occurs where the pressure is so large that the water level in a well drilled into the confined aquifer rises above the land surface—in other words, the well flows freely (if opened) without pumping. Note that a confined aquifer
does not have a water table—it is completely filled with groundwater.
. By definition, a perched water table is
a saturated groundwater zone separated from the aquifer below by a zone that is unsaturated or aerated
Springs form whereSprings form where
a water table intersects with the land surface
Springs form where a water table intersects with the land surface. This may occur in
in a depression of the land surface, particularly on hillsides
Springs form where a water table intersects with the land surface. This may occur in a depression of the land surface, particularly on hillsides. It can also occur where
two geologic (rock) formations outcrop at the land surface, if the lower one is less permeable than the upper one
A spring also may form when
back pressure forces water to the surface through a sinkhole, fracture, joint, or fault zone that acts as a conduit for water movement
The size, shape, regularity, and continuity of the pore space will determine
how much water can be extracted or stored in the subsurface, how fast water can move through the pore space, and how contaminants are distributed within the subsurface.
Groundwater fills
the entire pore space, but not all of that groundwater will be available for pumping
Groundwater fills the entire pore space, but not all of that groundwater will be available for pumping. Some pores may be
entirely isolated from the rest of the pores. Other pores may be so fine that water molecules are held tightly to the soil, particle, or mineral surfaces by adhesion (magnet-like forces on the surface of water molecules that attract them to the surfaces of mineral grains, especially clay).
Adhesion …………………………………..
immobilizes water molecules
Even if pores are not fine enough for adhesion
capillary forces can hold water back against gravity drainage
Different sediments and rocks not only have different ……………………. but also ……………………………………..
effective porosities, but also different specific yields
……………………………………… also have relatively high effective porosities
Clay, silt and fine sand
The period of time since groundwater fell as rain can now be estimated by a technique based on
the amount of tritium found in groundwater
Tritium is a
radioactive isotope of hydrogen,
Tritium is a radioactive isotope of hydrogen, believed to be formed
in the atmosphere from the action of cosmic rays on ordinary hydrogen
Tritium is a radioactive isotope of hydrogen, believed to be formed in the atmosphere from the action of cosmic rays on ordinary hydrogen. Thus, tritium is found in
all atmospheric water, such as rain and snow
. As a radioactive material, tritium
gradually decays or decomposes into simpler substances
As a radioactive material, tritium gradually decays or decomposes into simpler substances, and has a known
“half-life” of 12’/2 years. That is, one half of the radioactive form is dissipated in 12 1/2 years. An additional half is lost in the succeeding 121/2 years, and so on until the amount remaining is too small to be measured.
Isotopes
forms of atoms of an element which differ in their masses
Isotopes: forms of atoms of an element which differ in their masses, due to
variations in the numbers of mass particles in their nuclei
Hydrogen has three known isotopes: the most common form has
only a proton (a relative mass of one and a single positive electrical charge) in its nucleus
a second isotope known as
“deuterium” has one proton and a neutron (neutral in charge and also with a relative mass of one) in its nucleus, and thus a relative mass of two
a third isotope known as
“tritium” has two neutrons and a single proton in its nucleus, and thus has a relative mass of three.
As the approximate amount of tritium originally present in
water as it fell as rain is known and the amount remaining can be measured, the length of time underground can be calculated unless the amount remaining is too small to be detected by the instruments currently available.
Tests of this type on deep well water from several locations in Nebraska indicated underground water ages of about
14 to 61 years
