hydrogeology Flashcards
Hydrogeology
studies how water gets into the ground (recharge), how it flows in the subsurface (how groundwater flows through the aquifer), and how it interacts with the surrounding environment (soil sediments and rocks)- as well as contaminants.
Groundwater
water that exists underground in saturated zones beneath that land surface
Aquifers
water-bearing permeable geological formation (porous sediments and fractured rocks) that contain water or allow water to pass through them
Importance
jjuly 28th 2010: UN General Assembly saw that access to clean water and sanitation as a human right.
freshwater represents about 2.5% of all water on earth and is mostly trapped in glaciers and snow packs.
of the remaining 0.77%, 2% is available as surface water, while (*% is available as groundwater.
groundwater represents the main- the only source of drinking water- especially the surface water resources are contaminated.
groundwater plays a key role towards meeting sustainable development goals.
4 main components of Groundwater system
vadose zone: unsaturated zone, including the soil which may contain water within it but not too much- interconnected voids contains both water and air.
saturated zone: where all interconnected voids are completely filled with water.
water table: the surface on top of the saturated zone where the pressure of the water is equal to atmospheric pressure.
capillary fringe: variable thickness which is part of saturated zone, where groundwater seeps up by capillary action to fill interconnected voids. (in the vadose zone)
Aquifer system
a collection of aquifers, aquitards and aquicludes, which constitute the groundwater environment.
Saturated thickness
the distance from the water table to the base of the aquifer
Aquitard
geological formation that slow down the groundwater flow. they act as a barrier for groundwater by separating aquifers and partially disconnecting the flow of the water underground, although water cannot flow very fast through an aquitard, significant quantities of water can seep through aquitards in some conditions.
Recharge
the physical process whereby groundwater is replenished by “new” water entering the saturated zone
Recharge of confined aquifers
can only happen where they outcrop (usually at a higher elevation, many kilometres away from where they are confined) or from leakage (downward seepage) through upper aquitards/aquicludes (over a very long period of time)
Hydraulic Head
the work required to transform a unit mass of fluid from an arbitrarily chosen state to the current state
3 types of potentials
Pressure potential= work required to raise the water pressure.
Elevation potential= work required to raise the elevation.
Kinetic potential= work required to raise the velocity.
Darcy’s Law
Darcy’s Law can be used to describe the groundwater flow through porous media, and states that the discharge (q) per unit area is proportional to the difference in hydraulic head between 2 points, ad is a function of the permeability of the aquifer.
only valid for laminar flow.
Groundwater Flow
Groundwater always moves from where the total head is higher to where the total head is lower “regardless of the geometry of the aquifer in which it is flowing”.
Hydraulic Conductivity and Permeability
permeability is an intrinsic property of the aquifer, which depends on the aquifer’s solid skeleton and microstructure and is independent of the physical properties of the water.
this means that when considering saltwater or highly contaminated water with different fluid specific weight and fluid dynamic viscosity than pure water.
permeability remains constant whilst hydraulic conductivity changes.
Hydraulic Conductivity and Permeability. PT2
permeability decreases with depth- the pores and cracks in rocks and sediments at great depth are closed or greatly reduced in size because of the weight of overlying materials.
hydraulic conductivity is a function of the degree of saturation but in this case this can be ignored given that the aquifer is fully saturated.
Total and Effective Porosity
Total porosity includes the fraction of pore space that is interconnected (called “effective porosity”) and porosity due to isolated pores- theoretically range between 0 and 1.
HC and P are related to the porosity of the aquifer- but only interconnected pores through which groundwater move, are actually contributing to them.
ratio of the interconnected pore volume to the total bulk volume is defined as Effective Porosity
Specific Retention and Specific Yield
Effective Porosity= Specific Retention + Specific Yield.
Specific Retention: the volume fraction of water that is held back by adhesion and capillary forces, when an aquifer is drained.
Specific Yield: amount of water that is actually available for groundwater pumping, when sediments or rocks are drained due to lowering of the water table- depends on the distribution of pores, their shape and grain size.
Specific Retention and Specific Yield
larger the surface area of the rocks or sediments per unit volume, the ore water adheres to these surfaces- this decreases the specific yield in favour of specific retention.
fine-grained sediments have the largest specific surface area, highest specific retention and the lowest specific yield.
coarse-grained sediments have a very small specific surface area, hence the specific retention is small and the specific yield is large.
SY decreases with depth due to compaction of sediments.
Darcy and Seepage Velocity
based on the darcy law, it is possible to calculate the groundwater velocity, dividing the discharge by the cross sectional area perpendicular to the flow section
darcy velocity is not groundwater velocity since it is not flowing through cross sectional area but only through a portion of it
Transmissivity
productivity of an aquifer, defined as its potential to sustain various levels of groundwater flow and/or borehole abstraction, is a function of its transmissivity- transmissivity is a proxy for the amount of water that can be extracted from an aquifer
Lab and Field tests
hydraulic conductivity can be experimented through laboratory and/or field tests.
field methods provide more accurate and reliable measurements than lab tests and empirical formulas given that they allow to reproduce the physical processes of water movement- driven by Hydraulic Conductivity in the actual conditions in which it happens.
furthermore- in field tests, disturbance is kept to a minimum level and they can be performed over a larger, more representative area or number of samples
Lab and field test pt2
double ring infiltrometer= simple instrument used for determining water infiltration of the soil
rings are partially inserted into the soil and filled with water, after which the speed of infiltration is measured.
limits the lateral spread of the water during the infiltration
infiltration rate is expressed in terms of volume of water per ground surface and per unit of time
Piezometric surface
a piezometric surface represents the spatial distribution of hydraulic heads of an aquifer
Hydraulic Heads also often referred as Piezometric Heads or Piezometric Levels, can be measured in wells and/or piezometers reaching the saturated zone.
This is done by measuring the distance between the topographic surface (or the top of the well-casing) and the water table, and then subtracting it from the corresponding topographic elevation.
piezometers= small tubes
Potentiometric Surface
Confined (Artesian) Aquifers are under pressure exceeding that of atmospheric pressure, so when they are penetrated by wells the water rises above the point at which it was originally found and may even rise above the surface without the aid of a pump.
The piezometric surface may also be called the potentiometric surface 🡪 indicates the potential of the water in each point
Piezometric Campaign
Identify the scope (eg, conceptual model of the aquifer/guiding contaminat monitoring activities) and scale of the campaign (eg., regional/local).
Conduct an accurate literature review of the study area.
Identify already existing monitoring points that can be used.
Use startigraphic sections and well logs to identify the aquifer each monitoring points is referring to.
Make sure that the monitoring points are homogeneusly distributed (denser in areas of higher interest).
Measure the extension of the well and piezometer casings above the ground surface.
Switch-off any well-pump at least 15-minutes before taking any measurement (to make sure to measure the static level of the water).
