Hydrogeology Flashcards
Give examples of geofluids
gas, oil, brine, groundwater
What is the difference in pores in the unsaturated and saturated zones?
unsaturated - pores = water and air
saturated - pores = water
What is the pressure compared to atm in unsaturated and saturated pores?
and at gw table
unsaturated = less than atmospheric pressure
p less than atm in unsaturated pores
p =atm at gw table
p >atm in saturated pores
where is soil water found?
in the root zone
What is a cone of depression?
the shape formed around a groundwater extraction well
What are forms of input for the gw system?
recharge usually precipitation. Some of it goes to baseflow for rivers and the rest is extracted
What is a major consequence of groundwater extraction?
subsidence
Consequences of groundwater abstraction?
increase in irrigation needs
land subsidence in unconsolidated aquifers
increasing costs to lift water from deep aquifers
quality of water deteriorates
risk of saltwater intrusion increases
ecological detereoration
How does unsustainable groundwater abstraction impact salinity
the sea level increases with respect to the inland groundwater level and this will cause easier infiltration of brackish water
3 forms of recharge
artificial (managed aquifer recharge), natural (rainwater), indirect (aquitard leakage, cross formational flow)
pros and cons of surface water
accessible, plentiful, unsteady supply, prone to contamination
pros and cons of groundwater
steady, good quality, inaccessible, difficult ot find
aquifer
subsurface formation of a porous medium that contains and transmits significant amounts of groundwater
difference between aquifer and reservoir
reservoir is the word for hydrocarbons and aquifer for groundwater
aquitard
subsurface formation that can store water and has a low transmission capacity. It retards but does not prevent the flow of water to or from the adjacent aquifer.
aquiclude
very reduced storage, cannot transmit groundwater, impermeable.
unconfined aquifer
water table aquifer or phreatic aquifer
directly connected to water level, upper layer is the water table which is free to rise and fall.
confined aquifer
Clarify the pressure wrt atm
restricted by 2 layers of aquitard. Water pressure is >atm.
artesian well
if well is drilled through superposed aquitard into aquifer, water is under enough pressure to rise. If aquiclude is is above, pressure is even higher. A spring forms. An artesian well is when water rises till above the surface.
semi-confined aquifer (leaky)
bounded by aquitard that does transmit water when hydraulic head above and below the leaky boundary are in disequilibrium. The head difference exists between aquifer A and B so water passes through aquitard to aquifer B. Aquifer B is then semi-confined.
Karstic aquifer
controlled by rock dissolution
What are the 3 forms of energy in GW?
And corresponding head
kinetic energy –> velocity head
potential elevation energy –> datum head
pressure energy –> pressure head
Kinetic energy
formula for energy and for head
gained through motion/ velocity
E=1/2mv^2
velocity head = v^2/2g
Potential Energy
measured with respect to the datum (normally NAP)
z meters above datum, particle has z energy
pressure energy
by existing pressure, measured using piezometer
h = pressure/ weight (head)
Total energy/ head
teh sum of kinetic, potential and pressure.
Bernoulli’s equation
hydraulic head (sum of energies) is constant for an incompressible liquid
simplification of total energy/head
gw has small velocity, lots of P so velocity head is removed.
Porosity
the ratio of volume of void space to the total volume. Porosity determines how much watercan be held.
determines aquifer/tard classification
amount of water to saturate fixed volume
volume for porosity
void volume/ total volume
primary porosity
developed during rock formation
large in unconsolidated rock
small in consolidated rock
very small in highly consolidated rock
examples of unconsolidated rock
sand, silt, gravel
consolidated rock examples
sandstone, siltstone, conglomerate
2 types of forcing in rock formation
burial - new sediments over old
lithification - increased pressure and temperature
secondary porosity
voids formed due to subsequent tectonic processes. Mainly in consolidated rocks - fractures develop due to movement of earth’s crust. Widened by dissolving processes (Cavities) This can be dissolution, fracturing, faulting.
other names for primary and secondary porosity
matrix porosity - primary
fracture porosity - secondary
types of rocks and their primary porosity
metamorphic igneous etc.
crystalline rocks (metamorphic and igneous) have low primary porosity
volcanic rocks have higher primary porosity
in UNCONSOLIDATED rock, what determines primary porosity
size of grain/ rock fragments - independent of grain size if same pakcing
sorting of grains
arrangement - cubic or rhombus packing
shape of grain
how does sorting affect porosity?
high porosity - well sorted
low porosity - poorly sorted
in CONSOLIDATED rock, what determines the primary porosity?
all of the above and
compaction
cementation
Dual porosity
the existence of both primary and secondary porosity. This is fairly commone
What types of porosity occur in unconsolidated and consolidated rocks
unconsolidated - primary, never secondary
consolidated - mainly dual and secondary.
compare primary and secondary porosity
secondary porosity is often less than primary but has a large impact on groundwater flow.
Effective porosity
not all pores are connected so unconnected pores don’t haelp gw flowf
formula for effective porosity
effective porosity = volume of connected pores/ total volume
unconsolidated sediments and porosity
as grain size decreases, total porosity increase but effective porosity decreases
clay and porosity
clay minerals have plate shape. etted clay stores water between plates. High porosity but it holds the water so effective porosity is lower.
specific yield
ratio or % of total volume which can be drained by gravity. Also drainable porosity. property of unconfined aquifer. The part of the water thatcan be drained when water table is lowered.
specific yield term for confined aquifer
specific storage
specific retention
water left over from draining. porosity = specific retention + specific yield.
Water is retained by capillary forces.
split into meniscus filling and pendant droplets.
IF all pores are connected, ne = Sy + Sr
Porosity diagram
go check it on brightspace right now :) slide 26 lecture block 2
how does sorting affect porosity
well sorted = higher porosity
poorly sorted = lower porosity
porous sediment - high porosity
increasing depth - decreasing porosity
What determine groundwater flow?
hydraulic gradient and permeability/ conductivity
hydraulic gradient
the slope of the water table
permeability
ability of a rock to transmit a fluid.
relationship between discharge and hydraulic gradient
linear
Discharge vs. Flux
Discharge (Q) - volumetric flow rate, volume per time
hydraulic flux (q) - specific discharge/ darcy flux [m/d]
groundwater velocity formula
hydraulic flux/ effective porosity = average linear velocity
if tube were filled with only water, hydraulic flux would = velocity
maximum gw velocity
2v
Darcy assumption
only laminar flow, no turbulent –> slow flow is closer to laminar flow.
Darcy
hydraulic flux is proportional to the hydraulic gradient where proportionality constant K is the hydraulic conductivity
hydraulic conductivity properties
dependent on….
dependent on fluid properties (density, viscosity)
dependent on medium properties
m/d –> length/time
intrinsic permeability properties
independent of fluid peroperties.
only dependt on medium properties
m2 –> length squared
what is the similarity between intrinsic permeability and hydraulic conducitivity
connectivity of pores is important ans well as tortuosity of pore network
on which factors does K depend?
permeability, density, gravity, dynamic viscosity.
Hetero vs homogeneous k
heterogeneity –> K varies spatially
anistropy
k is dependent on direction
isotropy
k is independent of direction
3 types of (K) heterogeneity
layerd, discontinuous, trending
layered heterogeneity
each layer is homogeneous and isotropic
commo in sedimentary rocks (esp. interbedded deposits of clay and sand)
arithmetic (sum kb/b) and harmonic (sum b/sumb/k) average can be calculated
discontinuous heterogeneity
due to faults or large scale statigraphic features from tectonic movement. Like shifting Dutch flag down 1 half way.
Trending heterogeneity
sorting/ graing of deposits
deltas, alluvial fans, glacial outwash plains
transmissivity
capacity of an aquifer to transmit water
T=Kb
b is thickness of saturated aquifer
storativity
Specific storage represents the volume of water that an aquifer releases from storage per unit surface area of aquifer per unit decline in the
component of hydraulic head normal to that surface.
Specific yield but for a confined aquifer.
For a confined aquifer S = Ssb where Ss is the specific storage term.
Processes that cause hydraulic head differences
topography, compaction, density contrasts
topography and hydraulic head differences
Water tables that follow the topography develop with sufficient
recharge
transient topography, e.g. glaciation, mountain building
compaction and hydraulic head difference
at depth in geologically young subsiding sedimentary basins
density contrasts and hydraulic head differences
coastal areas cope with saline intrusion, e.g. Dutch lowland
- sea-level fluctuations
- (some types of contaminants)
Measuring hydraulic head
diver in a well –> pressure transducer
flow net
equipotential lines & flow lines
equipotential –> GW depth
flow lines are perpendicular to equipotential lines
lithology
the study of rocks
grain size and effective porosity correlation
the larger the grain size, the closer the total and effective porosity become, pores are better connected.
correlation between hydraulic conductvity and porosity
linear. lower porosity –> lower conducitivty (eg. shale)
accumulation during Pleistocene and holocene ages
> 10,000 pleistocene –> east and high in nl
<10,000 holocene –> west coast low in nl
West coast drinking water
low abstraction rate, many wells, parallel to coast,
convexivity of salt and freshwater
41m freshwater = 40m saltwater. So if you increase the height, you triple the depth of freshawter, so canals are created for convexivity.
overabstraction of water near west coast
salt/ brackish water intrusion occurs, can be reduced with artificical recharge. This can only happen with wide dunes so a larger convexivity, hence a deeper freshwater system.
Ghyben-Herzberg
calculates water bubble and freshwater/ saltwater intrusion
anomalous freshawater occurences & fluctuating sea level
- steady state for current sea level
- new steady state after regression
- unsteady state during transgression
pockets occur between transgression areas.
River plains in lowland areas
braided rivers
braided rivers: high flow velocity, variable flow velocity, sedientation causes route variation.
point bars
ocean pushes back sediment deposited by braided rivers.
Describe a braided river
arid/ arctic climate, little begetation, large hydraulic gradient, strongly fluctuating, flow rate, strongly fluctuating flow velocity, large sediment load
poorly sorted coarse sediment
describe a meandering river
humid climate, vegetation present, small hydraulic gradient, uniform flow rate, uniform and low flow velocity, small sediment load.
Fine grained sediment
sediment in meandering river
fine grained, large total porosity, low effectie porosity, low Ksat
erosion terraces and drinking water supply
older sediment is high, less flow. Younger porous sediment is lower and close to river, some contamination.
west and east maas
west is young, east is old
erosion terraces, factors for groundwater occurence
thickness of saturated layer, fragmentation of terraces, interaction of terraces with river
glacially influenced areas (list of morphological units)
glacial tongue basin, ice-pushed ridge, sandr, refilled erosion gulley, esker, kame terrace, ground moraine.
aquifer/ aquitard system glacial tongue basin
both aquifer and aquitard system (multiple)
aquifer/ aquitard system ice pushed ridge
phreatic aquifer
aquifer/ aquitard systems sandr
phreatic aquifer, coarse material
aquifer/ aquitard system refilled erosion gulley
phreatic/ semi confined aquifer, sometimes aquitard/clude (dependent on material)
aquifer/ aquitard system esker
phreatic aquifer, underglacial river, very narrow.
aquifer/ aquitard system kame terrace
phreatic aquifer, coarse sediment near glacier edge
aquifer/ aquitard system ground moraine
aquitard, transported upwards with force, boulder clay.
plateau and valley landscapes
horizontal flow: baseflow and springs
confined aquifer: fully saturated
unsaturated under aquitard: purged GW
springs - just under aquifer
Fold mountains
Drainage pattern and gw
Trellis drainage pattern, possibility of gw recharge
clines in fold mountains
anticline is the n shape
syncline is the u shape
Where do springs occur in fold mountains
where the aquifer is thinner, so has lower transmissivity
layer beds and bedding plates
layer beds - limestone, thicker and harder
bedding plates - softer and thinner
porosity and water flow in fold mountains
fracture flow - no porosity in layer beds so water flows through cracks. Most cracks in the middle, where infiltration occurs, and sharpest bending.
old mountains and groundwater
weathered cover when sandy, fractures, karstic (dissolving) limestone.
River valleys (erosion terraces), refilled glacial erosion gullies.
Basins
Areas developed due to load –> compaction of sediments (porosity reduction) During compaction in subsiding sedimentary basins, increasing overburden thickness (loading) will cause
sediments to compact (porosity reduction). Compaction can only proceed when fluids escape so allowing porosity reduction (is more efficient in easily compressible rocks).
Compaction is important in driving fluid flow within subsiding basins.
Which capitals lie on basins?
London and Paris
Horst and Graben Systems
Horst is the high lying part and Graben the low lying part of a system that has sunk, with a fault between the two. Graben has the higher porosity and conductivity. Steeper thermal gradient occurs due to faults, so warm water rises. Springs occur at the faults
separation of Horst and Graben hydrological systems
layers with different conductivity next to eachother. unconsolidated sediment (clay) smears along edges of fault. darker mineral oxidse along layer. Creates 2 separate hydrological systems.
faults in consolidated rock
instead of shifts with smeared clay, you have breaks –> increased fractures, stronger karstification, fractures cause higher k. More saline water intrusion.
Alluvial fans
occur near mountainous regions, flow velocity decreases with distance from mountains (carrying capacity decreases, coarser material deposited upstream, clay downstrea, sometimes you get layers of sand and clay due to fluctuating velocity deposits at varying locations.)
close to the mountain, you have an aquifer
some veritical flow if there’s a thin layer.
From aquifer to aquifer system, to aquitard to aquiclude.
verticla to horizontal flow away from the mountain.
Deep sand layers far away have artesian conditions.
The water has good quality and is frequently recharged.
Chalk vs. limestone
chalk - young and poorly consolidated
limestone - old and stronly consolidated.
Karstification
The process wherebt carbonate outcrops to the earth’s surface is exposed to leeching and dissolution by atmospheric water.
CaCO3 + CO2 + H2O –> Ca2+ + 2HCO3-
Factors to enhance kartsitifcation
precipitation adn evapotranspiration
CO2 production
decalcified cover layer
solutble rock
fracures
relatively thin layer beds, well developed bedding planes
intensive groundwater flow
TIME –> slow process
Dolines
collapsed dolines are found
cave system –> holes in teh subsurface - > you get domes that can collapse into the bowl underneath.
If you pump out GW, steeper gradient, more flow, more kartstification, more caves
can occur along faults and fissures.
Areas with magmatic (extrusive) rocks
acid rain + granite –> poorly weathered so pH is not compensated and it decreases dow to ~4
Brown coal powerplants cause acid rain.
physical weathering of surrounding granite to leave a pile fo less fractured granite, they are granite outcrops, remnant of erosion.
layers in magmatic (Extrusive) rocks
paleosol (underneath)
scrambled egg layer (water flow)
lava flow area (polygons)
intrusive magmatic rocks
melted solid inside the earth (eg. granite)
extrusive magmatic rocks
lava cooled outside earth’s surface
cooled down quicker, fewer minerals, eg. basalt.
pyroclastic rocks
deposited during fire events (eruption)
layers are due to pyroclastc eruption sequencing - cam be aquifer or aquitard, depending on rate of cooling
tuff layer
name of different pyroclastic layers
basalt formation
shrinks and breaks in polygons (in columns without porosity of conductivity, only flow between column through faults and fractures)
total prosity 1%
hydraulic conductivity quite large –> aquifer
scrambled egg layer definitely an aquifer
red layers metamorphic rock
ne lava bakes soil/ rock underneath –> iron comes out. Resembles tropical soils dut to conditions in tertiary.
dikes near basalt
dikes are an intrusive layer, lava squeezes through the fissures, left over lava in fissure cools to form a dike.
Basalt is an aquifer and dike prevents flow ( is an aquitard)
Springs often form near these dikes.
metamorphic rocks and phyllite
high pressure and temperature, phylliite is an aquiclude. difficult ot find water, you need dams.
Intro to the North Sea basin
sinking area of land, a lot of sediment is deposited, and follows ocean currents along Dutch coasts.
Short history
dunes along coast limit river water discharge to the sea. Water level rises and peat forms.
Significant amount of sediments carried by the rivers.
before:
less sediment so less dune formation
before:
Dry North Sea
Before: Weichselian, ice did not reach NL
Before
Interglacial, very high water levels, veenendaal underwater
before:
Saalian - ice-sheet
Before
Early Pleitocene
Aquifuge
no storage or flow of water, compact rock for example
Causes of Glacials in the pleistocene
caused by Milankovitch cycles: eccentricity, Tilt, precession
Marine Sediments
closer to the surface in the East than the west
considerable faults are present
older formations in the east are steeper. Each newer formation is less steep. (can be aquiclude)
thin and continuous layers mean simliar conditions over prolonged period and space.
West vs East sand and aquifer layers
West –> Thick aquifer with lots of sand which thins towards the East. This leads to drought in the East, ddeper hydrological base is not possible. Pre -quaternary remnants can only be found in the East.
Elsterian Ice Age in NL
Friesland and Groningen have some ice.
Peizer formation- deeper incisions related to glacier tongue patterns.
Gullies are filled with boulder clay.
Abstraction wells should be in confined conditions to avoid surface contamination.
Saalian Ice Age in NL
clay sheets are visible at the surace
Drenthe formation - perpendicular flow to expectations. Folding of clay sheets to impact water flow
glacial till - deposited at lower end of glacier
Very poorly sorted
Perched Aquifer Water level
water can infiltrate and goes towards gwl
A resistant layer exists so the water stagnates above this layer.
a sort of pseudo water level
you need low resisitivity on top of unsaturated aquifer layer above the actual gwl.
Bentolite
clay that swells by a factor of 2
coversand is from the
Weichselian
holocene in NL
Water flows to polders
k vs kD
capacity of soil// an aquifer to transport water
4 spatial scales
pore (microscopic, difficult to find k), core (can use darcy) , local (aquifer/ valley), regional (whole system)
gas permeatetry on dry outcrops
how easy it is to push air into an outcrop.
piezometer scale
single borehole -> layer aquifer/aquitard system
single piezometer testing
rapid introduction/ removal of water nad measure how long it takes for water to return –> Hupsel
measuring flow in a borehold
impeller (propeller device)
tmeperature techniques using distributed temperature sensing.
Distributed Temperature Sensing
NOT THE BOILING WATER
Shoot signals through fibre optic calbes
back scattered return signals are temperature dpeendent
– data is given for 20cm segmetns along 30km every second.
Cable can also be pressure dependent. –> Can be used for geothermal energy
DTS
distributed temperature sensing
Active DTS in fractured basement rock
insert warm water in one borehole and pump out water in the otherone. So you measure temp across entire depth of 2nd borehold.
But….
water in the field in large quantities at 60 degrees isn’t easy
temperature anomaly in 2nd hole is very small. (less than 1 degree)
Aquifer testing using a well fiel
large hydraulic head difference - low k
analystical solutions to aquifer testing using a well field
Thiem adn Cooper-JAcob method
5 methods to determine subsurface properties
geophysically derived properties
seismic methods
electrical resisitivty
electromognetic
ground penetrating radar
seismic methods for subsurface properties
What do you measure?
P-wave velocity -> mapping of geological subsurface faults, water table, aquitard location.
Electrical resistivity to determine subsurface aquifer properties
What can you determine?
aquifer zonation, water, anistropy estimation
electromagnetic methods to measure subsurface aquifer properties
How do you measure this?
helicopters, like electrical. EM transmitter and receiver are suspended.
ground penetrating radar
what do you measure, and frequency debate
dielectric constant values, mapping of stratigraphy.
Bang which bounces back from water table/ aquitard.
high frequency - low depth high detail
low frequency - high depth, low detail.
Electrical resistivity techniques
measures transport capacity of electricity through rocks, varies with hydrogeo properties.
rho is the resistance over a surface per unit distance
vertical electrical sounding - 1D profiling (vertical)
permafrost can also be measured
you do need prior knowledge of the area.
GRACE
gravity recovrey and climate experiment
They have measured the depletion in groundwater from gravity field across earth.
Groundwater Recharge
Water flux that replenishes the aquifer. It reaches the water table so the water table rises
vadose zone
unsaturated zone. Water that leaves this zone and heads towards the water table is also considered recharge
Field based techniques to estimate gw recharg
- lysimeter
- chloride mass balance.
- Historical Tracers
- Water Table Fluctuations
- Groundwater Age
- Temperature
- Darcinian Principles
Lysimeter usage
measure soil and seepage, deeper than the root zone, lateral flow is not recorded, incredibly expensive, point measurement, high maintenance required, scale weighs the changes in weight.
chloride mass balance how to
precip x [Cl in P] = drainage x [Cl in soil]
D= P x [Cl in P]/[Cl in soil]
assuming conservation of mass of chlorine in the soil and water. Low recharge means elevated [Cl in soil]
can’t be used when R > 400mm/y –> Cl is too low to be measured
Historical tracers how to
Pulses from historical events (eg. nuclear bomb, chernobyl) found back in gw. So if you measure tritium (‘63) in gw, you can trace the movement. R = vtheta
(theta= average moisture content)
using mass spectometry, you find peak and date is with transport and dispersion
Preferentaial flow paths in the vadose zone can cause complications when measuring.
Measuring at the water table how to
sustainable yield concept- look at varying gw tables.
Artesian well cannot have a sustainable yield - aquifer is deflated over time
unsustainable - more abstraction than recharge.
Water table fluctuations in aquifer how to
formula for recharge
Recharge = Specific yield * deltah/dealtat
you measure the charnge in water table
air movements, groundwater recharge, ET can cause fluctuations so method only woks for shallow systems.
Groundwater Age in aquifer how to
age - time since recharge
tritium for decade determination
14-C for 50,000 years (needs correction for carbon chemistry)
36-Cl for upto 10^6
Temperature in an aquifer
temperature profile of recharge and discharge
25 degrees per km
rising water is steeper than y=x, recharge water passes underneath y=x line
Climate change causes y=x to be shifted along the x axis.
Changes in temperature measurement in aquifers
How does CC change the steady state?
from steady state to transient stage to new steady state.
reacharge will be more strongly affected as high surface temp pushed down, Discharge points react slower to rising surface tmep
We measure a change in inflection point. You reduce temp-depth profile to the depth o finflection point dependent on q. Bigger q, deeper point.
Darcinian principles in aquifers
“look at the flow”
Numerical Vadoze zone model
Provide a physical-process based estimate of VZ recharge
fluxes.
* Computational demands usually prohibit regional scale
assessments using such models (meta-models, i.e. simplified
representations), although there is examples in the literature of
this (e.g. coupling Hydrus and MODFLOW).
* Data demands are high compared to surface water balance
methods.
* Hydraulic properties of VZ are uncertain, as well potential
existence of preferential flow paths (e.g., loess recharge
study).
Measuring groundwater concluding messages
-Groundwater recharge is the driver to groundwater flow, and
varies throughout the hydrogeological system (VZ recharge,
versus deep recharge)
* We reviewed a selection of the many (field)methods that can be
used in the field to assess shallow (unsaturated) and deep
(saturated) recharge fluxes.
* Global scale studies of VZ recharge usually rely on ‘simple’ water
balance models for surface hydrological fluxes.
* VZ recharge models that are physical process based are
computationally expensive
permafrost hydrology
acts as a hydrological seal –> no recharge
use historical data for thi
continuous permafrost
average annual soil temp <= 0 for a period of at least two years everywhere. Continuous refers to the permafrost on a spatial scale (>90%)
Discontinuous permafrost
permafrost but with gaps (10-90%)
isolated permafrost
small patches of permafrost
temperature in ice sheets
temperature increases with depth in an ice sheet until 0 is reached at the base/ surface of the ice. This happens with ice >1km thick.
The base of the ice sheet is the point where it melts. Sheet acts as an insulator tfor the soil surface. Current permafrost in Hudson Bay developed after the ice sheet retreated.
Active layer in permafrost
unfrozen during the summer. Soils thaws (doesn’t melt)
How to measure permafrost temp depth profile
using fibre cable. Temperature is proportional to hydraulic conductivity
GW flow and permafrost
a lid of permafrost acts as a seal, preventing recharge to underlying aquifer. Very little discharge from this aquifer. Deep gw flow in dormant aquifers is not considered in GW models.
Where can you find average annual soil temp on permafrost graphs?
if you take it as a reflection of a 3, so like E, then the middle line.
Permafrost dating
gap in gw dating, you have a recharge gap –> dat hte permafrost periods on a spatial scale.
GW discharge sources in areas of continuous permarfrost
Perenial spring/pingo -> hot springs,
Flattening of the hydrograph during Arctic warming
In permafrost areas baseflow is thought to be insignificant as the low- permeability permafrost restricts groundwater recharge and discharge. Total discharge = surface runoff + baseflow
In areas underlain by permeable catchments (in warmer areas) a larger
proportion of discharge will occur through subsurface discharge, i.e. groundwater flow, and be active throughout the season
Base flow recession
reduction of baseflow through winter. Gradient of recession is proportional to storage of basin.
Sharp gradient - little gw storage
gentle gradient - lots gw storage
Current permafrost and base flow recession
less recession than previously –> more active gw system –> more transmissivity –> subsurface holds more water due to thawing of permafrost.
Permafrost saturation, temperature and hydraulic head
- During permafrost retreat deeper flow paths develop
- Confined conditions for the lower aquifer evolve into unconfined conditions
- Groundwater inflow to the central depression increase while the ice-table retreats
Groundwater outflow (groundwater baseflow) through time
I III lower initial surface temperature : increasing initial permafrost thickness
Generic patterns emerge (system response) while the magnitude of groundwater outflow scales directly with aquifer permeability.
Late time acceleration of groundwater outflow increase due to shallow aquifer development and the late-time disappearance of remnant permafrost at depth.
Temperature as a control on aquifer architecture
Permafrost saturation (and permeability) is directly coupled to the unstable temperature distribution in high-latitude aquifers.
Numerical models to understand groundwater flow dynamics
during permafrost degradation
- Using a simple geometry in numerical models shows a clear hydrogeological system response to the removal of permafrost as a result of surface warming.
- The time-scale of changes in permafrost distributions as a result of changes in surface temperature conditions is in the order of 100s of years (for relative thin permafrost)
- Introducing thermally unstable permafrost, and more topographic complexity significantly reduces the easy by which patterns of discharge and recharge can be interpreted.
- Uptake of groundwater into elastic aquifer storage where hydraulic heads rise substantially during permafrost degradation impact recharge and discharge fluxes.
Conclusions
* Understanding permafrost hydrogeology is challenging:
– Time responses are such that long transient
processes have to be considered in the
interpretation of field data
– Data paucity for remote now-cold regions
– Complexity of coupled processes to be
considered (heat and fluid flow, freeze-thaw
dynamis)
Direct impacts of CC on GW
declines/ uncertainties in recharge (P&ET), sea level rise (change in boundary conditions)
Indirect Impacts
increased irrigation demands on gw due to stronger seasonality in stream flow, longer droughts due tu intensification of hydrological cycle –> response of system to changes (Feedback loop)
GW storage and reduced recharge rates
dropping water table
reduction in spring flow
overabstration more likely
Sea level rise and GW
melting ice caps, thermal expansion, freshwater outflow due to groundwater abstraction.
Consequences of sea level rise
greater potential for saline gw ingress –> encroachment
Mitigation measures CC & GW
Fresh water reduction in arid areas –> managed articificial recharge, soft engineering (sand dams)
how do sand dams work?
try to store surplus water from winter in the aquifer to use during the summer.
Dams in the middle of teh valley -> sand and sediment accumulate aganst the dam. These then act as a aquifer
Advantages of sand dams
less loss to runoff and evaporation
more stable water source
regeneration of ecosystems.
Modeling Regional Groundwater Flow systems driven by climate fluctuations
look at anomalous freshwater occurences in subsea bed.
recharge during glaciation: along sandstone and shale, deeper water underneath glacier is seawaer from Devonian. This is super ld water.
Connate water
liquids trapped in sedimentary rocks as tehy were deposited.
oxygen and temperature
In cold periods O16 evaporates easier than O18 and is fixed in the ice sheets. O18 is left in the sea water and thus a relatively high concentration compared to warmer conditions. In the ocean organic carbonate skeletons are examined that have accumulated on the ocean floor. The change in the ratio O18 / O16 in deep see sediments and ice sheets are measures of climate changes.
ice and climate change
Ice pushes down, forming an area of recharge (meltwater from ice sheet). This changes the system as that location used to be an area of disharge. Water that was pushed in during last ice age however can’t escape, hence the Devonian water in the basin. Glacial meltwater from long ago is very useful.
parsimonious
using as little energy as possible with the best results
topography driven flow
based on aquifer thickness and depth, transmissivity-based model.
