Hydro-geology and aquifer properties Flashcards
The emergence of groundwater is determined by?
- Lithology
- Regional geological structures
- Geomorphology of landforms
- Availability of recharge sources
How much of earths water is groundwater?
- 97.25 of earths water is in the sea
- 74.9 of fresh water is locked up in ice
- most of water left is ground water
Lithological factors are related to:
Physical properties of the material:
- Porosity
- Permeability
- Transmissivity
What do the physical properties of a rock affect? And what are they referred as?
The ability, and ease at which groundwater can pass through the subsurface.
- Aquifer properties
Define porosity:
The fraction of a given volume of material occupied by void space.
What is the equation for porosity?
n = 1 - known bulk mass density / particle mass density
void ratio (e) = Volume of void / Volume of solid
n = e / (1 - e)
n = Sy + Sr
What is primary porosity?
Determined by the inherent character of a soil/rock matrix, formed during rock formation
What is secondary porosity?
Develops from physical and chemical weathering along cleavage planes and in surface weathering.
Name the factors affecting porosity in sedimentary rock and sediments
- Packing (Cubic,0.48 and Rhombic, 0.26)
- Grain sorting
- Grain shape/orientation
- Diagenesis (compaction, removal/addition of material, mineral replacement/phase changes)
What is specific yield (Sy)?
ratio of water that drains from a saturated rock owing to the attraction of gravity to the total volume of rock - like water out of a sponge due to gravity.
What is specific retention (Sr)?
ratio of the volume of water in a rock can retain against gravity drainage to the total volume of the rock.
Sr increases with:
decreasing grain size
Coarse grain rocks have large pores therefore:
capillary films occupy very small proportion of n such that Sy is almost = n
n =
Sy + Sr
Fine grain rocks have small pores therefore:
capillary forces dominate such that Sr will almost = n
Particularly in fine grained aquifers:
Not all water contained in pores is viewed as available to groundwater.
Total porosity relates to:
the storage capability of the material
Effective porosity relates to:
the transmissive capability of the material (drainable)
Darcy studied the flow of water through porous material contained in a column and found:
the total flow, Q, is proportional to the difference in water level and the cross sectional area of flow; and inversely proportional to column length.
What is transmisivity?
the degree to which a medium allows something to pass through it.
Darcy’s law equation:
Q = -KA(dh/dl)
Q = Flow rate A = cross sectional area K = hydraulic conductivity dh/dl = hydraulic gradient dh = change in head between two points separated by small distance dl
What is the hydraulic conductivity (K or coefficient of permeability)?
Measure of the ease of movement of water through a porous medium. (Length/Time)
Whats the hydraulic conductivity (K) equation?
K = -Q / [A(dh/dl)] [L^3/T[L^2(L/L)] = L/T]
Whats specific discharge
A.K.A. Darcian velocity.
V = Q/A = -K(dh/dl)
Define an aquifer:
A layer of rock or soil sufficiently porous to store water and permeable enough to allow water to flow through them in economic quantities.
Aquitard:
An aquifer with low permeability strata that transmit water very slowly.
Aquiclude:
Impermeable strata that transmit negligible quantities of water
What are major/principle aquifers:
These are layers of rock or drift that have high intergranular and/or fracture permeability.
- high level water storage
What are minor/secondary aquifers:
Formations of variable permeability which support locally important abstractions
Define a non-aquifer/unproductive strata:
Have negligible permeability and support only very minor groundwater abstractions.
Give some examples of potential aquifers
- London clay (H Po, L Pe - non aquifer)
- Mercia mudstone (M Po, L Pe)
- Sherwood sandstone (L-M primary Po+Pe, H sec Pe - major aquifer)
- river terrace deposits (H Po+Pe - very small deposits)
- coal measures (tainted water from coal, variable primary, H secondaey Pe fractures)
- lower chalk (H Po, L primary Pe, H sec Pe - major aquifer)
Name the types of aquifer:
- Unconfined
- Perched
- Confined - completely saturated
- Leaky
- Artesian conditions - spring
Changes in what cause groundwater flow?
Head
What is affected by change in Head?
- Topography
- Aquifer properties
- Recharge (injection/rainfall)
- Discharge (surface water features/groundwater abstractions)
What can groundwater contour maps be used for?
- Determine groundwater flow direction & hydraulic gradient
- Examining recharge and discharge features
- Assessing groundwater - surface interactions
How do you plot groundwater?
- Select data (water table elevation or piezometric head)
- Draw the groundwater contours (lines of equipotential) between the points
- Construct a flow net (lines drawn perpendicular to contours show flow direction)
Define hydraulic gradient:
difference in head / distance between the two points
Difference in head =
difference in water table depth
What is the equation for hydraulic conductivity using head values
Q = -KA [ha-hb) / L] K = -Q / [A(dh / dl)]
Q = the discharge rate in cm^3/sec and is negative K = Hydraulic conductivity cm/sec L = Sample length (cm) A = Sample cross sectional area h = Hydraulic head (cm) dh/dl = hydraulic gradient
What is the equation for Transmissivity?
T = bK
T = transmissivity (m^2/d) b = saturated thickness of aquifer (m) K = Hydraulic conductivity (m/day)
Chemical and biochemical interactions between ground water and geological materials provide:
a wide variety of dissolved inorganic and organic constituents in natural ground waters.
What factors affect the chemical composition of ground water?
- Chemical and biochemical provide (in)organic constituents.
- Rainfall composition
- uptake by biological processes
- mixing with sea water
Name the 6 major constituents of groundwater:
Cations: - Ca^2+ - Mg^2+ - Na+ Anions: - Bicarbonate (HCO3^-) - Cl^- - Sulphate (SO4^2-)
What is the concentration of major ions?
> 5mgL^-1
What is the concentration for minor constituents? But what can increase it?
- 01-10mgL^-1
- Contamination from human activity
What are trace constituents?
They are the rest of the periodic table
- <0.1mgL^-1
What concentration are organic compounds and why?
<0.1mgL^-1
- due to oxidation of organic matter to CO2, CO2 acidifies the water dissolving carbonate
How do ions get into groundwater?
Water percolation through rocks, limestones aren’t soluble in water but in acids
CO2 + H2O = H2CO3
Evaporite minerals are soluble in water
NaCl(s) Na+(aq) + Cl-(aq)
What does TDS stand for and why does the calculated TDS not always = measured TDS?
Total Dissolved solids.
Bicarbonate (HCO3-) breaks down to CO3^2-, CO2(g) and H2O and this results in a weight loss
- CO2 and H2O escape
How can salinity of groundwater be measured?
- measure dry weight of solids after evaporation (mgL^-1)
- Electrical conductivity as groundwater is an electrolyte solution.
Salinity of groundwater classification using TDS:
Freshwater 0-1000
Brackish water 1000-10,000
Sea water 10,000-100,000 (35,000)
Brine water >100,000
- TDS > 2000 - 3000 is too salty to drink
What is water hardness?
Presence of Ca^2+ and Mg^2+ ions in water produce a scummy residue and scale. Expressed as mgL^-1 of CaCO3
What is used as a sensible method of ground water analysis?
A “routine” analysis includes all major constituents (excluding carbonic acid) + all minor ones (except B and Sr - as they are trace elements)
mgL^-1 is related to what other unit and how?
ppm or mgkg^-1. related through the density of the solution
Molar equivalents (meq), use the molecular weights of the ions and their valencies:
meqL^-1 = mg^L-1 x (valence / atomic weight)
23mgL^-1 of Ca^2+:
= 23 x (2/40.08) = 1.15meqL^-1
Aqueous solutions must be:
Electrically neutral
What is CBE and what is its equation?
Charge balance error.
CBE(%) = [(∑cations - ∑anions) / (∑cations + ∑anions)] x 100
What are the reasons for CBE
- An important anion or cation was not included in the analysis
- Sometimes this can point out the presence of a high concentration of a unusual anion or cation
- Human error
- Systematic error
What is a hydro chemical facies?
A distinct zone of groundwater that can be described as having cation and anion concentrations with definite limits, or “water type”.
Describe the Hanshaw and Black model:
- Carbonates emerge from marine and are flushed of seawater by freshwater
- Decreasing salinity + hydrochemical facies is Ca-HCO3 dominant
- Carbonates dissolve, recrystalised, cemented dolomitized to form an aquifer
- As recharge moves downgradient temp increases dissolving gypsum, dolomite and calcite
- Increasing ions of SO4^2-, Ca^2+, Mg2+
- Whilst mixing with saline water gypsum and dolomite dissolves and calcite precipitates out reducing ∑CO2
Describe the classic evolution of groundwater:
- Rainwater - dilute solution (NaCl of marine origin, dissolved CO2
- Into soil - addition of soil CO2 and fertilizer NO3, PO4, Cl, Na, K
- Unsaturated zone - solution of Ca and Mg carbonates Ca-Mg-HCO3 water
- Saturated zone - Bacterial reduction of O2, NO3, SO4
- FOLLOW INCREASING CONFINEMENT OF GROUNDWATER
- Leaching of iron oxides; precipitation of metal sulphides
- Ion exchange Na-HCO3 water (clays)
- Saline water Na-Cl water
- Static water; aquifer diagenesis
A range of CBE values for various samples may produce certain anomalies, if this is due to missing anions why might this be the case?
The presence of phosphates/nitrates in the sample
- If cations missing then possibly due to presence of silica/iron
Which to ions are often grouped together when calculating cation and anion percentages and plotting them on piper diagrams?
Na+ and K+
Define sustainable development:
A pattern of resource use that aims to meet human needs while preserving the environment so these needs can be met in the present and indefinite future.
- Meeting the needs of the present without compromising future generations ability to meet their needs.
Whats the size of Greenland?
2,166,086km^2
What percentage of Greenland is covered in ice?
81%
Whats the population of Greenland?
~60000
Facts about Greenland:
- Worlds largest island (2,166,086km^2)
- Self-governing protectorate of Denmark
- Dependant on fishing, many Inuit still subsistence hunters and fishermen
- Tourism becoming important, mining may follow.
What makes up 80% of Greenland’s economy?
Fishing/hunting
- Climate change is opening up land (increased natural resources and farming)
How many tourists do Greenland get per year
30,000 by cruise, 35,000 by air
Why is Greenland’s tourism market so small?
- seasons are short
- accomodations are limited
- Travel is expensive
Denmark subsidies Greenland how much per year?
$620M
How long has mining been ongoing in Greenland?
~150yrs
What challenges prohibit mineral exploration in Greenland?
- Weather
- Short seasons
- Transport
- Infrastructure
- no roads
- Shipping only in summer/ light aircraft normally
Name a currently productive mine in Greenland:
Seqi Olivine Mine
Exploration in Greenland is mainly done by which companies
- Greenland minerals and energy (REE)
- Platina (precious metals)
Give some BMP rules and regulations:
- Restrictions on flying over narwhal breeding areas and seabird nesting colonies
- All companies must undertake “environmental baseline studies” ascertain the levels of flora and fauna so they can restore the mine after closure.
What does BMP stand for?
Bureau of Minerals and Petroleum.
What benefits come from mining exploration in Greenland?
- Jobs and labourers, cooks, drivers etc.
- Opened up mining school to produce mining engineers and geologists for further work
- Sanaartornermik Ilinniarfik
In Greenland in 2007:
The goverment invited Oil and Gas companies to explore for hydrocarbons offshore - Cairn energy
Name the responsibilities Cairn Energy have to Greenland:
- A program of social and environmental responsibility
- Monitoring marine life in operational areas
- Utilising local Greenlandic contractors for transport ect.
- Greenpeace protested in 2010 on one of the rigs and stopped production for a few days
What is the future for Greenland?
- They need a stronger economy
- They are the ones most effected by climate change but need money from oil
- Declining income from their fisheries
- No other potential profitable resources
