Groundwater engineering pt 1

what is an aquifer?

an aquifer is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells

How do climate affect chemical weathering?

More precipitation + higher temperatures genrally give rise to more chemical weathering

what is the difference between mass and material properties?

Material properties is the rock’s texture, packing, porosity. For example magmatic rocks have crystalline granular texture with no porosity while a sedimentary rock is clastic with some porosity. In contrast, mass properties depend on scale and location. Over a large scale/volume of rock, inhomogenous and fractured rock is likely to be found meaning that there will be some porosity.

what is primary and secondary porosity?

Primary porosity is porosity existing from the creation of the rock, for example sedimentary rock with low grade of diagenesis will have high primary porosity. Secondary porosity has become after the creation of the inital rock. It can be tectonic activity, pressure release. It can also be chemical weathering.

Difference between porosity, effective porosity, specific yield

porosity is the total volume of pores, while the effective porosity is the volume of interconnected pores. Specific yield is the water that drains from a saturated rock due to gravity. It is almost equal to effective porosity, but a bit lower due to capillary forces and adsporption.

connection between effective porosity, permeability and hydraulic conductivity.

Effective porosity give rise to permeability, the geological formation’s ability to conduct gravitational flow, which is independent on the fluid. Hydraulic conductivity is permeability but it also takes into account the fluid’s properties.

What three properties is required in an aquifer?

an aquifer must have porosity, effective porosity and permeability,

which are 4 types of aquifers? which can give the highest yield?

poroous, fractured, dual porosity (both porous and fractures), karsts. Porous aquifers have usually highest yield.

explain aquifuge, aquiclude, aquitard, perched aquifer

aquifuge has no porosity nor permeability such as unfractured rock that acts as a barrier for water flow. Aquiclude has porosity but no permeability and acts as a barrier for water flow such as clay. Aquitard is similar to aquiclude but allows for small seepage, for example if we have silt. Perched aquifer is an aquifer in a local water table due to something (such as a aquiclude) hindering water from continoing downwards.

What is the gw. level for unconfined vs confined aquifers called?

water table and potentiometric surface.

How can darcy’s law be applied in confined vs unconfined aquifer?

In a confined aquifer the saturated thickness of the aquifer, b, does not change with the hydraulic gradient. thus q’ = Kb* dh/dl. In a unconfined aquifer, the saturated thickness is equal to the water table which makes it more complicated. however it can be approximated with dupuit equation

what is the dupuit assumptions?

assumptions in order to simplyfy calculations for flow in unconfined aquifer. thery are

• Hydraulic gradient is equal to slope of water table

• for Small water-table gradients, streamlines are horizontal and equipotential lines are vertical

how do groundwater level fluctate and why? in which geological units are the variations most/least noticable?

Groundwater level fluctates mainly due to variations in evatranspiration. Variations are largest for aquifers in consolidated fractured rocks, then comes till and lastly sand and gravel where fluctations are small.

how is storativity defined?

Is defined as the volume of water expelled from storage per unit surface area per unit change in head. When the head in confining or saturated aquifer changes, the water will be either stored or expelled.

specific storage Ss vs specific yield Sy

Sy is the specific yield, i.e the water that is drainable to gravity. Is dominating for unconfined aquifers, usually has a value of 0.02 to 0.3. Ss, is the storage due to the compressibility of the aquifer skeleton is dominating for confined aquifers, which is much smaller, usually on scale of 0.005 or less.

what is transmissivity?

T = flow of water that can be transmitted horizontally through an aquifer per unit width under a hydraulic gradient of 1. The transmissivity is the product of the hydraulic conductivity and the saturated thickness of the aquifer

T = bK

isotrophic vs anistrophic

isotrophic - permeability is the same in all direction, while in anistrophic material we’ll have different permabilities

how would flow lines be positioned in relation to the equipotential lines in isotrophic vs anistrophic material?

in isotrophic conditions the flow lines are perpendicular to the equipotential lines but in anistrophic it is not due to arrangement of the minerals.

mention three types of heterogeneous formations

A - Variations in thickness (even if K, ne same - S and T must be the same)’

B - Layering with different hydraulic properties between the layers

C - Sediments with different hydraulic conductivity lying next to each other.

give example of some assumptions for using flow net

• Homogeneous aquifer

• Fully saturated

• Isotrophic

• No change in the potential field with time

• Incompressible soil and water

• Laminar flow, darcy eq is valid

• All boundary conditions are known

when can we use Cooper-Jacob straight line time-drawdown method?

can be used for confined aquifers with transistens, for longer pumping times in a pumping test. Drawdown is plotted as y to time in a semilogarithmic plot, then a straight line is drawn on that. Then one uses the drawdown rate of the line and the point t where the line meets drawdown =0 to find S and T.

when can we use jacob straight line method?

is a modification of cooper-jacob method where we instead have simutanious observations of drawdown in three or more wells.

What governs how fast the groundwater flows?

velocity is a function of the hydraulic gradient and porosity. It will also increase if the sectional cross area decreases, the flow Q must be constant over an area.

What is dual porosity?

Porosity occuring in both fractures and pores, for example sandstone.

Storativity in confined vs unconfined aquifers

in unconfined aquifer the storativity is approxiamately equal to the drainable porosity, the specific yield. In confined aquifer, there is no drainable water due to gravity, and the storativity instead come from specific storage due to the compressibility of the mineral skeleton, which is much smaller than the specific yield.

Explain what volumetric water content and saturated water content is

volumetric water content is the total volume of water divided with total volume of soil. The saturated water content is equal to porosity n since the volume of water is equal to volume of void spaces

what is specific retention relation to field capacity

They are the same, the water content that can withstand gravitional pull.

what is Specific yield in relation to saturated water content and field capacity

Specific yield is water drainable by gravity, and saturated water content is when all void spaces are filled with water, field capacity is the water content that can resist gravity.

Saturated water content - Specific yield = field capacity

In pumping tests, how can we simulate recharge and barrier boundaries?

We can see the boundaries as either discharging wells or recharging wells that affect the actual well. The well should be “mirrored”, i.e be placed double the distance from the pumping well. Then Q for discharge/injection should be the same as the Q from pumping.

How can one detect recharge or barrier boundaries in a aquifer from a cooper-jacobs straight line method?

Cooper jacobs is used for long time pumping tests.  Theoretically, this curve should be linear. If we have a barrier boundary, the drawdown with time will be larger than it “should” (go below the line) and if there is a recharge boundary, the drawdown will instead be smaller (above the line)

describe the 3 phases that happens when we start to draw water from an unconfined aquifer

1.  pressure drops, water is released due to elastic storage, flow is horizontal - the time–drawdown follows theis curve. Ss dominates

2. decline of water table, Therefore we will have both vertical and horizontal flow components

3.  as time progresses, rate of drawdown decreases, flow is essentially horizontal again and again follows theis type curve. Specific yield Sy dominates

What is skin effect

 Positive skin is the smearing of clay and silt on the borehole well when we install a monitoring well that decreases the conductivity. Negative skin is improved permeability due to hard well development where water is pumped in and out of the well.

in a pumping test, what is the main parameter when it comes to the radius of the influence of the pumping well?

time. Longer time, larger radius of impact

what can be determined with single well tests? which disadvantages, sources of error are there?

Single well test

• Can estimate T

• Not S

• Must consider skin effects, wellborne storage

• Turbulent flow etc

what are slug tests good for?

Slug tests are good for

• Several testings possible within short time frame

• Suitable for high conductivity formations

• Low costs

• Simple and rapid

Specific retention relation to field capacity

They are approximately the same, water that is not drainable by gravity

what is soil moisture tension?

Fluid pressures above the water table, where the atmospheric pressure is larger than the fluid pressure is called soil moisture tension

How are these different units formed? Describe the permeability, porosity, effective porosity and grain size of 

1. Precambrian rocks

2. Organic deposit

3. Lake and sea deposits

4. Flood plain deposit

5. Beach deposit

6. Glacial till

7. Glaciofluvial deposits

1. No grains. 1-2% porosity. Are igneous and metamorphic rocks

2. Low permeability. Is bogs and fens.

3. fine grain size. low/no permeability. High porosity, no effective. Formed when Glaciers melt → deposits water “far away”, when water flow is low enough these small particles can settle. Forms For example glacial clay. We can have varved layer of clay and silt → due to variance in melting of glacial ice

4. Fine size, high organic content, clay/till. Flooded streams and rivers deposit fine grained material on the plain next to the stream channels

5. Land rise after glacier → temporary coast lines or where water has been standing → sorts and transports away the material in till. Sorted material. Coarse grain size, sand, gravel. Effective porosity of 30%. Permeable

6. formed by the movement of glaciers. mix of sizes, low permeability. content of the till reflects the bedrock beneth.

7. Melt water of ice within a glacier drainage in pathways capable of large scale sediment transport

I.e eskers, Grain size is coarse. Permeable layers