GEOL 3083 –  HYDROGEOLOGY –  FALL 2006

Study questions from first lecture (8/23/06)

Note: some of the questions below have overlap. That is, content of the answer to one question may also appear in the answer to another. This is common in all the sets of study questions.

1. Briefly describe 4 practical applications of hydrogeology.

2. When rain falls on the land, what are the basic factors that govern how much water will move along the surface as “overland flow”?

3. What is wrong with this statement?

Water that does not soak into the ground, but runs to a stream, will continue in the hydrologic cycle to eventually become vapor and fall again as rain/snow etc. But water that soaks into the ground will permanently leave the hydrologic cycle.

4. What are the typical components of the void system in the vadose zone.

5. What do the terms “saturated” and “unsaturated” have to do with defining “vadose zone” and “zone of groundwater”?

6. What is the term for lateral movement of water in the vadose zone, and what kind of condition could cause water to move laterally in the vadose zone?

7. What can cause upward movement of water in the vadose zone?

8. What is transpiration, and what is its significance in the hydrologic cycle?

9. What is evapotranspiration and what is its significance in the hydrologic cycle?

10. What is the “water table”?

11. How can we in general find the position of the water table?

This is so important, I’ll give you the answer here, even though I pointed it out in class: The water level in a well that penetrates the saturated zone will be at the elevation of the water table. (Technically to be absolutely sure, the water intake screen in the well should cross the water table. Under certain conditions a well that is screened only deep in the saturated zone may register a water level that is either above the level of the water table or below it. We’ll explain this later)

12. Groundwater will flow in the direction of __________________________________.

11. Explain why a river that is normally a gaining stream can become temporarily a losing stream during a flood condition.

12. During very high river stage conditions, such as after much heavy rainfall, “bank storage” can be created. What is “bank storage” and what happens to it after overland flow stops raising the level of a river?

13. What is an ephemeral stream and what makes it to be such?

14. What is an intermittent stream and what makes it to be such?

15. How are intermittent streams depicted on USGS 7.5 minute topographic maps?

16. What is a perennial stream and what makes it to be such?

17. How are perennial streams depicted on USGS 7.5 minute topographic maps?

18. How could you use a topographic map to determine the approximate boundaries of a “groundwater basin”.

19. State the basic relationship between surface topography and the configuration of the water table’s “topography”.

20. At a certain point along Hogwaller Creek the water level between rain falls in the spring is about 4.5 ft deep in the early spring (lots of water flowing through there). In the early fall it is less than 2 ft deep between rain falls and there is less water flowing in the stream. Regarding the local ground water situation, what can you say about the gradient of the water table in that vicinity at these two times of the year?

Study questions from 3rd lecture (Monday, August 28, 2006)

1. What is the difference between primary porosity from secondary porosity?

2. If you needed to estimate the total volume of water in a particular aquifer, what information would you need to obtain to make that estimate?

It should be pretty obvious that you need to know the aerial extent of the aquifer (that is, the extent of the aquifer in a horizontal sense) and the average thickness, and the porosity of the material that makes up the aquifer.

3. What is s porosity quantitatively? This is one simple “formula” you should know.

4. Of the following sediment characteristics, explain in what way they affect porosity of the sediment.

   packing [should contrast open vs close packing--which has better porosity?]

   grain size

   sorting [should contrast good sorting vs poor sorting--which has better porosity?]

5. How does sediment burial (i.e., deep burial vs. shallow burial) tend to affect sediment “packing”?

6. Of the following types of sand, which commonly has a lower porosity and why? [“Why?” has to do with which has better and which has poorer sorting]

--beach sand, river sand, wind-blown sand

I meant to point out that of the common ways that sand is deposited, beach processes and wind-blown dune processes tend to produce very well sorted sands. River sands are not as well sorted. Thus river sands would generally not have as high a porosity as a beach sand.

7. How about shales (rocks formed from clays)--do shales have generally high or generally low porosity as compared to sandstone? (Note that the most abundant bedrock type in the AR River Valley area and in the Ouachitas is shale. It generally is bedrock in the areas of lower elevation throughout these regions. Useful general knowledge.)

8. What kind of porosity is the main porosity of plutonic igneous and metamorphic rocks? Do these rock types in general have high porosity or low porosity?

9. In what kind of situation can limestone and dolomite have high porosities? What part of the state of Arkansas has these kinds of rocks?

(IMPORTANT NOTE: I forgot to speak of limestone and dolomite in lecture. These are important in northern Arkansas, constituting the bedrock of much of the northern part of the state, exclusive of far NE Arkansas. Both limestone and dolomite have mostly secondary porosity (fracture), though dolomite typically has some primary (including natural gas in deep subsurface in parts of the state!). So they typically will have <5% porosity. However!... because of the solubility of these rocks, dissolving of rock along fractures in many places will open up the porosity quite much, even making cavernous passage ways for water in the subsurface. ADD THIS TO YOUR NOTES.  

10. What is the difference between the "porosity" of a sediment and its "specific yield"? Also, is porosity normally > specific yield or < specific yield?

11. Know how grain size will effect specific yield. If two sediments have the same porosity but one is finer grained than the other, which will have the higher specific yield, the finer or the coarser--or no difference? (This was something I stated, but may not be as obvious in the tables—that is finer sediment will typically have lower specific yield.)

12. What prevents some water from draining from a porous medium such as sand?

Study questions from the 5th lecture (Friday, 9/1/06)

1. You should be able to write the Darcy's Law equation and know

• what each term of the equation represents
• and typical units for each parameter.

[There are very few “formulas” I will require you to learn. This, however, is one of the ones you should know.]

2. Of the various parameters in Darcy’s Law, which one reflects the water-transmitting properties of the medium (for example, sand) through which water flows?

3. Know how grain size, in general, affects hydraulic conductivity (K). (Do coarser sediments, like coarse sands and gravels, have higher K or lower K than finer sediments, like fine sands and silts?) [Note a brief glance at the handout on range of K for earth materials will tell you this. This is a very useful piece of knowledge to have in your head.]

4. Memorize the "typical" value of hydraulic conductivity (K) for "typical" fine sand (fine sand in the geologists sense, on the low end of fine sand in the engineer’s [ASTM] terminology). Know it in terms of m/d, ft/d, cm/s, and gpd/ft².

I did not give you this in class, but I will give it here. This would be an approximate value for a fairly typical fine sand. You would expect coarser sands to have higher K and very fine sands and silts to have lower):   K =

• 1 m/d
• 3 ft/d
• 1 X 10^-3 cm/sec
• 20 gpd/ft²

5. Compared to other physical parameters, does the hydraulic conductivity of natural earth materials tend to have a narrow range of values or a wide range of values?

6. Know how sorting, in general, affects hydraulic conductivity (K). (Do well sorted sediments tend to have higher or lower K-values than poorly sorted sediments

7. Would you expect a sand with well rounded grains to have better, poorer, or about the same hydraulic conductivity compared to a sand with angular grains?

8. If doing hydrogeologic work in a coastal region, what factor or factors will possibly affect the hydraulic conductivity parameter you use in calculations involved in groundwater flow? (that is, factors besides the nature of the sediment or rock through which the water flows)

9. For quantitative purposes, what is the best way to determine the hydraulic conductivity of an aquifer? Why is this method better?

10. Regarding that best method to get an accurate determination of K, what is the disadvantage of this method?

[I forgot to mention this. The problem with doing the pumping aquifer test is that it is rather expensive. There must be a well with a pump installed and a monitoring well also. Then someone needs to pay a hydrogeologist (hopefully) lots of bucks to be out there to perform the test and evaluate it.]

11. Which is generally a better means of obtaining hydraulic conductivity of an actual aquifer, a slug test or a pumping aquifer test? Explain why. [Remember that some questions I will put have overlap or get at same thing from a different wording.]

2. By what do you recognize an “artesian well”.

3. What is the difference between an "artisan well" and a "flowing well"?

4. What kind of situation with regard to the potentiometric surface will cause a well that is screened in a confined aquifer to be a “flowing well”?

[relate the flowing well to the potentiometric surface—that is, that the potentiometric surface is above the land surface at that point]

5. What is the relationship between the water table the potentiometric surface in an unconfined aquifer?

6. In constructing a potentiometric surface map of a water-table aquifer in a particular area...

a) how would you use existing wells to obtain data for the map of the water table?

b) how would you use surface streams to obtain data for the map?

7. How do we determine the potentiometric surface in a confined aquifer?

8. What is the minimum number of wells necessary to estimate the flow direction and hydraulic gradient of an aquifer in any given location?

[This directly relates to what I described as the ADEQ minimum requirement when contamination investigations are being done and to the homework problem we discussed.]

9. What is the difference between transmissivity and hydraulic conductivity?

10. You should also know the formula for transmissivity (T=Kb) and what each parameter is. 

11. When considering the transmissivity of aquifers, how does the thickness factor differ in the unconfined aquifer as compared to the confined aquifer?

 Study questions from the 8th lecture (Monday, 9/11/06)

1. In class we discussed transmissivity of an aquifer in terms of (1) isotropic vs anisotropic and (2) terms of homogeneous vs heterogeneous. Which of these two issues has to do with conditions at a single place, and which has to do with conditions over a range of places?

2. In considering an aquifer's transmissivity, it is important whether the aquifer is homogeneous or heterogeneous. What two main factors must be constant throughout the aquifer in order to consider the aquifer homogeneous with respect to transmissivity? [Consider the formula for T.]

3. Illustrate two different situations in which sedimentation factors can cause an aquifer in unconsolidated sediments to be heterogeneous in its K value (and hence in transmissivity).

[This question is not about the actual sedimentation process that causes the variation in K, but about the way in which the K values are unevenly distributed—we sketched a couple of examples on the board].

4. Two different geologic formations are described as sands. One was deposited as a beach deposit by a receding coast line. The other was deposited by a river system (fluvial). Which is likely to be more heterogeneous with regard to transmissivity?

[Note: I meant to mention in class that beach sands tend to be the same laterally over greater areas than river system sands. That is, river sands tend to be rather heterogeneous in a lateral sense as compared to beach sands.]

5. Describe a factor, other than original sedimentary factors, that can make a hard rock aquifer be heterogeneous?

6. Besides homogeneous vs heterogeneous, an aquifer may also be "isotropic" or "anisotropic" with regard to transmissivity. What does isotropic/anisotropic mean with regard to an aquifer's hydraulic properties?

7. In fractured rock aquifers, what is the most common factor that can introduce anisotropy to the aquifer’s transmissivity? [describe briefly]

8. Which normally have higher storage coefficients, confined aquifers or unconfined? [See the handout on Storativity.]

9. If equal amounts of water are withdrawn from a confined aquifer and a water-table aquifer, which will experience greater lowering of head?

10. What two factors are involved in releasing water from storage in a confined aquifer that is being pumped? That is, where is the water coming from, since the whole confined aquifer remains saturated while it is being pumped? [Note: see the handout and/or book.]

11. In an unconfined aquifer, the storage coefficient is for all practical purposes_______.

a) equal to the hydraulic conductivity

b) equal to the transmissivity

c) equal to the porosity

d) equal to the specific yield

[NOTE: I did not mention in class, but actually in unconfined aquifers there is also a minor amount of water that is released from storage by virtue of water expansion and aquifer “skeleton” compaction, just like in the confined aquifer, but it is essentially negligible because it is so little compared to the specific yield.]

12. Does the kinetic energy due to the momentum of moving groundwater contribute significantly to its total energy? If so, why? If not, why not?

13. What is meant by the term “elevation head”? [A simple definition: Elevation head is an expression of the potential energy of a parcel of water due to its elevation, from the force of gravity.]

6. If I schematically sketch a set of nested piezometers showing water levels in them, be able to tell me what the nested piezometers indicate about groundwater flow. [Note: If you understood the example discussed in class you will realize the implication here will be as to whether there is an upward component of flow (head in deeper piezometers is higher than shallower piezometers) or a downward component (head in deeper piezometers is lower) or no vertical flow (all piezometers have same total head).]

7. Why must piezometer measurements of hydraulic head in saline aquifers be adjusted by a correction factor? Also, what part of the total head must be adjusted? [In case you did not get it, we pointed out that the pressure head must be adjusted because the density of saline water is greater than “fresh” water.]

8. If a well is in an aquifer and has the lowermost 20 ft of the well “screened” (i.e. have slots for allowing water to enter the well pipe), and if we want to use the well to measure hydraulic head, how does the well differ from a piezometer in the head it measures?

This ends material

that will be covered in the first lecture test!

Still part of Lecture 9, but covered on next test:

9. Why must the equation for flow velocity of water in an aquifer (called average linear velocity) include the sediment’s porosity as a parameter?

Study questions from the 10th lecture (Monday, 9/18/06)

1. Considering the actual velocity of water moving in typical groundwater aquifers (such as fine to coarse sands) with common hydraulic gradients, what kind of rates are typical? ______

(a) fractions of a meter/day, (b) meters/day, (c) 10's of meters/day, (d) 100's of meters /day.

[The example problem on “average linear velocity” points to the answer for this.]

2. What kind of flow does Darcy's Law assume?______ (a) turbulent flow, (b) laminar flow

3. Why is it likely that the assumed condition for Darcy’s Law (mentioned in question 2) actually occurs in nature?

 [The answer to this question is that given the very slow movement of water demonstrated by the problem on average linear velocity, it is quite reasonable to assume that flow is laminar through the porous medium, rather than turbulent.]

4. What happens close to a pumping well that may cause the flow of water in the aquifer to not faithfully follow Darcy’s Law?

[In class I pointed out that very close to a pumping well the gradient can become very steep as water flows into the well. Your answer to this question should convey the understanding that a steep gradient will mean a faster flow velocity (take a look back at the equation and see the effect of gradient on velocity). Faster velocity can lead to turbulent flow, annulling the direct applicability of Darcy’s Law.]

5. In layered subsurface situations (such as sediments or sedimentary rocks) with alternating permeable (aquifers) and semi-permeable (aquitards) zones, what is the term used for the change in direction of flow where flow lines encounter the boundary between layers of different hydraulic conductivity?

6. Describe the typical movement of groundwater in aquifers and in aquitards in layered sedimentary sequences. (Describe the movement in terms of the relationship between flow lines and aquifer/aquitard boundaries).

[Here I am not looking for the mathematical expression, but the general, typical flow directions in aquifers and aquitards as related to aquifer/aquitard boundaries.]

7. Which of the statements below are true (T) and which are false (F).

_____The principle flow equations that fully model flow through porous media (such as groundwater) are algebraic expressions.

_____The principle flow equations that fully model flow through porous media (such as groundwater) are differential equations (calculus).

_____Most practicing hydrologists dealing with ground water work extensively with differential calculus to solve problems involving groundwater flow.

_____ Most practicing hydrologists dealing with ground water work mainly with algebraic expressions that have been derived from applying differential calculus to situations with limiting assumptions.

_____Flow nets are a method that applies principles of a graphical solution to a complex differential equation (the “LaPlace Equation”)

_____Flow nets are outdated applications that were developed by engineers that are so oversimplified that they bear little resemblance to real hydraulic flow conditions.

_____Computer software that solves hydrologic equations is the only method that is recognized today as having any value in solving hydrogeologic problems involving groundwater.

_____Computer software that solves hydrologic equations is valuable in that the software models can be used to solve for groundwater flow in complex situations that would be difficult to solve for because of the complexity of the calculations that would be required.

Study questions from the 11th lecture (Wednesday, 9/20/06)

1. Under steady-state conditions, in which kind of groundwater system (confined aquifer or unconfined aquifer) does the potentiometric surface tend to be more planar (ie. constant slope over an area)? And in which does the potentiometric surface tend to have curvature (ie. changing slope over an area)?

Planar (constant slope) ____________________________

Curved (changing slope)____________________________

2. Considering a water-table aquifer, how does the hydraulic gradient close to a discharge point normally compare to the hydraulic gradient of the same aquifer at a distance farther from the discharge point? (In another approach to the question, in which of the two areas does the slope of the water table tend to be steeper?)

3. _______In which kind of aquifer, confined or unconfined, can Darcy’s law can be applied to calculate the amount of flow through the aquifer if certain requirements are met?

a) confined aquifers

b) unconfined aquifers

c) both kinds of aquifers

d) Darcy’s law cannot be applied to any kind of aquifer no matter how many requirements are met

4. In the kind of aquifer that Darcy’s law can be applied to, which of the conditions below meet the requirements for applying Darcy’s law for calculating the amount of water flowing through?

a) a heterogeneous aquifer

b) a homogeneous aquifer

c) an isotropic aquifer

d) an anisotropic aquifer

e) aquifer of constant thickness

f) aquifer in which thickness varies

Study questions from the 12th lecture (Friday, 9/22/06)

1. What part of the subsurface is called the "vadose zone"?

 [Questions 2-6 relate to “Brief word about soils” handout.]

2. What mineral constituent that occurs in many different rock types is the most resistant to weathering and therefore is left behind in the soil profile (not referring here to clay minerals in shale).

3. What mineral is the most common constituent of sand and silt?

4. What group of minerals is the weathering product of most minerals other than quartz? [I did not mention this in class, but when we say “clay” here (the answer to this question), “clay” refers to a group of minerals called clay minerals. Different clays can form under different conditions and different starting minerals, but most minerals that constitute rock weather to some kind of clay mineral.]

5. What rock type(s) essentially dissolves away during the weathering process? What kind of mineral commonly exists in this (these) rock type in small quantities and becomes the main constituent of the soil profile?

6. For the questions listed here as #6, I am not referring to specific geographic locations, but to the general situation.

·        What typically comprises the solid material in the "A" horizon of a soil profile?

·        What typically comprises the solid material in the "B" horizon of a soil profile?

·        What typically comprises the solid material in the "C" horizon of a soil profile?

·        What typically comprises the solid material in the "D" horizon of a soil profile?

7. Which part or parts of the soil’s A, B, C, and D horizons are included in the vadose zone? Explain your answer.

8. If a soil in a certain place was called a "sandy loam", what is meant by the term "loam"? [See lower right of figures handout first page, and I also made a comment about the term “loam” in class.]

[Handout “Soils in different areas of Arkansas” will help with Q 9 & 10.]

9. The most common constituents of soil are sand, silt, and clay. These tend to be derived from weathering of the underlying bedrock and each tends to have greatly different hydraulic properties. Considering the following bedrock types, what kind of soils generally tend to develop?

Put correct letter in each blank below. (a) dominantly clay (± silt), (b) dominantly sand, (c) mixture of sand and clay (± silt)

____ Over limestone and dolomite bedrock

____ Over sandstone bedrock

____ Over shale

____Over interbedded thin sandstones and shales

____ Over the igneous rocks in Arkansas such as the syenites and related rocks near Benton and Little Rock and at Magnet Cove (east of Hot Springs).

10. If we know the answers to the previous question, it is then useful to know what are the most common rock types in various parts of the state in which we live, so that we can have an idea what kind of materials we might expect in the vadose zone at any particular location. Be prepared to answer the following questions about Arkansas.

a) What are the main rock types found in the northern part of the highlands region of Arkansas?

b) What are the main rock types found in the Arkansas River Valley region?

c) What are the main rock types found in the Ouachita Mountains region?

d) What are the typical subsurface materials of the “Cretaceous” outcrop region of Arkansas, just south of the Ouachitas?

e) What are the typical subsurface materials of the southernmost region and the eastern portion of Arkansas?

Answers to #10:

a) carbonates (dolomite and limestone)

b) shale + sandstone (in valleys and ridges respectively)

c)shale + sandstone (in valleys and ridges respectively) + chert (includes Arkansas Novaculite)

d) unconsolidated clay and unconsolidated sands (+ marl)

e) sands + clay/silt 

11. What are the major constituents that occur in typical vadose zone void spaces?

12.  In what way is soil “air” (soil gas) different from common atmospheric air?

13. Why does the percentage of CO₂ in soil gas tend to be much higher than in the atmosphere? [Note: this question is similar to Q 12, but is more on the “why” side of the issue. I spoke quickly here. The reason I gave is that any organic matter present in the soil when it is oxidized, that is decaying, produces CO₂ and H₂O]

 

Study Questions, Lecture #13 (Monday, 9/25/06)

1. What causes water to rise in the capillary fringe? (That is, what is the source of the forces that cause water to rise there?)

2. In general, will the capillary fringe be higher in finer-grained soil, in coarser grained soil, or no difference?

3. The strict definition of the water table involves water pressure. What is the general distribution of water pressure in the vicinity of the water table (i.e. below the water table, at the water table, and above it in the capillary fringe)?

4. Water in a well that is screened across the water table will have water standing in it_____

a) with the top of water in the well as high as the capillary fringe

b) with the top of water in the well at the level of the water table

c) with the top of water in the well at a level below the water table

[Note: The answer to this question can be obtained by studying the figure on the 2^nd page of the Vadose Zone lecture handout (figure from USGS Water Supply Paper 2220). IMPORTANT:  This question in no way deals with the situation covered in Fig 7.2 of the handout from the beginning of the Regional Groundwater Flow lecture that we began at the end of class. In Fig. 7.2, piezometers open only at their bottom are what are illustrated. In that figure, if the piezometers shown were instead wells screened across the water table (as in the question here) then the situation shown in the figure on the 2^nd page of the Vadose Zone lecture handout would apply to each of the “wells” that are shown—that is, the water level in the well would stand right at the water table elevation. I recommend that you add these comments into your notes in that section of the lecture.]

5. What is the "belt of soil moisture"?

6. What is the significance of a situation in which the capillary fringe intersects the belt of soil moisture?

7. In what kind of setting is the capillary fringe more likely to intersect the belt of soil moisture? (Name some factors that would increase the likelihood that the capillary fringe would intersect the belt of soil moisture).

[There are at least three factors—basically they will deal with the grain size of the sediment, the proximity to discharge areas, and whether the area is forested, or grassland or cropland. You need to comment on each of these factors and how they will affect whether the capillary fringe would intersect the belt of soil moisture.]

8. What is "field capacity" and what is the significance of the field capacity of the vadose zone in any particular area (i.e. with regard to water moving toward the water table)?

9. Does the groundwater system become recharged every time it rains? Give an explanation "why" with your answer. Your answer should include the role of “field capacity” and soil moisture content.

10. In the generally humid climate of the central through eastern US, during what general time period does groundwater recharge most commonly take place?

11. When do groundwater levels tend to decline in the generally humid climate of the central through eastern US?

12. What are the most significant factors that make certain seasons to be the time of groundwater recharge and other seasons not to be times of recharge? Include in what way these factors influence whether or not recharge will occur.

Note: this answer should deal with season influence on evaporation and on plant growth, which combined will influence water being able or not able to build up in the void system of the vadose zone.

13. If recharge stops, why do water levels in wells tend to start to drop, even in areas where no human extraction of water is occurring?

I stated this, but you may not have picked it up—water continues to discharge into streams, etc, as well as to plants that grow near the water.

14. If a liquid pollutant were spilled and began to soak into the ground, what would be the difference in its route downward through the vadose zone in a uniform medium-grained or coarse-grained sand as compared with a stratified sand with some layers of finer sand?

15. What is/are the main source of recharge to the ground water system in the arid western states in the areas of lower elevation?

[Note: In class I mentioned this, but I want to impress you with certain facts. In most areas, the water table is quite deep in this region. Consequently, the small amounts of rain fall and snow fall that blanket these lower elevation areas just does not build up the soil moisture through that thick vadose zone sufficiently to exceed field capacity, so you do not get aerial recharge. Rather you get water soaking into the ground through the beds of surface “loosing” streams that get their water from the mountains as the main source of recharge.]

16. Regarding water-table aquifers in areas of humid climate, indicate either a, b, or c for the following statements or questions. The choices are:

a) recharge areas

b) discharge areas

c) either recharge or discharge areas

d) neither recharge nor discharge areas

In ____ flow lines (for groundwater flow) generally converge.

In ____ flow lines (for groundwater flow) generally diverge.

____ are characterized by the presence of springs, streams, lakes, seeps, or generally moist ground.

In the central part of ____ there is a large vertical component to groundwater flow.

In ____ groundwater has a downward direction of flow.

In ____ groundwater has an upward direction of flow.

In ____ total head increases with depth.

In ____ total head decreases with depth.

In ____ the elevation of the water surface standing in a deep water-table piezometer is commonly lower than the actual water table. [an important thing to know]

In ____ the elevation of the water surface standing in a deep water-table piezometer is commonly higher than the actual water table.  [an important thing to know]                                            ADVANCE \d4

17. _____In humid climate zones, where do we usually find the depth from land surface to the water table to be the deepest?

a) in topographic high areas (b) in topographic lows (c) in either equally (on the average)

18. We normally consider "flowing" wells to only be associated with confined aquifers. In what situation can an unconfined aquifer produce a flowing well?

19. What is a “groundwater divide”?

10. What does the term “connate water” refer to?

 [In looking at my notes while preparing these study questions, I noticed that I forgot to cover this. You will see it mentioned on a figure in the handout (the one showing the kinds of residence times one can expect for water in shallow vs deep flow systems). Connate water, according to its original definition is water that was in the sediment at the time the sediment was deposited and is “trapped” along with the sediment. Some have shifted the meaning and said we should use “fossil water” for the definition above and use “connate water” for water that has been in the subsurface for substantial amounts of geologic time (no quantity placed on the amount of time—I would guess at least some thousands of years). However, according to the actual meaning of the word “connate” outside of geologic context, the original definition should be retained.]

11. Under what circumstance would a series of sediment layers that are defined as several geologic units ("formations") be lumped together as a single hydrologic unit?

12. Under what circumstance would a single package of sediment that is defined as one geologic unit ("formation") be divided into two or more hydrologic units?

13. Name a confined aquifer in southern Arkansas that is a single geologic unit, but has been split into two hydrologic units by hydrogeologists. (The same formation is also an important aquifer in parts of Louisiana and Mississippi).

[We discussed in class—an important unit to remember in Arkansas—the Sparta Sand.]

14. When there is a regional flow system involving multiple layers, including some layers of lower hydraulic conductivity, and involving a single regional discharge area (for example a river), which statement below is correct?_____

a) Water from the uppermost permeable layer in the system above any semi-confining layers will normally contribute the most water to the discharge zone.

b) Water from deeper, semi-confined layers will normally contribute the most water to the discharge zone.

c) Water from each layer in the system will contribute approximately the same amount to the regional discharge zone.

4. What is possibly wrong with this statement?: “We have put surface runoff barriers around that hazardous chemical spill site. Therefore we have eliminated any threat to that wetlands area that is nearby.” (Explain what exactly the remaining potential threat is. Don’t just jot down a couple of phrases.)

This concludes the material subject to the second lecture test

5. Concerning aquifers in an alluvial (river) valley hydrologic system, the drilling of a well may encounter aquifer and aquitard material. What part(s) of the alluvial system formed the aquifer deposits?__________  (choose from a, b, c)

and what kind(s) of sediment comprise the aquifer material? __________ (choose from d, e, f)

            a) channel deposit,  (b) point bar deposit,   (c) flood plain (overbank) deposit

d) silt and clay         (e) sand and gravel      (f) limestone and dolomite

6. In an alluvial (river) valley hydrologic system, the drilling of a well may encounter aquifer and aquitard material. What part(s) of the alluvial system formed the aquitard deposits? _______ and what kind of sediment comprises the aquitard material?_____

            a) channel deposit,  (b) point bar deposit,   (c) flood plain deposit

d) silt and clay         (e) sand and gravel      (f) limestone and dolomite

7. In the alluvial (river) valley systems, the aquifers mentioned in Q 5 tend to be _________

a) extensive in aerial extent,

b) limited in aerial extent,

c) irregular in aerial extent (i.e. some of them extensive, some of them limited)?

8. Geographically, where do we find "alluvial valley" hydrogeologic conditions?

Study questions from the 17th lecture (Wednesday, 10/4/06)

1. Where in Arkansas is "alluvial valley" hydrology most important?

2. Why are alluvial sediments in some areas of Arkansas so thick? Explain briefly how this came about.

[Note: this has to do with the Pleistocene glacial period, sea level, and post-Pleistocene developments]

3. What part of Arkansas’s "alluvial aquifer" has, on the average, the coarsest sediment?_____

a) the upper part

b) the lower part

c) on the average, the entire thickness is about the same.

4.  Considering the "basin and range" terrane of the southwestern US, in very general terms what are the subsurface materials of the two main components of this terrane, the basins and the ranges, like? (i.e. with regard to whether the subsurface materials are commonly solid, hard rock, or generally unconsolidated sediments).

5. In the alluvial basins of the US southwest, what part of an individual basin tends to have the best aquifer material? ______ And give a brief explanation why.

(a) the margins adjacent to mountain ranges,

(b) the center, at the farthest distance from the adjacent mountain ranges.

6. In the alluvial basins of the US southwest, what is the usual general direction of groundwater flow?_______

(a) from the basin margins toward the center

(b) from the basin center toward the margins.

[Note: in one of the figures I gave as a handout, on the overhead projection screen I had arrows showing flow of water in the subsurface from the basin margin toward the interior. This is the common general flow pattern. I  don’t know if the arrows are on your handout. I pointed out, however, that since the flow is from the coarse material into the fine, low K material, it is rather slow.]

7. Rancher Frank drills a well in an area of the alluvial basins of the US southwest. He gets a pretty good amount of water at 250 ft depth, but the water quality is not too great because of a high degree of dissolved solids. His friend advises him to have the driller keep going because he might get better water if he drills deeper. Tell him here what you know about water quality with depth in these areas so he can make an “educated decision” about whether to spend more money up front with deeper drilling with the prospect of getting better water deeper down.

8. In the alluvial basin terrane of the southwestern US, is the hydrologic system of each individual basin isolated from the hydrology of other basins? Explain.

9. In the alluvial basin terrane of the southwestern US what kind of streams are the most typical, gaining or losing streams? [Although we did not discuss this, most streams are loosing streams because the area has semi-arid climate. In the higher reaches of the alluvial fans adjacent to the mountain ranges the streams can be gaining for at least part of a year, but almost universally they don’t have to go far before they become loosing streams. In rare instances, like shown in a handout we did not discuss, this situation can be reversed for limited stretches of a stream.]

10. In the glaciated central region of the US (ie. previously glaciated), surficial geologic maps will show the distribution of "glacial till". What is glacial till like, and is it generally of low or of high permeability (does it usually constitute an aquifer or an aquitard)?

11. Some geologists have noted that the hydraulic conductivity of glacial tills is commonly (lower, higher--Choose one) in actual field conditions than when samples are measured by a permeameter in the lab.  Explain your answer.

[I forgot to mention in class that in a short course I attended there was an engineer who had worked on a project in which there were glacial deposits. They had taken samples of a glacial till and had permeability run on them using a permeameter in a lab. They designed their project using hydraulic conductivity determined in the lab—very low of course—but then when the well system was put in, it behaved as if the till were much more permeable (though not a bona-fide aquifer, still much leakier than they expected). The instructor was an expert who happened to be from Minnesota. He explained that often in tills a shrinkage fracture system (joints) can develop making the wider scale hydraulic conductivity to be much higher than an individual sample would indicate. That is, there often can be some “secondary porosity” associated with glacial tills.]

12. In the glaciated central region, some areas have extensive deposits of "glacial outwash" material. What did these sediments come from? What kind of materials are they generally made of? And are they in general good aquifers or are they generally aquitards?

13. _____Regarding glacial deposits in the "glaciated central region" of the US, which of the following three statements is more correct? Also, explain why.

a) In glacial deposits, aquifers and aquitards tend to be a fairly simple stacking of laterally continuous layers. Single aquifers are usually very extensive in lateral continuity.

b) In glacial deposits, aquifers and aquitards tend to be a fairly simple stacking of layers. However, single aquifers are usually very limited in lateral continuity.

c) In glacial deposits, aquifers and aquitards tend to be rather complex in their lateral extension. Some aquifers may be rather extensive, but others can be laterally discontinuous.

14. Which hydrogeologic region characterizes the hydrogeology in the deeper subsurface beneath the glacial deposits of the Glaciated Central Region?

15. The glacial deposits of the Glaciated Central Region are typically_____.

a) on the order of 20 ft or less thick

b) on the order of 100 ft or less thick

c) close to 500 ft thick on the average

d) over 1000 ft thick in most places

9. At the fresh-water/salt-water interface in the subsurface of coastal areas, does the fresh ground-water tend to overlie or underlie the saline ground-water? (Even though the interface is not horizontal, there is a common occurrence of one somewhat over the other.) Also explain why.

[This question is basically applying the concept illustrated in the previous one. As for why, you should point out how fresh water is less dense than salt water (which occurs by soaking in from the ocean) and that the head on land is at elevations above sea level while under the ocean it is just sea level.]

10. What is “passive” salt-water encroachment, and why can withdrawals of groundwater in the areas near the coast lead to passive salt-water encroachment?

11. What is “active” salt-water encroachment, and what condition is necessary for this process to take place?

12. In which situation is the fresh/salt water interface in the ground water system more likely to extend farther out to sea, in the case of an unconfined aquifer or in a confined aquifer?

13. What geologic formation is known as the "High Plains Aquifer"?

14. What kind of depositional processes deposited the sediment of what is known as the “High Plains Aquifer”? What kind of sediment is it? And is it consolidated or unconsolidated?

15. Is the "High Plains Aquifer" a confined aquifer or an unconfined aquifer for the most part?

[Note: I did not specifically state in class, but if you consider the figures that were shown, it should be evident that this aquifer is a water-table aquifer (unconfined).]

16. Briefly describe the hydrologic crisis that is occurring in the high plains area.