GLY 3882 exam 2

Specific discharge has been found to be 3.7 x 10-3 ft/day in an aquifer. Effective porosity is 0.14. What is the groundwater velocity (in ft/day)? Answer to 3 decimal places. Example for specific discharge = 1.5 x 10-3 ft/day and effective porosity of 0.11. velocity = specific discharge/effective porosity v=q/ne = 1.5 x 10-3 ft/day / 0.11 = 0.014 ft/day.

0.026

Specific discharge has been found to be 6.7 x 10-3 ft/day in an aquifer. Effective porosity is 0.05. What is the groundwater velocity (in ft/day)? Answer to 3 decimal places. Example for specific discharge = 1.5 x 10-3 ft/day and effective porosity of 0.11. velocity = specific discharge/effective porosity v=q/ne = 1.5 x 10-3 ft/day / 0.11 = 0.014 ft/day.

0.134

In a Darcy experiment, h1=14.4 cm, h2=11.7 cm and the distance between the two piezometers is 10.1 cm. What is the magnitude of the hydraulic gradient? Answer to two decimal places Example: h1=21 cm, h2=11 cm, and distance, d, = 20 cm. The magnitude of the hydraulic gradient=(21 cm-11 cm)/20 cm= 10 cm/20 cm =0.50.

0.27

In a Darcy experiment, h1=13.6 cm, h2=8.2 cm and the distance between the two piezometers is 12.5 cm. What is the magnitude of the hydraulic gradient? Answer to two decimal places Example: h1=21 cm, h2=11 cm, and distance, d, = 20 cm. The magnitude of the hydraulic gradient=(21 cm-11 cm)/20 cm= 10 cm/20 cm =0.50.

0.43

In a Darcy experiment, specific discharge is 0.08 cm/s and the hydraulic gradient is 0.17. What is the hydraulic conductivity (in cm/s)? Answer to two decimal places (for example 0.10 or 0.02) Example for q=0.005 cm/s and i=0.05 K=q/i = 0.005 cm/s /0.05 =0.10 cm/s

0.47

In a Darcy experiment, specific discharge is 0.07 cm/s and the hydraulic gradient is 0.13. What is the hydraulic conductivity (in cm/s)? Answer to two decimal places (for example 0.10 or 0.02) Example for q=0.005 cm/s and i=0.05 K=q/i = 0.005 cm/s /0.05 =0.10 cm/s

0.54

In a Darcy experiment, specific discharge is 0.17 cm/s and the hydraulic gradient is 0.2. What is the hydraulic conductivity (in cm/s)? Answer to two decimal places (for example 0.10 or 0.02) Example for q=0.005 cm/s and i=0.05 K=q/i = 0.005 cm/s /0.05 =0.10 cm/s

0.85

In a Darcy experiment, h1=17.2 cm, h2=10.3 cm and the distance between the two piezometers is 5.2 cm. What is the magnitude of the hydraulic gradient? Answer to two decimal places Example: h1=21 cm, h2=11 cm, and distance, d, = 20 cm. The magnitude of the hydraulic gradient=(21 cm-11 cm)/20 cm= 10 cm/20 cm =0.50.

1.33

If groundwater velocity is 6 ft/day, how many years will it take for the water to travel 24000 ft? Answer to 1 decimal place. Example: if groundwater velocity is 0.5 ft/day and distance is 30,000 ft: time = distance/velocity = 30,000 ft/0.5 ft/day =60,000 days To convert to years: 60,000 days divided by 365 day/year =164.4 years

11.0

Groundwater at a well located 67000 m from the recharge area of a confined aquifer is dated to be 5 000 years old. What is the groundwater velocity in m/yr? Answer to one decimal place. Example: If groundwater is 20,000 years old at 30,000 m from the recharge area, the velocity would be 30,000 m/20,000 years = 1.5 m/year.

13.4

If groundwater velocity is 4 ft/day, how many years will it take for the water to travel 20000 ft? Answer to 1 decimal place. Example: if groundwater velocity is 0.5 ft/day and distance is 30,000 ft: time = distance/velocity = 30,000 ft/0.5 ft/day =60,000 days To convert to years: 60,000 days divided by 365 day/year =164.4 years

13.7

An aquifer has a cross-sectional area of 34 km2. The specific discharge is 0.6 km/year. Calculate the discharge (Q) in km3/year. Provide your answer to 1 decimal place. Example for a cross sectional area of 45 km2 and a specific discharge of 0.1 km/year. Q=q x A =45 km2 x 0.1 km/yr = 4.5 km3/yr

20.4

Groundwater at a well located 63000 m from the recharge area of a confined aquifer is dated to be 8 000 years old. What is the groundwater velocity in m/yr? Answer to one decimal place. Example: If groundwater is 20,000 years old at 30,000 m from the recharge area, the velocity would be 30,000 m/20,000 years = 1.5 m/year.

7.9

A stream receives water from runoff and groundwater from an unconfined limestone aquifer. The stream has problems with bacteria that are likely from human waste. There are four septic systems that could be the possible source. Which of the following would be most effective in determining which septic system allows rapid flow to the stream?

Add a different dye tracer to each septic system and see how quickly they reach the stream. *Because the aquifer is limestone and unconfined, caves are likely. So, if possible, dye tracing is the most conclusive method to find a connection with the stream*

Darcy found that groundwater discharge increases proportionally (linearly) with:

All of these: Cross-sectional area hydraulic gradient hydraulic conductivity Discharge = hydraulic conductivity x area x hydraulic gradient

Which of the following is true concerning contaminants and contaminant movement?

All of these: Dissolved contaminants are carried by flowing groundwater through advection. Saltwater is not concerned a DNAPL because the ions are dissolved in water. Disperson is generally greater in the direction of flow than in other directions.

In the Bangladesh aquifer, it has been observed that extracted water becomes younger after a well starts to be pumped. Which of the following could provide a reasonable explanation for why that occurs?

As you pump water from a well, you increase the hydraulic gradient. This means that groundwater is flowing more quickly and younger water will enter the well. *Most likely, the pumping causes more rapid flow and brings young water downward. This can be a problem if there is contamination at or near the surface*

Which of the following would be least useful for dating groundwater in the Nubian aquifer system?

CFCs

An aquifer has a range of groundwater ages between 5 years and 5000 years. Which of the following radioactive isotopes or other tracers would NOT be useful for dating the age of recharge?

Chlorine-36 *Review the half lives and ages in Module 6*

Which of the following would be the most logical order for creating and using a groundwater model for prediction?

Collect field data and observations, build the groundwater model, calibrate the model using the data, use the model for predictions. *You'll need to know the area being modeling before you can build the model, so observations should be collected first. You'll also need hydraulic head data before you can calibrate a model. Calibration has to occur before prediction*

Which of the following would be likely to change the hydraulic conductivity of an aquifer?

Dissolution and precipitation of minerals through chemical reactions *Dissolution of minerals will remove some of the aquifer material and increase hydraulic conductivity. Precipitation of minerals could fill in pore space and reduce hydraulic conductivity*

If the countries of the Nile River basin followed the Prior Appropriations doctrine, which country would have priority during shortages?

Egypt *Egypt start diverting and using the water first, so would be considered a senior user*

If 200 cubic feet per second (cfs) of water enters an aquifer and 150 cfs leaves the aquifer, 150 cfs must be added to storage in the aquifer.

False *200 cfs-150 cfs = 50 cfs, Therefore 50 cfs is added to storage, not 150 cfs*

A collapse sinkhole forms gradually as the ground surface dissolves.

False. *A cover-collapse sinkhole forms when the roof above a dissolution-created cavity can no longer be supported and collapses suddenly into the cavity*

Plastic deformation occurs most commonly in sandstone aquifers when groundwater is mined.

False. *Clays compact a lot more than sand or gravel or lithified rock like sandstone. Groundwater mining of aquifers with clay layers/lenses will likely result in plastic deformation of those clay beds*

Eutrophic lakes indicate that humans have added nutrients.

False. *Eutrophic lakes can occur naturally and are not by themselves indicative of human involvement. But, some lakes can be turned into eutrophic lakes through an increase in nutrients and productivity (eutrophication)*

Unless you dive, climb, or drill into a cave, it is impossible to locate caves.

False. *Geophysical methods, such as seismic refraction, ground penetrating radar (GPR), or gravity studies can be used to locate caves underground*

After long-term groundwater mining ends and the hydraulic head rises, subsidence will cease and the land surface will return to its original elevation.

False. *Groundwater mining lowers hydraulic heads lower than ever before, increasing effective stress beyond past levels, and permanently altering the aquifer. Therefore, much of the subsidence will be permanent*

Because groundwater modeling has been developed, it is no longer necessary to collect data and observations from aquifers.

False. *Groundwater models can help with predictions for the future, but are constructed and calibrated using data and observations*

Old groundwater that has been isolated from the atmosphere generally has very low dissolved solids.

False. *If groundwater has been in an aquifer for a long time that means it has had a lot of interaction with the aquifer material and therefore likely has high dissolved solids*

In settling conflicts between states, prior appropriations governs water use in the west and whichever state started using the water first has senior rights.

False. *Interstate water compacts are usually made to solve conflicts between states. Permanent divisions are made based on beneficial use*

Groundwater mining refers to removing the aquifer sediment, bringing it to the surface, and squeezing the water from the sediment at the land surface.

False. *Mining of groundwater occurs when, over the long term, groundwater storage is decreasing due to pumping*

In most states, any water that has at some point been underground is considered groundwater and is subject to the groundwater allocation system rather than the system used for surface water.

False. *Most states consider any water on the surface to be "surface water" regardless of whether it spent time underground. In some locations, water underground might be allocated as surface water if it is in an underground river or "tributary groundwater"*

Subsidence is the most important consideration for planning sustainable groundwater use in a basalt aquifer.

False. *Most subsidence problems are related to compaction of clay or decay of organic soils. Neither situation applies to basalt aquifers*

Both primary and secondary drinking water standards are legally enforceable by federal law.

False. *Only primary drinking water standards are enforceable by federal law. Secondary standards are not enforceable by federal law, though states may decide on their own that they will enforce them through state laws*

As organic matter decays, it contributes more dissolved oxygen to the water.

False. *Organic matter decays lowers dissolved oxygen until it is depleted and then other electron acceptors (nitrates, etc) are used*

The main problem with human and animal waste is a high concentration of phosphates.

False. *Phosphate is generally not a major factor in waste. The most critical problems are generally bacteria and viruses. The next is eutrophication of surface water from nitrates*

Hydraulic heads in the Nubian aquifer have been dropping for the last 8000 years due to pumping.

False. *Pumping is very recent. For most of the 8000 years, hydraulic heads dropped because there was little recharge and groundwater continues to discharge to oases*

If Florida's groundwater were governed by the "rule of capture", residents of the Suwannee Basin could demand that Jacksonville reduce pumping because Jacksonville is capturing the Suwannee Basin water.

False. *Rule of capture allows users to pump from beneath their land and does not consider where the groundwater would have gone*

The Clean Water Act focused primarily on groundwater.

False. *The CWA focuses on surface water*

Hypoxic conditions are common in small, fast-moving streams due to the turbulence shaking the oxygen from the water.

False. *The turbulence keeps the water in contact with the atmosphere, so prevents hypoxic conditions*

Groundwater mining does not create any problems until the aquifer is empty.

False. *There are many problems that occur. Wells go dry and require more energy to pump. Water quality can be poorer at depth. Subsidence can occur*

Microscopic and macroscopic variations in flow velocity due to heterogeneity and branching of flow paths cause sorption of dissolved material.

False. *This causes dispersion of dissolved contaminants. Sorption is when contaminants with low solubility attach to (or within) solids in the aquifer*

Groundwater that connects to surface water is considered "tributary groundwater" and is treated as part of the surface water resource in some states that use the Riparian Doctrine.

False. *This is true for some states that use Prior Appropriations*

Best Management Practices (BMPs) were established to help agricultural areas minimize runoff or infiltration of water contaminated with synthetic organic compounds (TCE, PCE, DCE, VC).

False. *Water from agricultural areas is most likely contaminated with pesticides, herbicides, insecticides, and nutrients (N and P)*

Wells are often purged before sampling to ensure that the water is fully mixed with oxygen.

False. *Wells are purged to remove stagnant water from the well and to ensure the water sampled is representative of groundwater within the aquifer*

Under the riparian doctrine, senior users have priority for water rights.

False. This refers to the prior appropriations doctrine.

Chlorine-36 is a radioisotope useful for dating water that is 10,000 years old.

False. Chlorine-36 has a half-life of 300,000 years so wouldn't be useful for 10,000 year old water. Carbon-14 would be better.

If the hydraulic conductivity of an aquifer is 1 m/day, groundwater in the aquifer will travel 365 m in a year.

False. Hydraulic gradient must be taken into account, as well as porosity. The groundwater velocity depends on the effective porosity and the specific discharge. The specific discharge is a function of both hydraulic conductivity and hydraulic gradient.

Water flow in streams generally obeys Darcy's law.

False. Stream flow is generally turbulent and does not obey Darcy's law.

Because effective porosity of a sediment/rock is less than 1, specific discharge is always larger than velocity.

False. q=v*ne, so if ne is less than 1, v will be greater than q. For example, if q=10 m/day and ne=0.5, the velocity will be 10 m/day / 0.5 = 20 m/day

Which of the following best explains how groundwater in the confined upper Floridan aquifer could be thousands of time older than water at the springs along the Suwannee River?

Groundwater flows slowly through confining units.

Which of the following is true of water budgets and pumping?

If 300 cubic feet per second (cfs) of water enters an aquifer and 150 cfs leaves the aquifer, the volume of water in storage must increase by 150 cfs.

As water migrates from the land surface and into limestone, what would you expect to happen to its specific conductivity?

It would increase due to increasing total dissolved solids. *As the water slowly dissolves the surrounding calcite, the total dissolved solids will increase which will show as an increase in specific conductivity*

A lake receives water from runoff and groundwater from an unconfined alluvial aquifer. Water flows out of the lake into stream and is also lost to evapotranspiration. The lake has suffered eutrophication. Three years ago, all the farms in the watershed began best management practices to reduce nitrates. Unfortunately, there has been little improvement in the eutrophication. Which of the following is the most likely explanation?

Nitrates are probably entering with groundwater, which could have residence times of decades or more. *If the nitrates are entering with groundwater, there could be a significant lag time. Other possibilities exist. For example, the nitrates could be from other sources (lawns or septic tanks) or nitrates might not be the limiting nutrient in this lake*

Which of the follow is true of nutrients in surface water?

Nitrogen or phosphorus are commonly the limiting nutrient on productivity in natural systems.

When sampling a well, you should:

None of these

Which of the following are responsibilities of Florida's water management districts?

None of these

Which of the following is true concerning water contamination?

None of these: *Lawns have been found to be an insignificant contributor to water contamination. *The most common contaminant type introduced from leaky underground storage tanks is pharmaceuticals. *Superfund provides federal money to clean up all contamination sites. (none are true)

Which of the following is true of dams and levees?

None of these: (Levees have been found to increase the risk of earthquakes in the vicinity. Both dams and levees are used to store excess water to be used later. Dams often result in the accumulation of sediment downstream of the dam. )

The riparian doctrine allows users to depend on a predictable amount of available water.

Not true, because users may need to reduce use during shortages.

Which of the aquifers below could NOT have been dated with a combination of tritium, CFCs, and carbon-14?

Nubian *a million years old would be beyond 14C and require 36Cl or 81Kr*

Which of the following settings is most likely to have subsidence due to excessive pumping?

Pumping the upper Floridan aquifer where it is confined by the Hawthorn group. *Clay is the most compressible material, so look for the only answer choice that has some clay*

Which of the following is true of discharge, specific discharge, and velocity?

Q=specific discharge x cross sectional area (none)

To help artificially recharge an aquifer, treated wastewater is sprayed onto the land surface. Which of the following is true?

Some of the water will be lost to evapotranspiration.

As organic matter decays, how will dissolved oxygen be affected?

The concentration of dissolved oxygen will decrease. *The redox reactions associated with the decay of organic matter uses up oxygen as a first choice. Only once oxygen is depleted are other oxidizers used*

Water rights cannot be traded.

This is false. Some locations allow buying and selling of water rights.

Because some parts of Bangladesh have a water table that sometimes reaches the surface, there is the potential for recharge to increase due to pumping.

This is true, and increased recharge has been found to occur in some regions

If the countries of the Nile Basin followed the prior appropriations doctrine, Egypt would not have to reduce its use to allow Ethiopia to use more water.

This is true. *Because Egypt historically used most of the water, they established rights as the senior user. Note: International water law is very unclear on this topic and has some conflicting aspects*

If the Nubian aquifer's hydraulic gradient was larger in the past, the discharge would have been proportionally larger.

This is true. Darcy's law states that Q=KA x hydraulic gradient. So if the hydraulic gradient is larger, the discharge will be larger by the same amount. Note: Hydraulic conductivity and cross sectional area will not change.

Krypton-81 is a radioisotope useful for dating water that is a million years old.

This is true. The half life of 81Kr is 229,000 years.

Disperson is generally greater in the direction of flow than in other directions.

True

In Florida, the water management districts are responsible for monitoring and managing the water supply, including approval of consumptive use permits.

True

Oligotrophic lakes have low productivity.

True

Plastic deformation is irreversible and elastic deformation is reversible.

True

Confined aquifers elastically compress and expand as water is removed from or added to storage.

True *Adding water to storage increases fluid pressure and causes the aquifer to expand. Removing water from storage decreases fluid pressure and causes the aquifer material to compress*

If a well near a gaining stream is pumped at a high enough rate, this pumping can turn the gaining stream into a losing stream.

True *The pumping well will capture water that would have otherwise flowed to the stream. A reversal in hydraulic gradient will force stream water to flow into the aquifer and be captured by the well, making it a losing stream*

Darcy's Law might be incorrect for flow in a cave system.

True, if flow in caves is very fast, it will likely have turbulence and Darcy's Law may not be correct in this situation.

If everything else is the same, an aquifer with higher hydraulic conductivity would have a shallower and broader cone of depression than an aquifer with lower hydraulic conductivity.

True. *Because water can more easily reach the well, the cone of depression can spread out more. This results in a shallower and broader cone*

Groundwater models are based on Darcy's law and a water budget.

True. *Darcy's law is used to calculate flow between blocks of the model, and a water budget is used to compare inflow and outflow to each block to see how much water storage changes (and hydraulic head changes)*

Oxygen concentrations in newly recharged water tend to be higher than in older groundwater.

True. *Dissolved oxygen tends to decrease as groundwater ages*

Caves can form either due to carbonic acid in recharge, or due to sulfuric acid in upward flowing groundwater from below the aquifer.

True. *Epigenetic dissolution occurs near the surface when downward flowing water has high CO2 concentrations and results in carbonic acid. Hypogenetic dissolution occurs deeper when upward flowing groundwater contains hydrogen sulfides and results in sulfuric acid*

Pumping water into an aquifer and then removing it when needed is termed aquifer storage and recovery.

True. *Groundwater is stored in the aquifer when there is an abundance of rainfall/surface water and then recovered at a later time when there is a water shortage*

One purpose of an aquifer test is to test the hydraulic conductivity of the aquifer.

True. *Hydraulic conductivity and the storage properties of an aquifer are found by conducting aquifer tests*

Decreasing the pH of water can cause trace metal concentrations to increase because they are less likely to sorb to the solid phase.

True. *Metals sorb less strongly in more acidic waters*

Incongruent dissolution results in the formation of a new solid, while congruent dissolution removes a mineral and increases porosity.

True. *Minerals dissolved through incongruent dissolution will form new solids, typically clay minerals. Congruent dissolution only dissolves the mineral and leaves empty space behind*

Negative effects of pumping can include effects on surface water and subsidence.

True. *Pumping can capture water that would otherwise flow to surface water and negatively impact the water level in the surface water body. Reduction in hydraulic head due to pumping can cause subsidence. Other negative effects could include pulling in contaminants and effects on other pumping wells*

In some cases, water laws have evolved to encourage conservation and protect in-stream uses.

True. *The prior appropriations doctrine used to have a "use it or lose it" mentality but in some cases has since been updated to promote conservation. Also, "beneficial use" of prior appropriations, and "reasonable use" of the riparian doctrine now consider in-stream uses*

Under the riparian doctrine, you could be required to reduce usage if a new farm next door wanted water.

True. *The resource is shared, so if there are more users, each might have to reduce use*

As the reservoir behind a dam fills with water, the risk of landslides and earthquakes in the vicinity can increase due to rising hydraulic head.

True. *The rising water pressure will decrease effective stress and decrease shear strength*

New sources of water could include imported water, desalination, and reused wastewater.

True. *These are all examples of gaining "new" usable water, either through importing from elsewhere or cleaning up the water that is there*

Industry contributes contamination through spills and through waste disposal practices.

True. *This can be both legal and illegal/improper disposal. Spills can occur due to pipeline problems, failure of storage tanks, and during transfers*

High turbidity in well-water may be a sign that there is fine-grained sediment in the well.

True. *Turbidity is a measure of suspended solids in water. If groundwater from a well has high turbidiity it means there is too much fine-grained material in the well. Well-development can remove these fines from the well and surrounding filter pack*

Water infiltrating through waste produces leachate.

True. *Water infiltrating through waste will pick up any soluble chemicals within the trash. The resulting mixture of water and chemicals from the waste is called leachate*

Groundwater can flow from low to high elevation if the pressure difference is larger than the difference in elevation.

True. Groundwater flows from high to low hydraulic head. If the pressure difference is great enough, groundwater can flow from low to high elevation.

Specific discharge times cross-sectional area = total discharge

True. q=Q/A so Q=qA

You and your colleague are tasked with determining how long your city has until the regional aquifer will be depleted from pumping. The aquifer currently has a saturated thickness of 100 ft. Based on the aquifer storage properties and the current volume of water removed from the aquifer by pumping, your colleague calculates that hydraulic head will decline at a rate of 0.5 ft/year. Based on this, your colleague plans to tell your boss that there's nothing to worry about because the aquifer won't be depleted for another 500 years. Why is your boss incorrect?

all of these Population and water demand could increase into the future so the rate of drawdown will increase as well. The head decline will not be uniform throughout the aquifer but be localized in cones of depression. Therefore the aquifer may be depleted near pumping wells much sooner. You disagree with your colleague's math. You calculated 200 years until the aquifer is depleted.

Old groundwater that has been isolated from the atmosphere and taken deep flow paths is likely to have:

experienced warmer temperatures

A contaminant spill occurs in a basalt aquifer. In what part (or geologic material) within the aquifer would the contaminant advect most quickly?

flow tops

A contaminant spill occurs in a sandstone aquifer. In what part (or geologic material) within the aquifer would dissolved contaminants advect most quickly?

fractures *The material with the largest pore spaces will have the highest hydraulic conductivity and dissolved contaminants will move fastest*

A pipeline is being considered that would cross the High Plains aquifer carrying a dense non-aqueous phase liquid. A spill occurs. What is the primarily control on the migration direction of the DNAPL and key information should you gather?

gravity; you should determine the subsurface geology to detemine where the DNAPL might pool on top of clay layers.

Which of the following is NOT an advantage of surface water storage (reservoirs) over aquifer storage?

less likelihood of landslides or earthquakes *Both surface water and aquifer water storage can induce landslides and earthquakes.*

Which of the following is true of cones of depression due to pumping?

none of these *Unconfined aquifers release more water per change in hydraulic head because the pores can be drained. So the cone of depression will be shallower for an unconfined aquifer*

The barrel shows the spill location for a LNAPL. Where would it be most likely to pool?

none of these; it would stay near the water table

Mammoth Caves in Kentucky has multiple cave levels but is very far from the ocean. Cave formation is affected by changes in the river level rather than changes in sea level. The river has been eroding downward through time. Assuming that the Green River has consistently eroded a deeper channel as time passes, you should expect the cave ages in Mammoth Caves to be:

oldest at the top and youngest at the bottom *The water table would drop through time and deeper levels of caves would be younger than shallower caves*

In addition to being affected by water table elevation, cave formation would be most likely to focus in:

soluble layers that begin with higher hydraulic conductivity *Zones of high hydraulic conductivity will focus flow because it is the path of least resistance. This creates a feed-back loop since more dissolution will occur within these zones of higher flow, which increases the pore space and hydraulic conductivity even more*

Tritium is most useful for recharge <100 years.

true

Caves often form in recharge areas because that is where the groundwater is _______________________ with respect to calcite. Decay of organic matter can play an important role in dissolution because it contributes ________________ to the water, which forms carbonic acid and increases the solubility of calcite.

undersaturated; carbon dioxide *Newly recharged water contains very little dissolved calcite, which makes it undersaturated with respect to calcite. A solution that is undersaturated with respect to calcite has the power to dissolve more calcite until it reaches equilibrium. Carbon dioxide produced in the soil zone will dissolve in water and lower the pH which makes calcite more soluble*