GEOL3003 - Hydro Gradient and Hydraulic Engineering Geology

Assignment Task

Questions

1. Contaminant migration exercise

In the practical on Lake Gnangara, we found that groundwater discharges into the lake from all sides. As the proposed petrol station is located upgradient of the lake, there are concerns about the potential for organic contaminants from the petrol tanks to impact the lake (if a leak occurs). You are tasked with calculating some useful information so that remediation measures can be designed and implemented. The distance between the petrol station and the lake is 800 m. Assume the hydraulic gradient between the proposed station and the lake in the aquifer is the same as between bores 1s and 2s. The hydraulic conductivity of the superficial aquifer in the area is estimated to be 2.2 x 10 ^-6 m/s with an effective porosity of 0.2. The superficial aquifer has a saturated thickness of 8 m and can be regarded as homogeneous and isotropic.

• Compute the volumetric rate of groundwater discharge for 1m width as well as average linear velocity between petrol station and the lake in m/day.
• Calculate the time (in days) it would take for the contaminant to migrate from the petrol station to the lake. Note that there would be a spread in velocities in reality and the velocity might be slower considering retardation by aquifer materials. Equally, the velocity could be increased by pumping along the flow path. All that need not be taken into account but is spelled out here for your learning.
• The hydraulic conductivity for the same lithological unit can vary by an order of magnitude. Give two reasons for why this can be.
• You have been asked to conduct the same analysis and calculate the average linear velocity and travel time for two scenarios:

2. GW simulator

In the fishtank prac, all the experiments with red dye indicate a flow from right to left. But there was a moment I reverted the flow so that it went from left to right. How did I achieve that? Did flow reversal affect the lower or upper aquifer or both and why? Does it actually matter?

3. GW and mining

Remember we had a large diamond mine that was abandoned and a smaller diamond mine nearby being flooded by torrential rainfalls. When you really think about it, the larger mine should have equally been affected by climate events. So let’s re-do Q2e of the practical and ask again what the long-term evolution of water levels in the larger pit would be if it had been flooded right before the pumps were switched off. Assume similar conditions as for the small mine, i.e. same catchment area and rainfall but to spice things up, assume a runoff coefficient of 67% this time (i.e. lots of compacted soil around the large pit due to dense heavy vehicle traffic). For full marks I need to see (correct) calculations (3 marks) as well as some discussion along the lines of the original.

4. Seawater intrusion

A water supply well has been constructed at a location with a groundwater elevation of 5 m above sea level before pumping begins. The base of the well is 60 m below sea level. When the well is pumped at half its design rate, the groundwater level near the well stabilizes to 4 m above sea level. When the well is pumped at its design rate the groundwater level near the well stabilizes to 3 m above sea level. Use the Ghyben-Herzberg approximation introduced in the practical to answer the following questions about water supply in a thick unconfined aquifer in a coastal location.

• How far does the saltwater interface rise when the well is pumped at half its design rate?
• How far does the saltwater interface rise when the well is pumped at its full design rate?
• At the design pumping rate, how far below the base of the well is the saltwater interface?
• Assuming drawdown is proportional to the pumping rate, could the well be pumped at twice its design rate without the water produced from the well becoming saline? Reason your answer.

5. Well Testing

The thickness of a horizontal, confined, homogenous, isotropic aquifer of infinite areal extent is 60 m. A well fully penetrating the aquifer was continuously pumped at a constant rate of 100 m ³ /d. The drawdowns observed after 120 minutes of pumping in a series of fully penetrating observation wells are plotted below against the distance of each well from the production well.

• Use the distance-drawdown plot and the equations provided to calculate the transmissivity (T), hydraulic conductivity (K) and storativity (S) of the aquifer. Give the transmissivity in m ² day ^-1 .
• At what distance from the pumping well is the drawdown in the aquifer <2m>
• How many wells were observed? (1 mark)
• What is the maximum drawdown you could observe from this well set-up after 2h? 
• Name several measures to increase the drawdown within the time frame? 

6. VR app

As was seen from the VR app, pumping water out of a well creates a cone of depression (CoDe).

• How is a CoDe formed in an unconfined and confined aquifer, respectively? In both cases you pump water out but pumping a well has two different physical effects, depending on whether your aquifer is unconfined/confined.
• Which three parameters can you use to adjust the drawdown from a monitoring well close to a pumped well? (1 marks)
• What two parameters can you not influence but which are still important for the shape of a CoDe around a pumped well?
• What is the point of having two wells situated on opposite sides of a construction site? 
• Staying with the example of two wells, do we really need both of them if the aquifer is made of gravel? Reason your answer. 
• Which aquifer matrix requires more wells to achieve specific drawdown rates for a given pumping rate, sand or quartzite, and why?

7. Solute Transport

Back to our first exercise, you expect that the storage tank under the gasoline station has started leaking. The ensuing BTEX plume will migrate to the shallow aquifer and then be transported down-gradient. In the direction of the groundwater flow lies the Lake Gnangara. To gauge the threat of the BTEX plume for the lake, you take core samples from the aquifer and commission a soil lab to carry out sorption experiments (figure below). Based on the lab results, answer the following questions:

(a) What is the partition coefficient between aqueous and solid phase (provide numerical answer in scientific writing and with units)?

(b) Based on the equation below, calculate the retardation factor

where v _c is the velocity of the conservative solute, v _r is the velocity of the retarded solute, P _b is the dry bulk mass density of the soil (6 g/cm ³ ), and θ is the volumetric moisture content (0.3).

(c) You know from tracer studies with conservative solutes that the average groundwater velocity is 9.5·10 ^-5 m/s. How long will it take (in years) for the BTEX plume to reach the lake?

(d) Is there any danger for a potential BTEX contamination of the lake (and its significant cultural and heritage value)? 

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