Groundwater Investigation and Management

Part a Groundwater Investigations and Management

1. Consider the Mundubbera Geologic Map in Module 4.1. Explain why the regions have been highlighted and what it means to the linear fence diagram appearing below the map.
(2 marks)
2. Identify what information you need to develop an accurate potentiometric surface map.
(4 marks)
3. Using the Case Study: Regional Flow Systems in the Coastal Zone of the Southeastern United States (Fetter, pp255-262), write a 200-word summary of this Case Study. What effect do you believe saltwater intrusion from the Gulf of Mexico might have on groundwater extraction in the vicinity of the Tampa- St Petersburg region on the west coast of Florida?
(6 marks)
4. In approximately 200 words, discuss how artificial recharge could be used to prevent saltwater intrusion along any coastline where over-extraction of groundwater has become a problem. (4 marks)

5. Discuss how you might use hydrographs to estimate the safe yield of an aquifer. (2 marks)
Part b Groundwater Desktop Investigation
Learning Objective
Conduct a groundwater desktop investigation by applying a range of theoretical concepts taught in the topic

Module Learning Activities
An unconfined aquifer consisting of a well-sorted sand is bordered by the ocean to the east, a river to the north, and granite (no flow) to the west and south with a reservoir situated on the western boundary (Figure 1). The ocean has a constant head of 0 m AHD, the river has constant heads ranging from 8 m AHD at the inland boundary to 0 m AHD at the ocean discharge boundary, and the reservoir has a constant head of 40 m AHD. The top of the aquifer is at 50 m AHD and the bottom of the aquifer is at -10 m AHD. A network of 34 observation wells exists to monitor the groundwater levels. Assume the aquifer consists of only one material and is homogeneous and isotropic. The system is in steady state.

Figure 1 Diagram of the system

1. What are typical ranges for hydraulic properties of the aquifer based on the geology? List your reference/s. (2 marks)
Figure 2 Basic diagram of the system displaying the scale

A water budget for the system has the following long-term average annual values: precipitation 1,350 mm/yr; evapotranspiration 2,100 mm/yr; external river inflow to the reservoir 30.56 GL/yr; 2.978 ML/hr for 8 hours a day, 7 days a week is pumped from the reservoir; overland flow 120 mm/yr; inflow to the river 16.58 GL/yr; outflow from the river to the ocean 15.43 GL/yr; 420 kL/hr for 8 hours a day, 7 days a week is pumped from the river.

2. Conduct a water balance for the reservoir, river and aquifer to calculate the following (assume all hydrology average values):
a) Reservoir leakage to the aquifer (calculate the reservoir water balance)
b) Discharge from the aquifer to the river (calculate the river water balance)
c) Discharge from the aquifer to the ocean (calculate the aquifer water balance and assume that net recharge = 0 since recharge and evapotranspiration are approximately equal).
You may need to calculate/estimate the areas of the reservoir, river, and aquifer from
Figure 2. Please clearly define all parameters you are using and provide all calculations. (8 marks)

Figure 3 Groundwater levels from observation wells and example water level contours. River water levels are also indicated. All measurements are in m AHD.

3. Draw a water table contour map using the groundwater levels from observation wells in Figure 3. Example contours have been provided. (4 marks)

4. On your contour map, take three nearby, equally spaced hydraulic gradients next to the coastline and calculate the groundwater flow for each using Equation 4.59 (p. 141) from Fetter (2001), ensuring that hydraulic heads are measured from the bottom of the aquifer. You will need to select an appropriate hydraulic conductivity value based on the geology. Once you?ve made the groundwater flow calculations, take the average groundwater flow and multiply it by the length of the coastline. Compare this ocean discharge to the one calculated in the water balance and comment on your findings. Repeat these calculations next to the reservoir wall to compare reservoir discharge estimates to the aquifer. (6 marks)

5. Discuss errors in the above exercises including the water balance, water table contour map, hydraulic gradient and groundwater flow calculations. (4 marks)

Figure 4 New groundwater levels from observation wells after a recharge of 100 mm/yr was applied. The constant head boundaries remain the same as the initial scenario. All measurements are in m AHD.

Changes in climate and land use have produced a net aquifer recharge of 100 mm/yr; however the constant head boundaries remain unchanged. Steady state groundwater levels have been recorded from observation wells (Figure 4).

6. Provide some comments/illustrations/calculations for the new recharge scenario. (4 marks)


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