In this component of the project, you will apply the knowledge gained through this topic to assess the long term sustainability of a theoretical aquifer. The project requires you to assess the long term water availability of the area, the susceptibility to contamination and the susceptibility to sea-water intrusion. You will analyse existing data from the catchment, then subject to budgetary constraints, suggest additional monitoring and undertake the interpretation of the data obtained from this additional monitoring.
This is an open ended project. I strongly suggest that wherever you must make a difficult decision, that you clearly justify your reasoning for the approach taken.
The Garyville Aquifer system
1. Background
You are the new hydrogeologist at Hawke Consulting. Your task is to utilise the data below to understand the Grayville aquifer, a hypothetical aquifer system located in the region underlining Garyville and its surrounding areas.
The area is located on the coast extending approximately 30 km inland. Since the 1960s, the growth of the wine collecting middle class in the nearby capital city of A-town has led to an explosion of wineries in the area. These wineries form an important part of the economy of the region. This has resulted in extensive extraction from the upper quaternary aquifer and the tertiary aquifer. Apart form their deposition date, these two aquifers are considered continuous and can be treated as a single aquifer. These aquifers pinch out in the north, are bounded by the ocean to the west, a fault (hard rock) to the south and are underlain by an aquitard. The layout is shown in Figure 1.
2. Existing data
When initially assessing the feasibility of the site, the catchment board installed a monitoring network of 20 wells. The location for these are shown in Figure 1.
2.1 Well logs
The logs of these wells are included in a spreadsheet.
Figure 1 – Site map and location of monitoring Bores
Task 1 – Utilise existing data
Question 1 – What are the predominant sediments observed in these logs?
Question 2 – What values of hydraulic conductivity, porosity, Specific yield and Specific Storage would you expect to find? Please justify your suggested value ranges with references (e.g. text book references)
Question 3 – What is the range of thicknesses of the two aquifers (assuming that they are continuous)?
2.2 Monitoring Network
Attached is a spreadsheet of the monitoring data since 1966.
Question 4 – Plot the water levels from 3 representative monitoring bores (i.e. covering a spatial variation). Remember to label your wells and present the data appropriately (axes labels, figure captions).
Question 5 – What are the trends of the water levels?
Question 6 – Are the trends different for different areas of the aquifer?
2.3 Head contours
Question 7 – create head contours of the water levels in 1966
Question 8 – create head contours of the water levels in 2019
Question 9 – What notable changes are observable between 1966 and 2019?
2.4 Pump tests
Pump tests were also undertaken at three sites in the catchment and have been analysed. The resulting estimates of Transmissivity (T) and Storativity (S) are listed below.
Figure 2: location of pumping tests (data in Table 1).
Table 1: Results from pumping tests
| Well | T (m2/day) | S (-) |
| PUMP 1 | 39.5 | 0.19 |
| PUMP 2 | 150.6 | 0.30 |
| PUMP 3 | 95.8 | 0.69 |
Question 10 – Calculate the arithmetic and geometric mean of the T and S values
Question 11 – Are these likely to be representative of the aquifer (hint, comment on sample size, and quality of the tests)?
Question 12 – How do these T and S compare to the standard values based on Questions 2 and 3?
2.5 Recharge estimation
Recharge was calculated as a percentage of rainfall. An average rainfall of 650 mm/y was assumed with 10% of this being attributed to recharge. The catchment area was estimated to be roughly 136 km2. This gives a total recharge into the catchment of 8840 ML annually. It was decided that a maximum 10% of this value could be allocated for pumping, however average pump use is 651ML/year
2.6 Preliminary Water balance
The water balance for the site is given as:
Change in storage = recharge – discharge to the coast – pumping
Question 13 – Using, the average gradient near the coast from contour maps, the mean value of Transmissivity and an approximate width calculate the discharge to the coast in 1966 and 2019.
Question 14 – Estimate the change in storage volume of the aquifer. This can be done approximately by determining an average difference between the 1966 and 2019 heads, assuming a value for specific yield/ storage (give reason for assumption) and an approximate catchment area (given above).
Question 15 – Does the mass balance add up with the independent measurements (try average, 1966 and 2019 discharge to the coast)? Provide possible reasons why/ why not.
Question 16 – State and discuss the likely sources of error in these estimates
Task 2 – Future Work
As the newest member of the team at Hawke Consulting, you have joined the project since an additional field campaign was undertaken to collect further data.
In this next task, you are to:
- Provide an improved water balance of the Garyville Aquifer system.
- Address whether current extraction is sustainable (i.e. will aquifer dry out, whether there are serious risks of seawater intrusion etc.).
- To provide a description, with a budget (following the costings outlined in table 2), of the data that you would have requested if you were with the company prior to the second field/drilling campaign.
To perform this second task, you will be provided with an additional dataset that was collected within a $500,000 budget. Your job is to work with these additional data to see if you can improve upon the understanding of the system.
The investigation was constrained by a budget of $500,000 with the costs of individual items listed below:
Table 2: Costs of new data
| Item | Cost | Data* |
| New monitoring well | $100,000 | Lithology, thickness, current water level (not historical) |
| 100 day Pump test | $40,000 | Drawdown vs time data – need to analyse |
| Chloride concentration | $100 | Chloride concentration |
| Carbon 14 Age | $1,000 | Age of water |
* Note, you can only request carbon-14 or chloride data from existing wells, or new monitoring wells that are purchased.
The possible locations for each of the data types is pictured below:
Figure 3: Potential monitoring well locations.
Figure 4: Potential Pump test locations.
Project summary
To reiterate, your task is to:
- Answer the initial questions (Q1-16, i.e. Task 1)
Task 2 Analyse the new data,- Explain the data that you would have requested and why (subject to the
abovementioned budgetary constraints), - Produce a revised water balance, and
- Answer the question regarding the sustainability of the pumping.
It is important that you discuss why you chose the data you did as part of the process.
This “Future work” component should not exceed 2000 words.
Please find blank maps that you can use to draw your water level contours on the following pages.
Appendix 1
Blank maps
Appendix 2
Carbon-14 data in this project will be provided with the units of percent modern Carbon (pmC).
Depending on the source that you use, Carbon-14 has a reported half-life of between 5568 and 5730 years. i.e. after one half-life the amount of Carbon-14 will be half of its original value.
To convert your pmC data to a groundwater age (how long since the water recharged the groundwater system), you can use the equation below (based on half-life = 5730 years):
(1)
A key point to highlight is that in reality there are several factors that must be corrected for. We will not go to this level of detail here, however, it is important to highlight the accuracy, and potential error on your estimates. Please present your age calculations with the accuracy (column 2) and include errors (i.e. ±x years, column 3) of Table 1 below:
Table 1: Guidelines on presenting calculated ages from Carbon-14 analyses
| Calculated Age (years) | Round to the nearest (years) | Likely error range (years) |
| 0 – 1000 | nearest 10 | ±5 |
| 1000 – 10 000 | nearest 10 | ±10 |
| 10 000 – 25 000 | nearest 50 | ±10 |
| >25 000 | nearest 100 | ±50 |
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