Assessment Of Long-term Sustainability Of A Theoretical Aquifer

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)?
Question 4 – Plot the water levels from 3 representative monitoring bores (i.e. covering a spatial variation). Remember to label your wells appropriately (axes labels, titles, figure captions).
Question 5 – What are the trends of the water levels?
Question 6 – Are the trends different for different areas of the aquifer?

Utilizing Existing Data for Sustainability Assessment

In the project that ensues, an assessment on the long term sustainability of a theoretical aquifer is presented. The project entails determination of the sustainable water availability of the area, the susceptibility to contamination and the susceptibility to sea-water intrusion. Admittedly, the data provided is analysed against the backdrop of budgetary constraints, and the catchment data. Furthermore, additional monitoring is to be undertaken. Notably, the Garyville Aquifer System (theoretical aquifer) is bounded by an ocean to the west, a hard rock to the south, and a pinch out in the north. Besides, it is underlain by an aquitard such that it stretches (from the coast) 30km inland. The aquifer system is one of the crucial resources the people of Garyville town depend on; especially in wine production. Therefore, it is critical to examine the long term sustainability of the aquifer system.

In performing the assessment, the monitoring team installed a network of 20 wells and the individual locations are illustrated in figure 1. Consequently, the following tasks were undertaken:

Task 1 – Utilising existing data

• General
  • The predominant sediments observed in these logs

-Sandyclay and sandy limestone are most predominant

Based on the well logs and levels provided, the water levels were plotted for three selected monitoring wells. It should be noted that the random selection considered the geographical locations of the wells such that a fair balance in geographical symmetry was pursued hence wells number 2, 12 and 20 were the candidate wells for this case.

• Question 5 –What are the trends of the water levels?

Over the years, changes in water levels for the three selected wells seem to be fairly stable. The well with the highest water level is well number 20 located at the eastern edge next to the pinch out. The reduced levels are almost insignificant at least in the near future with well number 20 showing almost zero reduction in water level.

• Question 3 – What is the range of thicknesses of the two aquifers

Between 60 to 100 metres is ideal

Are the trends different for different areas of the aquifer?

     The trends seem to be similar as almost in all of the wells, there is a gradual reduction in water levels and it occurs in a uniform fashion

• What notable changes are observable between 1964 and 2017?

In 1964, the change in water levels were fairly stable than in the year 2017. The rate of reduction in water levels in 2017 translates to low recharge rates but high utilization rates hence potential dry out is high likely.

Pump tests were also undertaken at three sites in the catchment and have been analysed. The resulting estimates of Transmissivity (T) and Storativity (S)

Random Selection of Monitoring Wells

Question 11 – Are these likely to be representative of the aquifer

The sample size selected is too small making the final results to be more prone to inaccuracies and variation. Secondly, the test did not consider all corners of the aquifer to give the actual scenario on the ground. Hence from above, it can be said that these are not the likely representative of the aquifer.

Question 12 – How do these T and Scompare to the standard values based on Questions 2 and 3?

T and S are values used in the analysis of the parameters like hydraulic conductivity , porosity, specific yield and specific storage

Recharge was calculated as a percentage of rainfall. An average rainfall of 650 mm/y was assumed with10% 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 8840ML annually. It was decided that a maximum 10% of this value could be allocated for pumping, however average pump use is 651ML/year

The water balance for the site is given as:

Change in storage = recharge – discharge to the coast - pumping

2.6.1 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 1964 and 2017.

Average gradient = 80/800 = 0.1

Discharge to the coast= 22.31-14.997 = 7.313

2.6.2Question 14 –Estimate the change in storage volume of the aquifer. This can be done approximately by determining an average difference between the 1964 and 2017 heads, assuming a value for specific yield/ storage (give reason for assumption) and an approximate catchment area (given above).

In 1964, discharge to the coast= 0+recharge –pumping

= (8840-651)-95.3x12= 8 347.4

2.6.3 Question 15 – Does the mass balance add up with the independent measurements (try average, 1964 and 2017 discharge to the coast)? Provide possible reasons why/ why not.

Does not add up

2.6.4 Question 16 – Predict the errors in the mass balance by removing each of Change in storage, discharge to the coast and recharge independently, based on what the balance should be if the other components are known.

Tip, calculate:

A = 8840– 8347.4– 1143.6=-651

Then compared A to estimated storage.

B= 0 + 8347.4 + 1143.6= 9491

Then compared B to estimated recharge

C= 0 -651 + 1143.6= 492.6

Question 17 - State and discuss the likely sources of error in these estimates

Pump Tests and Analysis for Transmissivity and Storativity

Recharge was calculated as a percentage of rainfall. An average rainfall of 650 mm/y was assumed with10% 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 8840ML annually. It was decided that a maximum 10% of this value could be allocated for pumping, however average pump use is 651ML/year

The water balance for the site is given as:

Change in storage = recharge – discharge to the coast - pumping

2.6.1 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 1964 and 2017.

Average gradient = 80/800 = 0.1

Discharge to the coast= 22.31-14.997 = 7.313

2.6.2Question 14 –Estimate the change in storage volume of the aquifer. This can be done approximately by determining an average difference between the 1964 and 2017 heads, assuming a value for specific yield/ storage (give reason for assumption) and an approximate catchment area (given above).

In 1964, discharge to the coast= 0+recharge –pumping

= (8840-651)-95.3x12= 8 347.4

2.6.3 Question 15 – Does the mass balance add up with the independent measurements (try average, 1964 and 2017 discharge to the coast)? Provide possible reasons why/ why not.

Does not add up

Question 16 – Predict the errors in the mass balance by removing each of Change in storage, discharge to the coast and recharge independently, based on what the balance should be if the other components are known.

Tip, calculate:

A = 8840– 8347.4– 1143.6=-651

Then compared A to estimated storage.

B= 0 + 8347.4 + 1143.6= 9491

Then compared B to estimated recharge

C= 0 -651 + 1143.6= 492.6

Question 17 - State and discuss the likely sources of error in these estimates

Recharge estimation

Recharge was calculated as a percentage of rainfall. An average rainfall of 650 mm/y was assumed with10% 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 8840ML annually. It was decided that a maximum 10% of this value could be allocated for pumping, however average pump use is 651ML/year

The water balance for the site is given as:

Change in storage = recharge – discharge to the coast - pumping

2.6.1 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 1964 and 2017.

Calculation of Recharge and Water Balance

Average gradient = 80/800 = 0.1

Discharge to the coast= 22.31-14.997 = 7.313

2.6.2Question 14 –Estimate the change in storage volume of the aquifer. This can be done approximately by determining an average difference between the 1964 and 2017 heads, assuming a value for specific yield/ storage (give reason for assumption) and an approximate catchment area (given above).

In 1964, discharge to the coast= 0+recharge –pumping

= (8840-651)-95.3x12= 8 347.4

2.6.3 Question 15 – Does the mass balance add up with the independent measurements (try average, 1964 and 2017 discharge to the coast)? Provide possible reasons why/ why not.

Does not add up

2.6.4 Question 16 – Predict the errors in the mass balance by removing each of Change in storage, discharge to the coast and recharge independently, based on what the balance should be if the other components are known.

Tip, calculate:

A = 8840– 8347.4– 1143.6=-651

Then compared A to estimated storage.

B= 0 + 8347.4 + 1143.6= 9491

Then compared B to estimated recharge

C= 0 -651 + 1143.6= 492.6

2.6.5 Question 17 - State and discuss the likely sources of error in these estimates

Errors due to constrained sample size leading to a few areas being subjected to monitoring. Secondly, the geological make up in the area seem different in all corners hence we are not able to establish the truest picture of the aquifers.

Aquitard percolation error

Due to difference in percolation rates, the existing levels may not be very accurate as rates of discharge and recharge varies based on seasons. Besides, due to contamination from the nearby oceanic water, these levels are expected to be higher at the peripheral end where it is bounded by the ocean

Task 2 - Future Work

In this next task, the objectives are:

• To provide an improved water balance of the Garyville Aquifer system.
• To address whether current extraction is sustainable

The investigation is constrained by a budget of $500,000 with the costs of individual items listed below:

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

How the budgeting was done:

I considered the total number of wells to be monitored. I realized that by increasing the number of proposed data types and the pumping tests, we are likely to have the budget overshooting. For this case, the total costs exceeds by over 100% of the initial budget. However, it should be noted that in order to obtain more perfect results, the sample size must be expanded so as to cover the geographical and geological variations in a large span.  Therefore from the budget proposal, new data is hereby requested for further analysis.