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What are the predominant sediments observed in these logs What values of hydraulic conductivity, porosity, Specific yield and Specific Storage would you expect to find What is the range of thicknesses of the two aquifers (assume they are continuous) Attached is a spreadsheet of the monitoring data since 1960.Plot the water levels from 3 epresentative monitoring bores (i.e. covering a spatial variation).

What are the trends of the water levels Are the trends different for different areas of the aquifer Head contours
create head contours of the water levels in 1960 create head contours of the water levels in 2014 What notable changes are observable between 1960 and 2014 Pump tests were also undertak.

## Ground Properties

1. Predominant soil type is Sand
2. Aquifer properties:
1. Hydraulic conductivity: 9 * 10-7m-1– 8 * 10-3m-1.
2. Porosity: 0.25 – 0.5
3. Specific yield: 0.21 – 0.35
4. Specific Storage: 1 * 10-4 -5 * 10-4.
3. Range of aquifer thickness is between 58m and 206m.
4. Plots attached

1. Aquifer trends:
1. For GAR1, the water reduced at a steady rate between 1960 and 2014.
2. For GAR10, the water reduced at a steady rate too between 1960 and 2014.
3. For GAR20, the water increased and reduced at different stages such that there has only been a slight difference between 1960 and 2014.
2. From the graphs above, it is clear that, while the water levels dropped between 1960 and 2014, they did so at different rates indicating that different areas on the aquifer region have different water levels.

Contour attached

1. There is not major difference in the layout of the contours meaning a very slight variation in height across the station between 1960 and 2014.
2. Mean values of T and S.
1. T value mean
1. Arithmetic mean: (39.5 + 150.6 + 95.8)/3 = 95.3
2. Geometric mean: = 82.9
2. S value mean
1. Arithmetic mean: (0.19 + 0.30 + 0.69)/3 = 0.39
2. Geometric mean: = 0.34
3. They are not likely to be a representative sample because they have not been taken from points that cover a spatial variation as Question 4 also states.
4. The storativity of sand ranges from 0.21 to 0.35. While the arithmetic mean is out of this range, the geometric mean falls within this range. Transmitivity for sand ranges between 1.32*10-3and 1.32m2/day assuming an average thickness of 132m.
5. Taking K = 8.5 * 10-5m/s (value for a typical sandstone aquifer), H1and H2 = {82.046m and 1.13m respectively for 1960} and {82.048m and 1.06m respectively for 2014}; L=26000m; h = 132m and b = 5km (obtained from taking area, 136km2 and dividing by length, 26km)

For 1960:

Q = K. A (0.0031)

Q = 8.5* 10-5 * 132 * 5000 * 0.0031

Q = 0.17952m3/sec * 31536000

Q = 5484425.76m3/year = 5484.43ML/year

For 2014:

Q = K. A (0.0032)

Q = 8.5* 10-5 * 132 * 5000 * 0.0032

Q = 0.17952m3/sec * 31536000

Q = 5661342.72m3/year = 5661.35ML/year

1. Change in storage:

Total difference between heads = 529.492 – 499.836 = 29.656m

Average difference between the heads = 29.656/20 = 1.48m

Assumed specific storage and specific yield = 5 * 10-4 and 0.3 respectively

Change in storage = Sy * change in level * area

Change in storage = 0.35 * (1.48/1000) * 136 = 0.0704Km3 = 70448ML

Change in storage per year = 70448/ (2014 – 1960) = 1304.6ML/year

1. Mass Balance:
1. Average discharge = (5484.43 + 5661.35)/2 = 5572.89ML/year
2. Change = 8840 – 5572.89 – 651 = 2616.11
3. The mass balance does not add up to the independent calculations as the change in storage due to specific yield is less than half the change due to coastal discharge and pumping.
2. Predicting values:
1. Storage
1. Change in storage = recharge – discharge to coast – pumping
2. Change in storage = 2616.11
3. Compared to 1304.6 obtained from the specific yield, the value is more than double giving a significant error of 50%.
2. Recharge
1. recharge = Change in storage + discharge to coast + pumping
2. recharge = 1304.6 + 5572.89 + 651 = 7528.49ML/year
3. Compared to an estimated 8840ML/year, the error is 14.84% making it significant but not alarming.
3. Discharge
1. discharge = -recharge – Change in storage – pumping
2. discharge = -8840 + 651 + 1304.6 = 6884.4ML/year
3. Compared to an estimated 5572.89ML/year, the error is 19.05% making it also significant but not alarming.
4. The likely source of error from these estimates is the use of different parameters in the calculation of the values. Using the specific yield and change in water level gives a theoretical figure based on soil properties while using discharge to the coast and pumping gives a figure based on the expected use and slope. To get a near accurate figure, all these factors should be incorporated into the mathematical equation.

Budget:

Well monitoring: 3 wells * \$100000 = \$300000

100 day pump test: 4 pumps * 40000 = \$160000

Chloride concentration: 20 monitoring wells = 23 * \$100 = \$2300

Carbon 14 age: 20 monitoring wells = 23 * \$1000 = \$23000

Total budget costs: 485300

Request for data for:

Monitoring wells Prop 2, Prop 12 and Prop 16.

Pump: Pump 1, Pump 3, Pump 5, Pump 8.

Chloride concentration, wells: monitoring all proposed wells, GAR1, GAR14 and GAR20.

Carbon 14 age, wells: monitoring all proposed wells, GAR1, GAR14 and GAR20.

Data Analysis:

In preparation for the new data collection for the aquifer, a plan has been laid out in the budget above. The data points have been chosen primarily for their spatial distribution. The distribution has accounted for all the boundary conditions with as much effort put in place to distribute the samples in regions that have as much of the different properties. This is has been done in order to obtain the various chemical attributes of the well and soil in the well. The monitoring well test enables us to find the geotechnical properties of the soil in order to obtain the transmissivity, hydraulic conductivity, storativity, specific yield and specific storage (Gleeson et al., 2012). By extrapolating this information form data obtained, it is possible to assess the discharge and change of storage information.

Ground Properties:

After analysing the information in the well data chart, it is clear that the aquifer is predominantly in the fine sand part of the soil profile. This is indicated by the fact that sand and clay mix and therefore the sand is mostly fine. As such, the hydraulic conductivity is similar to that which is found in the new well in task 1 above and the hydraulic conductivity also falls in the recommended range provided. As such, it bears a heavy similarity to the results of the pumps above. However, the range of thickness of the well is different in the new aquifer tests. This is because, with an average thickness of 110m, and varying heads, the aquifer ranges from 15m to 102m. However, this can be attributed to a change in spatial distribution of the wells.

Hydraulic Conductivity and Transmisivity:

From pumping test calculations using the Cooper Jacobson formulae in the MS. Excel sheets provided, the following data has been derived:

## Hydraulic Conductivity and Transmisivity

NP1 A = T = 17.28m2/day, K = 0.00008m/min

NP3 A = T = 44.64m2/day, K = 0.0002m/min

NP5 A = T = 27.36m2/day, K = 0.00012m/min

NP8 A = T = 8.208m2/day, K = 0.000037m/min

Arithmetic mean of K = (0.00008 + 0.0002 + 0.00012 + 0.000037)/4 = 0.000109

Geometric mean of K = (0.00008 × 0.0002 × 0.00012 × 0.000037) ^ (1/4) = 0.0000918

This data is consistent with the data for the existing pumping tests recorded and owing to the fact that it is evenly spatially distributed, it can be considered accurate for this particular aquifer. The Storativity (S) data, owing to the consistency will be extrapolated from both tests and a figure of 0.32 is concluded.

The Transmissivity values of these new wells indicate a drop when compared to the other existing wells. This can be attributed to the finer nature of soil in the new wells compared to that of the older wells. The new wells primarily have finer sands with an addition of clays and this is expected to make the porosity and voids reduce. As such, any data requiting transsivity requires recalculation afresh. The value of K from this extrapolation is acceptable as it falls with the range specified for sandy soils generally and is also consistent with the K value used in calculation of the mass balance equation for the task 1 above. This as the K value is directly related to transmissivity, it is fair to conclude that the new test reveals consistency between the well test data.

Chemical Properties:

The chloride tests reveal that the salinity of the aquifer is beyond acceptable levels for human consumption which are required to not exceed 20mg/L. However, in this case, the average salinity is 721mg/L. The salinity levels increase with increase in depth of the aquifer with the highest at 1459.36mg/L and lowest at 443.74mg/L. This indicates that the water from the aquifer cannot be used for drinking straight from the well. The most probable reason for the salinity levels is the close proximity to the ocean which would also, naturally, have high levels of salinity as well.

This could also be attributed to the low levels of transmissivity observed which may lead to a soil retention. The chloride levels also indicate that the water is not suitable for extensive industrial use as this would lead to corrosion of machine parts due to oxidation and reduction as well as for agricultural purposes (Appelo and Postma, 2004). With the high chloride levels however, the water can still be used but this would require additional purification. With this in mind, it is important to consider the alternative purification methods.

The carbon dating test also indicates that the water’s age is similar in the two tests as the tests conducted on the new locations indicate some level of consistency when comparing to the new wells 1, 12 and 16 with existing wells in similar spatial locations on the corresponding map. The age of the water varies significantly but a pattern can be established from the graph. At present, the water at the upper levels of the aquifer shows more activity than that at the lower levels. The age of water is also higher closer to the ocean.

## Chemical Properties

This is indicative that ground water recharge is mostly by surface water and not the ocean. It is also indicative that the water at the upper levels above the sea level is more susceptible to evaporation in comparison to water in levels under the sea water level. This is also an indicator as to why the levels of salinity increase as the downward levels increase. With less evaporation, water retains the minerals while the water at the upper levels exposed to evaporation and recharge by surface runoff is likely to lose the chloride content.

Water Balance Equation

The data used in this calculation borrows from the new data taking into account the difference in head between heads of new wells 2 and 16 i.e. 0.178m and 59.8m respectively and the distance between them i.e. (21500m - 4000m). The K value used is the geometric mean of the K values for the proposed wells 1, 3, 5 and 8 whose hydraulic conductivity data is extrapolated from the pumping tests. Therefore, the discharge is given in the equation below.

For the new well data:

Q = K. A (0.0035)

Q = 9.2* 10-5 * 132 * 5000 * 0.0035

Q = 0.2125m3/sec * 31536000

Q = 6701400m3/year = 6701.4ML/year

Therefore, the new water balance equation, keeping all other things (ground recharge and pumping) constant, is as indicated below.

Change in storage = 8840 – 6701.4 – 651

Change in storage = 1487.6ML/year

The change in storage is significantly less than the change in storage from the above mass balance equation as it is almost half the change. It is however similar to the change obtained using data from specific yield which should be constant in both cases.

Mass Balance Sustainability

This balance equation factors in both the change in storage due to discharge to the ocean, pumping and ground recharge as well as the change in storage due to specific yield. The value of this change in storage due to specific yield is constant in both the new and old wells owing to the fact that the data is obtained over a duration time and is not bound to change significantly with the new wells. It is also assumed that, owing to the fact that the wells are similar in nature due to the geographical similarities of the points observed, and the fact that it is the same aquifer and the point data to be averaged in both sets of data varies only slightly.

Change in storage = 8840 – 6701.4 – 651 = 1487.6ML/year

Change in storage due to specific yield =1304.6ML/year

Difference = 1487.6 – 1304.6 = 183ML/year

At present, the project is sustainable when considering the discharge to the ocean alone. However, when we factor in discharge due to specific yield too, the change in specific yield over the course of the 54 years indicates that the change in storage in the next 10 years would be more than change in storage due discharge, pumping and recharge alone. This is primarily because, as the change in storage due to specific yield increases, it will reach a point where it is equal to the change in storage due to discharge and recharge and when that value goes lower, there will be an overdraft of the water with the current rate of recharge and pumping. As such, it is not sustainable

With the information, it is possible to conclude that this project is not sustainable when considering the new data. The head levels vary significantly compared to the old data making the change in storage also very different. It is therefore expedient to reduce pumping. This would help reduce the amount of water being drawn if all the other output discharges and recharge remain constant. At present, there is no way of increasing water to the aquifer without significantly increasing the costs which may be impractical in this case. The only other alternative is to bore deeper into the earth's surface to allow recharge from the esisting ground water table (Gleeson, et al., 2012).

Sea Water Intrusion

From the data provided, the likeliness of sea water intrusion is low to moderate risk. This is demonstrated by the fact that there is high chloride content in the water from the samples where chemical testing was conducted. This indicates a high likelihood of seawater influence as most surface runoff is usually free from salts and especially because of the filtration that happens as the water traverses the different layers of the soil profile (Appelo and Postma, 2004). Since no other agricultural or industrial activities have been highlighted, the likelihood is that the chloride content is a result of the water from the ocean seeping in.

The carbon 14 age test indicates that the water at the lower levels is older than the water at the upper levels. Owing to the fact that the water slopes towards the ocean, the higher levels are found away from the ocean and this is where the infiltration of ground recharge is bound to take place. However, as we look at the water near the coast, it is found to be older indicating a lower infiltration rate from the ocean.

With the soil being mostly fine however, it is also probable that the high retention rate would be responsible for the retained sea water. It is possible that, while the water may not infiltrate the aquifer from the ocean at a high rate, it is possible that the little water infiltrating is what is retained by the fine sands and clay mixtures. With the water retained leading to more chloride content in the water, the sea water’s influence is noticeable as it highly reduces the quality of the aquifer's water necessitating further filtration efforts in order to use it. This means that, while the water may not infiltrate at a high rate, its effect is unfortunately amplified due to the physical properties of the aquifer (Appelo and Postma, 2004).

References:

Appelo, C.A.J. and Postma, D., 2004. Geochemistry, groundwater and pollution. CRC press.

Gleeson, T., Wada, Y., Bierkens, M.F. and van Beek, L.P., 2012. Water balance of global aquifers revealed by groundwater footprint. Nature, 488(7410), pp.197-2

Cite This Work

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[Accessed 08 August 2024].

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