Aquifers

Problem 1: Storativity and Specific Storage in a Confined Aquifer

A confined aquifer consists of dense, sandy gravel with a thickness of 100 m and a porosity of 20%. The aquifer is underlain and overlain by low-permeability confining beds.

Given data:

• Aquifer thickness: b = 100 m
• Porosity: n = 0.20
• Water density: \rho_w = 1000 kg/m³
• Gravitational acceleration: g = 9.81 m/s²
• Water compressibility: \beta_w = 4.8 \times 10^{-10} m²/N
• Matrix compressibility (dense, sandy gravel): \beta_p = 5.2 \times 10^{-9} to 1.0 \times 10^{-8} m²/N

Tasks:

a) Calculate the specific storage due to water compressibility (let’s call that S_s^w).

b) Calculate the specific storage due to matrix compressibility (let’s call that S_s^M) using the range of values given.

c) Calculate the total specific storage (S_s) and the storativity (S) of the aquifer.

d) If the hydraulic head drops by 100 m over an area of 1 \times 10^9 m², calculate the volume of water released from storage.

e) Explain which component (water compressibility or matrix compressibility) contributes more to the total storage, and discuss the physical meaning of this result.

Problem 2: Specific Yield, Retention, and Porosity from a Drainage Experiment

A soil sample was collected and subjected to a drainage experiment to determine its aquifer properties. The following measurements were obtained:

Given data:

• Mass of sample after draining by gravity: 85 g
• Mass of oven-dried sample: 80 g
• Mass of water that drained by gravity: 20 g
• Bulk density of the wet soil: \rho_b = 1.65 g/cm³
• Density of water: \rho_w = 1.0 g/cm³

Tasks:

a) Calculate the total volume of the soil sample (V_T).

b) Calculate the volume of water retained in the sample after drainage (V_r).

c) Calculate the volume of water that drained by gravity (V_d).

d) Calculate the specific retention (S_r), specific yield (S_y), and porosity (n) of the sample.

e) Verify that the relationship n = S_y + S_r holds for your calculated values.

f) Based on your results, classify this material according to typical aquifer properties and explain what this means for its potential as a water-bearing formation.

Problem 3: Basin-Fill vs Fluvial Aquifer Systems

Compare and contrast basin-fill aquifers with fluvial aquifers, using specific examples from the lecture material.

Tasks:

a) Basin-Fill Aquifers:

• Describe the formation process of basin-fill aquifers, using the Basin and Range province as an example.
• Explain how the geometry of these aquifers differs from other types.
• Discuss the relationship between mountain ranges and basin-fill aquifer formation.

b) Fluvial Aquifers:

• Distinguish between braided and meandering river systems in terms of their aquifer-forming potential.
• Explain why braided river systems typically create more productive aquifers than meandering systems.
• Describe the sedimentological characteristics that make fluvial deposits good aquifers.

c) Comparison:

• Compare the hydraulic properties and heterogeneity of basin-fill versus fluvial aquifers.
• Discuss which type would be more suitable for large-scale groundwater development and why.
• Explain how the depositional environment controls the long-term sustainability of these aquifer systems.

Problem 4: Major Alluvial Aquifer Systems

Compare three major alluvial aquifer systems discussed in the lecture, focusing on their geological characteristics and importance.

Tasks:

a) High Plains Aquifer System:

• Describe the geographic extent and geological composition of this aquifer system.
• (Bonus Question) Explain the depositional history of the Ogallala Group and its relationship to the Rocky Mountains.
• Discuss why this aquifer is considered the largest in the United States and its current usage patterns.

b) Central Valley Aquifer System:

• Describe the unique surface water system of the Central Valley and its relationship to the Sierra Nevada.
• Explain how alluvial processes from the Sierra Nevada Mountains shape the aquifer characteristics.
• Discuss the variability in grain texture and hydraulic properties across different parts of the valley.

c) Indo-Gangetic Basin (IGB) Aquifer:

• Describe the geographic extent and importance of this aquifer system.
• Discuss why this aquifer is considered the world’s most important and the challenges it faces.

d) Comparative Analysis:

• Compare the depositional environments and sedimentological characteristics of these three systems.
• Discuss the factors that make each system particularly important for groundwater resources.
• Explain how geological history and current tectonic activity influence the sustainability of these aquifer systems.