AQUIFERS AND THEIR CHARACTERISTICS

1. AQUIFERS AND THEIR CHARACTERISTICS

2. I. General Groups A. Aquifer B. Aquiclude

3. I. General Groups A. Aquifer (def) A saturated, permeable, geologic unit that can transmit a significant amount of groundwater under an ordinary gradient.

4. I. General Groups A. Aquifer B. Aquiclude (def) A saturated geologic unit which does not transmit a significant quantity of groundwater under ordinary gradients.

5. I. General Groups A. Aquifer B. Aquiclude 1. Aquitard 2. Aquifuge

6. II. Aquifer Types

7. II. Aquifer Types A. Unconfined

8. II. Aquifer Types B. Confined

9. II. Aquifer Types A. Unconfined B. Confined C. Artesian

10. C. Artesian

11. II. Aquifer Types A. Unconfined B. Confined C. Artesian D. Perched

12. E. Potentiometric Surface and Water Table

13. III. Aquifer Characteristics

14. III. Aquifer Characteristics A. Transmissivity - measures the amount of water that can be transmitted horizontally by a full saturated thickness of aquifer. T

15. Storativity (storage coefficient)

16. Storativity (storage coefficient) Water is released from storage via: 1. decrease in fluid pressure 2. increase in pressure from overburden

17. B. Storativity (storage coefficient) (def): The volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in hydraulic head S

18. B. Storativity (storage coefficient)

19. B. Storativity (storage coefficient) S > 0.005 S = 0.02 to 0.3 S ~ Sy

20. B. Storativity (storage coefficient) Example Problem:An unconfined aquifer with a storativity of 0.13 has an area of 123 square miles. The water table drops 5.23 feet during a drought. How much water was lost from storage?

21. C. Specific Storage (elastic storage coeff.) (def): The volume of water that a unit volume of aquifer releases from storage under a unit decline in hydraulic head. Ss

22. C. Specific Storage (elastic storage coeff.) (def): The volume of water that a unit volume of aquifer releases from storage under a unit decline in hydraulic head. Ss * b = ? S

23. C. Specific Storage (elastic storage coeff.) (def): The volume of water that a unit volume of aquifer releases from storage under a unit decline in hydraulic head. Ss * b = S S

24. IV. Compressibility and Effective Stress A. Compressibility (general) “When pressure is applied to the aquifer, a reduction of volume can occur in 3 primary ways”

25. IV. Compressibility and Effective Stress A. Compressibility (general) • “When pressure is applied to the aquifer, a reduction of volume can • occur in 3 primary ways” • Compaction of water • Compression of individual sand grains • Rearrangement of sand grains into more closely-packed • configuration

26. IV. Compressibility and Effective Stress A. Compressibility (general) • “When pressure is applied to the aquifer, a reduction of volume can • occur in 3 primary ways” • Compaction of water (β) • Compression of individual sand grains • Rearrangement of sand grains into more closely-packed • configuration (α)

27. IV. Compressibility and Effective Stress A. Compressibility (general) Stress = Young’s Modulus of Elasticity Strain

28. IV. Compressibility and Effective Stress A. Compressibility (general) Strain = Compressibility Stress

29. B. Compressibility of Water (β) β = dVw/Vw dP Strain = Compressibility Stress

30. B. Compressibility of Water (β) β = dVw/Vw dP β = dρw/ ρw dP

31. IV. Compressibility of the Aquifer (α) and Effective Stress Compressibility of Porous Medium 1. “In general”….Terzaghi (1925) Stress Total Fluid Pressure Effective Stress

32. IV. Compressibility and Effective Stress Compressibility of Porous Medium 1. “In general”….Terzaghi (1925) Stress Total Fluid Pressure + Effective Stress σt P + σe

33. Stress Total Fluid Pressure + Effective Stress σt = P + σe

34. Stress Total Fluid Pressure + Effective Stress σt = P + σe dσt = dP + dσe

35. Stress Total Fluid Pressure + Effective Stress σt = P + σe dσt = dP + dσe dP = -dσe

36. Stress Total Fluid Pressure + Effective Stress σt = P + σe dσt = dP + dσe dP = -dσe dP = ρwgdh

37. IV. Compressibility and Effective Stress C. Compressibility of Porous Medium α= dVt/Vt dP

38. IV. Compressibility and Effective Stress C. Compressibility of Porous Medium α= dVt/Vt dP α = db/b dP

39. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer B. Water produced from expansion of water

40. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer B. Water produced from expansion of water dVwater = ρgα dVwater = ρgnβ

41. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer • Water produced from expansion of water C. The Link dVwater = ρgα dVwater = ρgnβ dVwater from α+dVwater from β= Ss

42. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer • Water produced from expansion of water C. The Link dVwater = ρgα dVwater = ρgnβ dVwater from α+dVwater from β= Ss ρgα+ ρgnβ= Ss

43. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer • Water produced from expansion of water C. The Link dVwater = ρgα dVwater = ρgnβ dVwater from α+dVwater from β= Ss ρgα+ ρgnβ= Ss ρg(α+nβ)= Ss

44. Linking the Parameters of α, β, Ss • Water produced by the compaction of the aquifer • Water produced from expansion of water C. The Link dVwater = ρgα dVwater = ρgnβ dVwater from α+dVwater from β= Ss ρgα+ ρgnβ= Ss ρg(α+nβ)= Ss S = Ss * ?

45. V. Linking the Parameters VI. Summary “ a problem to work….” A confined aquifer with initial thickness of 45 m compacts by 0.20 m when hydraulic head is lowered by 25m. a) what is the compressibility of the aquifer? b) If the porosity of the aquifer is 12% after compaction, what is the storativity of the aquifer?