Groundwater and Aquifer Properties: Understanding Hydrogeological Concepts

1. Baseflow Recession Q0

2. Baseflow recession • Q = Q0 e–at • Q = flow at time t after recession started (L3/T; ft3/s or m3/s). • Q0 = flow at the beginning of recession. • a = recession constant (1/T; d-1). • t = time since recession began. (T; d)

3. Meyboom method • Vtp = Q0t1/2.3 • Vtp = Volume of total potential groundwater discharge during a complete ground water recession (L3, ft3 or m3). • Q0 = baseflow at start of recession (L3/T, ft3/s or m3/s). • T1 = time it takes baseflow to go from Q0 to 0.1Q0

4. Increase of Recharge • find t1 • tc = 0.2144 t1 • find QA & QB • Vtp = QBt1/2.3 – QAt1/2.3 • G = 2 Vtp

5. Porosity • Porosity is percent of rock or soil that is void of material. • n = 100 Vv/V • n = porosity (percentage) • Vv= volume of void space (L3, cm3, m3) • V = unit volume of material including voids and solids.

6. Factors affecting porosity • Packing • Grain-size distribution - sorting

9. Sediment Classification • Sediments are classified on basis of size of individual grains • Grain size distribution curve • Uniformity coefficient Cu = d60/d10 • d60 = grain size that is 60% finer by weight. • d10 = grain size that is 10% finer by weight. • Cu = 4 => well sorted; Cu > 6 => poorly sorted.

10. d60 d10 d60 d10

11. d60 d10 d10 d60

12. Aquifer • Properties: Porosity, specific yield, specific retention. • Potential: Transmissivity, storativity. • Types: confined, unconfined. • Hydraulic conductivity, Physical Laws controlling water transport.

13. Specific Yield and Retention • Specific yield – Sy: ratio of volume of water that drains from a saturated rock owing to the attraction of gravity to the total volume of the rock. • Specific retention – Sr: ratio of the volume of water in a rock can retain against gravity drainage to the total volume of the rock. • n = Sy + Sr. • Sr increases with decreasing grain size.

19. Darcy’s Law

20. Darcy’s Experiment • Q  ha – hb. • Q 1/L. • Darcy’s Law: Q = -KA(ha-hb)/L. • Q = -KA(dh/dl). • dh/dl = Hydraulic gradient. • dh = change in head between two points separated by small distance dl.

21. Hydraulic conductivity • K = hydraulic conductivity (L/T). • K is also referred to as the coefficient of permeability. • K = -Q[A(dh/dl)] [ L3/T/[L2(L/L)] = L/T] • V = Q/A = -K(dh/dl) = specific discharge or Darcian velocity.

24. Factors influencing hydraulic conductivity • Porous medium. • Fluid passing through the medium.

25. Factors influencing hydraulic conductivity • Q  d2 • Q γ. • Q 1/μ. • d = mean pore diameter. • γ = specific weight. • μ = viscosity.

26. Darcy’s Law, cont. • Q = -[Cd2γA/ μ](dh/dl). • C = shape factor. • C, d = properties of porous media. • γ and μ = properties of the fluid.

27. Intrinsic Permeability • Intrinsic permeability Ki = Cd2 (L2). • K = Ki(γ/μ) or K = Ki(ρg/ μ) • Petroleum industry 1 Darcy = unit of intrinsic permeability Ki • 1 darcy = 1 cP x 1 cm3/s / (1 atm/ 1 cm). cP – centipoise - 0.01 dyn s/cm2 atm – atmospheric pressure – 1.0132 x 106 dyn/cm2 • 1 darcy = 9.87 x 10-9 cm2 ~ 10-8 cm2

29. Factors affecting permeability of sediments • Grain size increases permeability increases. • S. Dev. Of particle size increase poor sorting => permeability decrease. • Coarse samples show a greater decrease of permeability as S. Dev. Of particle size increases. • Unimodal samples (one dominant size) vs. bimodal samples.

31. Hazen method • Estimate hydraulic conductivity in sandy sediments. • K = C(d10)2. • K = hydraulic conductivity. • d10 = effective grain size (0.1 – 3.0 mm). • C = coefficient (see table on P 86).

33. Hazen method (General) • K = C(d50)j. • K = hydraulic conductivity. • d50 = effective grain size (mm). • C = coefficient. • j = an exponent (1.5 – 2).

37. Permeameters • Constant-head permeameter • Qt = -[KAt(ha-hb)]/L. • K = VL/Ath. • V = volume of water discharging in time. • L = length of the sample. • A = cross-sectional area of sample. • h = hydraulic head. • K = hydraulic conductivity

39. Falling head permeameter • K = [dt2L/dc2t]ln(h0/h). • K = Hydraulic conductivity. • L = sample length. • h0 = initial head in the falling tube. • h = final head in the falling tube. • t = time that it takes for head to go from h0 to h. • dt = inside diameter of falling head tube. • dc = inside diameter of sample chamber.

40. Aquifer • Aquifer – geologic unit that can store and transmit water at rates fast enough to supply amounts to wells. Usually, intrinsic permeability > 10-2 Darcy. • Confining layer – unit with little or no permeability … < 10-2 Darcy. aquifuge – absolutely impermeable unit. aquitard - a unit can store and transmit water slowly. Also called leaky confining layer. Raritan formation on Long Island. -- all these definitions are in a relative sense.

41. Aquifer – Cont. • Unconfined aquifer – water-table aquifer.

44. Aquifer – Cont. • Unconfined aquifer. • Confined or artesian aquifers.

45. Aquifer – Cont. • Unconfined aquifer. • Confined or artesian aquifers. • Potentiometric surface – surface at which water will rise in a well cased to the aquifer.

48. Aquifer – Cont. • Unconfined aquifer. • Confined or artesian aquifers. • Potentiometric surface – surface at which water will rise in a well cased to the aquifer. • Perched aquifer.

50. Aquifer