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Aquifer Nomenclature

Aquifer Nomenclature. Aquifer - a geologic unit that can store and transmit water at rates sufficient enough to supply exploitable quantities of water Confining Layer - a geologic unit having little or no intrinsic permeability Don’t Use Aquifuge - no water transmission

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Aquifer Nomenclature

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  1. Aquifer Nomenclature • Aquifer - a geologic unit that can store and transmit water at rates sufficient enough to supply exploitable quantities of water • Confining Layer - a geologic unit having little or no intrinsic permeability • Don’t Use • Aquifuge - no water transmission • Aquitard - stores water, little transmission • Aquiclude - aquifuge that forms upper boundary to aquifer • Leaky Confining Layer - a confining layer that leaks

  2. Unconfined Sand Semi-Confined Semi-Unconfined Clay Clay K’<<K K’<K K K Sand Sand Leaky Confining Layer - Storage Ignored Real Leaky Confining Layer - Storage cannot be ignored Types of Aquifers Confined Clay K’<10-7 Sand K>>K’ Rock • K = Horizontal Hydraulic Conductivity • K’ = Vertical Hydraulic Conductivity

  3. Perched Water Table

  4. Potentiometric Surface Water Table Well Water Table Artesian Well Flowing Well Confined Unconfined The same aquifer can be both confined and unconfined.

  5. Va = Vol of Air Total Vol. (Vt) Water Vw = Vol. of Water Vs = Vol of Solids Solid Variable No. 1 Porosity (n) = (Vv/Vt)x100 Expressed as % Basic Hydraulic Parameters Soil Air Va + Vw = Vv = Vol of Voids = Pore Space

  6. Saturate 1 m2 Example: 10 m column Determining Porosity Known Volume of Dry Soil Volume of Water Added = Vol of Voids Example: 100 cm3 soil, add 42 cm3 water = 42% porosity Add 3 m of water to saturate soil What is porosity?

  7. Volume Water Drained by Gravity x 100 Total Volume Sample = Specific Yield (Sy) Drain Variable No. 2 - Specific Yield Saturate Known Volume of Dry Soil

  8. Variable No. 3 – Specific Retention Volume Remaining on Soil Particles Total Volume = Specific Retention (Sr) Note: Specific Yield Dictates water bearing properties not porosity n = Sy + Sr

  9. Rocks Primary n % Secondary n % sandstone 5 - 30 Fractures increase overall n 2 to 5 % or more if weathered shale 0 - 10 crystalline < 5 Typical Values of n and Sy Unconsolidated n % Sy % Deposits Gravel 25 - 40 22 - 25 Sand 25 - 50 20 - 27 Silt 35 - 50 18 Clay 40 - 70 2

  10. Key Points • Specific Yield is the Important • Property for Flow • Smaller the grain size – lower the • Specific Yield • n = Sy + specific retention • Values usually estimated • Porosity varies only over two orders • of magnitude

  11. Fluid Pressure: a. closed tube w/ sand b. saturated & sealed c. under pressure d. no flow - static hp A Place Piezometer into tube to measure pressure “Water will rise in tube a height hp until Force produced by the weight of water in piezometer balances P being exerted in the pore space” Distribution of Water in Earth Materials Water in pore space exerts pressure on grains around pore space Define fluid pressure - P kg m/sec2 P º Force/Unit Area = m2 = N/m2 = Pa

  12. Unit Weight Define - rg as unit weight - Force exerted by one unit volume of water g = rg gw = 9820 N/m3 (metric) = 62.4 pcf (English) For water: P = rghp hp r = density of water g = accel. of gravity hp= ht. of water in well

  13. hp A Unit Weight can be determined for anything - gd = dry unit weight - solids gb = bulk unit weight - solids + moisture gs = saturated unit weight - solids + water at saturation Typical Application surface P@A = gwhp clay Note: taking measurements of water levels in in a well provides more than P Typical Values Pierre shale - 90-100 pcf Sandy Gravel 8% moisture - 125-135 pcf Limestone - 165 pcf

  14. Dh L D h1 h2 datum Measure Q Q µ Dh Q µ 1/L Q µ A Hydraulic Conductvity Darcy’s Experiment (1857) constant head reservoir Sand

  15. Hydraulic Gradient (Slope of the fluid pressure term) Slope = Hydraulic Gradient Dh L ft/ft or unitless Dh/L = I = dh/dL = i Q/A slope = K = Hydraulic Conductivity gradient Pulling Terms Together Q µ (Dh/L) A

  16. - sometimes see it written with negative sign b/c flow is in the direction of decreasing fluid pressure Conceptually, Gradient = 1 1 1 K = Flow in gpd per unit area under unit hydraulic gradient @ 25 C° 1 Unit Volume 1 1 Rewrite, KIA Q = Darcy’s Law Units - m/sec, cm/sec, m/day, ft/day gpd/ft2

  17. Important Points to Remember: K varies over 12 to 14 orders of magnitude Major control on rate at which contaminants move in subsurface Main parameter needed in modeling Varies spatially in response to geology Need to know how depositional/tectonic processes might influence spatial heterogeneity of K K1 K2 > K3 > K1 K2 K3 contaminant Typical Values sandy gravel 10-2 to 102 cm/sec silty clay 10-6 to 10-9 cm/sec

  18. Repeat Darcy’s Experiment Add Non-aqueous phase liquid Dh D L h1 Sand h2 datum Measure Q Q = KIA Q for water ¹ Q for NAPL Intrinsic Permeability Hold Dh, L and A constant K must vary with fluid properties -

  19. K µ unit weight (g) unit wt. = Force/unit volume = rg Pulling terms together: K µrg/ m or K = ki rg/ m Property of fluid Property of medium K µr (density) K µ 1/m(viscosity of fluid) ki = intrinsic permeability - property of just the medium K = hydraulic conductivity - property of the medium and of fluid

  20. Typical Values Material ki (cm2) K (cm/sec) Clay 10-12 to 10-15 10-6 to 10-9 Silt or Till 10-10 to 10-12 10-4 to 10-6 Fine Sand 10-9 to 10-11 10-3 to 10-5 Well Sorted Sand 10-7 to 10-9 10-1 to 10-3 Well Sorted Gravel 10-6 to 10-8 100 to 10-2 Define intrinsic permeability with lower case k with subscript i Hydraulic conductivity defined with a capital K Darcy’s Law Q = ki(rg/ m) (Dh/L) A K Units for Intrinsic Permeability - cm2, m2, ft2, etc

  21. K a property of the medium and fluid K through identical material will vary with density, viscosity and temperature of fluid Key Points ki property of the medium only

  22. Define Specific Storage Ss = rwg ( a + nb) rw = initial density of water g = acceleration due to gravity a = aquifer compressibility n = porosity b = fluid compressibility Ss = Specific Storage The volume of water either released from or taken into storage per unit volume of confined aquifer per unit change in fluid pressure

  23. Vol Out or In 1 unit 1 unit vol Conceptual meaning of Ss surface clay aquifer Ss = Volume of water released or taken into storage per unit volume of confined aq. per unit head change in fluid pressure Units - m3/m3/m = 1/m so units of 1/L

  24. x b Storage Coefficient Vol Out or In 1 unit area of aquifer surface 1 unit clay 1 unit vol Ss b = aquifer thickness b aquifer S = Ss x b Volume of water released or taken into storage from a vertical column of aquifer of height b, and unit basal area when subjected to a unit change in fluid pressure S is dimensionless

  25. surface clay I = 1 I = 1 1 unit vol K K T b aquifer K x b = T Transmissivity (T) K = volumetric flow per unit time per unit area of aquifer under a hydraulic gradient of one at 25 °C T = volumetric flow per unit time per one unit width of the aquifer extended over the entire thickness of the aquifer at 25 °C Units are: gpd/ft or m3/sec/m or m2/sec

  26. Q drop Pumping a Confined Aquifer clay aquifer Aquifer is still saturated - how can this be?

  27. rw P Vw Vt n storage P se Summary - In a confined aquifer, water is released from storage by: 1. Expansion of water 2. Compression of the Aquifer Two Ways Water is Removed from Storage in a Confined Aquifer 1. Pumping decreases fluid pressure, so …... , water expands as it is released Water Compressibility Component 2. Pumping decreases fluid pressure, so …… , water expelled by compression of aquifer Aquifer Compressibility

  28. Surface P = 0 drop When you pump water out from an unconfined aquifer - you literally dewater the pore spaces S is unitless Typical Values of S are 10-3 to 10-6 Removal of Water From Storage in an Unconfined Aquifer Water drains by gravity - in accordance w/ Sy

  29. Note: Storage actually = Sy + (Ss x b) Usually neglect any aquifer compression or water expansion b/c Sy is so much larger so, S = Sy for an Unconfined Aquifer Typical Values for Sy = 10-1 to 10-3 Key Points About Storage 1. Water released from storage in a confined aquifer by a) expansion of water and b) consolidation of aquifer material and is governed by S 2. Water is released from storage in an unconfined aquifer by dewatering the aquifer pores and is governed dominantly by Sy

  30. Q drop Specific Capacity (SC) clay aquifer • Pump well until steady drawdown in well is • achieved • Pumping Rate, Q / drawdown = Specific Capactiy • Units are L3/T/L, eg., gal/day/ft,

  31. General Relationship Between Specific Capacity and Transmissivity Transmissivity can be estimated by two empirical relationships and making some assumptions For a confined aquifer T = SC x 2000 Where, well radius = 0.5 feet pumping period = 1 day Initial T estimate = 30,000 gpd.ft Storage estimate = 10-3 For an unconfined aquifer T = SC x 1500 Where, same as above except storage is 7.5 x 10-2 Note: the effect of assuming a T value to estimate a T Value using this formula is not really a problem because It is derived from the Jacob modified non-equilibrium Equation and appears in a log term. So large variations In the assumed T has very little affect on the result.

  32. Bedrock Aquifers • Hydraulic Conductivity and Transmissivity of • bedrock wells can also be determined through • pumping tests • Average K values can be determined for entire • borehole • T is determined by multiplying the average K value • by the length of saturated uncased borehole • length • You can also set up packers and isolate individual • water-bearing fractures to determine the • K for an individual fracture

  33. Pump Well at 10 m3/min for 1 day Water Level drops 7 m over 1 ha Example Problem: What is the specific yield?

  34. Pump Well at 10 m3/min for 1 day Water Level drops 7 m over 1 ha Example Problem: What is the specific yield? 1. 10 m3/min x 60 min/hr x 24 hr/day x 1 day = 14400 m3 2. 14400 m3/10000 m2 = 1.44 m3/m2 = 1.44 m = Volume of water extracted 3. Change in water level was 7 m or 7 m3/m2 which is the total over which the change occurred 4. Therefore, 1.44/7 = 21 %

  35. Tectonic Alluvial Valley Alternating layers of Sand, Silt, Clay

  36. General Sequence Recent Deposits Lacustrine Deposits Sand and Gravel Till veneer (lodgement or ablation or both polished bedrock

  37. soil-water (root) zone Unsaturated Zone P<0 Capillary Zone Water Table P=0 Phreatic Surface Saturated Zone P>0 Ground- water Capillary Zone - combination of molecular attraction and surface tension between water and air capillarity Capillary zone can be saturated or nearly saturated but fluid pressure is negative surface intermediate zone (Aeration or Vadose Zone)

  38. root zone Recharge Happens Tension Saturated Zone Capillary Zone P=0 Water Table AWC Field Capacity PWP Soil Moisture Typical Water Profile in Soil Saturation 0 100% intermediate zone Depth (m) Saturated Zone (Specific Retention)

  39. 3. Low temperature fluids will move at different rate than high temperature fluids Recall, viscosity and density are temperature dependent Importance of K and ki Distinction 1. Different fluids will travel at different rates Water and Non-Aqueous phase liquids will move at different rates due to differences in density and viscosity 2. Brines and highly saline solutions will move at different rates due to higher density of saline waters over fresh water.

  40. Block of cement s s= Total Stress (psf) Spring z A Effective Stress & Storage Spring = soil matrix Now let’s place the spring in a cell

  41. imaginary piezometer 1. fill to base of block 2. water represents fluid in pore spaces h 3. no load carried by fluid Water rises in piezometer under its own weight - Hydrostatic Pressure P@A = rgh s closed cell A

  42. D s Excess fluid pressure 3. Additional load on fluid manifested in an increase in fluid pressure > hydrostatic Place additional load on spring s Start a Test z h A 1. Load applied matrix wants to consolidate and realign 2. Sealed tube, fluid has no where to go so additional load is borne by the fluid spring does not compress

  43. D s D s s h z’ 1. As water drains, excess fluid pressure dissipates 2. Load slowly transferred from fluid to matrix Drain s z h 3. Matrix responds by compressing - system consolidates and porosity decreases

  44. D s We write, st = se + P Fluid pressure returns to hydrostatic s Aquifer is consolidated h z’’ Total Stress, st (s + D s ) on system will resolve into 2 parts: P = Fluid Pressure load borne by the fluid se = Effective Stress load borne by the solids

  45. D s - At start of test, load borne by fluid - At completion of test load borne by solids - In between, load was shared by solids and fluid During the Test s Excess Fluid Pressure h z’ Total Stress is balanced by load borne by the solids (se) and the load borne by the fluid (P)

  46. Train Fluid Pressure rise + static 0 fall - Time Behavior in Confined Aquifers surface clay clay aquifer

  47. Train Train Stops rise + Train leaves static 0 fall - surface clay clay aquifer Fluid Pressure Time

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