ERT 246- HYDROLOGY AND WATER RESOURCES ENGINEERING GROUNDWATER Siti Kamariah Md Saat, PPK Bioprocess 2010

Groundwater Groundwater is part of the water cycle. It is water that is located beneath the earth’s surface in pores and crevices of rocks and soil. The process of water entering the ground to become groundwater is called recharge. The volume of recharge and how fast it enters groundwater is dependent on climate, the depth to the water table, the types of plants that use water in the soil and the types of soil and rock it must pass through.

Groundwater flow Because groundwater has to move between pores and crevices in soil and rock, it moves much more slowly than surface water. Water can move down a river in hours, days or perhaps weeks. Groundwater in an aquifer may take ten, one hundred or many thousand years to flow through an aquifer. Management of groundwater needs to consider the amount of water going into the aquifer, how much is stored in the aquifer, and how long it takes to move through it.

Groundwater Flow Groundwater velocity Depends on permeability and hydraulic gradient (slope of water table) Ranges from 100 m/day to mm/day

Groundwater problem and issue Groundwater is a complex resource. The unseen nature of groundwater makes it difficult to quantify, however careful monitoring and management of groundwater resources helps to guard against over-extraction and ensure reserves do not become stressed or drop below sustainable levels. It may also be impacted by climate change. However, by recording water levels and assessing recharge rates, it is possible to better-understand the resource and the impacts that climate change may have upon it.

Groundwater problem and issue While groundwater can be a reliable source of water, its overuse can result in failure of supply. The risk of loss of supply or changes to groundwater quality is increased as surface water systems become fully allocated and more people access groundwater to meet their water needs.

Water Profile Interstitial Zone Soil Water Intermediate Vadose Water Bound Water in Minerals Capillary Water Intermediate Vadose Water Water in Unconnected Pores Groundwater Soil Water Interstitial Zone Saturated Unsaturated

Subsurface Flow Infiltration Percolation Interflow Groundwater flow flow entering at the ground surface Percolation vertical downward unsaturated flow Interflow sub-horizontal unsaturated and perched saturated flow Groundwater flow sub-horizontal saturated flow

Groundwater zones Water in the ground is found in three general zones Vadose, or unsaturated zone (saturation <1) Saturated zone (saturation = 1) Capillary zone, lies above the water table

Aquifers Groundwater is the water found in an aquifer Aquifer: The saturated underground formation that will yield usable amounts of water to a well or spring. The formation can be sand, gravel, limestone or sandstone Any geologic unit through which water can move easily (i.e. it’s permeable) (= high permeability) Permeability: The ease with which water flows through porosity. Most important variable is grain / pore size.

Confined aquifer The saturated formation between low permeability layers that restrict movement of water vertically into or out of the saturated formation Water is confined under pressure similar to water in a pipeline In some areas confined aquifers produce water without pumps (flowing artesian well)

Unconfined aquifer The saturated formation in which the upper surface fluctuates with addition or subtraction of water The upper surface of an unconfined aquifer is called the water table Water, contained in an unconfined aquifer, is free to move laterally in response to differences in the water table elevations

Aquifer Types Unconfined - storage LARGE depends on specific yield Confined - storage SMALL depends on compressibilities

Confined and unconfined aquifer

Perched aquifer: Groundwater ponded above an impermeable layer within a larger unconfined system.

Ground Water and Surface Water These are almost always connected If a stream contributes water to the aquifer it’s called a “losing stream” If a stream receives water from the aquifer it’s called a “gaining stream” Same stream can be both at different places or at different times

Aquifer properties Porosity Specific retention Specific yield Permeability Transmissibility Storage coefficient

Porosity Percentage of open spaces in rock and sediment that can hold water. This determines the amount of water that a rock can contain. In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation.

Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space.

Specific retention, Sr Sr the ratio of the volume of water a geomaterial can retain against gravity drainage to the total volume of the geomaterial. Known as ability to retain water Sr= Wr/V, where Wr is a water retention volume and V is total volume

Specific yield, Sy Sy the ratio of the volume of water that drains from a saturated geomaterial flowing to the attraction of gravity to the total volume of the geomaterial Sy= Wy/V, where Wy= volume of water discharge V= total volume n= Sy + Sr

Specific Yield

Permeability A measure of the degree to which the pore spaces are interconnected, and the size of the interconnections. Low porosity usually results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with little or no interconnections between pores. A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability. Unit m/day or m/year

Storage coefficient Storativity(S) or Storage coefficient •The volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head.

Specific Storage Specific storage (Ss) or Elastic storage coefficient •The amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to compressibility of the mineral skeleton and the pore water unit change in head.

Specific Storage Ss = ρwg (α + nβ), where ρw= density of water g =the acceleration of gravity α = compressibility of aquifer skeleton (1/(M/LT2)) β = compressibility of water (1/(M/LT2)) n = porosity (L3/L3)

Specifics Unconfined Sy = n - Sr Confined S = b.Ss Ss = g.(a + n. b ) n porosity Sy specific yield (gravity drainage) Sr specific retention (like field capacity) Confined S = b.Ss b thickness Ss specific storage Ss = g.(a + n. b ) g specific weight a matrix compressibility b water compressibility

Transmissivity Amount of water that can be transmitted horizontally through a unit width by the full saturated thickness of the aquifer under a hydraulic gradient of 1. T= K B T: transmissivity(or m2/d) K: hydraulic conductivity (L/T) B: saturated thickness of the aquifer (L or m)

Groundwater Flow rate-Darcy Law Simple relationship that states that flow velocity is directly proportional to: hydraulic gradient: slope of the WT. hydraulic conductivity (K): parameter describing the permeability of the aquifer (also depends on the density and viscosity of the fluid). Typical rates are on the order of 1-10 cm/day for most aquifers.

Pollution of Groundwater Need a sense of ground water flow Warm up responses to the velocity of groundwater flow is dependent on: porosity and permeability 28% permeability and hydraulic gradient 61% porosity and hydraulic gradient 7% pressure gradient 4%

Applications Recharge surficial and deep aquifers Hazardous waste and landfill leachate Swale Design Percolation or Retention Pond Design Yield of an Aquifer Green Roof Design and Operation Seepage Through Reservoirs Exfiltration Design and Operation Yearly Volume Budgets

Darcy’s Law Darcy’s law provides an accurate description of the flow of ground water in almost all hydrogeologic environments. Flow rate determined by Head loss dh = h1 - h2

Darcy’s Law Henri Darcy established empirically that the flux of water through a permeable formation is proportional to the distance between top and bottom of the soil column. The constant of proportionality is called the hydraulic conductivity (K). V = Q/A, V  – ∆h, and V  1/∆L

V = – K (∆h/∆L) and since Q = VA (A = total area) Q = – KA (dh/dL)

Hydraulic Conductivity K represents a measure of the ability for flow through porous media: Gravels - 0.1 to 1 cm/sec Sands - 10-2 to 10-3 cm/sec Silts - 10-4 to 10-5 cm/sec Clays - 10-7 to 10-9 cm/sec

Conditions Darcy’s Law holds for: 1. Saturated flow and unsaturated flow 2. Steady-state and transient flow 3. Flow in aquifers and aquitards 4. Flow in homogeneous and heterogeneous systems 5. Flow in isotropic or anisotropic media 6. Flow in rocks and granular media

Example of Darcy’s Law A confined aquifer has a source of recharge. K for the aquifer is 50 m/day, and n is 0.2. The piezometric head in two wells 1000 m apart is 55 m and 50 m respectively, from a common datum. The average thickness of the aquifer is 30 m, and the average width of aquifer is 5 km.

Compute: a) the rate of flow through the aquifer (b) the average time of travel from the head of the aquifer to a point 4 km downstream *assume no dispersion or diffusion

The solution Cross-Sectional area= 30(5)(1000) = 15 x 104 m2 Hydraulic gradient = (55-50)/1000 = 5 x 10-3 Rate of Flow for K = 50 m/day Q = (50 m/day) (75 x 101 m2) = 37,500 m3/day Darcy Velocity: V = Q/A = (37,500m3/day) / (15 x 104 m2) = 0.25m/day

Seepage Velocity:. Vs = V/n = (0. 25) / (0. 2) =. 1. 25 m/day (about 4 Seepage Velocity: Vs = V/n = (0.25) / (0.2) = 1.25 m/day (about 4.1 ft/day) Time to travel 4 km downstream: T = 4(1000m) / (1.25m/day) = 3200 days or 8.77 years This example shows that water moves very slowly underground.

Limitations of the Darcian Approach 1. For Reynold’s Number, Re, > 10 or where the flow is turbulent, as in the immediate vicinity of pumped wells. 2. Where water flows through extremely fine-grained materials (colloidal clay)

Darcy’s Law: Example 2 A channel runs almost parallel to a river, and they are 2000 m apart. The water level in the river is at an elevation of 120 ft and 110m in the channel. A pervious formation averaging 30 m thick and with K of 0.25 m/hr joins them. Determine the rate of seepage or flow from the river to the channel.

Confined Aquifer Confining Layer Aquifer 30 ft m

Example 2 Consider a 1-m length of river (and channel). Q = KA [(h1 – h2) / L] Where: A = (30 x 1) = 30 m2 K = (0.25 m/hr) (24 hr/day) = 6 m/day Therefore, Q = [6 (30) (120 – 110)] / 2000 = 0.9 m3/day/ft length = 0.9 m2/day

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