Groundwater hydrology


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Groundwater hydrology

  1. 1. Hydrological Sciences Chapter 5: Groundwater Hydrology (10 hours) Prof. Dr. Hari Krishna Shrestha M. Sc. Interdisciplinary Water Resources Management Program nec Center for Postgraduate Studies Department of Civil Engineering Nepal Engineering College, Changunarayan VDC, Bhaktapur Director, Center for Disaster Risk Studies Contact: 1
  2. 2. Groundwater Hydrology Topics we will discuss in this Chapter • Occurrence and Movement of Groundwater • Water Bearing Strata, • Aquifer Characteristics Affecting Groundwater Yield, • Groundwater Recharge and Withdrawal, • Groundwater Movement, • Evaluation of Safe Yield; • Well Hydraulics- Steady and Unsteady Flow, • Partially and Fully Penetrating Wells, • Types of Wells, • Multiple Well System; • Groundwater Development in Nepal, • Conjunctive Use of Groundwater; • Groundwater Pollution- Point and Non-Point Sources, • Boundary Conditions and Transport Process in Aquifer System, • Problems of Groundwater Pollution in Nepal; • Groundwater Utilization in Nepal. 2
  3. 3. Occurrence and Movement of Groundwater • Groundwater: The water in the saturated zones of soil–rock systems. Aquifers are rock or sediment that act as storage reservoirs for groundwater and are typically characterized by high porosity and permeability. • Occurrence in the pores of geological formations • If pores are filled with water: saturated zone, otherwise unsaturated (vadose) zone • The vadose zone divided into (a) top soil, (b) intermediate and (c) capillary fringe • Capillary zone/fringe: The ground layer where water rises by capillary forces above the saturated layers. 3
  4. 4. Occurrence and Movement of Ground Water • Moves from higher overall energy (potential) to lower overall energy location • Groundwater movement is a part of the hydrological cycle. The rate of movement is rather slow. In some cases, the groundwater stays in the same location for several million years, like the groundwater in the deep aquifer in Kathmandu. • Path of Groundwater movement: (a) Infiltration ⇒ through flow ⇒ exit to water bodies (surface water) as river, lake, spring, ocean (b) Infiltration ⇒ percolation ⇒ storage in shallow and/or deep aquifer ⇒ very slow exit to water bodies at lower potential (movement may stop if path is blocked) 4
  5. 5. Occurrence of Groundwater in Unconfined (water table) Aquifer • What is a water table? What is a perched water table? • Why water table roughly follows topography, rather than getting flat? • What is the significance of water table regarding landslide potential? Unsaturated Zone Saturated Zone Vadose Zone Aeration zone 5
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  7. 7. Water Bearing Strata Types of water bearing strata • Aquifer: A groundwater reservoir in permeable geological formations which can release a considerable amount of water with relative ease (or economically viable) • Confined (Artesian) Aquifer: An aquifer where after drilling a fully-penetrating well the groundwater rises to the level which is equal to the elevation above a datum plus the pressure in the aquifer. • Unconfined (Water Table) Aquifer: An aquifer with confining layer only at the bottom; top is open to atmospheric pressure. Water table mimics topography. • Aquitard: Slower rate of flow compared to aquifer, so economically unviable. • Aquiclude: An aquiclude is rock or sediment that represents a barrier to groundwater flow • Aquifuge: negligible porosity &hydraulic conductivity • Perched Aquifer: clay lens above water table, may provide water even if water table goes down, may become dry if size too small 7
  8. 8. Aquifer Characteristics Affecting Groundwater Yield • Porosity, η = specific yield + specific retention • Permeability, k • Hydraulic Conductivity, K (Homogeneous, heterogeneous, isotropic, anisotropic): reflects combined effect of porous medium and fluid properties • Transmissivity / Transmissibility = KB • Storage Coefficient / Storativity: represents the volume of water released by a column of a confined aquifer of unit cross sectional area under a unit decrease in the piezometric head. • Specific Yield: actual volume of water that can be extracted by the force of gravity from a unit volume of initially saturated aquifer material • Specific Retention: The fraction of water held back in the aquifer against gravity • Stratification: yield more affected by high K strata if flow parallel to strata, yield more affected by low K strata if flow normal to strata • Interconnectedness of pores and/or fractures 8
  9. 9. Groundwater Recharge and Withdrawal • Natural vs. artificial groundwater recharge • Recharge quantity and rate depends on: Aquifer properties Hydraulic gradient Area of flow Groundwater recharge rate estimation using direct (tracer) and indirect methods. The indirect methods include: a) hydro-meteorological (G = Precipitation – Runoff - Evapotranspiration) b) water table fluctuation (G = Aquifer area x change in water table elevation), c) soil physics (q = - K [ (∂(ψ+z )/ ∂z]; G = q x t x aquifer area) • Withdrawal rate depends on: Pump capacity and Aquifer properties 9
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  11. 11. Groundwater Recharge and Withdrawal Sources of Natural Recharge • Rainfall • Reservoir • Rivers and streams • Canals • Lake Artificial Recharge method a) Conserve runoff b) Injection well c) Induced recharge from surface water bodies d) subsurface dykes e) percolation tank, check dams 11
  12. 12. Groundwater Movement • Moves from overall higher energy to overall lower energy area • Groundwater flow is governed by the porosity of the medium of flow and the head available for the flow. Higher the head available, higher the flow and higher the porosity of the medium of flow, higher the flow. Porosity of the material is taken care by the hydraulic conductivity of the material in Darcy’s law. Thus, Darcy’s law defines the groundwater flow in a porous medium. For one dimensional flow in a prism of porous material, the law can be stated as (Harbaugh, et al, 2000): • Q = - KA (h2 – h1) /L where • Q is the volumetric flow (m3 s-1 ); • K is the hydraulic conductivity of the material in the direction of flow (ms-1 ); • A is the cross-sectional area perpendicular to the flow (m2 ); • h1-h2 is the head difference across the prism parallel to flow (m); and • L is the length of the prism parallel to the flow path (m). 12
  13. 13. The partial-differential equation of groundwater flow is Kxx, Kyy, and Kzz are values of hydraulic conductivity along the x, y, and z coordinate axes, which are assumed to be parallel to the major axes of hydraulic conductivity (m/s); h is the potentiometric head (m); W is a volumetric flux per unit volume representing sources and/or sinks of water, with W<0.0 for flow out of the ground-water system, and W>0.0 for flow in (T-1 ); SS is the specific storage of the porous material (L-1 ); and t is time (T). (Harbaugh et al, 2000) The solution to the equation of the groundwater flow is obtained by numerical methods, using various solution techniques such as finite difference methods and finite element methods. t h SsW z h Kzz zy h Kyy yx h Kxx x ∂ ∂ =+      ∂ ∂ ∂ ∂ +      ∂ ∂ ∂ ∂ +      ∂ ∂ ∂ ∂ 13
  14. 14. Groundwater Movement Depends on hydraulic conductivity, hydraulic gradient, area of flow Basic Equation of groundwater movement: Darcy’s Equation: Q = VA = - K (dh/dL) A 14
  15. 15. Evaluation of Safe Yield of a well • Q = K0H • K0= specific capacity = discharge per unit drawdown • H = depression head • Critical depression head (higher depression head will result in dislodging of soil particles) • Critical or maximum yield • Yield rate associated with working head is safe yield Related Concepts: •Sustainable Yield •Groundwater Mining 15
  16. 16. Well Hydraulics - Steady and Unsteady Flow Case I: Steady flow in confined aquifer Vr = K (dh/dr) Q = A Vr = (2 π r B) K (dh/dr) (Q/ (2 π K B)) (dr/r) = dh (Q/ (2 π K B)) ln (r2/r1) = (h2 – h1) Q = [2 π K B (h2 – h1)] / ln (r2/r1) ⇐ Thiem’s Eq. Q = [2 π T (s1 – s2)] / ln (r2/r1) Q = [2 π T (sw)] / ln (R/rw) 16
  17. 17. Well Hydraulics- Steady and Unsteady Flow Case II: Steady flow in unconfined aquifer Vr = K (dh/dr) Q = A Vr = (2 π r h) K (dh/dr) (Q/ (2 π K)) (dr/r) = h dh Q = [ π K (h2 2 – h1 2 )] / ln (r2/r1) Q = [ π K (H2 – hw 2 )] / ln (R/rw) Approx. equations, to be used only when sw << H, or when no data available Q = [2 π T (sw)] / ln (R/rw) Q = [2 π T (s1 – s2)] / ln (r2/r1) 17
  18. 18. Well Hydraulics- Steady and Unsteady Flow Case III: Unsteady flow in confined aquifer Drawdown, s = (H – h) = (Q/ (4 π T)) ∫ ∞ u (e-u /u) du u = r2 S/ (4 T t) s =(Q/ (4 π T))W(u)=(Q/ (4 π T)) [-0.577216 – ln u + u – u2 /(2. 2!) + u3 /(3. 3!)] If u < 0.01, s =(Q/ (4 π T)) W(u)=(Q/ (4 π T)) [-0.577216 – ln u] (s1 – s2)] = (Q / (4 π T)) ln (t2/t1) S = 2.25 T t0/ r2 t0 obtained from time-drawdown plot 18
  19. 19. Partially and Fully Penetrating Wells • Partially penetrating wells pose additional dimension in estimating yield from wells due to vertical component of flow being more pronounced. 19
  20. 20. Types of Wells • Dug well, open well • Bored well • Deep well • Shallow well • Fully/Partially penetrating well • Horizontal well • Injection /recharge well • Private Wells • Public Wells 20
  21. 21. Multiple Well System • Local Overpumping Excessive withdrawal of groundwater at a regional scale may exceed recharge resulting in groundwater overdraft (see High Plains aquifer section). Overpumping of individual wells will not permanently deplete an aquifer but can impact the use of neighboring wells. Pumping of a well pulls down the local water table adjacent to the well as water is withdrawn faster than it can be replaced. The surface of the water table forms a cone of depression surrounding the well. The change in elevation of the water table (drawdown) decreases with distance from the well. Given sufficient time, the water table will be restored to its original level when pumping stops. Domestic wells rarely yield sufficient water to generate a significant cone of depression but the large volumes of water necessary for irrigation can result in the formation of a sizable cones of depression around irrigation wells during the growing season. • A simple animation of contrasting water levels in four wells as a result of excessive pumping of irrigation well (B). Water levels in lightly used domestic wells decrease due to the influence of heavy pumping of the irrigation well. • The cone of depression for any single well may affect neighboring wells prompting land owners to ask who owns the water below their property? Nearly fifty years ago, Tom Bristor found out the hard way that the state of Arizona considered groundwater to belong to the person who could pump it out of the ground. (For more on Bristor's story see Who owns the groundwater?) 21
  22. 22. Groundwater Development in Nepal • Different agencies (DOI,MOAC, DWSS, Private industries etc) involved in Groundwater development/utilization. • No institution with regulatory functions. • No policy of Groundwater regulation. • Purely utilitarian approach. • Groundwater development potential in Nepal • In Terai– • Annual recharge –6000 to 16000 MCM • Annual extraction ~ 1000 MCM • In Kathmandu Valley: • Annual recharge ~ 5.5 MCM • Abstraction for domestic use ~ 13.87 MCM • Abstraction for industrial use ~ 4.23 MCM • Groundwater Deficit ~ 12.6 MCM 22
  23. 23. The ability of households (of Kathmandu Valley) to continue to rely on private wells is in doubt. The total sustainable yield of the groundwater aquifer is approximately 26 million liters per day. Total groundwater extraction is currently about 59 million liters per day. As a result the groundwater table is falling, and contamination is increasing. Source: Dale Whittington, Subhrendu K. Pattanayak, Jui-Chen Yang, and Bal Kumar K.C., (Feb 2002), Do Households Want Privatized Municipal Water Services? Evidence from Kathmandu, Nepal 23
  24. 24. Conjunctive Use of Groundwater • Conjunctive use — Often used in discussing water supplies and water conservation. This phrase usually is used to describe the practice of storing surface water in a groundwater basin in wet years and withdrawing it from the basin in dry years. • Conjunctive use is the coordinated management of surface water and groundwater supplies to maximize the yield of the overall water resource. An active form of conjunctive use utilizes artificial recharge, where surface water is intentionally percolated or injected into aquifers for later use. A passive method is to simply rely on surface water in wet years (months) and use groundwater in dry years (months). • Conjunctive use is becoming a key part of the overall water management strategy in terms of coping with a growing population. In southern California basins, about 21.5 million acre-feet of additional conjunctive use potential is available, according to the Association of Ground Water Agencies. Source: 24
  25. 25. Groundwater Pollution: Point and Non-Point Sources Human development has added many potential pollution sources that may contaminate the groundwater supply. Pollution may be associated with specific identifiable point sources such as leaking storage tanks or may not be traceable to a single point of origin but may occur over a wide area (non- point source) such as croplands. Urban sources of groundwater pollution • LUST • Industrial dump • Septic tank • Sewage leakage • Polluted water bodies recharge • Landfill • Sea water intrusion • DNAPL / LNAPL 25
  26. 26. • Pollution that impacts water quality is divided into point and nonpoint sources. Point source pollution is discharged from a known source, such as a wastewater treatment plant or a factory. Point sources are monitored and regulated to control discharges. • By far the leading cause of water quality problems is nonpoint source pollution, the accumulation of runoff from city streets, construction sites and agricultural fields, spills and abandoned mines. Contaminants are picked up by rainfall, snowmelt and urban runoff and carried to creeks, rivers, lakes and even groundwater. Some examples of nonpoint source pollutants include fertilizers, herbicides, oil, grease and sediment. • It was once thought that polluted water would be naturally filtered as it seeped underground, but that is not the case. Many industrial chemicals are highly persistent and do not break down in soil. Consequently, more technologically advanced testing techniques are necessary to detect contamination. The full extent of groundwater contamination is not known, but the number of threats has increased, forcing the closure of thousands of wells. • Nonpoint sources are difficult to regulate because of their diffuse nature and so are dealt with through management measures that stress prevention and cost-effective, low-tech solutions. • Because of the collective impact of multiple pollutants on drinking water supplies, recreation, fisheries and wildlife, officials say the solution to the problem lies in educating people that they all have a part to play in minimizing the amount of pollutants that originate in a watershed. Among other things, this includes carefully following directions when applying lawn fertilizers, curbing pet waste and ensuring motor oil and other harmful chemicals are kept out of storm drains. Information about steps you can take to reduce runoff can be found at this website, Source: 26
  27. 27. Potential groundwater pollution sources in the U.S • approximately 3,000 landfills and thousands of illegal dumps that may leak a chemical soup of waste liquids; • 23 million domestic septic systems that serve homes beyond the reach of municipal sewer systems; • five million active and inactive underground storage tanks used to store products such as gasoline and industrial chemicals; • over one million abandoned and active oil and gas wells that produced crude oil mixed with brines and water/mud mixtures; • thousands of active and abandoned coal and metal mines many of which yield acidic run-off that percolates into the groundwater; • thousands of tons of animal wastes concentrated in areas of livestock (chicken, pig, cattle) farms; • millions of tons of fertilizers, pesticides, deicing salts and other materials added to the land surface annually. Is it different in Nepal? 27
  28. 28. Non-point pollution Non-point sources of pollution in agricultural regions are difficult to detect but can have some of the most far reaching effects as rural well waters are not monitored by municipal water treatment plants. Pollutants that may be present in rural wells include pesticides (herbicides, insecticides, fungicides) and nitrates that are products of fertilizers. Both pesticides and fertilizers are applied to crops and some are washed off into the groundwater and surface water systems. 28
  29. 29. The Environmental Protection Agency (EPA) is responsible for enforcing water quality standards for drinking water. Some of the common pollutants that the EPA recognizes in drinking water are listed below. • Contaminant Health Effect Selected sources Organic Chemicals • Benzene cancer leaking fuel tanks, industrial solvent • Toluene kidney disease chemical manufacture, industrial solvent • Trichloroethane cancer dry cleaning & industrial solvent Inorganic Chemicals • Arsenic cancer, skin lesions rocks, pesticides, industrial wastes • Nitrate blue baby syndrome fertilizers, feedlots, sewage • Lead nervous system damage corrosion of lead pipes Microbiological • Cryptosporidium stomach illness human/animal wastes What are the national water standards in Nepal? 29
  30. 30. Problems of Groundwater Pollution in Nepal Kathmandu • Fecal coliform bacteria in Kathmandu • In Kathmandu valley, iron, nitrate, phosphate concentration exceed drinking water standard in shallow tube wells and dug wells. Shallow aquifers are extensively polluted by sewage and polluted rivers. • High ammonia, methane concentration in deep aquifer. Terai • The overall groundwater quality of Terai is quite good except some high values of iron in most of the Terai region. • Recently, high (> 100 ppb) value of Arsenic were reported in some shallow aquifer of central and western part of the Terai district in Nepal. 30
  31. 31. Source: Sharma, C.K., 1987, Chemical pollution of the soil and groundwater in the Kingdom of Nepal 31
  32. 32. Arsenic (in groundwater) effect in Bangladesh 32
  33. 33. Nepal, Siraha District: forced to drink arsenic-contaminated water, October 14, 2009 • Thousands of people [in Siraha District, south-eastern Nepal] are forced to drink arsenic- contaminated water after the Drinking Water Office [failed to] provide [an] alternative to tube-wells with high concentration of arsenic. Three years ago, the District Drinking Water and Sanitation Division Office Lahan had detected arsenic beyond national standards in the water of 1234 tube- wells in the district and proved the water unfit for drinking. • The office had conducted arsenic tests in tube wells of 29 VDCs and detected arsenic beyond 50 ppb (parts per billion) in 2.83 percent out of 46,625 tube-wells in the district. Arsenic has hit hard the Hanuman Nagar, Khirauna and Hakpara VDCs the most. More than 50 ppb arsenic has been detected in 35 public tube-wells in Khirauna VDC. According to Nepal’s National Drinking Water Quality Standard 2063, water containing more than 50 ppb arsenic is considered unsafe for drinking. • Bramha Dev Kamati, former chairman of Hanuman Nagar VDC said, “It has been more than two years since the drinking water office prohibited us to drink water from the tube-wells after detecting arsenic in them but the office has not provided any alternative of tube-wells to us.” • “Alarmed by the arsenic presence, the Red Cross has distributed 1,100 Kanchan filters to the people in the arsenic-affected VDCs free of cost,” said Raj Dev Yadav, chairperson Red Cross Siraha branch. He also said that they have been launching awareness programmes to make people conscious about arsenic. • Source: By: Bharat Jardhamagar, Kantipur / NGO Forum, 20 Sep 2009 33
  34. 34. Boundary Conditions & Transport Process in Aquifer System Types of boundary conditions: • No flow • Variable flow • Head dependent flow • Flow along boundary • Flow along and across boundary • Constant head • Variable head • Sources and sinks (wells, drains, rivers, springs, lakes, ponds) • Transport process (the flow into and out of the aquifer ) in aquifer system in influenced by the boundary condition. Clear understanding of the boundary conditions is required for conceptual, analytical or numerical modeling of transport process in aquifer system. 34
  35. 35. Boundary conditions at the model site • East and West boundary cells set as GHB cells in top layer (Layer 1) • North and South boundary cells set as no flow cells • Known springs set as constant heads • 7 sets of horizontal drains set as drain cells • Vertical collection wells set as extraction wells • Surface drains set as drain cells • Cells representing river set as constant head (west boundary cells) and inactive (west of constant head cells • All other cells set as active Legend Springs Wells Drains Boundary Conditions (Example) 28 Model constructed with these BCs, and ran until calibrated. 35
  36. 36. Groundwater Utilization in Nepal GW Potential in Nepal Hills and Mountain Region • GW resources have not been investigated yet in the hills and mountains region. • The annual groundwater reserve in these regions is estimated to be at least 1,713 MCM (Kansakar, 2001). Terai Region • Various studies have estimated groundwater reserve or annual recharge, ranging from 8,800 MCM to 11,598 MCM in Terai region. Based on discharge and catchment area of river, total groundwater recharge estimated in Lesser Himalaya is1713 MCM (Kansakar, 2001). However, using average precipitation of 1500 mm, recharge area of 61345 km2 , and recharge rate of 5%, total recharge is about 4600 MCM. Kansakar’s study did not consider a number of factors such as subsurface flow. Hence, comprehensive hydrological study is needed for better approximation of annual recharge in Lesser Himalaya. Source: Khan, S. P. and Tater, P. S., 2006, Hydrogeology and Groundwater Resources of Nepal, GWRDP/DOI 36
  37. 37. Groundwater Uses in Nepal At present, there are • over 800,000 shallow drinking water wells (DWSS) • 70,000 govt. assisted shallow irrigation tube wells. • At least, another 30,000 STWs in private sector. • About 20,000 Treadle/Rower pumps. • About 1,000 DTWs are in operation for irrigation and drinking water supply. Source: Upadhyay, A. K., MoWR, GoN, 2008 37
  38. 38. • The amount of groundwater is 68 times the amount available in all the surface water bodies at any given time (rivers, lakes, reservoir). • 20% of all the global water use comes from groundwater resources. • Over 60% of the world population depend on groundwater for their drinking and domestic water uses. • Agriculture sector, which consumes over 80% of the total water used by man, is depending increasingly on groundwater. Global groundwater availability and use Source: Upadhyay, A. K., MoWR, GoN, 2008 38
  39. 39. Groundwater for drinking water and ecosystem in Nepal • Nearly 50% of the country’s population, who live in Terai, depend solely on groundwater for their domestic water supply. • A majority of the population living in the hills and mountains also meet their domestic water demands from spring water sources, which are the natural discharges of ground water. • Base flows in all the rivers are maintained by natural groundwater discharge and snowmelt. • Groundwater is an important element in maintaining the ecosystem. Source: Upadhyay, A. K., MoWR, GoN, 2008 39
  40. 40. • Groundwater Investigation initiated in 1967. • GW Development Potential Area * : • 7,26,000 ha STW(<50meter) • 3,05,000 ha STW (Marginal) • 1,90,000 ha DTW(>50meter) • Total Irrigated area through Groundwater: 253242 hector *Shallow Aquifer Investigation, UNDP-1987-1992, 20 districts terai bhawar Groundwater Investigation, tube wells and irrigation through groundwater in Nepal Source: Upadhyay, A. K., MoWR, GoN, 2008 40
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  43. 43. Groundwater Use Tera i 43
  44. 44. Groundwater and support to livelihood in Nepal • Over 20% of the irrigated area in Nepal, and over 30% of the irrigated area in Terai is served by groundwater resources through tubewell systems only (CBS 2001 and DoI 2008). (In India, it is 60-70%; Shah, 2006). • GW has the potential to irrigate 75% of the cultivated land in the Terai, year-round. APP and NWP has planned GW Irrigation development in 45% of area. • In the hills and mountains, drinking water supply and to some extent, irrigation, can be significantly improved through systematic development of GW resources. • It is the source of water for animal husbandry and poultry in the Terai, and to many in the hilly regions as well. • Most industries, in Terai or in other regions, rely on groundwater for their water needs. Source: Upadhyay, A. K., MoWR, GoN, 2008 44
  45. 45. GW in the Hills and Mountain Region Importance of GW to the hills people: An Example of Jhiku Khola basin, in Kavre District • Basin Area: 11,141 ha. • Population: 48,728 Stream Discharge: • Annual Mean Discharge: 1.559 m3 /s. • Minimum Discharge: 0.002 m3 /s. Spring Sources (Groundwater) • There are more than 400 spring sources. • 319 of them are used as community water supply sources. (Source: Shrestha et al., 2000). Source: Upadhyay, A. K., MoWR, GoN, 2008 45
  46. 46. Tasks Ahead - GW Management • Kathmandu valley is already a good example of groundwater mismanagement in Nepal. • Its shallow aquifers are over-exploited, unprotected, and uncontrolled. • Water wells and Dhunge Dharas have dried up in many areas. • Shallow groundwater is highly contaminated due to unregulated and unsafe sanitation practices. • Deep aquifers are increasingly under stress, and there is no mechanism to regulate new deep well construction, nor their operation. • Worse, the groundwater characteristics is poorly understood in Kathmandu valley, and there is no serious effort in this direction. Source: Upadhyay, A. K., MoWR, GoN, 2008 46