Characteristics of Groundwater Systems Session GWD 1_2: Characteristics of Groundwater Systems How groundwater moves Interconnection and permeability The interconnection of the pores in rock or soil allows water to: enter the groundwater, be stored in an aquifer below ground, flow through the subsurface, discharge naturally to a river, swamp, spring or the sea, be recovered in a well or bore How does all of this occur? - Again it goes back to the geology of the area and the different types of soil and rock. EXERCISE 2: Bore Data Analysis Exercise Hand out of Bore location map and Table of data Q: What features are shown on the map A: Ground elevation contours, a river, scale, locations of bores, direction indicator Things to note about the data – Different names of the wells – can occur because of different sources, different bore owners (eg Govt well, NGO, local community owned well etc) This is a good data set with depths and elevations of all bores /wells known from surveyed levels. If the elevations weren’t known, how could they be approximated (Answer: estimated from the top contour) Depth to water is the most easily measured piece of data (we’ll talk about that in Monitoring) Tasks: Using the data from the table, work out the water table elevation for June Groundwater Drilling and Development _Session GWD 1_2 Page 1 Characteristics of Groundwater Systems Plot the water table elevation at each bore / well on the map Contour the water table elevation - work out flow direction based on the groundwater elevations – Q: Which direction is the hydraulic gradient? A: Generally southeast What topographic feature is the likely discharge point for groundwater in this area – Work out the groundwater elevations for January Q: Are the levels the same? What are the differences? A: There is clearly a seasonal variation of around 1-2m lower seasonal variation. Do all bores show the same variation What are the differences and what could they mean? Bore 4: is 5.8m higher – this is possibly a bad measurement or the bore could be damaged Bore B has had no change – suggests an incorrect reading or missed when the measurements were taken so the June reading was copied HDW(2) has much greater decline than nearby bores/ wells (HDW(1) and bore 5) – could be due to much greater use, lower rate of recovery after use suggesting lower yield. in this particular case it would indicate limited potential for this well as a large volume source. Prepare a cross section of the water table and the ground surface – work out the depth of water in each bore at both June and January Q: How is the water table related to the topography What inferences can be drawn? A: The water table tends to mimic the ground surface and tends to flow towards drainage lines such as the river in this exercise Groundwater Drilling and Development _Session GWD 1_2 Page 2 Characteristics of Groundwater Systems Participants are to appreciate that it is important to collect good quality monitoring data (see later section) as erroneous measurements may be misinterpreted. This is needed for proper planning such as predicting if a resource is likely to run dry or change quality (by reversal of hydraulic gradient). Q&A: Different terrains in which groundwater might be possible source – how would you tell? Groundwater Drilling and Development _Session GWD 1_2 Page 3 Characteristics of Groundwater Systems Groundwater Drilling and Development _Session GWD 1_2 Page 4 Characteristics of Groundwater Systems Groundwater Movement through Aquifers The ease at which the groundwater flows is a function of the hydraulic gradient (mentioned above) and the properties of the aquifer. Movement of groundwater through the soil and within aquifers occurs by different pathways through different types of rocks as described in the section above. Depth of Groundwater Noted earlier that aquifers may occur at different depths below the ground in different aquifer layers The best (ie highest yielding, lowest salinity) aquifers are often hundreds or even thousands of metres below the ground surface. We mentioned this for Egypt and it is also visible on the cross sections that we discussed earlier Relevance to first phase emergency: This may not be helpful in a first phase emergency, unless the groundwater from these deep aquifers has already been drilled and developed for other purposes. As we will discuss later, deep aquifers require a substantial amount of effort to investigate and develop. So where these resources are known in some circumstances they may be able to be utilised. Permeability The permeability of a rock formation has an important impact on how groundwater flows to shallow and deep aquifers. Essentially, the more permeable layers allow more rapid and greater volume of groundwater flow. Some materials have little permeability and there is limited movement of groundwater - these are known as Aquitards. In the case where there is no ability for seepage to pass through a rock or soil this material is considered to comprise an aquiclude. The relative permeability of naturally occurring materials is shown in the following table (after Bear and Verruijt, 1987). Note that the permeability of different consolidated rock types is also reflected in the degree of fracturing that may occur as discussed previously which provides a secondary porosity. K (cm/s) 10² 101 Permeability 10−1 10−2 Pervious Aquifer potential Unconsolidated sediments / Soils 100=1 10−3 Good Clean Gravel 10−5 10−6 Semi-Pervious Clean Sand or Sand & Gravel 10−7 10−8 10−9 10−10 Impervious Poor None Very Fine Sand, Silt, Loess, Loam Peat Consolidated Rocks* 10−4 Stratified clay Oil Reservoir Rocks Groundwater Drilling and Development _Session GWD 1_2 Sandstone Unweathered Clay Limestone, Granite Page 5 Characteristics of Groundwater Systems Dolomite modified from Bear and Verruijt, (1987) Confined and Unconfined aquifers The presence of aquitards cause a build up of pressure in an aquifer as the groundwater is constrained from passing through. The water in the aquifer is therefore under pressure and is referred to as being confined . If the aquifer is tapped in a bore, the water level will rise up in response to the pressure. The distribution of pressure is called the potentiometric surface. The water table is at the same pressure as the atmosphere and is therefore said to be unconfined. When a well taps an unconfined aquifer, the water level in the well is the watertable. Unconfined aquifers are easier to establish bores or wells and are usually much shallower than confined aquifers. This will be discussed later in the session Groundwater Discharge Water will preferentially flow through aquifers that have higher permeability and will try to move to areas that have least restriction on flow. If the aquifer is confined by overlying aquitards this may require long flow times and the water may be contained subsurface until it eventually reaches a point where the constraint is released – this means the groundwater will discharge. Q & A : GROUNDWATER DISHARGE Q: if groundwater flows from areas of higher head to lower head, where do participants think it will ultimately end up? A: Ultimately the sea but also rivers or lakes Groundwater Drilling and Development _Session GWD 1_2 Page 6 Characteristics of Groundwater Systems A: Could also be at springs where the water table actually intersects the ground surface to allow the groundwater to seep or flow out at springs Follow up with the figure below to discuss concepts Based on Bear and Verruijt (1987) Discharge of Groundwater and Relationship of Groundwater to Topography Local and regional scale flow paths are affected by recharge location, discharge sites and aquifer types, with groundwater generally discharging at lowest points in the landscape. We saw this in the mapping of the water table – the water table followed the topography and graded towards the stream. Springs EXERCISE 3: Springs Go back to the cross section we drew – look at the water levels for January Now Sketch the watertable in bores 1 and A to be two metres below the surface Q: Would the water table intersect the ground surface? What would be the result of this? A: Springs would develop What are the implications as a water source for an emergency? Groundwater flows underground until the water table intersects the land surface and the flowing water becomes surface water in the form of springs, .In order to discharge as springs, the watertable in the recharge area must be higher than that in the area where it discharges. The Groundwater Drilling and Development _Session GWD 1_2 Page 7 Characteristics of Groundwater Systems more relief an area has, the more potential there is for groundwater to flow and thus to emerge as springs. Springs are a common source of groundwater arising from natural discharge. We saw in the section above that springs can seep out at the surface if there is a pathway for groundwater even before it reaches the water table – ie we have a perched water table Commonly the springs emanate from consolidated rocks such as limestone and crystalline rocks. Some springs are fed by shallow groundwater seepage out of the rock, others are fed by deep aquifer water discharged under artesian pressure that finds it way through the aquitard to the ground surface. Spring types under different groundwater conditions, After Davis and Lambert Springs can occur as individual identifiable “spring eyes” or as seepage across a less well defined area. . Springs can be perennial or seasonal depending on the particular circumstances. As water table (or water pressure in confined aquifers) falls and there is a decreasing rate of groundwater flow through the rocks forming the springs, the spring flow decreases. Interaction with Rivers Groundwater discharge to rivers occurs when the water table is at a higher elevation than the water in the river and there is a hydraulic gradient towards the river. The groundwater discharge contributes to baseflow in the river particularly in dry seasons From Reid et al 2009 Groundwater Drilling and Development _Session GWD 1_2 Page 8 Characteristics of Groundwater Systems At periods of higher flow in a river, the hydraulic gradient can reverse and there is recharge from the river into the groundwater. This is often a seasonal feature and in the case of some arid zones, provides the recharge that maintains the groundwater source for the future (see Case study 1) From Reid et al 2009 In some instances the water table may be naturally well below the river level and there is no direct connection. In this case, leakage through the bed of the river to teh aquifer can occur. From Reid et al 2009 Similar situations occur where there are lakes or wetlands. Relevance to an Emergency The conditions around streams can be important in an emergency where groundwater and stream water are both part of the water source. A reduction in the amount of groundwater migrating to the stream can limit the available supply particularly in dry periods. In wet seasons, flow of water in rivers (eg wadis in Africa) can be direct source of water, fill the layers of sand in the river bed to become a groundwater supply once the floods recede, and also allow seepage from the river into surrounding rocks to replenish groundwater resources taken from bedrock. Coastal Zones Under natural undisturbed conditions, there is a state of equilibrium in which sea water beneath the freshwater in coastal zones. This saline water is often referred to as the “salt water wedge”. Freshwater is less dense than salty water. To maintain the equilibrium and keep the salt water wedge from moving inland, the fresh water has to be at a higher elevation than the adjacent Groundwater Drilling and Development _Session GWD 1_2 Page 9 Characteristics of Groundwater Systems waterbody in order to stop the salty water displacing the fresh water. It is this principle that provides fresh water to many low-lying island systems. As the better quality groundwater is pumped, the head in the freshwater aquifer is lowered and there is the potential for saline water to move landward. Excessive pumping can result in total depletion of the fresh water, as the salt water mixes in with it. If a well is located above the interface, salt water can cone upwards towards the well. Care is needed to avoid contamination of these aquifers with salty water. This is a problem that needed to be dealt with in response to the Indian Oceam Tsunami (See Case Study2 – below). Groundwater Drilling and Development _Session GWD 1_2 Page 10 Characteristics of Groundwater Systems Case Study 2: Impacts of the Asian Tsunami on groundwater in Sri Lanka Summary The Indian Ocean Tsunami impacted coastal sandy aquifers were affected on the eastern side of SriLanka by causing inflow of salt water into wells. These aquifers contain a lens of fresh water sitting on top of the denser saline sea water. Recharge to these aquifers is typically rainfall infiltration. The Tsunami caused physical damage to many of the wells in the coastal areas, including filling with rubbish and even removal of well lining from the groundwater. When the tsunami struck salt water permeated into the entire aquifer, and thus all of the water supply being highly contaminated by salt. Immediately after the tsunami wells in the area were extensively pumped in the area by NGOs, civilians and armed forces as it was believed that this would clean the wells. Pumping on the wells led to increased EC. The groundwater wells often experienced an upconing effect when being ‘pumped clean’ after the tsunami. Many I/NGOs were attempting to pump out the salt water, however due to the increase in the size of the salt water wedge and excessive pumping resulted in the salt water being drawn up from the salt water wedge and increasing the salt water concentration at the borehole location Figure 23 - Groundwater Flow Pre-Tsunami (Lytton, 2008) Groundwater Drilling and Development _Session GWD 1_2 Page 11 Characteristics of Groundwater Systems Groundwater Flow Post-Tsunami (Lytton, 2008) Groundwater Drilling and Development _Session GWD 1_2 Page 12 Characteristics of Groundwater Systems Emergency Situation On the 26th of December 2004 a tsunami was triggered by a 9.0 magnitude earthquake off the coast of Sumatra. The wave caused destruction to many countries around the Indian Ocean. The tsunami struck the east and southern coast of Sri Lanka killing 40,000 people, and destroying numerous houses and water supply wells. Up to 450,000 people were displaced in Sri Lanka (Tamil Information Centre, 2006). This case study is prepared to outline some of the impacts of the Tsunami on the groundwater resource in coastal settings Aquifers in Sri Lanka: Aquifers in Sri Lanka are described by Panebokke and Perera ( 2005) The geological basement in much of Sri Lanka consists of high grade metamorphic rocks. The metamorphic rocks are low permeability crystalline materials although weathered material (regolith) on the upper exposed outcrop areas forms aquifers in some areas. Deep confined aquifers are located in the northwest of the island and Miocene limestone that occurs in the north of the country comprise karstic aquifers. The coastal zone on the eastern side of the country has extensive shallow coastal sand and alluvial aquifers that are highly permeable. These are the aquifers of greatest interest in this case study. Groundwater is highly utilised in many coastal areas from the coastal sand and alluvial aquifers. The majority of users are undocumented as they are private dug wells. Aquifers in SriLanka (after Panabokke and Perera, 2005) Groundwater Drilling and Development _Session GWD 1_2 Page 13 Characteristics of Groundwater Systems Coastal Sand Aquifers Along most of the coastline there are shallow aquifers located on the coastal sands. Groundwater recovered from these aquifers support agriculture (chilli and onions), domestic use by the local population and the tourist industry (Panabokke & Perera, 2005). These aquifers contain a lens of fresh water sitting on top of the denser saline sea water. Recharge to these aquifers is typically rainfall infiltration, which falls during the maha season (October – January). Due to the varying rainfall patterns the fresh and brackish water boundary expands and contacts over time (Panabokke & Perera, 2005). Over-extraction of the aquifers can cause a cone of upwards intrusion into the aquifer, resulting in a saline bore which can no longer be used (Cooray, 1967; Panabokke & Perera, 2005). Alluvial Aquifers Alluvial aquifers are located in or around current and old river beds, and vary in depth, extent and yield (Cooray, 1967; Panabokke & Perera, 2005). These aquifers are generally alluvial sediments, sands, clayey sands, and gravels which have been deposited, and then overlain with finer sediments. The beds extend several hundred meters either side of a river and up to 35m deep (Panabokke & Perera, 2005). Thus the water supplies to these aquifers are quickly recharged, and can support high extraction rates to varying degrees without significant drawdown effects. Types of Wells There are two key types of wells in Sri Lanka: tube and dug wells. Dug wells are generally shallow, water table aquifers which use the resources unconfined aquifers, often they are not more than 5 - 7m in depth and approximately 1-1.5m wide and have concrete casing. These are the most common types of wells in Sri Lanka, and are used for domestic water supply (Panabokke & Perera, 2005). The location and number of dug wells is not recorded in Sri Lanka. Tube wells are much smaller in diameter, and draw resources from much deeper in the aquifer. They often screen unconfined aquifers, not reachable by dug wells, and service both domestic, agricultural and industrial uses. They are often constructed with PVC pipe and fitted with a hand or motorised pump. Groundwater Drilling and Development _Session GWD 1_2 Page 14 Characteristics of Groundwater Systems Distribution of tube wells in Sri Lanka (from Panabokke and Perera, 2005) Impact of the Tsunami on Groundwater Damage to Wells The Tsunami caused physical damage to many of the wells in the coastal area, including filling with rubbish and even removal of well lining from the groundwater. When the tsunami struck salt water permeated into the entire aquifer, and thus all of the water supply being highly contaminated by salt. In a study by (Saltori & Giusti, 2006) it was noted that in the few day following the tsunami that the water within local drinking wells was up to 28,000μS/cm, however decreased to 3,000 – 8,000μS/cm in a few weeks. The Ampara district experienced three methods of salt water intrusion after the tsunami (Lytton, 2008): infiltration from sea water, the strength and pressure of the wave causing an underground saline wave into the aquifer (indicated by several witness reports of geysers beyond the wave extent), and the lagoons situated behind the dunes of the beach filling with saline water and slowly releasing it into the groundwater system (Lytton 2008). Lytton found EC decreased with distance from the coast line. Most wells, even those not inundated by the wave, still recorded values over that acceptable to the human palate of 1800uS/cm at the time of testing. Bacterial content of the wells recorded colonies too numerous to count. Leclerc et al.’s (2007) testing results concluded that the main cause of saline intrusion in the area was from direct inundation. Their testing regime involved wells in one local area, and then over an extended area, and comparing wells of similar conditions. Groundwater Drilling and Development _Session GWD 1_2 Page 15 Characteristics of Groundwater Systems As a result of the tsunami, large numbers of wells were without water due to well pollution due to saline intrusion or to pollution related to latrines and burial pits in the vicinity of wells. Saltori and Giusti also concluded that with the elimination of fish industry immediately after the tsunami that the agriculture industry increased and that with the electric pumps supplied by aid agencies that the agricultural industry could be contributing to the salinity problems in the area due to up coning, as well as contamination from nitrates and pesticides. Rehabilitation of wells Immediately after the tsunami wells in the area were extensively pumped in the area by NGOs, civilians and armed forces as it was believed that this would clean the wells. In addition, many members of the public believed that pumping need to take place in order to be able to use their well again. Various studies showed that pumping on the wells led to increases the EC. Well cleaning was taking place in this area by various organisations, however most did not take readings before or after their activities, and frequently the extracted water was dumped next to the well, allowing it to permeate back into the aquifer, causing a contamination loop (Lytton, 2008). It appears that pumping the well only destroys any remaining fresh water lens within the aquifer by either pumping it to its extent, the pumping causing mixing of fresh and saline water, or the recycling of the water causing in effect the same situation as another tsunami would do locally. The research concluded that there was no way to clean and restore the aquifers in the short term. As a result of the problems being produced in attempts to clean the wells, by the end of 2005 UNICEF produced guidelines on the cleaning of wells and the processes to be undertaken (UNICEF, 2005). The rain events in this area seem to have decreased the EC in the wells, however in some cases it caused an increase between events perhaps as salt stored in the soils leaching into the groundwater system (Piyadasa et al. 2006). The groundwater wells often experienced an upconing effect when being ‘pumped clean’ after the tsunami. Many I/NGOs were attempting to pump out the salt water, however due to the increase in the size of the salt water wedge and excessive pumping resulted in the salt water being drawn up from the salt water wedge and increasing the salt water concentration at the borehole location (Figure 24). The pumping was not beneficial for this reason, additionally many organisations when purging the bore let the contaminated water flow straight onto the ground next to the bore hole, causing recontamination (Lytton, 2008). In alluvial aquifer systems it was noted that while the salinity decreased over time, in places the salinity spiked after the first rain period. This was due to a flushing of the salts captured in the soils by the fresh water as it entered into the groundwater stream. References Bhattacharya, & al, e. (2008). Groundwater for Sustainable Development: Problems, Perspectives and Challenges. London: Taylor and Francis Group. Cooray, P. (1967). An Introduction To The Geology of Ceylon. Colombo: The Government Press. Leclerc, J.-P., Berger, C., Foulon, A., Sarraute, R., & Babert, L. (2007, May 7). Tsunami impact on shallow groundwater in the Ampara district in Eastern Sri Lanka: Conductivity measurements and qualitative interpretations. Retrieved March 10, 2010, from Science Direct: www.sciencedirect.com Lytton, L. (2008). Deep Impact: Why post-tsunami wells need a measured approach. ICE , 42-48. Groundwater Drilling and Development _Session GWD 1_2 Page 16 Characteristics of Groundwater Systems Panabokke, & Perera. (2005, January). Groundwater Resources of Sri Lanka. Retrieved March 2010, from http://tsunami.obeysekera.net/documents/Panabokke_Perera_2005_Sri_Lanka.pdf Piyadasa, R., Weerasinghe, K., Lakmal, H., & Maier, D. (2006). Groundwater quality changes in the tsunami affected coastal belt - Southern Sri Lanka. 32nd WEDC International Conference (pp. 320326). Colombo: SUSTAINABLE DEVELOPMENT OF WATER RESOURCES, WATER SUPPLY AND ENVIRONMENTAL SANITATION. Saltori, R., & Giusti, A. (2006). Challenges of tsunami and conflict affected rural water supply in Sri Lanka. 32nd WEDC International Conference (pp. 523-529). Colombo: SUSTAINABLE DEVELOPMENT OF WATER RESOURCES, WATER SUPPLY AND ENVIRONMENTAL SANITATION. Tamil Information Centre. (2006, March 15). Sri Lankan Tsunami Situation Report: Report Number 6. Retrieved April 2010, from UNEP: http://www.unep.org/tsunami/reports/Tsunami_Report_No_6.pdf UNICEF. (2005). Guidelines for the rehabilitation of tsunami effected wells. Galle: UNICEF. Villholth, K., Manamperi, A., & Buergi, N. (2006). Chemical Characteristics of Tsunami-Affected Groundwater and Lagoon on the East Coast of Sri Lanka. 32nd WEDC International Conference (pp. 334-340). Colombo: SUSTAINABLE DEVELOPMENT OF WATER RESOURCES, WATER SUPPLY AND ENVIRONMENTAL SANITATION. Groundwater Drilling and Development _Session GWD 1_2 Page 17 Characteristics of Groundwater Systems Climatic influences: Influences on aquifers Climatic influences including seasonal variations in amount of water that can infiltrate. As discussed earlier, rainfall and evaporation are key influences in the amount of water that may enter the soil. This can also be a seasonal factor. To illustrate this Dillon et al (2009) plotted the ratio of rainfall in the driest six months /total rainfall against the ratio of evaporation / rainfall for a range of locations. This shows the huge difference in the potential for recharge across the world Source: Dillon et al 2009 The impact on aquifers at a particular location can also change with time, with influence on groundwater availability. The seasonal recharge that occurs from flood waters is a desired consequence of the wet season in many arid environments. There is a potential impact of climatic events (floods particularly) on water quality with the introduction of fresh water in areas of groundater recharge The effect of seasonal climate impact on aquifers can be seen from plots of groundwater level over time (this is a bore hydrograph), especially when compared to climatic events. An example of a representative bore from the Murray Basin in Australia (after Macumber 2009) shows the rise in the water level in the bore after a wet period (1973) when extensive recharge occurred, followed by seasonal fluctuations due to groundwater abstraction pumping. Of particular note is the decline of groundwater level from 1992- 2006 which coincides with a combination of groundwater pumping and a prolonged period of below average rainfall particularly from 2000 onwards. Groundwater Drilling and Development _Session GWD 1_2 Page 18 Characteristics of Groundwater Systems After: Macumber, P. G., 1991. Interaction between groundwater and surface systems in northern Victoria. Prepared for Department of Conservation and Environment – Victoria. Macumber, P. G., 2007. Groundwater Occurrence and Process in the Mid-Loddon WSPA. Report prepared for GoulburnMurray Water, June 2007. Group Discussion: What are the potential implications of climate change on groundwater ? Discussion points: Reduced recharge in areas where there is lower rainfall o lower availability o also potential for increased demand o perhaps increase in evaporation Some wetter areas to have higher rainfall and greater intensity o perhaps increased recharge o not necessarily increased groundwater recharge in the immediate area because of increased surface runoff o May be more recharge remotely from increased stream flows and leakage o with higher surface runoff, possible drainage into existing wells (will mention again later in pollution section), Changing water demand o as agriculture changes with climate change o perhaps as population moves Groundwater Drilling and Development _Session GWD 1_2 Page 19 Characteristics of Groundwater Systems Variability of Water quality Groundwater is a solvent that is in contact with a wide range of earth materials (Fetter, 1992). The chemistry of groundwater is complex and varies on location. Even in the same aquifer there are differences depending on the time that water takes to flow through the aquifer, the recharge rate of fresh water, the rocks that the groundwater flows through and whether there are reactions of the water with the mineral particles in the rocks. This is discussed later. Groundwater salinity is a primary indicator of water quality and there can be a wide variation in groundwater salinity in aquifers. Naturally occurring groundwater can be less than 100mg/L (milligrams per litre) to more than 50,000 mg/L (Hem, 1985). Salinity greater that 600mg/L becomes increasingly unpalatable (World Health Organisation), so not all groundwater is drinkable. There are numerous maps of groundwater salinity, and there is a series by Water Aid and the British Geological Survey describing salinity in the countries in which Water Aid works. An example of Aquifer salinity in two sedimentary aquifers in Sudan near Khartoum, which appears to be related to recharge from the Nile. Lower salinity (<500mg/L) is found near the rivers. From Elrail et al 2009 In addition to salinity, there are naturally occurring chemicals eg Arsenic, Fluoride, Iron that are dissolved in groundwater and that make it unfit for or undesirable for drinking. (We will discuss these and contamination from anthropogenic pollutants later). Relevance to Emergencies The variation in salinity means that even if there is groundwater, the quality may be unsuitable for drinking. Groundwater Drilling and Development _Session GWD 1_2 Page 20 Characteristics of Groundwater Systems There are also naturally occurring chemicals which can make groundwater unfit for drinking Groundwater Drilling and Development _Session GWD 1_2 Page 21