Issues In Urban Hydrogeology

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Issues In Urban Hydrogeology

  1. 1. Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. Michael J. Dobbins Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254 USA ABSTRACT As the world’s population continues to grow, it has been estimated that most of the world’s population has moved to the cities; as of 2009 over 50% already live in cities. This growth in urban populations and resultant increase in water demand; increase in contamination, intensification of development, and other issues will amplify stresses on water resources. There are numerous issues unique to the urban groundwater interface; simply the introduction of man- made structures creates a substantial disruption to the normative water cycle model. The concept of an agglomeration of urban regions into what is now being called a Megaregion will cause increases in impervious cover, deep foundations and structures, trenches and tunnels, transportation and impervious cover captured runoff pollutants, interruptions in recharge, excessive recharge, overdrafting, aquifer recharge bypass and others. Problems caused by the development of Megaregions will not necessarily be new or even unique but they will be more numerous, spatially more diverse, and holistically more important. These stresses on groundwater supplies world wide and more specifically in Megaregions within the US will increase with time; yet with foresight, planning, and ‘best practice’ guidance and political will these problems can be managed and urban prosperity maintained. INTRODUCTION Populations world wide are growing at an accelerating rate. In 1990 Foster estimated that by 2000 over 50 percent of the worlds population (3.2 Billion) will live in urban areas. In fact, in 2008 half of the world’s population of 3.3 billion people lived in urban areas, and that number is estimated to grow to 6.4 billion by 2050 (UN-HABITAT, 2008); as of 2009 we have already 1 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  2. 2. exceeded Fosters estimate of 3.2 billion. In 2008 the Texas state demographer estimated that by 2040 the population in Texas will grow between 57 to 155 percent and that the vast majority of this growth will be in the Houston, Austin, Dallas/Ft. Worth areas with El Paso, the Rio Grande Valley, and San Antonio coming in a close second (Texas State Data Center and Office of the State Demographer, 2009; Sharp, 1997). The global trend is for populations to move from rural regions into more urbanized areas, which strain urban infrastructures. Historically, the location of towns and cities has been determined by three factors; an adequate and consistent food supply, availability of trade and most importantly proximity to potable and navigable water (Foster and Chilton, 2003). This conjunction of a population center, agriculture, livestock, and water has always been a tenuous proposition with the potential for contamination, overuse and abuse, politicization, or even outright destruction leading to political instability, for example, as we see today in the countries adjoining Lake Chad (Waititu, 2009). Little has changed, prosperous urban areas still need food, trade and water but increasingly these have to come from longer distances and involve some mode of transportation and infrastructure. As urban populations increasingly densify, water issues will increase in scope, become more diverse, and increase spatially. Most of these issues exist today and there will be the addition of many new issues, some hydrologic, some technical and some political. But these problems are going to become increasingly more urgent with more potential dangers as our populations increase, compact into higher density urban settings and put increasing demands specifically on groundwater resources and the structure of the aquifer providing that water. Life and more specifically urban life cannot exist without a constant and reliable supply of clean and safe potable water. Megaregions Members of the Community and Regional Planning community along with sociologists and geographers have over years recognized a trend and unwittingly revived an idea of Robert A. Heinlein from his 1940 short story “The Roads Must Roll”. What Heinlein called “Roadtowns” are today being called Megaregions. The definition of a Megaregion is still evolving but there are some basic characteristics that are held in common; 1) spatial continuity, 2) shared history or culture, 3) transportation connectivity, and 4) economic and commercial compatibility (Lang and Neilson, 2009; Ross, et al. 2009, America 2050a, Dewar and Epstein, 2009). Examples of currently developing Megaregions in North America are the Texas Triangle of Dallas/Ft. Worth, 2 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  3. 3. Houston and San Antonio; the I-35 Corridor that runs from Kansas City through Wichita to Oklahoma City, Dallas/Ft. Worth, Austin and San Antonio; Cascadia running from Vancouver, Canada to Portland, Oregon and on to Southern California; the Great Lakes region running from Duluth to Chicago through Cleveland Buffalo and Rochester and potentially on to Montréal but excluding Ottawa. On a smaller scale there is the Twin Cities of Minneapolis and St. Paul, Dallas and Ft. Worth and the Research Triangle of Raleigh-Durham-Chapel Hill (Lang and Neilson, 2009; Ross, et al. 2009, America 2050a). Examples of Asian Megaregions are Tokyo- Nagoya-Osaka, Hong Kong-Perl River Delta (Dewar and Epstein, 2009). The primary commonalities between these are based on cultural similarities, economic synchronicity and transportation connectivity; no large-scale or regional consideration has been specifically given to development, ecological impact or resource management. However in defining the Piedmont- Atlantic Megaregion (PAM) - which runs from the Research Triangle to Charlotte to Spartanburg to Atlanta and ending at Birmingham, Alabama - ecologic and specifically water availability was a major consideration. Population centers in the PAM share a common transportation corridor, culture and industries; yet the actual measure used to delineate the boundaries of the PAM was watersheds (Ross et al., 2009). In this case, thought was given to water resources as well as economic and social factors. America 2050, which is a leader in development of the Megaregion concept, is beginning to recognize the role of water and the environment in Megaregions. They have begun incorporating water resources into several of their forums (America 2050b); it is also beginning to appear as part of standardized planning criteria (Dewar and Epstein, 2009). An important note; Megaregions are not created by urban planners but are the result of geographic, economic and sociological trends. Urban planners have just recognized that metropolitan regions tend towards agglomeration and now they are attempting to quantify and qualify how this agglomeration is proceeding, and to determining ‘best practices’ for the future. Anthropogenic Issues Historically it has been the nature of things that as humans expand and populate land they indelibly change the land. Agriculture, development, harvesting of forests, planting of monocultures, building, digging and growing by humans have left their marks. Megaregions will compact and intensify these anthropogenic changes creating indelible effects on groundwater and the aquifers storing this water. 3 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  4. 4. Impervious cover The advent of urban densification, ‘just-in-time’ manufacturing, and other factors have brought an increased need for transportation (specifically automotive transportation), living space, and retail and commercial space. It has been estimated (Ferguson, 2005) there were 1000,000 hectares of pavement in America (Frazer, 2005) Figure 1 shows the increase in the amount built or “impervious” cover as population density increases. Since 1947, America has been, in essence, evolving into a suburban society, growing ever outward requiring an increasing transportation capability and transportation infrastructure. Transportation in America has been focused almost exclusively on the automobile and the semi- tractor and trailer with the later as the primary mover of raw and finished goods and the former as the primary people mover. Roadways are the essential arteries for commerce and economic growth, with the advent of Megaregions along with increasing densification transportation requirements will increase; if Ferguson’s estimates hold true then transportation requirements will increase dramatically. It is also worth noting that impervious cover includes roofs and buildings as well as pavements, parking lots and roads. In fact it has been estimated that impervious cover can run to 80 percent of the land surface in urban and industrial areas (Foster, 1990). The problems with impervious cover are many; one of them is it provides a capture mechanism for particulates, phosphates, nitrates, antifreeze, organic and inorganic compounds, pathogens, metals, NAPLs, hydrocarbons, etc. that are concentrated then washed off with the next rain entering into the local surface and/or groundwater supply; it diverts recharge, increases sheet Figure 1: from Ferguson, 2005, p. 4 flow and encourages flash flooding (Ellis, 1997; Foster, 1990; Naik et al., 2008; Hall and Ellis, 1985; Garcia-Fresca and Sharp, 2005; Remmler and Hütter, 1997). Yet a more important issue exists; primarily that impervious cover can intercept and divert surface recharge away from the local aquifer (Foster, 1990; Frazer, 2005; Grischek et al., 2002). 4 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  5. 5. Lake Wingra in Madison, Wisconsin is a small .53 hectare lake that approximately 100 years ago was fed by 35 different springs. Today the area around Lake Wingra is fully developed and now the lake is being fed by only four streams; it suffers from algal blooms, turbidity and closure due to bacterial contamination (Frazer, 2005). Urbanization and the resulting increase in impervious cover appears to have intercepted recharge to the local aquifer leading to the drying up of the springs, which shows a transition in the local baseflow from aquifer fed springs to runoff and baseflow fed streams. In addition the flushing of the pavements during rains has concentrated pollutants into the lake. The hydrogeology of an aquifer has been significantly altered in this case and may not be recoverable; a known supply of groundwater has potentially been eliminated (Frazer, 2005). Recharge interception and diversion, as at Lake Wingra, can lead to recharge transference. Interception of meteoric water by impervious surfaces; channeled into storm water sewers; emptied into channelized and in all probability concrete lined streams and channels are diverted to far off locations. This interception and diversion into impervious collection systems creates a rain surge of the surface runoff to a point outside an aquifer’s recharge zone. Removing a potentially significant amount of recharge from one aquifer and transferring it to another and changing the water balance equation for both aquifers. New work has been done to determine just how “impervious” impervious cover is. Wiles (2008) evaluated the hydraulic conductivity and transmisivity of various paving surfaces in Austin, Texas. His evaluation in to the permeability of expansion joints and fractures in pavements found a generalized hydraulic conductivity (K) numbers 10-5cm/s to 10-2cm/s for urban pavements. This is equivalent to the hydraulic conductivity of glacial tills, silts or clean sorted sands. These are fairly significant conductivity values and it demonstrates that even though impervious cover does intercept recharge it may not be to the extent previously believed. It may be possible that significant recharge is possible, under the proper conditions, through impervious cover. Follow up investigations should be done to better determine permeability and hydraulic conductivity values and real world effects across a wider diversity of localities. A regional analysis of infiltration through this secondary permeability might yield some interesting recharge data. 5 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  6. 6. Generally impervious surfaces act as a collector of numerous contaminants which then are flushed into the local water cycle with the next rain. Research has found chlorides, nitrogen compounds, heavy metals, organic carbon, hydrocarbons, chlorinated-aromatic-halogenated hydrocarbons, fecal coliform and fecal streptococci, phosphorus, pesticides, coal tar pitch, calcium magnesium acetate (a de-icer), ethylene glycol and many other contaminants items as constituents of runoff from urban impervious surfaces (Foster and Chilton, 2004; Foster and Chilton, 2003; Foster, 2001; Hall and Ellis, 1985; Frazer, 2005; Remmler and Hütter, 1997; Fellman and Barker, 1997; Li and Barrett, 2008; Li et al., 2008). There is a litany of other hydrologic problems arising from impervious cover such as; flash floods and stream wash-out destroying local streams, elimination or increase of evapotranspiration, reduction in wildlife, heat island effects and reduced quality-of-life (Frazer, 2005; Hall and Ellis, 1985; Ellis, 1997). Ironically one unintended advantage of impervious cover is the significant reduction in transpiration of groundwater (Foster, 1990). With population and urban growth come more jobs and more need for supporting industries like grocery stores, airports, malls, office buildings and factories. With the advent of Megaregions the amount of impervious surface is going to increase significantly so the potential problems are also going to increase, maybe not in number but most assuredly in scale. Water Quality With the increasing population densities of the Megaregions point-source and nonpoint-source pollution will in all probability increase. Nonpoint-source pollution sources may even begin to aggregate to where they may be considered point or area-source contributors and that point- source sites may proliferate to the point that they are considered non-point or area-source polluters. The issues facing Megaregion aquifer(s) are no different then are being faced today; they just increase in scope, complexity, and diversity. Pollutants from waste water facilities, septic systems and septic collection networks are of a special concern and recently the issue of medicines exiting or escaping the waste stream is of growing importance. Current and historical industrial contaminants are always a concern. Surface water or runoff water can carry numerous contaminants from legal and illegal sources; dumping, leaky tanks, accidental spills and even construction are major sources of pollution as shown in Figure 2 (Foster, S. et al., 1997; Foster, 2001; Foster and Chilton, 2003: Garcia-Fresca and Sharp, 2005; Li and Barrett, 2008; Barrett et al., 1998). 6 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  7. 7. Much work has been done investigating the infiltration of human wastes, both fecal and household, into aquifers. Endemic to developing countries is the use of sewage ponds or septic tanks or even direct waste dumping; the developed world uses various types of multistage septic system that include physical waste removal and biologic waste removal capabilities. However even the best systems in the developed world have weaknesses. Research has shown that septic tanks and ponds tend to leak and/or overflow effluent (Chadah et al. 1997; Faye, et al. 2004; Pandit, et al. 2009) and that between 1971 and 1978 in the U.S. “… overflow or seepage of Figure 2: Quality and sustainability factors for urban groundwater (Eiswirth, 2002) sewage primarily from septic tanks or cesspools was responsible for 41% of the outbreaks and 66% of the illness caused by contaminated underground water…” (Craun, 1981). In the developing world and specifically in India and China, it is a common practice to use grey water to irrigate crops and turf. While the grey water may have been purged of solids and may have even had some biological purification, the odds are that it still has a significant colloidal load and has significant chemical and pathogenic content. The developing world also has a large problem with raw sewage seeping into local aquifers due to leaks in aging or poorly designed or poorly installed municipal sewage lines (Naik, 2008). Yet the developed world isn’t immune to this either. Vázquez-Suné et al. in 2005 determined that most of the aquifer recharge under Barcelona, Spain is due to leakage of both the fresh water and waste water networks while in 1997 Eiswirth and Hötzland found that the main source of groundwater pollution in the 7 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  8. 8. Plitterdorf/Rastatt region in Germany is due to leaking sewage lines. Of even greater concern is the amount of pharmaceuticals, endocrine mimics and other xenobiotics (chemical compounds found in extrinsic settings, such as human estrogen in fish) are being found in surface and groundwaters. Loehnert (2002) found significant amounts of clofibric acid, used to regulate blood lipids, and gadolinium, a rare earth metal used as an organic complex in diagnostic medical treatment, in Berlin’s groundwater Strauch et al. (2008) found several xenobiotics in the Leipzig and Halle/Saale areas. Currently, there are no standards for xenobiotics in drinking water, nor are there wastewater treatments that can remove pharmaceuticals and endocrine mimics. Some will breakdown over time, but others like BPA and t-Nonylphenol (used as a surfactant, an anti-oxidant in processed foods and pharmaceuticals, a hardener and stabilizer in plastics and pharmaceuticals) are persistent. Surface runoff can have a significant effect on water quality as has been discussed in the prior section on impervious cover. Surface flows are an area where surface water and groundwater begin to interface. In many localities surface runoff is captured in retention ponds and allowed to infiltrate naturally, but more commonly runoff is funneled to local streams or canals. This channelization of runoff is typified by high flux – short duration events, essentially flash floods (Hall and Ellis, 1985; Foster, 1990; Fellman and Barker, 1997; Pandit, 2009). This type of event means surface runoff and stream waters never have a chance to infiltrate into storage, so recharge waters are lost to the aquifer and the contaminants in the runoff waters then are then concentrated downstream (Foster and Chilton, 2004). Hydrogeologic Issues The concept of a Megaregion implies many things that may seem mutually exclusive, such as high density development and metropolitan sprawl, but the most important implication for this section is spatial extent. Megaregions tend to be defined by transportation corridors, cultural and industrial commonality, politics, and spatial adjacency. This means that a single Megaregion may span several different and hydrogeologically distinct aquifers and even several individual watersheds. The management and problems of a shallow riverine aquifer might be different then a gravel or a deep crystalline or a karstic aquifer. Yet they all could be part of a single water management area for a Megaregion. Thus, there are many issues in common and can be considered together. Aquifer recharge, impervious cover, overuse, physical effects on an aquifer, effects of surface water and groundwater flow are common issues to all aquifers. 8 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  9. 9. Aquifer Recharge The textbook case of aquifer recharge is shown in Figure 3, meteoric water infiltrates as direct recharge, surface runoff to surface water and water lost as evapotranspiration. In an urban environment this ideal cycle is broken and many new factors and sources of recharge are introduced (Figure 4). Conventional logic says that as an urban area increasingly pumps its ground water then dewatering will take place and the water table will decrease, but this isn’t always the case. It is generally accepted that an increase in impervious cover inhibits infiltration and encourages surface runoff and decreases aquifer recharge (Garcia-Fresca and Sharp, 2005; Coldewey and Meβer, 1997; Foster, 2001; Foster, 1990). In a Megaregion with an increasing amount of impervious cover there is the possibility that recharge will be all but eliminated from some aquifers and that a significant change in groundwater flow will occur and be exacerbated Figure 3: Normal aquifer recharge cycle (Lerner, 1997) by the already limited recharge due to arid conditions. The issue of aquifer recharge is of specific concern in arid regions and in areas where groundwater is essentially “mined”. This is the case in Las Vegas, what used to be known as the Las Vegas Wash, Rainbow Wash and other arroyos that ran through town have now become fully concreted capture channels, they run through the extent of the city and exit to the southeast into Lake Las Vegas, then into Lake Meade. This whole channelized drainage route is down gradient and for all intents and purposes bypasses the aquifer in the Las Vegas valley and empties into the Lake Meade surface water system. Another recharge related problem is excessive recharging to an aquifer which can be more detrimental then recharge bypass. Urbanization brings turf irrigation, storm water retention 9 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  10. 10. systems, fresh water, grey water, and sewage distribution systems along with standalone sewage systems each which supplement natural recharge. Urbanization today and the Megaregions of the future will also need to import a portion of its water and even acquire water from surface sources. All of this water becomes part of the fresh (potable) water distribution network where loss of water from leaking pressurized fresh water pipes has been estimated to be 15 to 30 percent typically and could be as high as 70 percent in antiquated or ill maintained water systems (Naik et al. 2007; Jiao, 2008; Foster, 1990; Eiswirth, 2002; Grischek et al., 2002; Lerner, 1997). Storm sewers and foul waste sewers can be both a supply of recharge and also a conduit for discharge. Both types of sewers are unpressurized; if the water table is lower then the sewer line then the sewer line can lose water to the aquifer at a rate estimated to be from 5 to 20 percent of the flow. If the water table is equal to or above the sewer pipe level, then groundwater can leak into the pipe and drain away. (Lerner, 1997; Vázquez-Suné et al., 1997) While leaking septic tanks are a concern the inherent design of a septic tank means that as it fills up with water or waste (or both), over time leaching out wastewater from overflows or laterals at the top of the tank. The implication is that wastewater will always discharge from a septic tank unless it is used at a very low level. The unintended effect of these various leaks is a consistent recharge of the aquifer underlying the urban area, leading to a rising water table. There can be many side effects of this such as is happening to Perth, Australia. Perth is underlain by four aquifers, one shallow Quaternary unconfined aquifer and three deep confined or partly confined aquifers. The municipality uses the deep aquifer for municipal water while the shallow aquifer is generally used by individuals. The confined aquifers are gradually being dewatered and much of this water ends up via leaking pipes, irrigation, sewage disposal, etc. into the shallow aquifer. There is some connectivity between aquifers, but not enough to drain the shallow unconfined aquifer into the deeper partly confined aquifers. This rising water table has caused flooding in some areas, and allowed more saline water from both the ocean and a deep saline aquifer to intrude into the deeper aquifers (Appleyard, 1999). Liquefaction can also be a concern; this was first investigated as a result of the 1971 earthquake in Burdur, Turkey. Because of the water saturation of the soils beneath Burdur, the approximately 6.0 magnitude earthquake caused sand and water fountains, liquefaction of soils, 10 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  11. 11. land slides, and associated damage to life and property and even flooding (Murck et al., 1996; Davraz, 2003). Another issue with a rising water table is structural; flooding of basements tunnels, subways, and corrosion of building structures, degradation of concrete, and ground swell with lifting and buckling of structures are just a few of the issues. Figure 4: Urban recharge schema (Vázquez-Suné et al., 2005). Aquifer Discharge Overdraft of an aquifer leads to a lowering water table and associated issues. There have been several articles written on various aspects of the dewatering of urban aquifers in major metropolitan areas such as Houston (USA), Mexico City (Mexico), Bangkok (Thailand), Osaka (Japan), San Jose (USA), Shanghai (China), Tokyo (Japan) and of course Venice (Italy). All these cities are in geologically different settings with various types of aquifers but have the same problem: land subsidence due to dewatering and matrix compaction and collapse (Holzer and 11 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  12. 12. Johnson, 1985). Subsidence can be quite a dangerous and costly problem; in Tbilisi (Georgia) several major buildings have sustained major structural damage due to foundation shifting. Around Moscow (Russia) (Dzhamalov and Zlobina, 2002) and throughout the Ural sink holes are appearing (Gayev et al., 1999). Around Tomsk (Russia) as head levels decrease, numerous perched and disconnected aquifers are appearing in place of the contiguous regional aquifer (Pokrovsky et al., 1999). In Mexico City there are “rising” buildings where the land around buildings is collapsing, in Houston and San Jose there are sinking buildings and lands that are now at or below sea level that still regularly flood and require dikes and pumps to stay dry (Holzer and Johnson, 1985). In various areas, hydraulic gradients have significantly changed introducing saline waters into groundwater water supplies (Garcia-Fresca and Sharp, 2005; Sharp, 1997). The compaction of the aquifer as part of subsidence does unrecoverable damage to the aquifer; while there is an elastic factor to compaction there is also an inelastic factor. Even with repressurization of the aquifer loss of both porosity and permeability results in the loss of storage (Holzer and Johnson, 1985). Megaregions may expect both problems of excessive recharge and too little recharge of aquifers that are side by side. While the deficit and excess recharge may be spatially separated both would be part of the same groundwater management region and same demand source. In a Megaregion the problems won’t be different; there just will be more variety. Urban Structures Typically as a city matures and density increases, the construction of large and deep structures - such as high rises, subways, utilities, and parking garages follow. A Megaregion will be the same but with more. What happens to the aquifer and how is groundwater flow affected, with an increase in these massive structures that tend to have deep support structures. Several of the issues discussed already have a direct effect on structural engineering. Subsidence, water table rise, liquefaction, water pollution are all problems for large engineered buildings. Large Substructures. When constructing a 60 story building, a pit is excavated to house the massive substructure that may be a simple pier foundation, or it may consist of several subfloors and then the foundation. Either way a large amount of material is dug up and replaced with concrete support structures. As a single structure this probably has little effect on groundwater, but in a Megaregion there will be clusters of these types of buildings, 10 or 20 12 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  13. 13. Manhattans or downtown Dallas’s clustered together over a 518 square kilometer area with all the requisite roadways, trenches with buried utilities, subways, tunnels of various types and sizes, construction projects. Numerous impediments, both large and small, have been introduced to the local - and regional - groundwater flow, and many new channels of higher conductivity (K) have altered the groundwater flow, both horizontally and vertically. One procedure for building a large foundation for a building entails installing a diaphragm around the whole foundation and then digging caissons into the ground as the supporting foundation. This impermeable diaphragm is in all essence a cofferdam with the intent of blocking groundwater flow from coming in contact with the foundation members and is visible in Figure 5 (Jiao et al., 2008). Figure 5: Impermeable foundation (Jiao, 2008) Another technique is to dig the pit or shafts and line it with a reinforced clay, chemical or cement grout sealant, to block water penetration and provide temporary wall support, then the foundation is poured within the lining (Forth and Thorley, 1997). This large 40-80 meter wide or wider obstruction can be 50 meters deep or deeper can act as a groundwater dam (Vázquez-Suné 13 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  14. 14. et al., 2005). When clustered together there is a similarity to a classic crystalline fractured rock problem, just on a much larger scale. One area that hasn’t yet been researched: what happens vertically with these structures? At 50 meters or deeper a foundation can effectively penetrate a shallow aquifer and through a confining or more/less conductive layer into another deeper aquifer. An immediate concern is the potential effects on a construction project, as discussed by Shaquor and Hasan (2008) in their report, about how the dewatering protocol for a particular project in Kuwait had to be reworked. In this case conductivity values (K) were incorrectly chosen and the excavation was consistently flooding. This project required more pumping to keep the excavation pit dewatered and re-analysis of the effects of the higher then expected conductivity (K). Other problems encountered was the possibility of a deleterious effect on the concrete, would the substructure require occasional or constant pumping to prevent flooding, its resulting drawdown cone, effects on local aquifer and what to do with the pumped water? What happens if you don’t pump the water from the sub structure? It seems that there would be a ‘ponding’ effect where the backfill material and drainage systems become storage. While this may be bad for the foundation could it be beneficial for the aquifer? In each of these cases it is fairly evident that the structure is an impediment to horizontal water flow but there seems to be no research on the effects of these structures on vertical flow or storage effects. A caisson drilled 50 to 70 meters into the ground can quite easily penetrate through a shallow aquifer and into a deeper possibly confined aquifer creating the potential for a hydraulic transmission. This needs further research. Linear Structures: Trenches and Tunnels. In the urban environment, utilities such as fresh and sewage water mains are buried for both practicality and safety. This burial can range from .3 meters deep in the southern US to approximately 2.5 meters deep in northern Minnesota and North Dakota to avoid freezing in the winter (Frankel, 2002, p. 5.29, Figure 5.3). But in major metropolitan areas utilities many be buried much deeper depending on grade and avoidance of other utilities. When these utility trenches are filled the backfill material generally runs to pea gravel aggregate on the bottom, sand beneath and around the pipe then more gravel then the original soil (Frankel, 2002, p. 6.68). In areas of shallow rock formations a trench is cut, drilled or chipped out of the rock layer then the utilities are then laid and buried in the trench. The use of sand and gravel cushions and protects the utility pipes during reburial and provides 14 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  15. 15. some pliability in the soil to protect from soil shifts or compaction. The sand and gravel in the trench also create soakways that provide drainage to prevent the pooling of fluids around the utility pipes and also drains off leaks from broken water pipes (Krothe, 2002). The same scenario is applicable for large storm drains, underground roadways, trains and subways. In each case a long linear structure in a trench filled with an almost impermeable core surrounded by a highly conductive material cuts across an aquifer, interrupting the base flow. Krothe conducted field tests on existing utility trenches in Austin, Texas, and found that the conductivity values (K) in the utility trenches were from one to 10 orders of magnitude greater then the native soil (Krothe, 2002). The effect is the creation of a large linear pore space which allows diversion of flow. Today in every city there are large utility trenches that house various utilities that run for miles; many cities even have larger tunnel structures. These tunnels could cause serious divergence in recharge and/or groundwater flow; in fact it could capture local groundwater flow diverting it into areas from where it is needed and expected (Krothe, 2002). Krothe found that these trench pathways can so effectively capture flow that flow paths can develop at right angles to the regional hydraulic gradient. The ease of flow through trenches means that pollution plumes can move down the trenches at 56 to 1175 percent faster then through native soils (Krothe, 2002). On the other hand, having these trenches means that pollution from pipeline breaks most likely would be retained in the trench, making remediation much more predictable and simpler. Unique Structures. London has always had always had problems with tidal flooding, so in 1974 the construction of the Thames Barrier commenced with the intention of blocking extremely high tides and storm surges to prevent the flooding of London and the London Plain. While the Barrier has been successful, it also has the potential of causing unintentional flooding by the blockage of the Thames and the raising of the adjacent water table (Gray and Foster, 1972). As Grey and Foster studied this issue; they also found that river walls installed along the Thames blocked much of the interflow between the Thames and the local groundwater baseflow. While it eliminated much of the influx of water from the river into bank storage during high tide it also prevented the drainage of the aquifer into the Thames during heavy rains causing localized flooding as shown in Figure 6. This is of specific concern in The City - London proper – because all of the old and many of the newer buildings rest on the London Clay. Various foundation methods have been used but they all essentially rely upon “floating” upon a stable but 15 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  16. 16. soft London Clay (Adams et al., 2000). In the past a lowering water table along with lower porewater pressures has strengthened the clay but a rising water table as is being seen today is increasing porewater pressure in the clay, causing swelling and lowering the strength and stability of the London Clay( Adams et al, 2000). Dewatering schemes are being considered to control the stability of the clay. Figure 6: Effects of the Thames on London groundwater (Grey and Foster, 1972). There is also the issue of urban areas, even countries, which are effectively below sea level. This introduces a whole new concern where the local hydraulic gradient is from the sea inland instead of from land to sea. This case presents essentially a reversed Ghyben-Hertzberg freshwater lens with an inland directed seepage face and a reversed Kohout convection regimen. Concomitant to this is the risk of salinization of groundwater supplies, forcing of contaminant plumes inland into regions of freshwater supplies, and flooding (Jacobs, 1999). All underground structures will have to be designed as if they were underwater. This is the case in The Netherlands. Amsterdam is profiled in Figure 7, showing the Metro subway is 4 meters below 16 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  17. 17. mean sea level (MSL) and that several other buildings are at the same level or lower, including the airport. Exacerbating problems massive and deep building foundations, shallow and/or deep trenches and tunnels, and locally unique factors such as flood barriers and river walls and sea level can have deleterious effects have on groundwater and aquifers. Megaregions will have the same problems but scaled up. Consider a Manhattan or a London with all their foundations, tunnels, and trenches compressed into a small area; for a Megaregion there could potentially be clusters of dozens of these high density areas within the Megaregion. The effect on the local and regional Figure 7: Sea levels in relationship to Amsterdam structures (Jacobs, 1999). groundwater flow(s) will need to be determined. Other Issues Taniguchi et al. found in 2008 that with variations in groundwater, in specific the lowering of the water table in urban areas and the resulting lowering soil water content, the heat island effect is exacerbated, enhancing low level ozone creation and generally reducing air quality, increasing the energy demand for cooling. It has also been found that in Cairo rising groundwater tables are menacing several antiquities (Amer et al., 1997). Climate change and its effect upon the hydrologic cycle are of a specific concern. While the exact effects are still being debated, some generalities such as sea level rise can safely be assumed. As these effects are further refined they, will need to be incorporated into groundwater models for Megaregions. 17 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  18. 18. Politics of Groundwater While a Megaregion may begin to appear as a single organism with like needs and complementary capacities, it is composed of independent political entities each with its own individual constituencies. Just like Garrett Hardens (1968) “Tragedy of the Commons”, there are renewable but finite resources to be divided amongst users. There is also the burden of historical precedent, state, federal and local laws, and even social and cultural restrictions. The politics of water can be a minefield for the unwary, the question of ownership, private and commercial use and allocation are all sensitive subjects but ones that will need to be dealt with in the future. Even if every technical problem is defined and a solution found for each, there still needs to be a social and political will and desire to solve or prevent the problems and institute good management practices. The problem is that there inevitably is a conflict of needs, wants and perspectives in the social sphere and even conflicts in the science. There is also the inevitable conflict over costs and money. Groundwater tends to be more susceptible to abuse then surface water, it seems to be a ‘out of sight-out of mind’ thought process, you can see pollution in a river or a dry river but you cannot see groundwater pollution or a dry aquifer. Water issues also can be tied up with significant emotional content, in the Southwest and Western United States the issue of water is one of individual and personal rights, ownership and in some cases survival. Getting community buy-in is a critical aspect of defining aquifer use. To be able to have the business community, residents, the environmental community and others agree to what they want the future to look like is the challenge. A Megaregion will have many more stakeholders with many more viewpoints spanning several aquifers, each with their own unique properties and capacities. The eventual goal is to be able to make a scientifically and politically sound decision on water use. Decision Support Systems (DSS), which are computer based logic systems, have been used for many things in the past. Recently a DSS system was integrated with MODFLOW to provide decisions based on aquifer capacities and community wants and needs (Pierce, 2006). To understand how a DSS works Figure 8 shows “The sustainable cycle” as defined by the National Rivers Authority in the United Kingdom. A very important point is that this is a decision cycle with no beginning or ending, meaning that decision making is not static (Newson, 2000). It is a constant cycle or examination, determination and decision making, in other words change is constant. Decision making tools can be extremely useful, but dealing with humans also means dealing with emotions and some irrationality. If a DSS system produces inconclusive 18 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  19. 19. results or results that run counter to stakeholder expectations, then the whole process can become tainted (Pierce, 2006). Yet if done well, and understood as a process of constant change; then a DSS system such as this can become an indispensable tool for making decisions across a large spatial region with numerous stakeholders and constituencies and multiple aquifers. Again more research and focus group evaluations need to be done in this area. Figure 8: Decision Support System model (Adams et al., 2000). DISSCUSSION The Future of Megaregions and Groundwater The lists of potential groundwater problems presented in this paper are only a fraction of the dilemmas that will need to be addressed in a Megaregion. It may appear that the problems outnumber and overwhelm any possible solutions and that some of the complications are essentially unsolvable; but many of these have already been researched before and workable 19 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  20. 20. solutions developed. Other issues will require more research and evaluation such as the DSS system; such a multi-disciplinary project may also require input from sociologists, public policy and legal as well as hydrogeologists, engineers, and computer scientists. Much of the solution to the added complexity is going to come from some sort of governmental body with the authority to make the required decisions; this body may even have to work across state boundaries where legal rights may differ. But this shouldn’t be a new idea as aquifers don’t abide state boundaries and states have had to work together in the past. This governmental agency will have to meld stakeholder needs and wants against aquifer capabilities and provide a management plan that is sustainable, realistic, and fulfills expectations. Again this is not a new concept; the Texas Groundwater Conservation Districts are exactly this sort of entity. These entities generally cover a large special area but usually they only cover a single aquifer. Best Practices Academia is unrivaled at research and at investigating the minutia of a topic which are the bricks and mortar of the world. Yet academia isn’t reputed for actually using those bricks and mortar to build a house. As Megaregions begin to develop the time to get things right is in the beginning and what is missing are standards of ‘best practices’ for aquifer management. There are too many unknowns to develop absolute rules, instead process and procedures can be developed by those in academia that are the most knowledgeable and unbiased. This would allow decision makers to evaluate a situation before it happens and make an educated and reasoned decision. This is a tall order but it is something that needs to be done. SUMMARY AND CONLCLUSION Much research has been done into groundwater issues in various areas. Figure 9 gives a breakdown of areas researched and it also shows how little work has been done or is still to be done in many areas. Topics such as system linkages, land cover change, water requirements are areas that have seen little involvement but are of great importance. In a Megaregion there will be significant linkage between groundwater and surface water, linkage between water distribution, collection, storage, purification, inputs and outputs. In a Megaregion large scale and spatially broad changes can be expected in land cover and land use; water demand, usage, distribution, 20 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  21. 21. capacity and distribution requirements still need definition. This is important to note because as population densities increase and Megaregions begin to develop if the knowledge and protocols are not in place then development will happen as it always has; off the cuff design by engineers, politicians and planners with little thought given to groundwater resources. As development begins and has already begun, hydrogeologists need to be part of the discussion because there will only be one shot to get development right. Once the zoning rules are in place, building plans are approved, roads laid and trenches dug its too late; at that point groundwater becomes a remediation issue and not a management or development issue. Figure 9: Hydrogeological research (Pierce, 2006) More research needs to be done and hydrogeologists are beginning to work outside the confines of their scientific training as water issues are a synthesis; a multidisciplinary arena where science meets politics, meets finance, meets public policy, meets urban planning, meets sociology, meets activism, meets business, meets human frailty. Hydrogeologists will need to play a role as mediators and facilitators between all the disparate disciplines providing good science to fashion best practices for groundwater management in the burgeoning Megaregions of tomorrow. 21 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
  22. 22. As Megaregions come into existence, the complexity of groundwater issues will blossom. Instead of dealing with a few hundred trenches and a few deep foundations, there will be thousands of trenches and tunnels of all shapes and sizes and hundreds of deep foundations of all sizes, kinds and designs. Now is the time to improve our understanding and leverage what time we have into synthesizing knowledge into useable forms so urban planners and engineers can plan properly from the beginning, instead of spending money later on remediation, repair and redesign. And in the end we will all have enough water. ACKNOWLEDGMENTS I would like to thank Kristin Vollman who provided me with many extremely valuable comments and edits. To John (Jack) Sharp who has earned my respect and admiration and provided me with a role model. I would like to thank S.S.S Foster, P.J. Chilton for developing the concepts we use today in urban hydrogeology and for asking the questions and writing the papers that have now hooked me on hydrogeology. To Luna Leopold who understood the interconnection of modern society and water resources long before others. But I want to especially thank my wife Lisa Loftus-Otway for her tolerance, support and her welding of the red pen of editor that made this paper something I can be proud of. 22 of 28 Issues in Urban Hydrogeology: Anthropogenic Effects on Groundwater and the Megaregion. - Dobbins
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