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
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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,
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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.
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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).
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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.
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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).
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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
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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.
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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
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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,
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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,
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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.
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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.
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