Sandstone Aquifers
2.7.2 Sandstone Aquifers
Sandstone aquifers in the United States are more widespread than those in all other types of consolidated rocks. Although generally less permeable, and usually with a lower natural recharge rate than surficial unconsolidated sand and gravel aquifers, sandstone aquifers in large sedimentary basins are one of the most important sources of water supply both in the United States and worldwide. Loosely cemented sandstone retains significant primary (intergranular) porosity, whereas secondary fracture porosity may be more important for well-cemented and older sandstone (Fig. 2.77). In either case, storage capacity of such deposits is high because of the thickness of major sandstone basins.
Sandstone aquifers are highly productive in many places and provide large vol- umes of water for all uses. The Cambrian-Ordovician aquifer system in the north-central United States is composed of large-scale, predominantly sandstone aquifers that extend over parts of seven states. The aquifer system consists of layered rocks that are deeply buried where they dip into large structural basins. It is a classic confined, or artesian, system and contains three aquifers. In descending order, these are the St. Peter-Prairie du Chien-Jordan aquifer (sandstone with some dolomite), the Ironton-Galesville aquifer (sandstone), and the Mount Simon aquifer (sandstone). Confining units of poorly perme- able sandstone and dolomite separate the aquifers. Low-permeability shale and dolomite compose the Maquoketa confining unit that overlies the uppermost aquifer and is con- sidered to be part of the aquifer system. Wells that penetrate the Cambrian-Ordovician aquifer system commonly are open to all three aquifers, which are collectively called the sandstone aquifer in many reports.
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F IGURE 2.77 Conglomerate of the lower sandstone of the Dakota group about 2 mi north of Bellvue, resting unconformably on Morrison shale. (Larimer County, CO, 1922. Photograph courtesy of USGS Photographic Library, 2007.)
The rocks of the aquifer system are exposed in large areas of northern Wisconsin and eastern Minnesota. Regionally, groundwater in the system flows from these topo- graphically high recharge areas eastward and southeastward toward the Michigan and Illinois Basins. Subregionally, groundwater flows toward major streams, such as the Mississippi and the Wisconsin Rivers, and toward major withdrawal centers, such as those at Chicago, IL, and Green Bay and Milwaukee, WI. One of the most dramatic effects of groundwater withdrawals known in the United States is shown in Fig. 2.78. Withdrawals from the Cambrian-Ordovician aquifer system, primarily for industrial use in Milwaukee, WI, and Chicago, IL, caused declines in water levels of more than 375 ft in Milwaukee and more than 800 ft in Chicago from 1864 to 1980, with the pump- ing influence extending over 70 mi. Beginning in the early 1980s, withdrawals from the aquifer system decreased as some users, including the city of Chicago, switched to Lake
162 ChapterTwo
MC HENRY
DU PAGE
DE KALB
LA SALLE
0 25 50 miles 0 25 50 km
F IGURE 2.78 Decline of water levels in the Cambrian-Ordovician aquifer system from 1864 to 1980, as a result of large groundwater withdrawals centered at Chicago and Milwaukee. Contour lines are in feet; dashed where approximately located. (Modified from Miller, 1999, and Young,
Michigan as a source of supply. Water levels in the aquifer system began to rise in 1985 as a result of decreased withdrawals (Miller, 1999).
The chemical quality of the water in large parts of the aquifer system is suitable for most uses. The water is not highly mineralized in areas where the aquifers crop out or are buried to shallow depths, but mineralization generally increases as the water moves downgradient toward the structural basins. The deeply buried parts of the aquifer system contain saline water.
Other large layered sandstone aquifers that are exposed adjacent to domes and uplifts or that extend into large structural basins or both are the Colorado Plateau aquifers,
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the Denver Basin aquifer system, Upper and Lower Cretaceous aquifers in North and South Dakota, Wyoming, and Montana, the Wyoming Tertiary aquifers, the Mississippian aquifer of Michigan, and the New York sandstone aquifers (Miller, 1999).
Examples of continental-scale sandstone aquifers include the Guaran´ı aquifer sys- tem in South America, the Nubian Sandstone Aquifer System in Africa, and the Great Artesian Basin in Australia. The Guaran´ı Aquifer System (also called Botucatu aquifer) includes areas of Brazil, Uruguay, Paraguay, and Argentina. Water of very good quality is exploited for urban supply, industry, and irrigation as well as for thermal, mineral, and tourist purposes. This aquifer is one of the most important fresh groundwater reser-
voirs in the world, due to its vast extension (about 1,200,000 km 2 ), and volume (about 40,000 km 3 ). The aquifer storage volume could supply a total population of 5.5 bil- lion people for 200 years at a rate of 100 L/d/person (Puri et al., 2001). The gigantic aquifer is located in the Paran´a and Chaco-Paran´a Basins of southern South America. It is developed in consolidated aeolian and fluvial sands (now sandstones) from the Triassic-Jurassic, usually covered by thick basalt flows (Serra Geral Formation) from the Cretaceous, which provide a high confinement degree. Its thickness ranges from a few
meters to 800 m. The specific capacities of wells vary from 4 m 3 /h/meter of drawdown to more than 30 m 3 /h/m. The total dissolved solids (TDS) contents are generally less than 200 mg/L. The production costs per cubic meter of water from wells of depths between 500 and 1000 m and yielding between 300 and 500 m 3 /h vary from US$ 0.01 to US$ 0.08, representing only 10 to 20 percent of the cost of storing and treating surface water sources (Rebou¸cas and Mente, 2004).
The rocks of the Nubian Sandstone Aquifer System in northern Africa (NSAS; Fig. 2.79), which is shared by Egypt, Libya, Sudan, and Chad, vary in thickness from zero in outcrop areas to more than 3000 meters in the central part of the Kufra and Dakhla Basins, and range in age from Cambrian to Neogene. The main productive aquifers, separated by regional confining units, are (from land surface down) Miocene sandstone, Mesozoic (Nubian) sandstone, Upper Paleozoic-Mesozoic sandstone, and Lower Pale- ozoic (Cambrian-Ordovician) sandstone (Salem and Pallas, 2001). In some locations, the confined portions of the system provide water to high-yielding artesian wells such as the one shown in Fig. 2.80. The groundwater of the Nubian Basin is generally of high quality. TDS range from 100 to 1000 parts per million, with an increased salinity northward toward the Mediterranean sea where the freshwater saline water interface passes through the Qattara depression in Egypt. In Libya, the TDS of the deep Nubian aquifers ranges from 160 to 480 mg/L and from 1000 to 4000 mg/L in the shallow aquifers (Khouri, 2004).
Because of the semiarid to arid climate, the present-day natural recharge of the NSAS is negligible and countries in the region have formed a joint commission for assessment and management of this crucial, nonrenewable source of water supply. Table 2.1 shows comparison of the total recoverable freshwater resources stored in the system and the present-day annual withdrawals (Bakhbakhi, 2006).
Groundwater withdrawals from the system have been increasing each year for the past 40 years. During this time period over 40 billion m 3 of water has been extracted from the system in Libya and Egypt. This has produced a maximum drawdown of about
60 m. All but 3 percent of the free flowing wells and springs have been replaced by deep wells. Until recently, almost all the water extracted was used for agriculture, either for development projects in Libya or for private farms located in old traditional oases in Egypt. With completion of the first phase of the so-called “Great Manmade River”
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MEDITERRANEAN SEA Benghazi
Tobruq
Port Said Ajdablya
Oasis Dakhla
The Great
EGYPT
Oasis Aswan
Kufrah
LIBYA
I JABAL
TS
AWAYNAT
I B Selima
Dongola River
Nile Faya
Directions of groundwater flow
0 200 km
I 70 Nubian Post-Nubian
Nubian System outcrop
Groundwater Extraction
Extent of Post-Nubian System
Major from Nubian
Major from Post-Nubian 100
Mountain ranges
Average annual rainfall (mm)
Transport of extracted groundwater
F IGURE 2.79 Nubian Sandstone Aquifer System (NSAS) in North Africa. (Modified from Bakhbakhi, 2006.)
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F IGURE 2.80 Flowing well at Kharga Oasis, Egypt, 1961. (Photograph courtesy of USGS Photographic Library, 2007.)
project in Libya, 2 million m 3 of groundwater extracted from NSAS is transported daily via buried large-diameter reinforced concrete pipes for some 2000 km to the coastal cities in the north. Other phases of the project include groundwater extraction from another nonrenewable aquifer west of NSAS called North Western Sahara Aquifer System (NWSAS). The combined groundwater extraction from the two aquifer systems in Libya for centralized water supply of various users along the coast is reportedly 6.5 million
m 3 /d. It has been estimated that the cost of this megaproject is more than 25 billion US dollars (Wikipedia, 2007). The estimated remaining volume of freshwater that can be extracted from the entire NSAS in all four countries (Egypt, Libya, Chad, and Sudan) is
about 14,500 km 3 (Bakhbakhi, 2006). The Great Artesian Basin in Australia covers 1.7 million km 2 and is one of the largest groundwater basins in the world. It underlies parts of Queensland, New South Wales, South Australia, and the Northern Territory. The basin is up to 3000 m thick and contains
a multilayered confined aquifer system, with the main aquifers occurring in Mesozoic sandstones interbedded with mudstone (Jacobson et al., 2004). Groundwater in the Great Artesian Basin has been exploited from flowing wells since artesian water was discovered in 1878, allowing an important pastoral industry to
be established. Wells are up to 2000 m deep, but average about 500 m. Artesian flows
Present Total Nubian System
Present
Post-Nubian System
Extraction Present
Volume in
Volume in
Volume in
Recoverable
from
from Extraction
Post-Nubian from the Region
(km 3 ) NSAS (km 3 ) Egypt
Area (km 2 )
(km 3 )
Area (km 2 )
0.570 2.170 From Bakhbakhi, 2006.
T ABLE 2.1 Freshwater Stored in the Nubian Sandstone Aquifer System (NSAS) and Present-Day Groundwater Withdrawals per Region
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total number of flowing
total discharge
artesian boreholes drilled
of flowing 3 /a)
artesian boreholes
(Mm 3000
400 g artesian boreholes boreholes
number of artesian
Rate of discharge from 200
boreholes still flowing
artesian 1000 number of flowin
2000 F IGURE 2.81 Trends in artesian overflow from wells in the Great Artesian Basin of Australia. (From
from individual wells exceed 10 million L/d (more than 100 L/s), but the majority have much smaller flows. About 3100 of the 4700 flowing artesian wells drilled in the basin remain flowing. The accumulated discharge of these wells (including water supply wells in about 70 towns, as in most cases the artesian groundwater supply is the only source of water) is about 1200 million L/d, compared to the maximum flow rate of about 2000 million L/d from about 1500 flowing artesian wells around 1918 (Fig. 2.81). Nonflowing artesian wells, about 20,000, are generally shallow—several tens to hundreds of meters deep. It is estimated that these generally windmill-operated pumped wells supply on average 0.01 million L/d per well and produce a total of about 300 million L/d. High initial flow rates and pressures of artesian wells have diminished as a result of the release of water from elastic storage in the groundwater reservoir. Exploitation of the aquifers has caused significant changes in the rate of natural aquifer discharge. Spring yields have declined as a result of well development in many parts of the basin during the last 120 years, and in some areas springs have ceased to flow (Habermehl, 2006).