Groundwater, Minerals, and Mining
The purpose of this article is to examine the origin of some of our natural physical resources and outline some methods used in extracting and processing them. The extraction and processing of groundwater will be considered first, followed by several types of minerals.
The way in which these resources are extracted are dependent on factors such as geologic setting, economic value, and the impact extraction has on the environment and society. Legal issues such as of ownership of the resource and environmental impacts to air and water commonly also enter the picture. This is especially true of groundwater extraction, which will be examined first.
Groundwater Occurrence and Extraction
Technically, groundwater is water stored between the sediment grains in soil, or in space between mineral grains in bedrock. It is also stored in bedrock fractures. The underground storage area of water is called an aquifer. Groundwater collects in aquifers after water percolates downward through soil or bedrock and reaches the aquifer area.
Aquifers have been extensively harvested in the United States over the past 100 years or so for agricultural purposes. Further, the economic value and demand for groundwater as bottled water has skyrocketed over the past 20 years, due primarily to pollution of surface waters, which are generally the sources of water for many cities.
Many people are now wary of drinking water provided by city or town reservoirs. This is because they are open and susceptible to both particulate matter from air pollution and contamination from other surface waters that lead into them. Groundwater, however, is naturally filtered by the soil/rock that water percolates through. Therefore, it is a desirable source for clean drinking water, and does not have to be extensively treated.
Finding a suitable aquifer for extraction requires research. The water volume contained within an aquifer is dependent on several factors, such as grain size of the sediment or rock, and the percent of voids between the grains (porosity). Further, the permeability, or ease of flow of fluid through the host aquifer, is important. Finally, the total size of the aquifer plays a role in how promising extraction might be.
Testing the quantity and quality of an aquifer involves research of geologic maps and reports to find a potentially suitable aquifer area. Once a promising area with suitable geologic materials is found, test borings are performed. Several in-situ tests can be performed using the test-boring hole to determine if the aquifer has sufficient permeability for its desired use. Once a potentially suitable area has been drilled, several in-situ tests are performed in the test borings.
One such test is known as a “slug test”. This involves the abrupt removal, addition, or displacement of a known volume of water. Subsequent monitoring and changes in water level are performed to determine how well or poorly the conductivity of the soil or rock formation near the test hole is (1). More extensive tests include single and multiple well pumping tests, which help to determine the suitability of the entire aquifer area. Here, the test well water level is drawn down to determine the potential volume and permeability of the aquifer (1).
A serious problem with using aquifers lies in extracting more water than is naturally replenished in the aquifer by precipitation. This is especially true in water withdrawal associated with livestock and agriculture. An example of this is the Ogallala Aquifer, a far-reaching feature that stretches from Texas to Wyoming beneath the High Plains. It has been extensively used for agriculture to irrigate surface areas that other wise would be dry. It is inexorably being depleted by massive extraction with not enough natural recharge, and will someday “run dry”. This will cause a great shift in the way crops are grown for livestock and humans.
Further problems with groundwater extraction lie in answering the legal questions as to who owns the resource and who is to profit from it. For example, the water drawn up from a well might originate beneath a neighbors land, and might take away from that neighbor’s resource. Taking this a step further, extracting significant amounts of water from large aquifers may affect many neighbors. The legal party who has rights to the water might be dependent on the state the aquifer is located in. Further, proving a resource has been diminished in your well by another party is extremely difficult to prove, and can be blamed on many other extraneous factors such as precipitation amounts, overuse of the resource by others, acts of God, etc.
Many cities throughout the United States, especially in the western, more arid states have begun to take measures to try and conserve ground water. For example, Denver, Colorado has rules meant to conserve water, such as limiting summer lawn watering to cooler times of the day and using a hose nozzle with a shut off value while washing a car (2). Hopefully, measures such as these will aid in conserving this precious resource.
Quartz: Use and Mining
Now we move onto mineral resources, the first being quartz.
Quartz is one of the most abundant minerals found in rocks near the Earth’s surface. Its many uses include components in cell phones, watches, and other electronics. In addition, it is used in glass making, construction, and jewelry (3). It is chemically inert, has desirable electrical properties, and is quite heat resistant. In non-crystal form, it is also used in the construction industry.
Quartz is found in igneous, sedimentary and metamorphic rocks. It is very durable in the environment and is found in almost any geologic setting. It can occur as well-formed crystals in caves, in the sedimentary rock sandstone, or in the metamorphic rock quartzite. Quartz is also found in almost all igneous rocks. Further, it can occur as loose sand deposits, or as veins contained in other rocks. The challenge is finding a deposit with the desired qualities, along with a sufficient concentration, in a location conducive to extraction. It generally occurs in nature as well-formed crystals or jagged, irregular grains.
Gemstone quality quartz is hexagonal in shape and is formed in nature when hydrothermal fluids rich in mineral ions percolate through unconfined spaces, where a crystal can grow without impediments such as other minerals or rocks crowding it. The unconfined spaces are usually fluid filled fractures or caves beneath the Earth. Over millions of years, crystals grow in the fluids. Eventually, tectonic shifts in the rocks then cause the fluid to drain, and a cave or fracture containing well-formed quartz crystals remain. Most states have small commercial privately owned quartz-collecting caverns/mines open to the general public. These are generally small sample of gemstone quality quartz in the mine or associated tailings. However, the serious collector would have to do a large amount of legwork to find accessible areas of high quality, large gemstone quartz.
Quartz crystals can also be grown synthetically. This plays a key role in technology, as synthetic quartz emulates the electrical properties of natural quartz crystals. This is of high practical use in watches, clocks and communications equipment (4).
In non-cave environments, quartz can become concentrated in sedimentary rock deposits. This is because quartz is very durable in the environment and is not highly susceptible to chemical weathering. It can therefore be transported long distances by streams and be worked and reworked near ocean shores. Other minerals tend to be pounded away by abrasion or chemically dissolved, whereas quartz is sturdy. When solidified into a sedimentary rock, the resulting sandstone may contain up to 99% quartz. By crushing the rock, and passing the material through a network of fine sieves, silica sand is produced which is used in glass making and the petroleum industry. Larger particles of crushed sandstone and quartzite are also used as construction materials for roads and buildings.
Sand deposits associated with ancient glacial or beach environments also provide sources for quartz. If these are found in areas conducive to mining, some of the issues associated with mining quartz from bedrock, such as blasting, may be alleviated.
In terms of environmental impacts, the fact that quartz can be made synthetically under controlled conditions in a laboratory mitigates many public concerns. Further, collecting gemstones commonly has little environmental impact. As a crushed stone source, sandstone and quartzite production represent about 4% of the total annual crushed stone production in the United States (5). Here, open pit mining to produce crushed stone can have severe complications with regard to air, noise, and water quality around the mine. These impacts will be discussed in more detail when considering the mining of calcium carbonate, which represents a whopping 66% of the total annual crushed stone production in the United States (5).
Calcium: Use and Mining
As with quartz, calcium is contained in mineral compounds, and is abundant in the Earth’s crust. Calcium is used by society in compound form. Calcium provides a critical role in providing structure and strength to the human skeleton. Further, it is used as an alloying agent in the production of some metals, such as lead calcium alloy used in car batteries (6). In addition, calcium compounds are used in construction, cement manufacture, insecticides, animal feed, and fertilizers (7).
Calcium is incorporated in mineral compounds that occur primarily in the sedimentary rocks limestone, dolostone, gypsum and anhydrite. Limestone (calcium carbonate) is formed in shallow marine environments when calcium carbonate muds accumulate in the form of shell debris, fecal matter, coral fragments and carbonate precipitates. Dolostone, which is magnesium rich calcium carbonate, is created by the post-depositional alteration of limestone by magnesium rich groundwater (8).
The other two calcium bearing rocks, gypsum and anhydrite, are called evaporites. These type of rocks form by the evaporation of mineral rich waters which once occupied ancient inland seas or lakes. An example of the type of deposit is the Michigan Basin, an area formerly occupied by an inland sea in the water portion evaporated, leaving behind rock salt, gypsum, and other types of evaporites.
As stated in a previous section, in the United States, limestone comprises about 66% of all crushed stone types used for construction purposes. This is mined from open pit strip mines, which can lead to all sorts of environmental impact problems. Commonly these types of mines were originally set up on the periphery of cities or towns, far enough away from neighbors to minimize impacts. In the modern world, many of these cities and towns are direct neighbors with the mine due to urban growth. As a result serious concerns associated with mining impacts arise. These are related to the blasting or rock affecting nearby structures by their shock waves, air pollution from dust particulates and noise pollution from trucks and heavy equipment. Further concerns are generally related to trucks and traffic patterns, and water pollution.
To help alleviate these concerns, most states have mining laws addressing these concerns. New mines are required to prepare environmental impact statements and mining permits to allow operations. Existing mines have to abide by state and federal blasting regulations, air and water discharge regulations, and other related laws and regulations.
Of all the natural resources mentioned in this article, diamonds are the most prized. It is a rare mineral that is composed entirely of carbon. It is chemically simple substance, but also the hardest (most resistant to abrasion) of all known minerals. Diamonds are used either as gemstones or as industrial diamonds.
In nature, diamonds occur in a rare geologic setting. They can only be formed at high temperatures and pressures, like that of the Earth’s mantle 100 miles down. The majority of the diamonds that have been discovered were delivered from such depths by volcanic upwelling. The most well known diamond deposits occur in the “Kimberlite pipes” of South Africa (9). Kimberlite is an igneous rock that is commonly emplaced as a carrot-shaped, vertical intrusion. Diamonds occur as inclusions in the Kimberlite, and also as placer deposits from the erosion of Kimberlites. In the world, there are some 6400 known Kimberlite pipes, of which about 30 have been deemed economically feasible to mine (9).
“Industrial” diamonds are sometimes crushed to produce abrasive powders. These include gemstone quality diamonds that are too small to be sold as jewelry. The crushed diamonds are embedded on cutting tools, such as diamond studded drilling bits, and other similar tools (10).
Gemstone diamond quality is determined by color, the cut, clarity of the diamond, and carats (a unit of weight). The optical properties of diamonds make it very popular. It has a high luster and disperses white light into the colors of the spectrum, which gives diamonds “fire”.
Diamond mining is performed in surface strip mines, underground mines and alluvial placer deposits. As a result plethora of legal and environmental challenges can be encountered in removing, drilling and processing rock or alluvium. With open pit and underground mining, large quantities of surplus waste rock can accumulate and needs to be managed. When alluvial mining occurs, waste overburden is spread out and smoothed over mined areas and revegetated.
However, as with gold panning, informal alluvial diamond digging is allowed on non-regulated private and government lands if small quantities are involved and environmental impacts are minimal (11).
Many gemstones other than quartz and diamonds exist in nature and are mined. The entire concept of gemstones is based on value, either monetary or social. A gemstone is generally defined as an attractive and valuable piece of cut and polished mineral or minerals used in jewelry or other decorations (12). As there are over two thousand known minerals, the list of possible gemstones is extensive. More valued “precious” gemstones include Emeralds, Sapphire, and Rubies. Semi-precious gemstones include Amethyst, Beryl, and Fluorite.
As there are a large number of gemstones, the geologic circumstances that create them can be widely varied. A few environments of formation are as follows.
Caves or caverns are generally created by the action of water either dissolving or wearing away rocks underground. Many caves are submerged for extensive periods of time, creating an environment where waters saturated with various dissolved minerals flow through. Over time extensive crystals of different gemstone minerals can occur. If and when the water drains, gem quality minerals may be left behind. An extreme example of this are the giant gypsum crystals in the Cave of Crystals, Naica, Chihuahua, Mexico. These crystals can measure up to 30 in length, and may be several feet in diameter (13).
Igneous and Metamorphic terrains may be conducive to some types of gem formation. Garnet is a good example. It can occur in silica rich igneous rocks such as granite or pegmatite and it is also typical of high-grade metamorphic rocks, where it can grow to several centimeters in diameter. Gore Mountain, NY is famous for its large garnet crystals.
A third gem forming environment involves geodes and agates, both of which are round shaped rocks which, when opened, may reveal beautiful crystals. They form when “bubbles” are trapped in lava or sediments, and a void in the rock is formed. These voids generally occur in igneous rocks as a result of escaping gases or in sedimentary rocks when organic matter buried in sediment weathers away over time. When mineral rich waters later percolate through the cavities minerals such as quartz, dolomite, calcite and pyrite (fools gold) may be left behind.
There are many “pay to prospect” semi-precious gemstone mines scattered throughout the United States where a person can shovel, sieve, or pan for gems for a fee. It is also possible to prospect for gems on certain public lands, such as the Bureau of Land Management properties in the western United States. On public lands where it is allowed, certain limits are in place and only small areas can be lightly disturbed, and collecting is not for resale.
Commercial gem mines in the United States are generally small with a few employees that tend to be part time. As with any type of mine, state laws and regulations have to be followed, and concerns are given to real or potential environmental impacts associated with mines.
To summarize, groundwater and the minerals discussed here are found in a variety of geologic settings throughout the United States and the world. Some are used as major resources by many, such as groundwater and crushed stone from open pit mining. Others have more personal use and value, such as gemstones. Whatever the use, the origin, processing, and use of these resources must be performed in an environmentally responsible manner for the sake of future generations.
(1) TECHNICAL GUIDANCE MANUAL FOR HYDROGEOLOGIC INVESTIGATIONS AND GROUND WATER MONITORING CHAPTER 4 SLUG AND PUMPING TESTS February 1995.
(2) DENVER WATER: 2016 Rules for Outdoor Water Use.
(3) Quartz. Geology.com.
Retrieved from: http://geology.com/minerals/quartz.shtml
(4) An Introduction to Synthetic Gem Materials. Gemological Institute of America, Inc.
Retrieved from: http://www.gia.edu/gem-synthetic
(5) Geology.com. Crushed Stone.
Retrieved from: http://geology.com/articles/crushed-stone/
(6) Westbrook Resources Ltd. Calcium Alloys.
Retrieved from: http://www.wbrl.co.uk/calcium-alloys.html
(7) WIKIPEDIA The Free Encyclopedia: Calcium.
Retrieved from: https://en.wikipedia.org/wiki/Calcium#Occurrence
(8) Dolomite. Geology.com.
Retrieved from: http://geology.com/rocks/dolomite.shtml
(9) WIKIPEDIA:The Free Encyclopedia: Kimberlite.
Retrieved from: https://en.wikipedia.org/wiki/Kimberlite#Morphology_and_volcanology
(10): Geology.com. Diamond.
Retrieved from: http://geology.com/minerals/diamond.shtml
(11) World Diamond Council.DIAMONDFACTS.ORG. DIAMOND MINING AND THE ENVIRONMENT FACT SHEET.
(12) GEMSELECT. Natural Gems Only.
Retrieved from: http://www.gemselect.com/other-info/minerals-rocks-gems.php
(13) ScienceDaily. Initial stages by which giant gypsum crystals form.
Retrieved from: https://www.sciencedaily.com/releases/2012/04/120405142152.htm