GROUND WATER

GROUNDWATER BY USMAN TEE JULY, 2010 INTRODUCTION: Water that occurs below the ground and is brought to the land surface by wells or springs is referred to as groundwater. Groundwater is a significant part of the hydrologic cycle, containing 21 percent of Earth's freshwater. Groundwater comprises 97 percent of fresh water not tied up as ice and snow in polar ice sheets, glaciers, and snowfields. This greatly exceeds the amount of water in streams, rivers, and lakes. Groundwater is critically important in supplying water to streams and wetlands, and in providing water for irrigation, manufacturing, and other uses. Groundwater once was thought to be of unlimited quantity and naturally protected by the soils above it. It is, however, significantly vulnerable to overuse and the improper use and disposal of chemicals at the land surface. The proper use and protection of this resource requires an understanding of how the groundwater system works. In particular, water managers need to know what groundwater is, where it comes from, how it occurs in the subsurface, and how it moves below the ground. Groundwater can form distinctive geologic features, such as caves, sinkholes, and petrified wood. It also can appear as hot springs and geysers. Hot groundwater can be used to generate geothermal energy. DISTRIBUTION OF WATER IN THE HYDROSPHERE LOCATION PERCENTAGE (%) Oceans 97.20 Glaciers and other Ice 2.15 Groundwater 0.61 Lakes Fresh 0.009 Saline 0.008 Soil moisture 0.005 Atmosphere 0.001 Rivers 0.0001 GROUNDWATER ORIGIN AND OCCURRENCE Groundwater is water that occurs below the Earth's surface at depths where all the pore (open) space in the soil, sediment, or rock is completely filled with water. In the unsaturated zone, pore spaces contain air; hence, no groundwater can be pumped from this zone. Usable groundwater occurs in the saturated zone, where pore spaces are completely filled with water. Groundwater is part of the hydrologic cycle, originating when part of the precipitation that falls on the Earth's surface sinks (infiltrates) through the soil and percolates (seeps) downward to become groundwater. Groundwater will eventually come back to the surface, discharging to streams, springs, lakes, or the oceans, to complete the hydrologic cycle. GROUNDWATER ZONES A well that is drilled will first pass through a zone called the unsaturated (vadose) zone where the openings in the soil, sediment, or rock are primarily filled with air. Water exists here only in transit downward. The thickness of this zone depends on such factors as climate, elevation, season of the year, and area-wide groundwater withdrawals through pumping. In the rainy season of humid areas, the unsaturated zone may be a fairly thin layer, extending from the land surface to only a few meters (10 feet or so) below the surface. But in drier months of the year, the unsaturated zone may extend deeper as recharge to the aquifer declines and withdrawals increase. In arid regions, the unsaturated zone may be a thick layer, extending from the land surface to 300 meters (1,000 feet) or more below it. Further drilling will reach a zone called the saturated zone where all of the openings are filled with water, and where the water is known as groundwater. If the saturated zone is permeable enough to supply a well with water under normal hydraulic gradients, this saturated zone is called an aquifer. Importantly, an aquifer is not an underground river, lake, or pool. Rather it consists of geologic materials whose open spaces (pore spaces) are filled with water that moves down a pressure gradient, and which can be tapped productively by wells. The top of the saturated zone is called either the water table (if the aquifer is unconfined) or the potentiometric surface (if the aquifer is confined). To visualize the zones, imagine a bucket filled with gravel. Ample pore space exists between the individual pieces of gravel. If water is poured on top of the gravel, the water will percolate down through the pore spaces and begin to fill these spaces from the bottom up. The water in pore spaces at the bottom of the bucket represents groundwater; that is, all the pore spaces are filled with water. If holes were punched in the bottom of the bucket, water would flow out. Using this analogy, the bucket of gravel is like an aquifer: water is stored within in it and will move through it toward a discharge point—in this case, the hole in the bucket. Permeability is determined by the size of pores and the degree to which they are interconnected, and hence, the ease by which water can flow through the material. Highly permeable aquifers, such as those comprised primarily of coarse sand and gravel, can supply more water than less permeable aquifers, such as those comprised of silts or clays. In this example, the pores in sand and gravel are larger than those in silt and clay, so water moves through sand and gravel more quickly. In some aquifers, especially in sedimentary bedrock, water occurs in fractures (cracks) instead of pore spaces in sediments. The yield from a fractured rock aquifer can vary from less than 1 liter per minute, or about 0.3 gallons per minute (if the well encounters few fractures) to large quantities of groundwater—for example, more than 300 liters per minute, or about 100 gallons per minute (if the fractures are numerous and large). The water table defines the top of an unconfined aquifer. Water in a well penetrating an unconfined aquifer will remain at the elevation of the water table. Some streams and lakes intercept the water table, allowing direct groundwater–surface water exchange. The potentiometric surface reflects the water pressure of a confined aquifer, and is the level to which water in a well will naturally rise (i.e., to an elevation above the confined aquifer it penetrates). All wells in confined aquifers are considered artesian wells. If the elevation of the potentiometric surface is above than the elevation of the land surface, groundwater will flow naturally (without pumping) from the well, known as a flowing artesian well. Recharge to unconfined aquifers occurs over a wide area of the unsaturated zone, directly above the aquifer. Recharge to confined aquifers occurs only where there is a pathway (e.g. a fracture) through the confining layers, or where the confined aquifer is exposed at the surface and becomes unconfined. Generally, the recharge area for a confined aquifer is at a higher elevation than the aquifer itself, and may be many kilometers from the well. AQUIFER AND WELL TYPES Aquifers are divided into two types: unconfined and confined. An unconfined aquifer is often shallow, and the vadose zone above it primarily contains permeable material. The top of the aquifer is the water table. The water table moves up and down on a seasonal basis. It is highest during the wet season owing to higher recharge and lower pumping rates (e.g. no irrigation), and lowest during the dry season because of limited recharge and higher use (e.g. a high rate of irrigation). Confined aquifers may be shallow or deep, and are characterized by being separated from the surface by low-permeability strata (e.g. geologic layers) that confines the groundwater below it. In a confined aquifer, groundwater is generally under pressure. This water pressure may vary seasonally, similar to the water table in an unconfined aquifer. Because groundwater in a confined aquifer is under pressure, it will rise in a well bore above the level of the aquifer penetrated by that well. One way to visualize this is to squeeze a milk or juice pouch that is punctured at the top by a straw. If the straw fits firmly into the squeezed pouch, the liquid will rise up into the straw, above its level inside the pouch. Artesian and flowing artesian wells are typical of wells drilled into confined aquifers. An artesian well is one in which the groundwater rises above the level of the penetrated aquifer. The water in an artesian well will rise to an elevation at which the pressure of the water in the aquifer is matched by the pressure reflected by the elevation of the water in the well; this level is known as the hydrostatic level. If groundwater reaches all the way to the surface under its own pressure, the well is called a flowing artesian well. GROUNDWATER MOVEMENT The water table (or the potentiometric surface of a confined aquifer) is not a flat surface: rather, there are high areas and low areas just like the hills and valleys found on land. Just as surface water tends to flow downhill, groundwater tends to move downgradient from water-table areas (or potentiometric regions) of higher elevation to water-table areas (or potentiometric regions) of lower elevation. Normally, but not exclusively, the higher water-table areas of unconfined aquifers coincide with higher elevation at the land surface, and the lower water-table areas coincide with low areas. As a result, groundwater in unconfined aquifers tends to flow towards, and discharges to, streams, lakes, and wetlands; because these water bodies often occur in low points of the watershed. Even groundwater from confined aquifers tends to discharge to larger area-wide rivers. Two other common discharge areas for groundwater are springs and wells. A spring is an area where groundwater has access to the land surface. In some cases, precipitation infiltrating downward from the ground surface encounters a relatively impermeable rock or sediment layer as it moves down toward the underlying aquifer. The groundwater, which cannot pass through the low-permeability layer, moves along the top of the layer until the layer is exposed at the ground surface and the water can emerge as a spring. In this typical "gravity spring", the most common form of spring, gravity is the driving force for water movement. Such springs commonly occur at the side of a hill, or at an outcrop such as a bluff or canyon wall. In other cases, fractures allow groundwater to move from the aquifer to the surface. Groundwater from a spring can issue onto the land surface, or directly into a stream, lake, or ocean. A well also provides a connection between groundwater and the land surface. In general, a pump is used to draw the groundwater up to the land surface where it can then be used.  Flow Rate When referring to how fast surface water moves, hydrologists generally talk in terms of either meters or feet per second. Groundwater moves much more slowly than water in streams, often at rates of only a few centimeters (or inches) per day. Groundwater velocity is controlled by the permeability of the aquifer and steepness of the water table (or potentiometric surface). The more permeable the aquifer and the steeper the slope of the water table or potentiometric surface (i.e., the pressure gradient), the faster groundwater moves. In highly permeable gravels or in fractures, groundwater may move 10 meters (33 feet) per day or more.  Recharge Recharge, or replenishment, of an unconfined aquifer occurs at the ground surface directly above the aquifer. In contrast, recharge to a confined aquifer may occur many miles away, typically at a higher elevation where the aquifer is no longer confined; that is where the overlying materials are permeable and allow percolating rainfall to reach the confined aquifer. Once recharged, the groundwater flow downgradient to where the aquifer is confined. CONTAMINATION OF GROUNDWATER A groundwater pollutant is any substance that, when it reaches an aquifer, makes the water unclean or otherwise unsuitable for a particular purpose. Sometimes the substance is a manufactured chemical, but just as often it might be microbial contamination. Contamination also can occur from naturally occurring mineral and metallic deposits in rock and soil. Groundwater in its natural state tends to be relatively free of contaminants in most areas. Because it is a widely used source of drinking water, the contamination of groundwater can be a very serious problem. Rain leaches fertilizers, pesticides applied to agricultural crops and pollutants from city landfills into the groundwater. Liquid and solid wastes from septic tanks, sewage plants and animal feedlots and slaughter houses may contain bacteria, viruses and parasites that can contaminate groundwater. Liquid waste from industries and military bases can be highly toxic, containing high concentrations of heavy metals and compounds which are widely used in industries. All these in one way or the other goes into the ground causing the contamination of the groundwater there present. In summary, groundwater can be contaminated by city landfills, agriculture, industry, or sewage disposal. Some pollutants can be filtered out by passage of the water through moderately permeable geologic materials. CLEANING UP CONTAMINATED GROUNDWATER Groundwater typically becomes polluted when rainfall soaks into the ground comes in contact with buried waste or other sources of contamination, picks up chemicals, and carries them into groundwater. Sometimes the volume of a spill or leak is large enough that the chemical itself can reach groundwater without the help of infiltrating water. Groundwater tends to move very slowly and with little turbulence, dilution, or mixing. Therefore, once contaminants reach groundwater, they tend to form a concentrated plume that flows along with groundwater. Despite the slow movement of contamination through an aquifer, groundwater pollution often goes undetected for years, and as a result can spread over a large area. HOT WATER UNDERGROUND Hot springs are springs in which the water is warmer than human body temperature. Water can gain heat in two ways while it is underground. First, and more commonly, groundwater may circulate near a magma chamber or a body of cooling igneous rock. Most hot springs are found in the areas associated with relatively recent volcanism. Groundwater can also gain heat if it circulates unusually deeply in the earth, perhaps along joints or faults. The normal geothermal gradient (the increase in temperature with depth) is 25oC/kilometer (about 75oF/mile). Water circulating to a depth of 2 or 3km is warmed substantially above normal surface water temperature. Warm water, regardless of its origin, is lighter than cold water and readily rises to the surface. A Geyser is a type of hot spring that periodically erupt hot water and steam. The water is generally near boiling (100oC). Eruptions may be caused by a constriction in the underground “plumbing” of a geyser, which prevents the water from rising and cooling. THE WATER TABLE Responding to the pull of gravity, water percolates down into the ground through the soil and through cracks and pores in the rock. Several kilometers down in the crust, percolation stops. With increasing depth, sedimentary rock pores tend to be closed by increasing amounts of cement and the weight of the overlying rock. Moreover, sedimentary rock overlies igneous and metamorphic crystalline basement rock, which usually has very low porosity. The depth where downward percolation stops is generally about 5 kilometers below the surface, although in thick wedges of sedimentary rock, percolation may reach a depth of 10km or more. The subsurface zone in which all rock openings are filled with water is called the saturated zone. If wells were drilled downward into this zone, groundwater will fill the lower part of the well. The water level inside the well marks the upper surface of the saturated zone; this surface is the Water table. Most rivers and lakes intersect the saturated zone. Rivers and lakes occupy low places on the land surface, and groundwater flows out of the saturated zone into these surface depressions. The water level at the surface of most lakes and rivers coincides with the water table. Groundwater also flows into mines and quarries cut below the water table. Above the water table is a zone that is generally unsaturated and is referred to as the vadose zone. Within the vadose zone, surface tension causes water to be held above the water table. The capillary fringe is a transition zone with higher moisture content at the base of the vadose zone just above the water table. Some of the water in the capillary fringe has been drawn or wicked upward from the water table (much like water rising up a paper towel if the corner is dipped in water). The capillary fringe is generally less than a meter thick but may be much thicker in fine-grained sediments and thinner in coarse-grained sediments such as sand and gravel. A Perched water table is the top of a body of groundwater separated from the main water table beneath it by a zone that is not saturated. REFERENCES 1. www.mhhe.com/plummer10e 2. www.waterencyclopedia.com 3. www.wikipedia.com 4. Physical Geology -Plummer, McGeary and Carlson