Hydrogeology is the study of groundwater and its interaction with the geologic framework, landscape, surface-water bodies, and the human environment. It is both a qualitative and quantitative science having many applications to the modern environment. Among the more prominent of these are: identifying the geologic characteristics of groundwater resources beneath any given point in the landscape, and quantitatively determining how much groundwater is available for different kinds of end uses without compromising the resource; detailing the role of groundwater discharge in maintaining the base flows of surface-water bodies, such as the White River, and the hydrology of natural areas, such as natural lakes and wetlands; and assessing the relative sensitivity of groundwater to contamination across the landscape, based on a combination of the geologic framework and empirical case studies of groundwater quality.
These and other applications broadly highlight the concept of hydrogeologic framework — how different combinations of geologic, topographic, and hydrologic conditions control the occurrence, abundance, chemistry, and movement of groundwater beneath different geologic terrains. Seemingly subtle differences in the geologic framework can cause groundwater characteristics to vary enormously from place to place, even within the confines of a relatively small area such as Marion County. This Web page presents a primer on the hydrogeologic framework of Marion County, and highlights how differences in hydrogeologic conditions are manifested from place to place, both in terms of meeting human needs as well as in the natural landscape and water bodies of the county.
An abundance of both groundwater and surface-water resources has been instrumental to the historical growth of Marion County as a center of population and commerce. The City of Indianapolis has long been served by a centralized public water supply that uses a combination of surface water sources, derived from Eagle Creek, Geist, and Morse Reservoirs (the latter is in Hamilton County), along with several wellfields situated in favorable hydrogeologic settings along Fall Creek and the White River valley.
Most outlying parts of the county, as well as some large industrial and commercial water users within the area served by public water supplies, have traditionally used individual wells to provide groundwater for residential, commercial, and industrial purposes. The construction records from many thousands of such wells, now available online at the Indiana Department of Natural Resources, Division of Water, served as a key source of subsurface information in the preparation of the geologic and hydrogeologic interpretations presented in the original report upon which this Web site is based.
Aquifers, Confining Units, and Aquifer Systems and Complexes
Groundwater is defined as all water below the surface of the Earth. Major sources of groundwater are concentrated in bodies of permeable rock and sediment called aquifers — a term that has traditionally meant any body of rock or unconsolidated sediment capable of producing groundwater to a well in an economically useful amount (Maxey, 1964). From a contemporary perspective, this definition could also include any geologic unit capable of transmitting contaminants at a rate significantly greater than surrounding materials, as well as one that yields environmentally significant amounts of water, an example being the discharge of groundwater to an ecologically important wetland. In contrast, a confining unit generally is a poorly permeable body of rock or sediment that restricts groundwater flow, helps to retard contaminant migration, and typically is not capable of yielding significant amounts of water to a well or supporting an ecologically significant habitat.
It is clear from these definitions that all the sand and gravel units in Marion County, as well as the sandstone and limestone bedrock units, constitute aquifers, whereas shale , siltstone , till , and related fine-grained sediments are confining units. Indeed, virtually every sand and gravel body of any meaningful size functions or could potentially serve as a groundwater source for domestic wells in the county. Many sand and gravel aquifers and all bedrock aquifers beneath the county are confined or semiconfined by overlying till units, while the large valley aquifers along the White River and Fall Creek directly underlie the land surface at most places and are, thus, mostly unconfined .
As illustrated in Figure 1, aquifers occur along a variety of horizons and in many configurations beneath Marion County. They include sand and gravel bodies throughout the glacial section as well as limestone and sandstone bedrock. The major confining units consist of sequences of glacial till of late Wisconsin and pre-Wisconsin age, along with shale and siltstone bedrock. The majority of wells tap the shallow aquifer system , which comprises a hydraulically integrated system of unconfined, semiconfined, and confined aquifers along and above the pre-Wisconsin surface (heavy red line), including the large outwash complex associated with the White River valley. Several other sizable aquifer complexes and numerous smaller aquifers occur at even greater depth, some of which are concentrated in and above buried bedrock valleys. (Diagram adapted from Brown and Laudick, 2003.)
An aquifer system, on the other hand, is defined as "a heterogeneous body of permeable and poorly permeable materials that functions regionally as a water-yielding unit; it consists of two or more aquifers separated at least locally by confining units that impede groundwater movement, but do not affect the overall hydraulic continuity of the system" (Poland and others, 1972). Water-level data available from many water supply wells and groundwater monitoring wells, coupled with the subsurface geologic information regarding the origin and geometry of aquifers and confining units, indicate that the sand and gravel units along the pre-Wisconsin surface and within the overlying late Wisconsin sequences collectively form a coherent shallow aquifer system. Permeable sandstone of the Borden Group lies at relatively shallow depth and commonly forms the pre-Wisconsin surface in parts of southwestern Marion County, and is also considered to be part of the shallow aquifer system in that area. This system is simply referred to as the "shallow aquifer system," and for the most part, is distinct from deeper sand and gravel or bedrock aquifers. The shallow aquifer system typically is the first groundwater source encountered below the surface; it is the predominant water supply for a majority of wells in the county and is the primary source of water that supports the base flows of streams and ecologically sensitive wetlands and riparian areas. The shallow aquifer system is the primary focus of this Web page.
The shallow aquifer system also includes the large bodies of highly transmissive, saturated surface outwash along the major drainages, whose middle and lower parts are frequently in direct contact with, and thus hydraulically connected to, other aquifers in the shallow system, as well as to several deeper aquifers and aquifer complexes. Although the White River itself is relatively small, its valley overlies a massive complex of sand and gravel (fig. 1) that contains an enormous volume of groundwater. This valley aquifer system is the preeminent source of water for the greater Indianapolis area and supports the preponderance of manufacturing, energy-generation, and other water-intensive activities in the valley.
Other aquifers and aquifer systems are also present at greater depth below the shallow aquifer system. These deeper water-bearing units include a variety of both large and small sand and gravel bodies as well as transmissive zones within the Silurian and Devonian carbonate rocks. Over most of the county, these deeper units generally are not physically connected to the shallow aquifer system, although some direct hydraulic interconnection may locally exist where large channels extend downward from the pre-Wisconsin surface, and where the deeper units are truncated along that surface. Many of the deeper sand and gravel aquifers are local features of relatively limited areal extent. The major exceptions include two very large aquifer complexes associated with thick, extensive bodies of pre-Wisconsin sand and gravel — the Fall Creek aquifer complex, centered on the Fall Creek tunnel valley in Lawrence, and the southwestern aquifer complex, which is situated along the flanks of the ancient pre-Wisconsin White River valley in the southern part of the county.
Water Table, Potentiometric Surface, and Groundwater Flow Systems
The water table (fig. 2) represents the level below which all openings in the bedrock or unconsolidated sediments are completely filled with groundwater, producing what is called a "saturated" condition. In contrast, materials above the water table are under unsaturated conditions because the openings are filled with a mixture of air and water. The unsaturated zone above the water table also is commonly referred to as the " vadose zone ."
Relationship between the water table, piezometric surface, and water levels in wells developed in confined and unconfined aquifers.
The water table is present both in aquifers and confining units. Unconfined aquifers are also known as "water table aquifers" because they usually have a significant vadose zone between the land surface (in the upper, unsaturated part of the aquifer) and the water table; the depth of the water table, and thus the thickness of the unsaturated zone, fluctuates seasonally in response to precipitation patterns, groundwater recharge, and water levels in adjacent streams. This relationship highlights the strong connection that is typically present between surface water and groundwater in adjacent aquifers: during periods of flooding, surface water commonly leaks into the adjacent aquifer and raises groundwater levels; whereas in dry periods, the continuous discharge of groundwater from adjacent aquifers maintains the base flow of surface streams. An important implication of this relationship is that the elevation of a surface water body in contact with an unconfined aquifer also represents the elevation of the water table at that location.
On the other hand, it is very common for a shallow water table to be present where glacial till or other fine-grained sediments are at the land surface, because they typically are poorly permeable and retard downward movement of water into underlying aquifers. It might be recalled that most of the central till plain was a wetland prior to settlement and the advent of artificial drainage, so the appearance of a seasonally high water table in the modern landscape (fig. 3) is scarcely surprising, and is simply a reminder of the natural history of the region.
In some cases, a continuous saturated condition may persist downward through the confining unit and into the underlying aquifer. The water level in such a confined aquifer, when measured in a well, typically will be higher than the top of the aquifer (fig. 2), reflecting the hydraulic head in the aquifer. This is referred to as an artesian condition, or artesian aquifer . The existence of artesian conditions is a major part of the definition of a confined aquifer. In rare cases, artesian pressure may be sufficiently great that such a well will flow freely without the aid of a pump. Such flowing artesian wells typically occur in low parts of the landscape in aquifers that receive recharge from considerably higher elevations.
In any case, the level to which water rises when measured at any given point in a confined aquifer is referred to as the potentiometric (or piezometric) surface. By measuring water levels at many locations in a confined aquifer or aquifer system, a map showing the potentiometric surface can be produced. The potentiometric surface is somewhat of an abstract concept because, in the absence of wells, the overlying confining unit prevents the water level from rising above the top of the aquifer; instead, the aquifer is under pressure. The elevations of the potentiometric surface and water table are typically different at any given point in the landscape because they are related to different bodies of rock or sediment. The water table is almost always higher than the potentiometric surface below major uplands, while the reverse is usually true beneath major lowlands, such as river valleys.
Still another common condition occurs when there is sufficient contrast in permeability between a confining unit and the underlying aquifer to cause water table conditions to exist in both units, that is, the aquifer is not completely saturated and a secondary vadose zone exists in the upper part of the aquifer below a saturated confining unit. In this situation, the water table in the confining unit is referred to as a " perched water table " (fig. 3) because its presence is a function of the slow permeability of the confining unit, whereas the underlying aquifer is sufficiently permeable to allow groundwater to flow laterally outward and drain partly out of the aquifer, and is thus referred to as semiconfined. Such conditions are quite common in Marion County in heavily dissected parts of the landscape, notably near the high bluffs along the major valleys, which intercept confined aquifers and allow them to drain to the surface. Many springs and seeps occur at such places.
Schematic diagram illustrating the distribution of hydraulic head, the groundwater flow system, and the distribution of groundwater recharge and discharge in relation to surface topography for a simple water-table aquifer (from Fleming, 1994).
The preceding discussion calls attention to the fact that groundwater is a dynamic resource that flows from one place to another in the subsurface in response to lateral and vertical differences in hydraulic head. Hydraulic head is a function of pressure and elevation. The former is the predominant driver of hydraulic head differences in confined, artesian aquifers, as reflected by the configuration of the potentiometric surface, whereas the latter is more important in unconfined (water-table) aquifers; however, both components are present in all types of aquifers. Because of this, neither the water table nor the potentiometric surface are flat, rather they are slightly to strongly sloping surfaces whose configurations broadly mimic the topography of the overlying landscape (fig. 4). Regional uplands are underlain by sizable water-table and potentiometric highs, while large valleys coincide with similarly shaped valleys in the water table and potentiometric surfaces of subjacent aquifers. This relationship reflects the fact that most uplands act as groundwater recharge areas — places where precipitation percolates downward through the soil and unsaturated zone and into aquifers — whereas most streams are the recipients of groundwater discharge, whose ongoing leakage to the surface in the form of springs, seeps, and direct discharge in streambeds sustains the base flow of streams throughout the year, especially during extended periods of little to no precipitation.
The slope of the water table or potentiometric surface makes up the horizontal component of the hydraulic gradient , which determines the compass direction of groundwater flow. The hydraulic gradient also has a strong vertical component at places, however, which causes groundwater flow to be three-dimensional and is largely responsible for the crucial processes of groundwater recharge and discharge. Vertical components of groundwater flow may occur within an aquifer or they may entail flow from one aquifer to another, across an intervening confining unit.
Schematic diagram of a simple aquifer — confining unit situation along a major valley such as the White River, showing confined and unconfined conditions and the relationship between the water table and potentiometric surface. Blue arrows indicate the direction of groundwater flow in aquifer and confining unit. Groundwater flow is much slower, and predominantly vertical, in the much less permeable till confining unit, as indicated schematically by the dashed arrows, whereas flow is faster and predominantly horizontal in the permeable sand and gravel aquifer. (Adapted from Brown and Laudick, 2003.)
In general, horizontal flow is predominant in most large aquifers, whereas vertical flow is typically greater across thick confining units (fig. 5). Locations where different aquifers come into contact or are superposed, such as along major paleosurfaces where intervening confining units have been cut out, or where sand- and gravel-filled channels have cut through a confining unit, are often of great importance because they act as preferred conduits that enhance recharge (or discharge) into and out of deep aquifers, and may act as pathways for the migration of contaminants into aquifers that are otherwise confined and generally well protected from surface contamination.
The subject of groundwater recharge — how fast it occurs, how much occurs and at what times of year, where it occurs, and what specific mechanisms are involved — is complex, because the process takes place almost entirely out of sight and is very challenging to accurately measure at a single point in the landscape, much less over the area of an entire county. Nevertheless, the overall motion of groundwater through aquifers and confining units involves much recharge and discharge, and constitutes the groundwater flow system in any given area. Groundwater flow systems are typically represented by maps and cross sections that show the elevation of the water table and potentiometric surface(s) of major aquifers.
A generalized water-level map of the shallow aquifer system in Marion County (which may be viewed in the IMS portion of this Web site and is recommended while viewing the remainder of this web page) was made by using water-level data reported from wells developed in aquifers along and above the pre-Wisconsin surface. In upland areas, the shallow aquifer system is mostly overlain by sheets of till up to several tens of feet thick and is, thus, under confined conditions, whereas in and near the White River valley, this system is largely under unconfined (water table) or semiconfined conditions (see fig. 1). In the latter area, aquifers in the upper part of the system lie immediately below the modern land surface and are unconfined, whereas deeper parts of the system may be locally confined or semiconfined by extensive till sheets. Because of this range in conditions, the water-level contours are a composite feature representing the potentiometric surface in confined parts of the system and the water table for parts of the system that are largely unconfined. The difference between these areas is clearly indicated by the hydrogeologic settings on the map, but for simplicity, the entire contoured surface is referred to as the "potentiometric surface." Also, in some areas, there are few wells developed in the shallow aquifer system, consequently the elevation and configuration of the potentiometric surface were largely inferred from a few widely scattered water levels, surface topography, and the elevations of perennial streams in contact with the aquifer system. For all these reasons, these maps are intended to show only broad, regional trends of the potentiometric surface, and cannot be reliably used to infer local, small-scale flow directions within any particular aquifer.
Despite these limitations, the potentiometric contours reveal several important features. The most striking trend is the parallelism between the elevations of the water-level contours and modern surface topography. Elevated areas of the potentiometric surface coincide with topographically high areas, while major potentiometric valleys parallel the large topographic valleys associated with the major drainages. The contours broadly represent the hydraulic head at any given place in the aquifer system, thus the potentiometric map can be thought of as a two-dimensional representation of the distribution of hydraulic head within the aquifer system. As groundwater always flows from higher to lower head, the map clearly shows that groundwater originates at the potentiometric highs below uplands in the county, and flows towards the major drainages, such as the White River, Fall Creek, Eagle Creek, and other perennial streams. From this pattern, we can infer that these uplands are places where the shallow aquifer system receives considerable groundwater recharge, whereas much discharge from this system occurs in the major stream valleys. Indeed, the fact that the major streams continue to flow (albeit at a reduced rate) during periods of low or no precipitation indicates that their base flows are supported by abundant groundwater discharge. Most of that discharge comes from the shallow aquifer system, which is commonly in contact with surface streams and frequently underlies their beds. By the same token, periods of heavy precipitation and high stream flows cause surface water to leak back into the aquifer system along the surface streams, recharging the valley aquifers and contributing to the overall hydrologic balance that maintains stream flow throughout the year.
Another interesting feature of the potentiometric map is the relationship between the slope of the hydraulic gradient and surface topography. Potentiometric contours tend to be relatively widely spaced beneath both the large uplands and the broad stream valleys, reflecting a gentle horizontal hydraulic gradient. Lateral groundwater flow is likely to be comparatively slow beneath such places. To an extent, however, vertical groundwater flow is potentially of greater importance in such places, as demonstrated by large differences in water levels between wells developed in aquifers at varying depths. Beneath most uplands, for example, water levels become progressively lower in aquifers at greater depth, indicating downward flow (groundwater recharge), whereas the opposite is true beneath major stream valleys, in which water levels in deep aquifers are commonly greater than those in shallow aquifers, indicating upwelling of groundwater towards the surface. In contrast, the potentiometric contours are typically bunched closely together near the walls of the major valleys, indicating steep hydraulic gradients and strong inward flow of groundwater towards the modern streams. This process is visibly manifested at the modern land surface by lines of springs and seeps near the bases of large bluffs. Depending on the size of the aquifer involved and the strength of the hydraulic gradient, some of these springs and seepage areas may discharge groundwater at rates of hundreds to thousands of gallons per day within a relatively compact area, producing large seepage swamps and other kinds of ecologically valuable wetlands. Holliday Park, located along the White River bluff in north-central Marion County, preserves excellent examples of such features in a natural state.
The preceding discussion highlights the wide range of subsurface conditions that occur in Marion County. Differences in groundwater flow conditions coupled with much diversity in the geologic framework of aquifers and confining units means that the occurrence and behavior of groundwater varies greatly from place to place, often at a local scale. Although it is virtually impossible to capture all of the local variation at the level of a county-scale map, the concept of hydrogeologic settings offers one way of organizing some of this hydrologic and geologic variability into a geographic framework useful for understanding how major geologic terrains within the county function in terms of their surface hydrology and groundwater resources.
Hydrogeologic settings incorporate a combination of basic hydraulic data (for example, the potentiometric surface map and recharge-discharge relations inferred from it) and geologic framework information (for example, glacial terrain s, thickness and continuity of confining units, types of aquifers prevalent in a given area) to characterize the range of interaction between groundwater, surface water, and the geologic framework within a given region. Thirteen hydrogeologic settings are defined in Marion County (see the IMS), based on known and inferred combinations of geologic and groundwater flow system conditions. Undoubtedly a greater number of such settings could be differentiated at a more localized scale utilizing site-specific data. Figure 6 schematically illustrates the majority of the settings in relation to subsurface geology, groundwater flow patterns, surface water bodies, and their position in the regional landscape.
Schematic hydrogeologic cross section across southern Marion County, showing the configuration of the shallow aquifer system, relationship between the potentiometric surfaces of the shallow aquifer system and deeper aquifers, generalized groundwater flow directions, and hydrogeologic settings (labeled across the top of the diagram) (from Brown and Laudick, 2003, adapted from fig. 11 in Fleming and others, 1993).
Hydrogeologic settings take into account the following elements: (1) the sequence, or geometric arrangement, of aquifers and confining units, also known as hydrostratigraphy ; (2) the physical attributes of the geologic materials that comprise the sequence, which in turn control their abilities to transmit and store groundwater; (3) position in the groundwater flow system, particularly with respect to recharge and discharge areas, and determined mainly by the configuration of the potentiometric surface or water table, and its relationship to surface water and other landscape elements; (4) the degree of confinement of the system as a whole, as determined both by water-level data and by the thickness and extent of confining units; (5) estimated rate of surface infiltration, based on the geologic material at the land surface, the characteristics of the soil developed on that material, and the typical slope and shape of the land surface; (6) typical thickness range of the vadose (unsaturated) zone; and (7) estimated strength and direction of the predominant hydraulic gradient (lateral or vertical) within a particular setting. These features are summarized here for the thirteen hydrogeologic settings mapped in Marion County.
Sensitivity to Contamination
The term "sensitivity to contamination" refers to the relative ease with which contaminants can migrate into the groundwater flow system at any given place on the landscape, and the potential impact on groundwater quality once they enter the system. A variety of contaminants can potentially migrate into aquifers and negatively affect water quality. Although some contaminants have densities greater or less than water, and thus may sink or float in a column of water, most are soluble in water and thus behave similarly to water in terms of their ability to enter the groundwater flow system and their subsequent migration in the subsurface. A good example of such a "generic" contaminant is nitrate, a widely occurring ion that comes from a variety of sources likely to be found in Marion County — fertilizer, sewage and septic effluent, animal wastes, as well as natural soil processes.
Most contaminant sources are situated on or just below the soil surface, so the same geologic and hydrologic characteristics that define a given hydrogeologic setting also influence its relative sensitivity to contamination. Among the most important of these are position in the groundwater flow system and recharge potential. Groundwater discharge areas are inherently less sensitive to contamination because the movement of groundwater (often upward) into surface-water bodies tends to limit the impact and areal extent of most contaminants that may be introduced at the surface. Unless the contaminant is heavier than water and sinks through the aquifer, the impact is likely to be restricted to the uppermost part of the aquifer close to the water table. This isn't meant to downplay the issue or write off the top of the aquifer system in these places, only to point out that the chance of contamination spreading into deeper parts of the system is considerably less than in settings characterized by strong downward hydraulic gradients and high rates of groundwater recharge.
On the other hand, not all recharge areas, as defined by elevated regions on the potentiometric surface, are necessarily at heightened risk of contamination. Actual recharge rates are likely to be reduced on uplands where aquifers are confined by thick till, whose relatively low hydraulic conductivities physically limit the rate at which water can be transmitted into subjacent aquifers. The recharge potential is greater where the till-confining units are thin or breached by sand and gravel bodies, and greater still where aquifers are unconfined. The differing combinations of conditions that define the respective hydrogeologic settings allow the relative recharge potential of each setting to be qualitatively estimated. Recharge potential can be thought of as an indicator of the relative amount of total precipitation that ultimately becomes groundwater, and the rate at which it reaches the saturated zone. At least some recharge is required for most potential contaminants to actually reach the groundwater flow system. Therefore, recharge potential is closely related to the sensitivity of groundwater to contamination from surface or near-surface sources, and can act as a proxy for sensitivity to contamination. Using the example of a groundwater discharge area, even though the White River valley is a regional discharge area, it is also characterized by high recharge potential, because thick sections of sand and gravel commonly underlie the surface. The sand and gravel can rapidly absorb and transmit large amounts of water to the top of the aquifer system, even though groundwater flow may be upward in deeper parts of the system.
The different hydrogeologic settings in Marion County can be qualitatively ranked according to their relative sensitivities to contamination. This ranking is essentially an estimate of the relative rates at which contaminants might enter the shallow groundwater flow system, based on the combinations of geologic and hydrogeologic attributes that define each setting. The sensitivity of groundwater within each hydrogeologic setting is depicted as a range that is generally indicative of the range of geologic conditions within the setting. These ranges also reflect the fact that contaminants having different physical properties (for example, heavier or lighter than water) are likely to exhibit radically different aqueous behavior in the same hydrogeologic setting. In view of these considerations, the sensitivity of groundwater in Marion County constitutes a series of overlapping ranges, with large and obvious distinctions between some hydrogeologic settings, but little or no distinction between others.
One other significant risk to groundwater quality unrelated to any geologic or hydraulic property is posed by the thousands of old wells present in the county. Many of these are still in service, but as the area served by public water supplies expands outward into enclaves of homes and businesses currently served by individual wells, the disposition of those abandoned wells becomes critically important. Under the Indiana well code, wells that have been abandoned and no longer have any reasonable prospect of further service are required to be properly sealed by plugging them with a grout material. This is normally accomplished by pouring a granular bentonite product down the well casing until the borehole is filled. As the bentonite becomes hydrated, it expands to form a seal that prevents polluted runoff and other surface-derived contaminants from migrating down the well casing into the aquifer. Few people other than water-well drillers are aware of this procedure, and it is very common to see old wells still standing on properties that have been connected to public water supplies for many years. Many older well casings are steel, which will eventually corrode and rust out, providing a ready-made-and largely unseen-conduit for polluted runoff, lawn chemicals, septic effluent, and other pollutants to find their way into the groundwater resource, where they may eventually show up in other wells in the form of bacterial or chemical contamination. Additional information about what constitutes an abandoned well, the environmental risks it poses, and how to properly seal it, can be found here.
Brown, S. E., and Laudick, A. J., eds., 2003, Hydrogeologic framework of Marion County, Indiana — a digital atlas illustrating hydrogeologic terrain and sequence: Indiana Geological Survey Open-File Study 00-14, CD-ROM.
Fleming, A. H., 1994, The hydrogeology of Allen County, Indiana — A geologic and ground-water atlas: Indiana Geological Survey Special Report 57, 111 p.
Fleming, A. H., Brown, S. E., and Ferguson, V. R., 1993, Hydrogeologic framework of Marion County, Indiana: Indiana Geological Survey Open-File Study 93-05, 67 p.
Maxey, G. B., 1964, Hydrostratigraphic units: Journal of Hydrology, v. 2, p. 124-129.
Poland, J. F., Lofgren, E. E., and Riley, F. S., 1972, Glossary of selected terms useful in studies of the mechanics of aquifer systems and land subsidence due to fluid withdrawals: U.S. Geological Survey Water-Supply Paper 2025, 9 p.