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GW2003.02 Terminology Used in Studies of the Source of Water to Wells Under Steady-State Conditions
The Office of Ground Water has received many recent inquiries about the
appropriate terminology to use when describing the source of water to wells.
This memorandum discusses these terminology issues and offers standard
operational definitions of selected terms. As is standard practice in
technical writing, any terms used in a U.S. Geological Survey report should
be clearly defined to state exactly what is meant and how relevant
calculations were made.
As discussed by Reilly and Pollock (1995), the term 'source of water to wells' has been used in the hydrologic literature in two distinct and quite different contexts: a water-budget (or water-balance) context and a transport context. Theis (1940) discussed the source of water to wells in a water-budget context. The water-budget context addresses the water-budget components affected by water withdrawn from the ground-water system (for example, water withdrawn from a well caused a net decrease in ground-water discharge to a stream). The water-budget context does not address the paths of the water that discharges to the well, but addresses the effect on the system-wide water budget. This memorandum addresses the terminology associated with the use of the term 'source of water to wells' in a transport context. The source of water in a transport context represents the location where the water discharging from a well originally entered the ground-water system (for example, see Franke and others, 1998). The transport context focuses on the flow paths of water to the actual point of discharge.
In discussions to date, three aspects of the source of water to wells have been considered significant from a transport or water-quality perspective under steady-state conditions. These three aspects are: (1) a surface area located on the boundary of the three-dimensional ground-water system indicating where the water that eventually discharges at a specific well enters the ground-water system, (2) the three-dimensional volumetric part of an aquifer through which ground water flows to a discharging well from the area contributing recharge, and (3) the projection of the three-dimensional volumetric part of an aquifer to a two-dimensional map or cross-sectional view. Some terms used in the literature to define these and other aspects include capture area, source area, contributing area, contributing recharge area, zone of contribution, area contributing recharge to wells, zone of influence, area of diversion, and area of influence (for example, see Brown, 1963; USEPA, 1987; Morrissey, 1989; Risser and Madden, 1994; Schalk, 1996; Domenico and Schwartz, 1998; Zheng and Bennett, 2002). Some of these terms have been defined differently by different investigators and some terms are defined differently for two-dimensional and three-dimensional systems, thereby causing confusion. In addition, the analysis of the source of water under transient hydraulic conditions adds another level of complexity because the areas and volumes change through time as a function of the stresses, traveltimes, and flow paths of the water (see Masterson and others, 2002; Reilly and Pollock, 1995; Pollock, 1994). Thus, it is important that any term used in a U.S. Geological Survey report be clearly defined to state exactly what is meant.
Some useful terms and definitions for steady-state conditions are provided below as an aid to investigators reporting on the results of studies on sources of water to wells. The terms in the list below are considered standard terminology for USGS reports. The Office of Ground Water recommends the use of these terms when they meet the objective of the investigation.
1. The "area contributing recharge to a discharging well" (or the shorter form "area contributing recharge") for three-dimensional systems, is defined as the surface area on the three-dimensional boundary of the ground-water system that delineates the location of the water entering the ground-water system that eventually flows to the well and discharges (modified from Reilly and Pollock, 1993). This boundary is usually located on the water table and along surface-water boundary features. However, depending upon the definition of the ground-water system under investigation, it can be located along any boundary. For example, a continuous confining unit might be considered the boundary for a study of an isolated confined aquifer system.
2. The "zone of contribution to a discharging well" (or the shorter form "zone of contribution") is defined as the three-dimensional volumetric part of an aquifer through which ground water flows to a discharging well from the area contributing recharge (modified from Morrissey, 1989). The zone of contribution can be visualized as a three-dimensional streamtube through the aquifer.
3. A term that is related to the "zone of contribution to a discharging well" is the projection of the three-dimensional volume of water flowing to a discharging well to a two-dimensional map or cross-sectional view. The areal projection may be important because of concerns about the potential for sources of contamination above the zone of contribution to enter the zone of contribution; for example, such sources of contamination might be dense non-aqueous fluids or short-circuiting long-screened wells. We recommend using the term, "areal extent of the zone of contribution to a discharging well" to clearly state that the two-dimensional map area is the projection of the three-dimensional volume of water flowing to a discharging well. Similarly, for cross-sectional views, we recommend that the view be simply identified as a "cross-sectional view of the zone of contribution to a discharging well."
Even under steady-state conditions, the water discharging from a pumped well is a blend of water of different ages or traveltimes. Many investigators have identified time-related areas contributing recharge to a discharging well. Time-related areas and volumes can be calculated for steady-state or transient-state hydraulic conditions (for example, see Pollock, 1994, p. 6-17 through 6-22). These traveltime delineations often are used to relate water quality observed in wells to the quality of water entering as recharge and to the quality of water already in the zone of contribution when the well began pumping. If important to the analysis, these time-related areas should be explained and identified clearly in reports and presentations. The analysis and presentation of transient time-related areas and volumes is complex and problem-dependent, and the definition of terms for these areas and volumes is beyond the scope of this memorandum.
The recommended terms are readily definable for steady-state conditions. If the system is transient in nature, then care must be taken to adequately define in a report or presentation exactly what characteristic is being calculated at a specific time. In summary, there have been many definitions of terms used to define areas and volumes that are the source of water to discharging wells. Investigators must carefully describe their objective and approach to these problems and clearly define the terms they use.
William M. Alley /signed/ Chief, Office of Ground Water
Distribution: All WRD Employees
References:
Brown, R.H., 1963, The cone of depression and the area of diversion around a discharging well in an infinite strip aquifer subject to uniform recharge: U.S. Geological Survey Water-Supply Paper 1545-C, p. C69-C85.
Domenico, P.A., and Schwartz, F.W., 1998, Physical and chemical hydrogeology, second edition: John Wiley and Sons, Inc., New York, NY, 506 p.
Franke, O. L., Reilly, T. E., Pollock, D. W., and LaBaugh, J. W., 1998, Estimating areas contributing recharge to wells – Lessons from previous studies: U.S. Geological Survey Circular 1174, 14 p (Third printing, 1999).
Masterson, J.P., Hess, K.M., Walter, D.A., and LeBlanc, D.R., 2002, Simulated changes in the sources of ground water for public-supply wells, ponds, streams, and coastal areas on western Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 02-4143, 12 p.
Morrissey, D.J., 1989, Estimation of the recharge area contributing water to a pumped well in a glacial-drift, river-valley aquifer: U.S Geological Survey Water-Supply Paper 2338, 41 p.
Pollock, D.W., 1994, User's Guide for MODPATH/MODPATH-PLOT, Version 3: A particle tracking post-processing package for MODFLOW, the U.S. Geological Survey finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 94-464, 6 ch.
Reilly, T.E., and Pollock, D.W., 1993, Factors affecting areas contributing recharge to wells in shallow aquifers: U.S. Geological Survey Water-Supply Paper 2412, 21 p.
Reilly, T. E., and Pollock, D. W., 1995, Effect of seasonal and long-term changes in stress on sources of water to wells: U.S. Geological Survey Water-Supply Paper 2445, 25 p.
Risser, D.W., and Madden, T.M., Jr., 1994, Evaluation of methods for delineating areas that contribute water to wells completed in valley-fill aquifers in Pennsylvania: U.S. Geological Survey Open-File Report 92-635, 82 p.
Schalk, C.W., 1996, Estimation of the recharge areas contributing water to the south well field, Columbus, Ohio: U.S. Geological Survey Water-Resources Investigations Report 96-4039, 26 p.
Theis, C.V., 1940, The source of water derived from wells -- essential factors controlling the response of an aquifer to development: Civil Engineering Magazine, May 1940, p. 277-280.
U. S. Environmental Protection Agency, 1987, Guidelines for delineation of wellhead protection areas: USEPA, Office of Ground-Water Protection, Wash. D.C., June 1987.
Zheng, Chunmiao, and Bennett, G.D., 2002, Applied contaminant transport modeling, second edition: John Wiley and Sons, Inc., New York, NY, 621 p.
Carolyn L. Wakelee Office of Ground Water, WRD
U.S. Geological Survey 12201 Sunrise Valley Dr. Mail Stop 411 Reston, Virginia 20192 Phone: 703/648-5001 Fax: 703/648-6693
As discussed by Reilly and Pollock (1995), the term 'source of water to wells' has been used in the hydrologic literature in two distinct and quite different contexts: a water-budget (or water-balance) context and a transport context. Theis (1940) discussed the source of water to wells in a water-budget context. The water-budget context addresses the water-budget components affected by water withdrawn from the ground-water system (for example, water withdrawn from a well caused a net decrease in ground-water discharge to a stream). The water-budget context does not address the paths of the water that discharges to the well, but addresses the effect on the system-wide water budget. This memorandum addresses the terminology associated with the use of the term 'source of water to wells' in a transport context. The source of water in a transport context represents the location where the water discharging from a well originally entered the ground-water system (for example, see Franke and others, 1998). The transport context focuses on the flow paths of water to the actual point of discharge.
In discussions to date, three aspects of the source of water to wells have been considered significant from a transport or water-quality perspective under steady-state conditions. These three aspects are: (1) a surface area located on the boundary of the three-dimensional ground-water system indicating where the water that eventually discharges at a specific well enters the ground-water system, (2) the three-dimensional volumetric part of an aquifer through which ground water flows to a discharging well from the area contributing recharge, and (3) the projection of the three-dimensional volumetric part of an aquifer to a two-dimensional map or cross-sectional view. Some terms used in the literature to define these and other aspects include capture area, source area, contributing area, contributing recharge area, zone of contribution, area contributing recharge to wells, zone of influence, area of diversion, and area of influence (for example, see Brown, 1963; USEPA, 1987; Morrissey, 1989; Risser and Madden, 1994; Schalk, 1996; Domenico and Schwartz, 1998; Zheng and Bennett, 2002). Some of these terms have been defined differently by different investigators and some terms are defined differently for two-dimensional and three-dimensional systems, thereby causing confusion. In addition, the analysis of the source of water under transient hydraulic conditions adds another level of complexity because the areas and volumes change through time as a function of the stresses, traveltimes, and flow paths of the water (see Masterson and others, 2002; Reilly and Pollock, 1995; Pollock, 1994). Thus, it is important that any term used in a U.S. Geological Survey report be clearly defined to state exactly what is meant.
Some useful terms and definitions for steady-state conditions are provided below as an aid to investigators reporting on the results of studies on sources of water to wells. The terms in the list below are considered standard terminology for USGS reports. The Office of Ground Water recommends the use of these terms when they meet the objective of the investigation.
1. The "area contributing recharge to a discharging well" (or the shorter form "area contributing recharge") for three-dimensional systems, is defined as the surface area on the three-dimensional boundary of the ground-water system that delineates the location of the water entering the ground-water system that eventually flows to the well and discharges (modified from Reilly and Pollock, 1993). This boundary is usually located on the water table and along surface-water boundary features. However, depending upon the definition of the ground-water system under investigation, it can be located along any boundary. For example, a continuous confining unit might be considered the boundary for a study of an isolated confined aquifer system.
2. The "zone of contribution to a discharging well" (or the shorter form "zone of contribution") is defined as the three-dimensional volumetric part of an aquifer through which ground water flows to a discharging well from the area contributing recharge (modified from Morrissey, 1989). The zone of contribution can be visualized as a three-dimensional streamtube through the aquifer.
3. A term that is related to the "zone of contribution to a discharging well" is the projection of the three-dimensional volume of water flowing to a discharging well to a two-dimensional map or cross-sectional view. The areal projection may be important because of concerns about the potential for sources of contamination above the zone of contribution to enter the zone of contribution; for example, such sources of contamination might be dense non-aqueous fluids or short-circuiting long-screened wells. We recommend using the term, "areal extent of the zone of contribution to a discharging well" to clearly state that the two-dimensional map area is the projection of the three-dimensional volume of water flowing to a discharging well. Similarly, for cross-sectional views, we recommend that the view be simply identified as a "cross-sectional view of the zone of contribution to a discharging well."
Even under steady-state conditions, the water discharging from a pumped well is a blend of water of different ages or traveltimes. Many investigators have identified time-related areas contributing recharge to a discharging well. Time-related areas and volumes can be calculated for steady-state or transient-state hydraulic conditions (for example, see Pollock, 1994, p. 6-17 through 6-22). These traveltime delineations often are used to relate water quality observed in wells to the quality of water entering as recharge and to the quality of water already in the zone of contribution when the well began pumping. If important to the analysis, these time-related areas should be explained and identified clearly in reports and presentations. The analysis and presentation of transient time-related areas and volumes is complex and problem-dependent, and the definition of terms for these areas and volumes is beyond the scope of this memorandum.
The recommended terms are readily definable for steady-state conditions. If the system is transient in nature, then care must be taken to adequately define in a report or presentation exactly what characteristic is being calculated at a specific time. In summary, there have been many definitions of terms used to define areas and volumes that are the source of water to discharging wells. Investigators must carefully describe their objective and approach to these problems and clearly define the terms they use.
William M. Alley /signed/ Chief, Office of Ground Water
Distribution: All WRD Employees
References:
Brown, R.H., 1963, The cone of depression and the area of diversion around a discharging well in an infinite strip aquifer subject to uniform recharge: U.S. Geological Survey Water-Supply Paper 1545-C, p. C69-C85.
Domenico, P.A., and Schwartz, F.W., 1998, Physical and chemical hydrogeology, second edition: John Wiley and Sons, Inc., New York, NY, 506 p.
Franke, O. L., Reilly, T. E., Pollock, D. W., and LaBaugh, J. W., 1998, Estimating areas contributing recharge to wells – Lessons from previous studies: U.S. Geological Survey Circular 1174, 14 p (Third printing, 1999).
Masterson, J.P., Hess, K.M., Walter, D.A., and LeBlanc, D.R., 2002, Simulated changes in the sources of ground water for public-supply wells, ponds, streams, and coastal areas on western Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 02-4143, 12 p.
Morrissey, D.J., 1989, Estimation of the recharge area contributing water to a pumped well in a glacial-drift, river-valley aquifer: U.S Geological Survey Water-Supply Paper 2338, 41 p.
Pollock, D.W., 1994, User's Guide for MODPATH/MODPATH-PLOT, Version 3: A particle tracking post-processing package for MODFLOW, the U.S. Geological Survey finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 94-464, 6 ch.
Reilly, T.E., and Pollock, D.W., 1993, Factors affecting areas contributing recharge to wells in shallow aquifers: U.S. Geological Survey Water-Supply Paper 2412, 21 p.
Reilly, T. E., and Pollock, D. W., 1995, Effect of seasonal and long-term changes in stress on sources of water to wells: U.S. Geological Survey Water-Supply Paper 2445, 25 p.
Risser, D.W., and Madden, T.M., Jr., 1994, Evaluation of methods for delineating areas that contribute water to wells completed in valley-fill aquifers in Pennsylvania: U.S. Geological Survey Open-File Report 92-635, 82 p.
Schalk, C.W., 1996, Estimation of the recharge areas contributing water to the south well field, Columbus, Ohio: U.S. Geological Survey Water-Resources Investigations Report 96-4039, 26 p.
Theis, C.V., 1940, The source of water derived from wells -- essential factors controlling the response of an aquifer to development: Civil Engineering Magazine, May 1940, p. 277-280.
U. S. Environmental Protection Agency, 1987, Guidelines for delineation of wellhead protection areas: USEPA, Office of Ground-Water Protection, Wash. D.C., June 1987.
Zheng, Chunmiao, and Bennett, G.D., 2002, Applied contaminant transport modeling, second edition: John Wiley and Sons, Inc., New York, NY, 621 p.
Carolyn L. Wakelee Office of Ground Water, WRD
U.S. Geological Survey 12201 Sunrise Valley Dr. Mail Stop 411 Reston, Virginia 20192 Phone: 703/648-5001 Fax: 703/648-6693