Terminology Used in Studies of the Source of Water to Wells Under Steady-State Conditions

In Reply Refer To: 
Mail Stop 411                                                             May 28, 2003 



OFFICE OF GROUND WATER TECHNICAL MEMORANDUM NO. 2003.02 


Subject: 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components affected by water withdrawn from the ground-water system (for 
example, water withdrawn from a well caused a net decrease in ground-water 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