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components affected by water withdrawn from the ground-water system (for
example, water withdrawn from a well caused a net decrease in ground-water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