Environmental Engineering Reference
In-Depth Information
et al . ( 2007 ) used specific conductance, a water-
quality characteristic that is measured continu-
ously at hundreds of stream-gauging stations
across the United States. Radon, an inert gas
emanating from source rock, has been used as a
naturally occurring tracer for groundwater dis-
charge to streams and coastal waters; virtually
all radon measured in surface water comes from
groundwater (Gunderson and Wanty, 1991 ).
Rhodamine dye, bromide, chloride, and lithium
are often used in tracer-injection studies.
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4.6.1 End-member mixing analysis
The first step in developing a tracer mass-budget
equation for a stream is to construct a water-
budget equation for that stream (Equation ( 4.2 )).
The water-budget equation can be simplified
by application over a stream reach that has no
tributaries and for a time period within which
precipitation and evapotranspiration of stream
water can be neglected. With the additional
assumption that stream stage is constant (and,
hence, change in storage is 0), the water-budget
equation takes the form:
Figure 4.13 Average monthly recharge (as percent of
annual recharge) calculated by RORA for 197 streamflow
sites in Pennsylvania (after Risser et al ., 2005a ).
4.6 Chemical and isotopic
streamflow hydrograph analysis
Tracers are used in surface-water studies for a
variety of purposes (Kendall and McDonnell,
1998 ), including determining velocity and
residence times in streams (Kimball et al .,
1994 ), identifying flow paths in karst terrains
(Katz et al ., 1997 ), and identifying natural and
human-induced biogeochemical reactions
within watersheds (Soulsby et al ., 2004 ). Tracer
methods are also used to quantify base flow
over gaining reaches of streams. The most
common approach, end-member mixing ana-
lysis, consists of constructing a mass balance
for the chemical or isotopic tracer. Tracer-
injection methods measure tracer concentra-
tions in stream water at multiple distances
downstream from the injection point; the rate
of groundwater discharge is then inferred
from the rate of tracer dilution. The use of
tracers in the subsurface to estimate recharge
is addressed in Chapter 7 .
Tracers should be conservative (i.e. nonre-
active) and have no unknown sources in the
watershed. Naturally occurring tracers, such as
chloride, and tracers such as tritium that have
been introduced to the groundwater system
through the actions of humans are commonly
used in end-member mixing analyses. Stewart
Q
sw
= ++
Q
sw
RQ
seep
(4.14)
out
in
off
where Q sw out is stream discharge at the down-
stream end of the reach, Q sw in is streamflow at
the upstream end of the reach, R off is direct sur-
face runoff, and Q seep is base flow. We can then
write a mass balance equation for any tracer
and denote that tracer's concentration as C ,
where subscripts refer to one of the four terms
in Equation ( 4.14 ):
C Q
sw
=
CQ
sw
+
C
R
+
C Q
seep
(4.15)
out
out
in
in
Roff
off
gw
For the end-member mixing analysis (EMMA),
stream discharge and stream and groundwater
concentrations must be measured. C Roff must be
measured or estimated; sometimes it is assumed
equal to stream concentration at a time when
runoff dominates streamflow. At other times
C Roff is set equal to tracer concentration in pre-
cipitation (the validity of either approach should
be examined). Base flow is determined by com-
bining Equations ( 4.14 ) and ( 4.15 ):
Q
=
[(
C Q
CQ
)
C
sw
sw
seep
out
out
in
in
Roff
(4.16)
(
Q
Q
)] /(
CC
)
sw
sw
out
in
gw
Roff
 
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