IDENTIFYING SOURCES OF BASEFLOW IN THE SANTA ANA RIVER, CALIFORNIA

Gregory O. Mendez and Kenneth Belitz[^1]

# 

ABSTRACT: The Santa Ana River is an important source of groundwater
recharge for aquifers in Orange County, California. Under base-flow
conditions, a substantial percentage of the Santa Ana River consists of
treated wastewater. The objectives of this study were to characterize
the interaction between the Santa Ana River and the shallow groundwater
system, and to quantify the percentage of wastewater and other sources
of flow in the Santa Ana River. To accomplish these objectives, stream
discharge and dye-tracer concentrations were measured in the Santa Ana
River and its tributaries. The study area was divided into an upper
reach (8 mi) and lower reach (10 mi). Discharge measurements were made
at 10 sites along the river, including a USGS gaging station (Santa Ana
River at MWD Crossing), and on 5 tributaries. Water samples were
collected every 5 to 15 minutes at seven sites to measure Rhodamine WT
dye concentrations. The dye concentrations allow for a Lagrangian
interpretation of the discharge data. By measuring the discharge
associated with same mass of water, the percentage of wastewater can be
quantified. The flow of the Santa Ana River at MWD Crossing, the end of
the upper reach, was about 68 percent wastewater an at the Powerline
site, the end of the lower reach, wastewater increased to approximately
77 percent of total flow.

KEY TERMS: surface-water/groundwater interaction; infiltration

# INTRODUCTION

The Santa Ana River basin is being studied as part of the U. S.
Geological Survey's (USGS) National Water Quality Assessment (NAWQA)
Program. The Santa Ana River drains an extensively urbanized area of
about 2,700 mi^2^ and more than 4 million people. The Santa Ana River is
the largest stream system in southern California, extending from its
headwaters in the San Bernardino Mountains over 100 mi to its mouth at
the Pacific Ocean. The climate is Mediterranean having hot, dry summers,
and cool, wet winters. Average annual precipitation ranges from 12
in/year in the coastal plain, 18 in/year in the inland alluvial valleys,
and 40 in/year in the San Bernardino Mountains. Most of the
precipitation occurs between November and March. Base flow in the Santa
Ana River is maintained almost entirely by treated wastewater discharge.

The study area is located along an 18-mile reach of the Santa Ana River
in San Bernardino and Riverside Counties (Fig. 1). Upstream from the
study reach, the Santa Ana River is often dry. At the beginning of the
study reach, discharges from the Rialto Waste Water Treatment Plant
(WWTP) and the Rapid Infiltration and Extraction (RIX) Facility release
flow to the Santa Ana River. The study area was divided into an upper
8-mile reach and lower 10-mile reach. The two reaches have a common
point at a USGS gaging station 11066460 (Santa Ana River at MWD
Crossing) where stage is continuously monitored. Three tributaries in
the upper reach contribute flow directly to the river. There are two
tributaries in the lower reach, which consists of flow from Riverside
Water Quality Control Plant (WQCP) and runoff from Hole Lake, that merge
and flow parallel to the river. This combined flow is separated from the
river by a dirt levee that allows percolation to the river while it
flows to the Hidden Valley Wetlands. This tributary flow goes through a
series of ponds in the wetlands that allows water to percolate into the
ground and flow to the river at many locations.

METHODS

Measurements of discharge and dye concentrations were used to quantify
the exchange of surface water and groundwater. Discharge measurements
were made at 10 sites along the Santa Ana River and on 5 tributaries
(Fig. 1). Discharge measurements were made about every 2 hours at most
sites using techniques outlined by Carter and Davidian, 1968. Rhodamine
WT Dye was injected into the Santa Ana River and dye samples were
collected at seven sites downstream, three of which were in the upper
reach. Dye samples were collected and analyzed every 10 minutes until
dye arrived, every 5 minutes through the peak, and every 15 minutes for
several hours following the peak. A detailed explanation of the analysis
and presentation of time-of-travel data are covered in Kilpatrick and
Wilson (1989).

The ideal Lagrangian scheme for monitoring rivers is to make an initial
discharge measurement and then follow the same mass of water, making
additional discharge measurements in a downstream sequence, so that
physical and hydrologic changes in the water mass can be investigated.
The flow at one site can be related to the flow at an upstream site by
comparing flow since time of peak dye concentration at each site.
Because travel times between sites were unknown initially,

![](media/image1.png){width="6.93125in" height="4.027083333333334in"}

**Figure 1.** Location of Santa Ana River dye tracer sites, southern
California.

discharge-measurement times at upstream and downstream sites may not
correspond to the same mass of water and therefore were adjusted to
correspond to designated travel times since peak dye concentration. This
method allows a point for point comparison of the discharge associated
with the same water mass as it moves down stream. Tributary flow between
sites was added to the upstream flow, when applicable, to compute
differences between sites. In addition to computing differences in
discharge for several points in time, the average difference in
discharge for the set of points was calculated. This provides an
estimate of net gains or losses in the Santa Ana River integrated over
periods ranging from 6 to 14 hours. This approach assumes that the
travel time remains constant between sites during the 6 to 14 hour time
period.

There are several potential sources of error associated with flow
measurements (Rantz, 1982). A discharge measurement is rated using
several criteria pertaining to the channel characteristics and number of
sections measured, by the person making the measurement. Most flow
measurements made during this study were rated "good;" but some were
rated "fair," indicating possible errors of 5 to 8 percent. Therefore,
some of the observed changes in flow along the river may not be related
to exchanges with groundwater. Additional sources of error are
variability due to measurements made by more than one person, fatigue,
and limited visibility during the night. For the purposes of this study,
an error associated with any single discharge measurement was specified
as 5 percent. Given this, error bars were computed for the differences
between measurements. When one plots the average difference in discharge
as a line on the time series plots, the error bars provide constraints
on the suitability of the average as a summary of the individual points.
A smaller error associated with an individual discharge measurement
creates smaller error bars and, consequently more stringent constraints
on the tolerance for the line of average difference.

Dye Injection and sampling

Rhodamine WT dye was used as a tracer because it is (1) water soluble,
(2) highly detectable (3) not susceptible to background fluorescence,
(4) nontoxic in low concentrations, (5) relatively inexpensive, and (6)
reasonably stable in a normal water environment (Wilson and others,
1986). Rhodamine WT dye and fluorometers have been described in detail
by Wilson and others (1986) for their use in dye-tracing studies

Containers of Rhodamine WT dye were weighed before and after releasing
dye into the river to determine the amount used. The equation used for
estimating the quantity of dye necessary for Rhodamine WT 20-percent dye
is from Kilpatrick (1970):

> V*s* = 3.4x10^-4^(Q*m*L/*v*)^0.94^ C*p,*

where

V*s* = volume of stock Rhodamine WT 20-percent dye, in liters;

Q*m =* maximum stream discharge at downstream site, in cubic feet per
second;

L = distance to downstream site, in miles;

*V* = mean stream velocity, in feet per second; and

C*p* = peak concentration at downstream sampling site, in micrograms per
liter.

Grab samples were collected at each site by wading into the main flow of
the channel and filling a 250-ml amber glass bottle. Dye concentrations
were measured in the field using a fluorometer calibrated using
standards of known concentrations. Also, sample bottles were labeled and
sealed for subsequent measurements at a constant temperature using a
calibrated fluorometer in the USGS laboratory in Sacramento, California.
A water sample was collected at each site before the first arrival of
the tracer to determine the background fluorescence, which was
subtracted from the tracer fluorescence. Dye concentrations were used to
construct time-concentration curves and estimate travel times.

RESULTS

The dye-discharge study in the upper reach began on May 21, 2001 at
21:50 with the release of 2.0 liters of dye into the Santa Ana River at
Riverside Ave., the second site. Time-concentration curves (Fig. 2A)
show the travel and dispersion of the dye cloud as it moves downstream
from Riverside Ave. to MWD Crossing. The computed travel times reflect
the movement of the peak dye concentration from site to site. The
approximate travel times were 46 minutes from RIX to Riverside Ave.
(estimated), 145 minutes from Riverside Ave. to Mission Blvd., 145
minutes from Mission Blvd. to Railroad Crossing, and 55 minutes from
Railroad crossing to MWD Crossing.

![](media/image2.png){width="6.93125in" height="3.81875in"}

Figure 2. Time-concentration curves at Santa Ana River sampling sites in
southern California (A, upper reach; B, lower reach).

During the first day of the tracer test, discharge measurements were
made at five sites along the Santa Ana River and on three tributaries
(Fig. 1). At the first site, below the confluence of RIX and Rialto
WWTP, six discharge measurements were made over 9 hours. Because dye was
released at the second site, the travel time was estimated from the
measured velocity and distance between sites; these values ranged from
80 to 650 minutes after the peak. At the second site, located

![](media/image3.png){width="6.93125in" height="4.104166666666667in"}

Figure 3. Flow plots for sites on the upper reach, Santa Ana River (SAR)
in southern California.

under the Riverside Ave. Bridge, six discharge measurements were made
over 10 hours ranging from 35 minutes before dye release to 560 minutes
after the release. At the third site, located downstream of the Mission
Blvd. bridge, six discharge measurements were made over 10 hours ranging
from 90 to 540 minutes after the peak. At the fourth site, Railroad
Crossing, five discharge measurements were made over 8 hours ranging
from 90 to 540 minutes after the peak. At the last site on the upper
reach, MWD Crossing, two check measurements were made during the study.
Stage is continuously monitored at MWD Crossing with values recorded
every 15 minutes, so no interpolation of discharge is needed. The
interpolated and computed discharges for the upper reach are plotted as
a function of time since the peak of dye concentration in figure 3. For
the purpose of comparing flow, discharges for the upper reach were
interpolated at 90, 180, 270, 360, 450, and 540 minutes past the dye
peak.

At the site below RIX, the uppermost site in the reach, the average
interpolated discharge was 58.5 ft^3^/s, which was entirely wastewater.
Discharge at Riverside Ave, the next site down stream was 52.6 ft^3^/s,
a net loss of 5.9 ft^3^/s. Because there were no observable gains along
this reach, discharge remained 100 percent wastewater. A net gain of 4.2
ft^3^/s of non-wastewater was measured at the Mission Blvd site (3.7
ft^3^/s input from Lake Evans plus a channel gain of 0.5 ft^3^/s); the
average interpolated discharge was 56.8 ft^3^/s. This gain of
non-wastewater reduces the wastewater at Mission Blvd. to about 93
percent (52.6 ft^3^/s) of total discharge. At the Railroad Crossing
site, the average interpolated discharge was 60.8 ft^3^/s, a gain of 4
ft^3^/s (non-wastewater gains of 3.3 ft^3^/s from Sunnyslope Creek plus
0.7 ft^3^/s likely from groundwater seepage). This gain of
non-wastewater decreases the wastewater at this site to about 86 percent
(52.6 ft^3^/s) of total discharge. At the MWD Crossing site, the average
flow was 71.5 ft^3^/s. However, this net increase of 10.7 ft^3^/s
represents the combined non-wastewater gain of 15.3 ft^3^/s from
Tesquesquito Creek and stream channel losses of 4.6 ft^3^/s between
Railroad Crossing and MWD Crossing. Determining the resulting percentage
of wastewater flow requires proportioning the loss according to the
percentage of wastewater at the upstream station. That is, 86 percent of
the 4.6 ft^3^/s loss was wastewater and all of the gain was non-waste
water. Therefore, the wastewater flow at MWD is the combined wastewater
at Railroad crossing (52.6 ft^3^/s) minus 4.0 ft^3^/s (87 percent of the
4.6 ft^3^/s loss), or about 68 percent (48.6 ft^3^/s) of the total flow
at MWD Crossing.

The dye-discharge study in the lower reach began on May 22, 2001, the
second day of the tracer study, at 20:00 by releasing 2.6 liters of dye
at MWD crossing. Time-concentration curves (Fig. 2B) show the travel and
dispersion of the dye cloud as it moves downstream from MWD Crossing to
Powerline. The approximate travel times were 225 minutes from MWD
Crossing to Hidden Valley, 125 minutes from Hidden Valley to Grulla
Court, 125 minutes from Grulla Court to I-15, and 245 minutes from I-15
to Powerline.

In the lower reach, discharge measurements were made at 6 sites along
the Santa Ana River and on 2 tributaries (Fig. 1). Discharges for these
sites are shown in figure 4 as a function of time since the peak of dye
concentration.

![](media/image4.png){width="6.93125in" height="5.526388888888889in"}

Figure 4. Flow plots for sites on the lower reach, Santa Ana River (SAR)
in southern California.

Interpolated and measured discharges for the lower reach are plotted at
30, 120, 210, 300, 390, 480, and 570 minutes past the dye peak, except
for the last site where data was not available for entire period. At the
first site, data are available continuously so no interpolation was
needed. At the second site, Van Buren Blvd., seven measurements were
made over 12 hours ranging from 150 minutes before the dye peak to 695
minutes after the peak. At the third site, Hidden Valley Wetlands, seven
measurements were made over 12 hours ranging from 25 minutes before peak
to 700 minutes after peak. At the fourth site, Grulla Court, seven
measurements were made over 12 hours ranging from 50 minutes before peak
to 640 minutes after peak. At the fifth site, I-15, seven measurements
were made over 12 hours ranging from 160 minutes before peak to 600
minutes after peak. At the last site, Powerline, four measurements were
made over 6 hours ranging from 35 to 600 minutes after peak.

When the tracer was started in this reach, the flow at MWD Crossing was
about 67 ft^3^/s. The percentage of wastewater and non-wastewater
sources are used from upper flow calculations, making the flow at MWD
Crossing about 68 percent (45.6 ft^3^/s) wastewater. At Van Buren Blvd.,
the average interpolated discharge was 73.6 ft^3^/s; the average net
gain was 6.6 ft^3^/s. The source of this gain is assumed wastewater
percolating to the river from the Riverside Water Quality Control Plant
(WQCP). Therefore, flow at Van Buren is about 71 percent (52.2 ft^3^/s)
of total discharge at the station. From Van Buren Blvd. downstream to
Hidden Valley Wetlands, flow from Riverside WQCP (46 ft^3^/s) and Hole
Lake outflow (3 ft^3^/s) combine and flow parallel to the river behind
an earthen levee that allows percolation to the river. Therefore, gains
in the river below Van Buren Blvd. were assumed 94 percent (46 ft^3^/s)
wastewater and 6 percent (3 ft^3^/s) non-wastewater (Hole Lake outflow).
At Hidden Valley Wetlands, the average interpolated discharge was 106.2
ft^3^/s, an average net gain of 32.6 ft^3^/s, which is 94 percent (30.6
ft^3^/s) wastewater. Therefore, the flow at Hidden Valley Wetlands was
about 78 percent (82.8 ft^3^/s) wastewater. At Grulla Court, the average
interpolated discharge was 112.6 ft^3^/s, an average net gain of 6.4
ft^3^/s. Again assuming that 94 percent of this increase is wastewater,
flow at Grulla Court is about 79 percent (88.8 ft^3^/s) wastewater. At
I-15, the average interpolated discharge was 116 ft^3^/s, a net gain of
3.4 ft^3^/s non-wastewater (there are no wastewater inputs in this
reach); therefore flow at I-15 was about 77 percent (88.8 ft^3^/s)
wastewater. At the last site, Powerline, the average interpolated
discharge was 112 ft^3^/s; with an average net loss of 4 ft^3^/s.
Therefore, the percentages of flow at Powerline did not change, making
the flow at this site approximately 77 percent (85.7 ft^3^/s)
wastewater.

SUMMARY

The discharge of the Santa Ana River, measured below Prado Dam located
5.5 mi downstream of the study area, is about equal to the total flow
from wastewater-treatment facilities upstream (Hamlin and others, 1999).
However, the interaction between the Santa Ana River and groundwater is
unknown. In particular, the proportion of base flow that is treated
wastewater or non-wastewater is unknown. To estimate sources of flow in
the Santa Ana River we conducted a Lagrangian discharge study. Rhodamine
WT dye was used as a tracer and discharge was measured at 10 sites on
the Santa Ana River and on 5 tributaries. Travel times were calculated
based on dye samples collected at each site. Flow was measured
approximately every two hours for periods ranging from 6 to 14 hours at
each Santa Ana River site. The flow at each site was interpolated to a
common time since peak dye concentration to allow for analyses of the
same mass of water as it moved downstream, and gains and losses between
stations were attributed to sources and sinks. The analysis showed that
flow in the Santa Ana River at MWD Crossing, the end of the upper reach,
was 68 percent wastewater and 31 percent other sources. At the Powerline
site, the end of the lower reach, the wastewater increased to
approximately 77 percent of the flow.

REFERENCES

Carter, R.W., and Davidian, Jacob, 1968, General procedure for gaging
streams: U.S. Geological Survey Techniques of Water-Resources
Investigations, book 3, chap. A6, 13 p.

Hamlin, S.N., Belitz, K., and Paybins, K.S., 1999, Santa Ana Basin: U.S.
Geological Survey Fact Sheet 054-99, 4 p.

Kilpatrick, F.A., 1970, Dosage requirements for slug injections of
Rhodamine BA and WT dyes, in Geological Survey research 1970, Chapter B:
U.S. Geological Survey Professional Paper 700-B, p. 250-253.

Kilpatrick, F.A., and Wilson, J.F., Jr., 1989, Measurement of travel and
dispersion in streams by dye tracing: U.S. Geological Survey Techniques
of Water-Resources Investigations, book 3, chap. A9, 27 p.

Rantz, S.E., and others, 1982, Measurement and computation of
streamflow: Volume 1. Measurement of stage and discharge: U.S.
Geological Survey Water-Supply Paper 2175, 284 p.

Wilson, J.F., Jr., Cobb, E.D., and Kilpatrick, F.A., 1986, Fluorometric
procedures for dye tracing: U.S. Geological Survey Techniques of
Water-Resources Investigations, book 3, chap. A12, 34 p.

[^1]: U.S. Geological Survey, 5735 Kearny Villa Rd. Suite O, San Diego,
    CA 92123, Phone: (858) 637-9005, Fax: (858) 637-9201, E-Mail:
    gomendez@usgs.gov or kbelitz \@usgs.gov