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Erosion Control and Natural Degradation of Pesticides Leads to a
Decline in Pollutants in the Middle Gila River
Waterbodies Improved
Historical pesticide use in the Middle Gila River watershed
contaminated aquatic life, making it unsafe for consumption.
Based on these conditions, 12 segments within the Middle Gila River watershed were added
to Arizona's 2002 Clean Water Act (CWA) section 303(d) list for pesticide impairments (i.e.,
dichlorodiphenyltrichloroethane [DDT], chlordane and toxaphene). The natural degradation of the
chemical constituents within the ecosystem, combined with landowner implementation of best
management practices (BMPs), decreased pesticide levels in the watershed overtime. Based on
this improvement, Arizona removed 12 segments within the Middle Gila River watershed from its
impaired waters list in 2014.
Problem
The Middle Gila watershed is in central Arizona around
Phoenix. It contains a total of 1,786 miles (mi) of
major streams and canals (Figure 1). The Gila River is
the longest river in the watershed at 263 mi. Major
tributaries to the Gila River in the study area include
the Salt River, the Agua Fria River, and the Hassayampa
River, all of which are intermittent in flow.
Widespread pesticide applications in western
Maricopa County, such as DDT and its metabolites
(DDTr) (1945 to 1969), toxaphene (1960s to 1982) and
chlordane (1970s to 1988), caused biological impair-
ment In the Middle Giia watershed. Agricultural runoff,,
sedimentation/erosion into waterways, and possible
instances of Inadvertent direct application over water-
bodies allowed legacy pesticides to enter the food
chain and rise through trophic levels over time.
Multiple reaches of the Gila, Salt, and Hassayampa
rivers were listed on Arizona's CWA section 303(d) list
of impaired waters in 2002 by the U.S. Environmental
Protection Agency (EPA) for excessive levels of DDTr,
chlordane and toxaphene in fish tissue. The listed seg-
ments included:
•	Hassayampa River (AZ15070103-001B, 2.3 mi)
•	Salt River (AZ15060106B, 14.1 mi)
•	Painted Rock Borrow Pit Lake (AZ15070201-1010,
190 acres)
Figure 1. Many of central Arizona's Middle Gila River
watershed segments were impaired for pesticides.
•	Painted Rock Reservoir (AZ15070101-1020, size
varies)
•	Eight Gila River segments (AZ15070101-001, 18.7
mi; AZ15070101-005, 16.9 mi; AZ15070101-007, 5.1
mi; AZ15070101-008, 5.3 mi; AZ15070101-009, 7 mi;
AZ15070101-010, 13.9 mi; AZ15070101-014, 11.9
mi; AZ15070101-015, 3.7 mi).
No official state standards for these impairments exist.
Fish tissue levels in periodic studies were compared
to EPA-recommended thresholds for subsistence and
recreational fishing for ali three pesticides. Exceedance
of thresholds led to posting of fish consumption
AZ15070101-014
Waterman Wash ti
Agua Fria River
River
Buckeye Canal
Hassayampa River-
Gila River to Buckeye Canal
AZ15070101-009
Centennial Wash to r—
Hassayampa River (
AZ1S070101-010
Hassayampa River to
Waterman Wash
1999 Sampling Locations
Gila River
AZ15070101-008
Gillespie Dam to
Centennial Wash
AZ15070101-007
Rainbow Wash to
Gillespie Dam
AZL15070201-1010
Painted Rock
Borrow Pit Lake
AZ15070101-001
PR Reservoir Dam to
Sand Tank Wash
AZ15070101-005
Sand Tank Wash ti
Rainbow Wash
AZ15070101.1020 G
Painted Rock Reservoir
(Dry - Inundabon Area)
Middle Gila
Pesticide-Impaired
Reaches
Pesticide-Impaired Reaches
and Water Bodies
• Fish Collection Sites
/ Reach Divisions
Agua Fria River
Phoenix
AZ15070101-015
Agua Fria River to
Salt River salt River
AZ150601063-001D
Salt River -
Gila River to
CoP 23rd Ave WWTP

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advisories in 1991. Advisories were used as
the basis of the CWA section 303(d) listings
by EPA in 2002.
Project Highlights
Historic Temporal Trends, DDTr in Fish Tissue,
Gila River System
c a
o o
U JC
Due to the extent of the affected area exhib-
iting pesticide contamination of aquatic
life, the previous banning of the use of
these pesticides, and the relatively recent
efforts In assessment, major activities have
consisted of conducting periodic surveys of
aquatic life contamination whiie waiting for
the pesticides to break down and be elimi-
nated from the food chain. The area has
been surveyed several times over the last
35 years by various agencies and individuals,
including in 1980 by Clark and Krynitsky, in
1984-1985 by the U.S. Fish and Wildlife
Service (USFWS), In 1994 by USFWS and
the Arizona Department of Environmental
Quality (ADEQ), in 1999 by ADEQ, and in
2012. by ADEQ and USFWS. Contamination
levels consistently declined from survey to survey,
following a half-life decay pattern that allowed for
rough projections of the dates the problem would take
care of Itself. Fish consumption advisories were first
posted In 1991 and finally removed in 2015 (in the
first Arizona action consisting of lifting consumption
advisories). Because pesticide contamination of water
and sediment had long since passed by the time ADEQ
began investigating the problem, not much additional
action could be taken.
Area farmers were already aware of the economic
benefits of soil conservation activities, and many
have implemented soil retention BMPs in recent
decades. Although not necessarily implemented to
protect water quality at the time, these measures
likely prevented further liberation of sequestered
pesticides in the soils of the area. In a related matter,
the Arizona State Parks system, which served as the
former administrator of the Painted Rock Borrow Pit
Lake, was forced to close down the park in 1989 due
to rising pesticide levels The Borrow Pit Lake lease
was relinquished and returned to the Army Corps of
Engineers (Corps), Los Angeles district office, in the
early to mid-1990s. It is not known what the Corps'
plans are with regard to the lake now, but they have
been advised that lake is no longer under a fish con-
sumption advisory.
¦	1980
¦	1985
¦	1994-95
¦	1999
¦	2012
-2012 SV
-Linear (2012 SV)
Reaches/Water Bodies
Figure 2. Decreasing levels of DDT and its metabolites (DDTr) levels in
fish tissue over time. Black bar represents screening value of 0.117. This
value is calculated as a function of a designated fish consumption rate,
carcinogenic risk level and body weight.
Results
Sampling by ADEQand the USFWS in 2011 and 2012
Indicated that DDT, toxaphene and chlordane levels
have substantially decreased over time. Toxaphene
was not detected in any 2012 samples (54 total fish
collected). Chlordane was detected in just about half
the USFWS samples from Painted Rock Borrow Pit
Lake (6 of 13), but was not detected in the samples
collected by ADEQ. DDT derivatives were detected
in about half (23 of 54) of samples collected by both
ADEQ and the USFWS. The geometric mean concentra-
tions of these data met screening values (Figure 2).
On the basis of these data, Arizona has removed 12
segments from its 2014 impaired waters list for DDT,
toxaphene and chlordane impairment. ADEQ lifted the
fish consumption advisories after consulting with the
Arizona Department of Health Services.
Partners and Funding
The restoration of the Middle Gila Watershed was sup-
ported by the USFWS-Arizona Ecological Services Field
Office and the Arizona Game and Fish Department.
The USFWS-Arizona Ecological Services Field Office
supported sampling efforts. The Arizona Game and
Fish Department was the consulted agency for the
lifting of the fish consumption advisory.
&
V PROt*°
2
0
U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-16-001EE
November 2016
For additional information contact:
Doug McCarty
Arizona Department of Environmental Quality
602-771-4521 • dm4@azdeq.gov
Sam Rector
Arizona Department of Environmental Quality
602-771-4536 • smr@azdeq.gov

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