7046 ‎(2)‎

• UCREFRP Catalog Number: 7046
• Author: Valdez, R. A., W. J. Masslich and A. Wasowicz.
• Title: Final Report\
• USFW Year: 1992.
• Copyright Material: NO

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indirect effect on the toxicity of pollutants. Total alkalinity in the Dolores River ranged from 92.1
mg CaCOJl on Trip 3 to 3,424 mgll on Trip 5. Dissolved alkalinity was measured only during Trips
5 and 6, and ranged from 62.7 to 146 mgll. There were no consistent differences in alkalinity, total
or dissolved, between study reaches. Historically, the range of alkalinity measured in the Dolores
River in 1960 (82-850 mgll) was lower than measured by BIO/WEST in 1991 (165-3,424 mgll). A
similar comparison for the San Miguel River showed little difference in total alkalinity. The EP A
criteria for freshwater aquatic life for alkalinity is a minimum of 20 mg CaCOJl except where natural
concentrations are less.

4.3.2.2 Hardness. Water hardness in the Dolores River varied from 138.5 (Trip 2) to 912 mg
CaC03Jl (Trips 2 and 4). Although hardness varied substantially between reaches, no distinct trends
were apparent. In 1991, hardness in the San Miguel River was lower than any Dolores River reach
for all trips, ranging from 129 - 165 mgll. High values during Trip 2 probably reflected a high
noncarbonate hardness fraction, since alkalinity was relatively low for the same samples. Dolores
River water was classified as moderately hard to very hard based on the classification used by Sawyer
(1960). The effect of hardness on freshwater fish and other aquatic life is often related to the ionic
concentration rather than carbonate, therefore no EP A criteria exist (EP A 1986).

4.3.2.3 DH Units. pH varied from 7.7 during Trips 2 and 4 to 8.5 during Trip 3. No distinct
trends or differences were apparent between trips or study reaches, except for slightly lower values
in all reaches for Trip 5. In 1960, measurements of pH in the Slick Rock area and near Gateway did
not exceed 8.0 and were as low as 7.5 (USPHS 1961). The pH of the San Miguel River near the
confluence ranged from 7.6 to 8.5 in 1990-1991, compared to 7.6 in 1960. pH as low as 4.3 was
measured in the main channel several miles below Uravan (Sigler et ale 1966) while emuent with a
pH as low as 2.3 was being discharged into the San Miguel River from a uranium mill in 1960
(USPHS 1961). A pH of 5 to 9 is not directly lethal to freshwater fish (European Inland Fisheries
Advisory Commission 1969), however, the toxicity of several common pollutants is markedly affected
by pH changes within this range, and increasing acidity or alkalinity may make these poisons more
toxic (EP A 1986). The EP A criteria is set at 6.5 -9.0 for freshwater aquatic life.

4.3.2.4 TDS. Levels of total dissolved solids (IDS) in the Dolores River ranged from 220 (Trip
4) - 6,320 mgll (Trip 1) during 1990-1991. IDS were generally higher on Trip 1 because of low flows
which concentrated dissolved solids. IDS decreased below the confluence of the San Miguel on Trip
1, because to dilution by flows from the San Miguel River. High IDS above the confluence of the
San Miguel River where related to saline groundwater inflow into the Dolores River across Paradox
Valley. The San Miguel River had a noticeable diluting effect on IDS throughout the study.
Maximum levels of IDS recorded below the confluence of the San Miguel were 2,595 mgll on Trip
1 in 1990, compared to 3,822 mgll in 1975 and 3,020 mgll in 1960 (Miller 1976, USPHS 1961). Thus,
IDS levels have decreased in the Dolores River since the 1960's and 70's (Table 42). Rawson and
Moore (1944) found that several common freshwater fish species survived exposure to 10,000 mgll
IDS. Pimentel and Bulkley (1983) determined that Colorado squawfish avoided IDS concentrations
greater than 4,400 mgll. No criteria have been set for IDS by the EP A

4.3.2.5 Ammonia. Ammonia varied from 0.10 mg NH3-NJl on Trip 2 to 0.963 on Trip 1. High
values associated with Trip 1 were probably related to extremely low flows during this period. No
pattern was found between study reaches. Ammonia in the San Miguel River ranged from <0.2 to
0.44 mgll. Levels of ammonia declined in the Dolores River in the last 15 years from a high of 9.0
mg NH3Jl below the San Miguel confluence (Miller 1976) in 1975 to <0.2 to 0.963 mgll during 1990-
1991 (Table 42). Ammonia levels of up to 23.5 mg NH3Jl were observed in the San Miguel River
below Uravan in 1975. Ammonia is acutely toxic to freshwater organisms at concentrations ranging