7729

• UCREFRP Catalog Number: 7729
• Author: Valdez, R. A., W. J. Masslich and A. Wasowicz.
• Title: Annual Summary Report - 1990\
• USFW Year: 1991.
• Copyright Material: NO

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In addition to water quality samples for laboratory analysis, field measurements were taken for
conductivity, salinity, pH, alkalinity and dissolved oxygen. Water quality data collected afield are
presented in Table 29.

Alkalinity

Alkalinity is a measure of the buffering capacity of water. Buffering capacity is important to water
quality (EP A 1986) since pH has a direct effect on organisms as well as an indirect effect on the
toxicity of certain other pollutants. Alkalinity in the Dolores River was moderate, ranging from 92.1
on Trip 3 to 165.5 mg CaCOJL on Trip 1. Overall, alkalinity was slightly higher on the first trip than
the following two trips, probably due to base flow conditions. Samples from individual trips showed
little difference between study reaches. Historically, alkalinity levels in the Dolores River near Slick
Rock in 1960 were similar to levels found in 1990 (United States Public Health Service 1961).
Alkalinity levels in the San Miguel River just above the Dolores River confluence are slightly higher
than in 1960. The EP A Criteria for freshwater aquatic life for alkalinity is a minimum of 20 mg
CaCOJL except where natural concentrations are less.

Hardness

Water hardness varied from 138.5 to 912 mg CaC03/L, both values taken from Trip 2. Although
hardness varied substantially between reaches, no distinct trend was apparent. High values during
Trip 2 may reflect a high noncarbonate hardness fraction, since alkalinity levels were relatively low
for the same samples. Values did not differ greatly between trips although, again, the variance was
large. Dolores River water is classified as moderately hard to very hard based on the classification
used by Sawyer (1960). In 1960, water hardness ranged from 760 - 2200 mg/L around the Slick Rock
area, and went as high as 2,360 mg/L above the mouth of the San Miguel River (USPHS 1961).
Water hardness in the San Miguel just above the confluence decreased from 900 mg CaC03/L in 1960
to 528 mg/L on Trip 3 in 1990. The effects of hardness on freshwater fish and other aquatic life
appear to be related to the ions causing the hardness rather than the hardness itself, therefore no
EPA criteria exists (EPA 1986).

pH Units

Values for pH varied only slightly, from 7.67 on Trip 2 to 8.45 on Trip 3, with most values close to
8.3. No distinct trends or differences were apparent between trips or study reaches. In 1960,
measurements of pH levels in the Slick Rock area and around Gateway never exceeded 8.0 and went
as low as 7.5 (USPHS 1%1). The pH of the San Miguel River near the confluence was 8.45 in 1990,
compared to 7.6 in 1960 and pH values as low as 4.3 were measured in the main channel several
miles below Uravan. Effluent with a pH as low as 2.3 was being discharged into the San Miguel
River from a Uravan uranium mill in 1960. A review of the effects of pH on freshwater fish by the
European Inland Fisheries Advisory Commission (1969) found the pH ranges (5 - 9) not directly
lethal to fish. 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.

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