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Table 2. Summary of Wilcoxon-Mann-Whitney rank-sum tests comparing flow-adjusted physical measurements, constituent
<br />concentrations, and thermodynamic properties of water samples collected from Yampa River near Maybell during 1950-74
<br />with those measurements, concentrations, and properties collected during 1975-99
<br />[Tests were made with atwo-tailed significance level of 0.05 and a null hypothesis (Ho) defined as no significant difference between distributions of
<br />measurements, concentrations, and properties in samples collected during 1950-74 and 1975-99 and an alternative hypothesis (H~) defined as a significant
<br />difference between measurements, concentrations, and properties; E, earlier period, 1950-74; L, later period, 1975-99; N, number of samples; p-value,
<br />significance level; ft3/s, cubic feet per second; ° C, degrees Celsius; µ5/cm, microsiemens per centimeter at 25 degrees Celsius; >, greater than; <, less than;
<br />mg/L, milligrams per liter; percent, percentage of cation (calcium, magnesium, sodium, and potassium) equivalents relative to total cation equivalents or
<br />percentage of anion (chloride, sulfate, and alkalinity) equivalents relative to total anion equivalents; saturation index, logarithm of ratio of ion activity
<br />product to solubility constant; PCp ,effective pressure of carbon dioxide gas; saturation factor, number of times solution is saturated with carbon dioxide
<br />gas compared to saturation at ambrent atmospheric pressure]
<br />
<br />Test variable
<br />Unit 1950-74 (E)
<br />
<br />Median N 1975-99 (L)
<br />
<br />Median N Test
<br />
<br />result Test
<br />
<br />~rvalue
<br />Specific conductance µS/cm 469 472 530 181 Ht:L>E <.001
<br />pH Standard units 7.7 473 8.2 183 Ht:L>E <.001
<br />Alkalinity mg/L as CaC03 138 473 137 183 Ho .659
<br />Dissolved solids, mg/L 287 473 317 183 Hl:L>E <.001
<br />sum of constituents
<br />Dissolved solids, tons/day 280 473 371 183 Ht:L>E <.001
<br />total load
<br />Calcium mg/L 38 473 41 183 Ht:L>E .002
<br />Magnesium mg/L 16 473 20 183 Ht:L>E <.001
<br />Sodium mg/L 36 473 39 183 H1:L>E .008
<br />Potassium mg/L 2.5 434 2.5 183 Ho .226
<br />Chloride mg/L 18 473 14 183 Ht:E>L <.001
<br />Sulfate mg/L 72 473 110 183 Ht:L>E <.001
<br />Fluoride mg/L 0.3 183 0.2 183 HI:E>L <.001
<br />Silica mg/L 10 472 6.9 181 H1:E>L <.001
<br />1Nitrate mg/L as N .23 448 .06 124 H1:E>L <.001
<br />Calcium, fraction Percent 40.0 473 37.4 183 Hl:E>L <.001
<br />Magnesium, fraction Percent 26.4 473 31.1 183 H1:L>E <.001
<br />Sodium, fraction Percent 32.0 473 29.7 183 HI:E>L .008
<br />Potassium, fraction Percent 1.4 434 1.2 183 H1:E>L <.001
<br />Chloride, fraction Percent 10.6 473 6.4 183 H1:E>L <.001
<br />Sulfate, fraction Percent 29.1 473 40.7 183 HI:L>E <.001
<br />Alkalinity, fraction Percent 58.3 473 51.9 183 Ht:E>L <.001
<br />ZCalcite saturation Saturation index -0.27 473 .26 183 Ht:L>E <.001
<br />~PCp of solution Saturation factor 10.2 473 2.5 183 HI:E>L <.001
<br />ZpH at atmospheric Pao
<br />2 Standard units 8.66 473 8.66 183 Ho .773
<br />t Late period for nitrate concentrations is 1975-94.
<br />2 Calculated with computer program PHREEQC (Parkhurst, 1995)
<br />Trends in Dissolved Solids and Major Ions
<br />Specific conductance (a measure of dissolved-
<br />solids concentration) and concentration of dissolved
<br />solids (fig. 18) were significantly greater during 1975-
<br />99 than during 1950-74 (table 2). Dissolved-solids
<br />load (discharge times dissolved-solids concentration,
<br />in tons per day) also had a significantly greater median
<br />value during this period. A significant increase
<br />(p-value <0.001) in measured pH values (fig. 18)
<br />occurred between the two periods. To a significant
<br />degree, the 1975-99 period had greater concentrations
<br />of calcium (fig. 19), magnesium (fig. 19), sodium
<br />(fig. 19), and sulfate (fig. 20). To a significant degree,
<br />26 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000
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