Expectations
A. Background
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Metals figure in Colorado’s evaluation of streams for possible Outstanding Waters status.
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Seven of the 12 parameters considered are dissolved metals concentrations.
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The concern is how they compare with standards for the protection of aquatic life.
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Metals can find their way into streams from dissolution of geologic material.
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There is believed to be no other metals input entering the candidate Dolores streams.

B. Questions
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So, what concentrations of dissolved metals can be anticipated from dissolution of geologic material?
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Would those concentrations vary seasonally?
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How do the metals concentrations measured compare with what is expected?
C. Source of constituents
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Constituents in solution result from water from rainfall and snowmelt in contact with minerals.
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In particular, this would be the rainfall and snowmelt that infiltrates...
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And moves relatively slowly across below-ground exposed minerals surfaces...
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Carrying the dissolved constituents it acquires from that contact into surface water flow.
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Dissolved oxygen and carbon dioxide are present in surface water from exposure to the atmosphere.
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Due to dissolution of carbonate-bearing minerals, stream water is slightly basic, pH 8.0-8.8.
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This despite rainwater itself being slightly acidic, pH 5.0-5.5.
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The buffering capacity of carbonates in solution makes stream pH resilient to change.
D. Metals solubility
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Dissolved metals, on the other hand, are less forthcoming despite mineral contact with water.
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That is, metals-bearing minerals typically have low solubility.
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Exceptions can occur as a result of sulfide-bearing minerals.
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Their contact with water can make it acidic.
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Acidic water can aggressively leach metals from minerals.
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As mentioned, however, stream water monitored in the study area is basic.
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There is no indication that sulfide-bearing minerals are affecting water quality.
E. Hardness effect
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Hardness is the concentration of calcium and magnesium ions in solution.
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By convention, it is expressed as the concentration of calcium carbonate.
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Calcium and magnesium can occur, for example, from the dissolution of limestone...
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A common geologic material, which also accounts for carbonate ions in solution.
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Carbonate ions can form dissolved molecules with metals, referred to as speciation.
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The toxic form of dissolved metals, however, is unspeciated, that is, the metals ion alone.
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Therefore, the standards for dissolved metals...
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Must be adjusted on a case-by-case basis for the speciation caused by hardness.
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Hardness measured in the sampled streams is 51-191 mg/l.
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As an example of the hardness effect, a copper standard of 5 ug/l at 50 mg/l hardness...
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Becomes 16.2 ug/l at 200 mg/l hardness, a 324 percent increase.
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At 50 and 200 mg/l hardness, the lead standard is 1.2 and 5.3 ug/l.
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The zinc standard is 65 and 228 ug/l at those hardness values.
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The standards shown in Metals results have been adjusted for hardness.
F. Ionic strength effect
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Separate from hardness, the presence of non-metals ions can raise metals concentrations.
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For the streams, a maximum increase is estimated to be about 40 percent.
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It is due to an ionic strength effect.
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That is, metals ions in the presence of other ions (negatively charged) have their movement, their reactivity, restricted.
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That restriction means metals ions stay in solution in larger concentrations.
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In particular, negatively charged carbonate ions are attracted to the positively charged metals ions...
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And collect around them, reducing the opportunity for the metal ions to return to a solid form.
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The carbonate ions causing the effect are relatively abundant, as compared with the dissolved metals.
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Considering that the solubility of metals-bearing minerals is low, as already noted...
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And the pH of the study area stream water is slightly basic, instead of aggressively acidic...
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The result is that even an increase up to 40 percent results still in a very low concentrations of the metals.
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In fact, for some, they are too low to be detected by routinely applied laboratory methods.
G. Basis for ionic strength correction
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As a matter of detail, the 40 percent estimate is based on 200 mg/l stream water hardness.
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That is conservative, as water hardness measurements are lower, 51-191 mg/l, as noted above.
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The 200 mg/l is a concentration of 0.002 moles/l expressed as calcium carbonate, as usual.
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The ionic strength of the solution is 0.008 moles/l, or conservatively rounded up, 0.01 moles/l.
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The activity coefficient for a metal in a solution of 0.01 moles/l ionic strength is approximately 0.7.
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It means that, for example, instead of 3 ug/l, an expected metal concentration is 4.3 ug/l.
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Expressed as parts per billion (ppb), that would be 4.3 ppb, instead of 3 ppb.
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Again, even with ionic strength correction, metals concentrations are expected to be very low.
H. Temperature response
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In general, the solubility of metals in water from metals-bearing minerals increases with temperature.
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Calcium solubility, on the other hand, decreases with temperature.
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This is due to thermodynamics and, specifically, to the enthalpy of the dissolution reactions.
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The enthalpies of dissolution are positive, in general, for metals-bearing minerals.
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And negative for calcium-bearing minerals, such as calcite, aragonite, and gypsum.
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Of particular interest in this study are concentration differences due to normal and high temperatures.
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Manganese was detected in all the candidate streams except Coal.
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So manganese concentrations are used in the example that follows.
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Based on solubility, a concentration of 5.37 ppb could be anticipated at 57 F, and 5.75 ppb at 77 F.
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This would be from dissolution of the manganese-bearing mineral rhodochrosite.
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The 57 F is a moderate July-August water temperature in the study area, and 77 F is a high temperature.
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Clearly, the calculated concentration difference that results from those temperatures is very small.
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The log solubility products used in this example were -10.481 for 57 F (14 C) and -10.539 F for 77 F (25 C).
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For comparison, the manganese concentrations measured at the candidate sites were 0.6-7.1 ppb (or ug/l).
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Those results included summer sampling, with observed water temperatures at 50-63 F.
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Overall, both metal concentrations and differences due to temperature variation are small.
I. In-stream water
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The stream water sampled in the study area is not acidic and is well oxygenated. (See non-metals results.)
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As a result, any dissolved metals entering will not stay in solution.
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They will be precipitated as metal hydroxides or will be adsorbed onto streambed surfaces.
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While that removes them from the water column, they still are in the aquatic ecosystem.
J. Summary
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Metals concentrations in the streams are reasonably expected to be small.
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This is based on the low solubility of metals-bearing minerals likely prevailing in the study area.
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There is no evidence of acid-producing materials.
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That is, stream pH is slightly basic and no elevated metals concentrations are observed.
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Neither hardness nor ionic strength effects on dissolved metals concentrations...
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From higher-concentration constituents, that is, calcium, magnesium, and carbonate ions...
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Nor higher warm-weather water temperatures...
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Are expected to cause elevated metals concentrations in the study area.
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And there is no apparent scientific reason for that to change in the future.
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As well, any small concentrations of metals that might enter the stream water...
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Will be removed from the water column by natural processes of precipitation and adsorption.
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Results so far from sampling and analysis for dissolved metals are shown here, along with standards
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It is evident from sampling results that dissolved metals either are not detected in the stream water...
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Or are at concentrations well below stream water standards...
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As applied in the evaluation for possible classification by Colorado as Outstanding Waters.