Reasonable expectations

A. Background

  1. Metals figure in Colorado’s evaluation of streams for possible Outstanding Waters status.

  2. Seven of the 12 parameters considered are dissolved metals concentrations.

  3. The concern is how they compare with standards for the protection of aquatic life.

  4. Metals can find their way into streams from dissolution of geologic material.

  5. There is believed to be no other metals input entering the candidate Dolores streams.

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B. Questions

  1. So, what concentrations of dissolved metals can be anticipated from dissolution of geologic material?

  2. Would those concentrations vary seasonally?

  3. How do the metals concentrations measured compare with what is expected?

C. Source of constituents

  1. Constituents in solution result from water from rainfall and snowmelt in contact with minerals.

  2. In particular, this would be the rainfall and snowmelt that infiltrates...

  3. And moves relatively slowly across below-ground exposed minerals surfaces...

  4. Carrying the dissolved constituents it acquires from that contact into surface water flow.

  5. Dissolved oxygen and carbon dioxide are present in surface water from exposure to the atmosphere.

  6. Due to dissolution of carbonate-bearing minerals, stream water is slightly basic, pH 8.0-8.8.

  7. This despite rainwater itself being slightly acidic, pH 5.0-5.5.

  8. The buffering capacity of carbonates in solution makes stream pH resilient to change.

D. Metals solubility

  1. Dissolved metals, on the other hand, are less forthcoming despite mineral contact with water.

  2. That is, metals-bearing minerals typically have low solubility.

  3. Exceptions can occur as a result of sulfide-bearing minerals.

  4. Their contact with water can make it acidic.

  5. Acidic water can aggressively leach metals from minerals.

  6. As mentioned, however, stream water monitored in the study area is basic.

  7. There is no indication that sulfide-bearing minerals are affecting water quality.

E. Hardness effect

  1. Hardness is the concentration of calcium and magnesium ions in solution.

  2. By convention, it is expressed as the concentration of calcium carbonate.

  3. Calcium and magnesium can occur, for example, from the dissolution of limestone...

  4. A common geologic material, which also accounts for carbonate ions in solution.

  5. Carbonate ions can form dissolved molecules with metals, referred to as speciation.

  6. The toxic form of dissolved metals, however, is unspeciated, that is, the metals ion alone.

  7. Therefore, the standards for dissolved metals...

  8. Must be adjusted on a case-by-case basis for the speciation caused by hardness.

  9. Hardness measured in the sampled streams is 51-191 mg/l.

  10. As an example of the hardness effect, a copper standard of 5 ug/l at 50 mg/l hardness...

  11. Becomes 16.2 ug/l at 200 mg/l hardness, a 324 percent increase.

  12. At 50 and 200 mg/l hardness, the lead standard is 1.2 and 5.3 ug/l.

  13. The zinc standard is 65 and 228 ug/l at those hardness values.

  14. The standards shown in Metals results have been adjusted for hardness.

F. Ionic strength​ effect

  1. Separate from hardness, the presence of non-metals ions can raise metals concentrations.

  2. For the streams, a maximum increase is estimated to be about 40 percent.

  3. It is due to an ionic strength effect.

  4. That is, metals ions in the presence of other ions (negatively charged) have their movement, their reactivity, restricted.

  5. That restriction means metals ions stay in solution in larger concentrations.

  6. In particular, negatively charged carbonate ions are attracted to the positively charged metals ions...

  7. And collect around them, reducing the opportunity for the metal ions to return to a solid form.

  8. The carbonate ions causing the effect are relatively abundant, as compared with the dissolved metals.

  9. Considering that the solubility of metals-bearing minerals is low, as already noted...

  10. And the pH of the study area stream water is slightly basic, instead of aggressively acidic...

  11. The result is that even an increase up to 40 percent results still in a very low concentrations of the metals.

  12. In fact, for most, they are too low to be detected by routinely applied laboratory methods.

G. Basis for ionic strength correction​

  1. As a matter of detail, the 40 percent estimate is based on 200 mg/l stream water hardness.

  2. That is conservative, as water hardness measurements are lower, 51-191 mg/l, as noted above.

  3. The 200 mg/l is a concentration of 0.002 moles/l expressed as calcium carbonate, as usual.

  4. The ionic strength of the solution is 0.008 moles/l, or conservatively rounded up, 0.01 moles/l.

  5. The activity coefficient for a metal in a solution of 0.01 moles/l ionic strength is approximately 0.7.

  6. It means that, for example, instead of 3 ug/l, an expected metal concentration is 4.3 ug/l.

  7. Expressed as parts per billion (ppb), that would be 4.3 ppb, instead of 3 ppb.

  8. Those concentrations are below detection by routine laboratory analysis.

  9. Again, even with ionic strength correction, metals concentrations are expected to be very low.

H. Temperature response​

  1. In general, the solubility of metals in water from metals-bearing minerals increases with temperature.

  2. Calcium solubility, on the other hand, decreases with temperature.

  3. This is due to thermodynamics and, specifically, to the enthalpy of the dissolution reactions.

  4. The enthalpies of dissolution are positive, in general, for metals-bearing minerals.

  5. And negative for calcium-bearing minerals, such as calcite, aragonite, and gypsum.

  6. Of particular interest in this study are concentration differences due to normal and high temperatures.

  7. Manganese was detected in all the candidate streams except Coal.

  8. So manganese concentrations are used in the example that follows.

  9. Based on solubility, a concentration of 5.37 ppb could be anticipated at 57 F, and 5.75 ppb at 77 F.

  10. This would be from dissolution of the manganese-bearing mineral rhodochrosite.

  11. The 57 F is a moderate July-August water temperature in the study area, and 77 F is a high temperature.

  12. Clearly, the calculated concentration difference that results from those temperatures is very small.

  13. 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).

  14. For comparison, the manganese concentrations measured at the candidate sites were 0.6-7.1 ppb (or ug/l).

  15. Those results included summer sampling, with observed water temperatures at 50-63 F.

  16. Overall, both metal concentrations and differences due to temperature variation are small.

I. In-stream water

  1. The stream water sampled in the study area is not acidic and is well oxygenated. (See Non-metals results.)

  2. As a result, any dissolved metals entering will not stay in solution.

  3. They will be precipitated as metal hydroxides or will be adsorbed onto streambed surfaces.

  4. While that removes them from the water column, they still are in the aquatic ecosystem.

J. Summary

  1. Metals concentrations in the streams are reasonably expected to be small.

  2. This is based on the low solubility of metals-bearing minerals likely prevailing in the study area.

  3. There is no evidence of acid-producing materials.

  4. That is, stream pH is slightly basic and no elevated metals concentrations are observed.

  5. Neither hardness nor ionic strength effects on dissolved metals concentrations...

  6. From higher-concentration constituents, that is, calcium, magnesium, and carbonate ions...

  7. Nor higher warm-weather water temperatures...

  8. Are expected to cause elevated metals concentrations in the study area.

  9. And there is no apparent scientific reason for that to change in the future.

  10. As well, any small concentrations of metals that might enter the stream water...

  11. Will be removed from the water column by natural processes of precipitation and adsorption.

  12. Results so far from sampling and analysis for dissolved metals are shown here, along with standards

  13. It is evident from sampling results that dissolved metals either are not detected in the stream water...

  14. Or are at concentrations well below stream water standards...

  15. As applied in the evaluation for possible classification by Colorado as Outstanding Waters.

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