Calculation of the Contribution to the Total Amount of Metal in Bed Sediments from Various Sources

         The partitioning of copper, lead, and zinc between the water, colloidal and bed sediments as a function of pH is very evident from the combined water, colloid, and bed-sediment data in this report. In Cement Creek at Memorial Park, where the pH at low flow was 3.89, copper concentrations were high in the dissolved phase and labile copper in the bed sediments upstream from Memorial Park was in the range of 50-80 ppm (fig. 24D). In the upper Animas River above Silverton, where the pH at low flow was 6.82, copper concentration in the water were low whereas labile copper concentrations in the bed sediments were in the range of 300-500 ppm (fig. 26D). Smith and others (1992) demonstrated that the partitioning of metals in acidic, iron-rich waters followed a predictable path that could be modeled using sorbtion partitioning data and the iron hydroxide content of these waters (fig. 32A and B). As the colloidal content of the water decreases, the pH at which metals are sorbed by the colloids increases. We interpret the data from our study to indicate that copper is partitioned to the bed sediments through the sorption of copper to the colloids as a function of increasing pH. The aggregation and settling of the colloidal phase to the bed sediments results in a partitioning of the metal load from the dissolved and suspended colloids to storage in a static blanket of colloidal material in the bed sediments.

         Dissolved and colloidal lead concentrations in the waters of Mineral and Cement Creeks and the upper Animas River are all less than the limit of detection indicating that lead is partitioned into the colloidal phase at low pH and precipitated to the bed sediments as soon as it reaches the streams. The dramatic differences in the lead concentrations in the bed sediments between Cement Creek (a gradual decrease from 600 to 300 ppm fig. 24E), the upper Animas River (generally greater than 1,000 ppm fig. 26E), and Mineral Creek (1,000 ppm at the headwaters decreasing to about 200 ppm at the confluence, fig. 22E) demonstrates how lead in bed sediments responds over a short distance to the changes in the sources of lead as was shown previously in the study of acidic streams at Mount Emmons near Crested Butte, Colo. (Gulson and others, 1992). This concept is particularly dramatic in the changes in the lead-isotopic compositions seen in bed sediments from the profile of Mineral Creek (fig. 23A and B).

         Zinc concentrations in Cement Creek, where pH at low flow was 3.89, were 1,338 ppb, and zinc in the bed sediments is about 200 ppm above Memorial Park (fig. 24G). In the upper Animas River above Silverton, where the pH at low flow was 6.62, the dissolved zinc concentration was 319 ppb and zinc in the bed sediments was in the range of 3,500-1,000 ppm (fig. 26G) indicating that much of the dissolved zinc has been partitioned to the colloidal phase in the bed sediments as described above. This process also continues through the Animas Canyon reach (fig. 13A). An understanding of this dynamic process of partitioning metals between the dissolved, colloidal, and bed sediment compartments is necessary to evaluate the process of metal transport over the hydrograph in the Animas River watershed (fig. 9) and their impact on water quality.

         The geochemical profiles showing the dispersion and dilution of metals in the bed sediments of the Animas River below Silverton are convincing evidence that the source of these ore metals in the Animas River watershed is above Silverton. A very important question that should be addressed is:


         Can we isolate the contributions of the three major tributaries, Mineral and Cement Creeks
         and the upper Animas River, to determine which tributary provides what portion of the metals
         in the bed sediments?


There are two basic approaches to this fundamental question: mass balance calculations like those outlined in equation 1 and lead-isotope calculations as outlined in equation 2. We will use a combination of these two approaches to arrive at an estimate of the sediment added by Mineral and Cement Creeks to the total mass of the bed sediment in the Animas River below Silverton.

          The mass balance or mixing calculations are straight forward. The total concentration of any given metal in the bed sediments is defined by:

where:         T(Me)_AB is the total concentration (ppm) of the metal Me in the bed sediments of the Animas River below Silverton (site A-72),

                   X is the fraction of the total metal concentration contributed by bed sediments from Mineral Creek,

                   (Me)_MC is the concentration of the metal Me in the bed sediments from Mineral Creek above the confluence,

                   Y is the fraction of the total metal concentration contributed by bed sediments from Cement Creek,

                   (Me)_CC is the concentration of the metal Me in the bed sediments from Cement Creek above the confluence,

                   Z is the fraction of the total metal concentration contributed by bed sediments from the upper Animas River above the confluence with Cement Creek, and

                   (Me)_UAR is the concentration of the metal Me in the bed sediments from the upper Animas River above the confluence.

The solution to this equation requires either an independent determination of one of the variables X, Y, or Z, or a series of equations which can be solved simultaneously. The critical question in this approach to ask is:

         What metal concentrations have sufficient variation and provide mathematical leverage in
         this calculation?

          Since there is no significant variation in the geology of the subbasins, there is no significant variation in any of the lithophile element concentrations that can be used to derive percentages of materials added to the Animas River by either Mineral or Cement Creeks. Labile copper and zinc are actively precipitated from solution immediately below the confluence of Mineral and Cement Creeks, so the concentrations of these metals in the bed sediments are highly variable and cannot be used in the calculations. Labile lead concentrations, however, are stable as lead is present in both the colloidal phase and bed sediments in all three drainages.

          Lead-isotopic data provide a powerful tool in evaluating the relative contributions of various sources to the bed-sediment concentrations of various metals. As the primary focus of this study was to evaluate the transport and partitioning of metals between the water column and the bed sediments from the Silverton area downstream, it is essential that we have a measure of the amount of metal added to the bed sediments by tributaries along the course of the Animas River. The measurement of the mass of sediment added by individual tributaries is possible using the lead-isotope ratios. The lead-isotope ratios are a function of mass rather than a function of concentration, allowing us to calculate the relative mass of lead added by individual tributaries that causes changes in the lead-isotopic ratios. These percentages are calculated using the following equation:

where:         PC         is the percent of the metal derived from the contaminant source,

                   R_B          is the ²⁰⁶Pb/²⁰⁴Pb value determined in stream sediments in tributaries draining a specific geologic terrain,

                   R_T          is the ²⁰⁶Pb/²⁰⁴Pb value determined in the contaminated bed sediment of the Animas River today at a distance downstream from the source of the contaminant, and

                   R_C          is the ²⁰⁶Pb/²⁰⁴Pb value determined at the source of the contaminant.

Similar calculations can be made using the ²⁰⁸Pb/²⁰⁴Pb. Generally, there is not sufficient analytical resolution to make accurate calculations using the ²⁰⁷Pb/²⁰⁴Pb value unless the source rocks are more than two billion years old because of the short half-life of the parent isotope for ²⁰⁷Pb, ²³⁵U.

          The amount of sediment added to the Animas River by Cement Creek cannot be determined using the lead-isotopic data because there is no lead-isotopic difference between the lead-isotopic composition in the labile phase of the bed sediments of Cement Creek and that in the Animas River. There is, however, a difference between the lead in the labile phase of the bed sediments of Mineral Creek and the Animas River. Since there is virtually no change in the isotopic composition of labile lead of the Animas River between site 95ABS113 above the confluence of Cement Creek and site A-72 below the confluence of Mineral Creek, it follows that the amount of labile lead added by Mineral Creek is small. We used equation 2 to determine the value of X in equation 1. The lead-isotope calculation indicates that about 3 percent of the total mass of the labile lead in the bed sediments at site A-72 was derived from Mineral Creek. This number is subject to significant measurement error due to the very small difference in the lead-isotopic values measured in the sediments of the Animas River. Given these analytical limits, we can use the lead-isotopic data to assign an upper limit on the contribution of metals from the bed sediments of Mineral Creek of less than 10 percent of the total labile lead in the Animas River at site A-72. In reality, the number may be closer to 5 percent.

          The chemical data can then be used to estimate the contribution of Cement Creek to the Animas River from equation 1. As was shown earlier, both zinc and copper are being actively precipitated from the water below both the confluence of Cement and Mineral Creeks with the Animas River. Thus, the kinetics of the settling process and the variability that this introduces into the sampling precludes the use of either the copper or zinc data from the 2M HCl-1%H₂O₂ digestion in such a calculation. Only the zinc in the sphalerite component of the sediments is a stable chemical variable in this mixing zone. Unfortunately, there is little variance in the zinc data. The zinc in sphalerite from the Animas River is 1,200 ppm, from Cement Creek is 1,000 ppm, and in the Animas River below Silverton at site A-72 is also 1,000 ppm. There is, however, significant leverage in the lead concentration data which, when combined with the determination of the limits of the sediment contribution from Mineral Creek using the lead-isotopic data, can be used to make a mass balance calculation. The lead concentrations are as follows:

          The lead-isotopic data can also be used to calculate the contributions of metals to the labile phase of the bed sediments for the Animas River below Silverton. We will use the lead-isotopic data from site A-72 below Silverton as the composition of the contaminant, R_C. Below Silverton, the composition of the lead supplied by the tributaries is different for the areas underlain by Precambrian, Paleozoic and Mesozoic, and Tertiary rocks. The areas are shown by the model for this calculation in figure 33A values used for R_B and R_T are given in table 4. The results expressed in percent of the mass of the contaminant present PC at the point along the river course at R_T, are presented in figure 33B and in table 4. Through the Animas Canyon reach, there is little dilution due to the highly resistant nature of the Precambrian rocks to erosion. Major changes in the total mass of metal in the bed sediments of the Animas River take place downstream as stream sediments derived from the areas underlain by Paleozoic, Mesozoic, and Tertiary rocks are added by tributaries draining different sized basins and carrying differing bed sediment materials. Hermosa Creek and the Florida River are major sediment contributors to the bed sediments of the Animas River as indicated by the steep changes in the data in figure 33B. Furthermore, the gradient changes (fig. 33C) which reduces the carrying capacity of the Animas River causing sediment to be deposited in the stream bed.

Table 4. Lead-isotope parameters for calculation of dilution of the metals in the Animas River from Silverton¹