2011-09-26_PERMIT FILE - C1981012 (24)

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55 • dissolved iron concentration (0.70 mg/1), and the absence of any iron dis- coloration suggest that sulfuric acid is not being produced. Deterioration Of Stream Quality Not Related To Maxwell And Allen Mines The six year mean discharge of the Purgatoire at Madrid is 40 cfs, 1 or 29,980 ac-ft per year. The mean dissolved solids concentration at tladrid is 226 mg/1. The corresponding concentration at Stonewall is 166 mg/1, in- dicating a mean annual increase of dissolved solids of about 60 mg/l. A corresponding 6-year mean discharge at Stonewall is not available, so it is not possible to accurately compute the increase in salt load between the two stations. However, the mean for the approximately two years of record at Stonewall is 14,220 ac-ft per year and, for the same period, the mean at N Madrid is 59,460 ac-ft per year., These limited data suggest that mean annual flow at Stonewall is about 24 percent of the Madrid discharge. Using a mean annual discharge at Madrid of 28,980 ac-ft, 6955 ac-ft at Stonewall and the above indicated values for dissolved solids concentrations, there is a total • pickup of salts equal to 8895 - 1568 = 7327 tans. Of the total, the combined T discharges of the Allen Mine, the active waste dump, and the current Max- well discharge of 15 gpm could contribute 157 tons, or about 2 percent. No attempt was made in this study to account for the source of the re- maining salt pickup. However, even with no contributions related to man's activities, a degradation of water quality should be expected. Plate 4 shows the profile of the Purgatoire River from above Stonewall to near the Trinidad Dam. The slope is decreasing and becomes even less east of Trinidad. The upper portion of the watershed is composed of crystalline rocks, while the reach between Stonewall and Trinidad is through terrestrial sediments. Because of the slope, the residence time of channel water in the lower reaches is larger than in the steep reach in the crystalline rocks, even if one dces not consider the relative lengths of each reach. This fact, coupled with the observation that sedimentary rocks often contain readily soluble materials in larger concentrations than do crystalline rocks, causes one to anticipate a natural degradation in water quality. It is unfortunate that the natural deterioration cannot be determined more quantitatively. Nevertheless, there • ~ seems to be little justification for regarding the quality of waters at Stonewall as being the standard against which all the waters along the stream