WSP10328

• File Number: 8273.400
• Description: Colorado River Basin Salinity Control - Federal Agencies - Bureau of Land Management
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/27/2002
• Author: Schumm and Gregory
• Title: Diffuse-Source Salinity -- Mancos Shale Terrain
• Water Supply Pro - Doc Type: Report/Study

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carbonates and suI fates were detected (Table 2-1). All samples of
alluvium were composed primarily of quartz and feldspars, and shale
samples additionally contained appreciable quantities of clay minerals -
montmorillbnite, illite, chlorite, and mica. Highly soluble evaporites
such as the chlorides and nitrates were not present in detectable
amounts, with one exception. Gypsum (Ca504 2H20) comprised the bulk of
the moderately soluble minerals in most instances, with smaller
quantities, of hydrated Na and Mg sulfates.
The mineralogic character of the sampled alluvium and shales
indicates that the highly soluble fraction of these surficial materials
has been leached. In fact, the most soluble evaporites would have
remained in situ only under extremely arid conditions.
X-ray ,di ffraction maxima of samples collected from a Mancos Shale
hills lope in Indian Wash indicate the prevalence of gypsum on the
surface and in underlying material, primarily as large (> 2mm) crystals
Although the average size of gypsum crystals is smaller in channel bed
materials than on the hillslopes a considerable quantity of granule-
sized soluble mineral is still present in the bed of first order
channels, although most gypsum crystals dissolve further downstream
(Shen et aI, 1981).
The predominance of gypsum, Na-Mg sulfates and carbonates indicates
that their dissolution will yield a relatively saline water of the Ca
(Na, Mg)- 504 (HC03) type. Indeed, hydrologic investigations of
Colorado River water extending from the mountains downstream to the
mouth. of the Colorado River have shown that these are the primary
inorganic soluble constituents (Iorns, Hembree and Oakland, 1965;
Mundorff, 1972; U.S.8ureau of Reclamation, 1974, Shen et al, 1981).

Solubility

One aspect of chemical kinetics, which is the study of solute
release rate, is relevant to an understanding of diffuse-source
salinity. The rate of release may determine which process is
responsible for the bulk of solute yields. For instance, a slow
dissolution rate may imply that only subsurface flow that is in contact
with soil and rocks for a long time is responsible for most of the
solute yield. Over land flow, usually short-lived, may be a dominant
solute producing mechanism if the dissolution rate is high.

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