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Dames & Moore, 1999 - USDA Forest Service

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Weathering of iron-bearing sulfide minerals (including pyrite, pyrrhotite and chalcopyrite) produces water<br />

containing acid, sulfate and iron. This water is strongly acidic and oxidizing, and can oxidize and dissolve<br />

other sulfide minerals (such as sphalerite). The acidic water also reacts with other minerals, particularly<br />

carbonate and silicates. 'Ihese minerals consume acid, releasing their elements to solution. Reaction with<br />

silicates commonly releases aluminum, sodium, potassium, calcium and magnesium. The water produced<br />

by reaction with rocks commonly contains a mixture of elements reflecting these processes.<br />

As mine-influenced water mixes with other more dilute waters, concentrations can be diluted. Metals can<br />

also be removed from solution by a variety of processes including pH and Eh increases (resulting in<br />

precipitation), efflorescence (removal by evapo-concentration), co-precipitation (removal of metals by<br />

precipitation), and sorption (removal by precipitation on secondary minerals). These processes can<br />

significantly remove metals before the water enters Railroad Creek.<br />

6.4 EVIDENCE AND IMPLICATIONS OF GENERAL CHEMICAL PROCESSES AT THE<br />

HOLDEN MINE SITE<br />

As a first step in understanding weathering processes at individual hyd rock metal mine sites, it is useful<br />

to examine water chemistry for evidence of overall chemical controls. The exposed walls, waste rock and<br />

tailings all originate from a narrow geological sequence containing specific minerals as described in<br />

Section 6.1.2. It is expected that the chemistry of waters at the Site have some common characteristics<br />

that reflect:<br />

The presence of certain abundant reactive sulfide minerals (pyrite, pyrrhotite,<br />

chalcopyrite, sphalerite) and silicate minerals (principally chlorite, biotite and sericite) in<br />

.the ore and host rocks<br />

The chemical changes resulting from oxygenation and mixing of these waters with the<br />

dilute runoff, groundwater, or surface water<br />

. The following subsections describe overall site-specific evidence for these reactions. The interpretative<br />

tool used is described in Section 6.2.1. Bivariate scatter plots showing concentrations of metals versus<br />

sulfate and each other were examined. If the data points fall on a straight line with a constant slope and<br />

intercept near 0 (arithmetic axes) or a slope of 1 (logarithmic axes), a constant ratio of the two parameters .<br />

is implied. This indicates a common mineralogical source. For example, dissolution of sphalerite (ZnS)<br />

would produce a water with zinc/sulfate (Zn/S04)=l (molar concentrations), regardless of the quantity of<br />

water contacting the mineral. A constant ratio can be preserved for several different waters all influenced<br />

by the same process. If the data lie on a straight line with a nori-zero intercept (arithmetic axes) or a<br />

curvilinear trend (logarithmic axes), mixing of two different waters is indicated.<br />

The data used in these interpretations are shown in Section 5 and include all surface water and seep<br />

samples collected in 1997 and 1998.<br />

\\oM~S~I\VOLI\COMMOMWP\~~Uwldm-2~n760.doc<br />

6- 1 8<br />

17693-005419Uuly 27,<strong>1999</strong>;4:11 PMDMFINAL RI REPORT

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