SEASONAL TRENDS IN METAL TRANSPORT IN A MINING-IMPACTED STREAM: NORTH FORK CLEAR CREEK (NFCC), COLORADO

An important source of metals to aquatic ecosystems is acid-mine drainage (AMD) from active or abandoned sulfidic-ore mining sites. Natural processes and remedial activities both lead to the neutralization of acidity, which results in the removal of dissolved metals from the water column and the production of finely-grained sediments. These sediments can remain suspended in the water column or settle out to the streambed and are suspected to be the key to metal transport, fate, and toxicity in these aquatic systems.

NFCC receives AMD that is high in iron, manganese, copper, and zinc. Once the metal-laden water mixes with the upstream NFCC water of higher pH, there is a transformation from ferrous to ferric iron, with visible precipitation of colloidal iron oxyhydroxides (HFO). These colloids can scavenge other metals present in the water column and can either be transported downstream or aggregate, and then settle to the bed sediment. Additionally, some metals may remain in the dissolved phase during transport until conditions are more favorable for sorption and/or precipitation (e.g. higher pH or an alternate sorbent).

Sorption of heavy metals to solid phases (amorphous oxyhydroxides, organic material, and/or microbial surfaces) present in an AMD impacted stream may represent a temporary storage of these metals. However, the mobility of these solids may change depending on the hydrologic conditions of the creek. For example, a higher flow event, such as a rainstorm, could result in re-suspension and transport of the metal-laden solids further downstream. Additionally, there could be dissolution or release of weakly adsorbed metals back into the dissolved phase. In such cases, the solids would then represent a source of metals to downstream sites.

The most downstream site on NFCC, just above the confluence with Clear Creek, has been monitored for metals bi-weekly from May 2002 to present. This paper presents seasonal trends in copper, iron, manganese, and zinc behavior (dissolved and particulate metal) at this site from May 2002 to May 2004. Results indicate that both manganese and zinc have seasonal trends, but that copper and iron do not have observable seasonal trends. Additionally, metal transport behavior is influenced by strong storm events (such as localized snow and rain storms).