2000 Progress Report: Photochemical Processes Controlling Manganese Chemistry in Pristine and Contaminated Mountain StreamsEPA Grant Number: R826649
Title: Photochemical Processes Controlling Manganese Chemistry in Pristine and Contaminated Mountain Streams
Investigators: McKnight, Diane M. , Hrncir, Duane
Institution: University of Colorado at Boulder , Mesa State College
Current Institution: University of Colorado at Boulder , The University of Texas at Dallas
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $351,175
RFA: Exploratory Research - Environmental Chemistry (1998) RFA Text | Recipients Lists
Research Category: Sustainability , Land and Waste Management , Air , Engineering and Environmental Chemistry
Objective:The study of in-stream manganese concentrations and fluxes on varying time scales can reveal periods of high toxicity to biota, seasonal processes controlling transport through the watershed, and conditions influencing metal cycling within the stream. We hypothesize that Mn-photoreduction can be an important reaction controlling the speciation and diel cycles of manganese within the water column. To investigate this, a conservative tracer was injected (on two occasions) for 36-hours into Lake Fork, a neutral pH stream containing large amounts of MnOx on the streambed. It is also hypothesized that seasonal trends exist and are controlled by temporal changes in manganese loadings from various sources within the watershed. Sources were located and sampled, along with ten other sites in the surrounding area and stream, at a consistent rate for an entire year. The purpose was to identify critical times of high loading into the stream and associated downstream response.
Two main sources for manganese were identified in the abandoned mine area adjacent to Lake Fork. Discharge from the mine addit represented the majority of the manganese loading into the stream during most of the year. The manganese flux from this source steadily decreased during the winter, eventually reaching a rate approximately half of that during summer months. At the beginning of spring snowmelt, the dissolved manganese flux increased from the winter low of 400 g/hr to a high of 900 g/hr during a one-month period. Also during snowmelt, mine tailings located below the Dinero Tunnel became a major source of manganese. The loading from these tailings peaked at about 2000 g/hr and varying amounts of manganese were flushed from the tailings for three months. This high loading of manganese during springtime, combined with low streamflow due to an upstream dam, resulted in elevated concentrations of dissolved manganese (greater than 3 mg/L). During this time, high concentrations of manganese in the water column were associated with high precipitation rates of MnOx. Over half of the dissolved manganese was removed from the water column in a 2 kilometer stream reach. The release of water from Sugarloaf Dam in early summer increased the flow in Lake Fork Creek to over 100 times baseflow, but this delayed snowmelt pulse only lasted for two weeks. The dilution during this pulse decreased the manganese concentrations drastically (to less than 0.030 mg/L) and shifted the dominant processes in the stream from precipitation to dissolution of oxides accumulated on the streambed. These studies of the annual pattern of MnOx precipitation and dissolution help to elucidate seasonal controls on MnOx photoreactivity.
The two conservative tracer experiments were conducted approximately two months after the flow in Lake Fork returned to baseflow. This permitted fresh metal oxides to accumulate on the streambed and become more representative of average conditions, as opposed to more transient conditions inherent during snowmelt. Samples were collected hourly from the stream at three different sites ? Mn2+, H2O2, dissolved organic carbon (DOC), total and dissolved cations and anions, and pH were measured. A significant increase in Mn2+and dissolved Mn occurred during the day, which indicates that MnOx photoreduction is an important process in this system. The difference between daytime and nighttime concentrations of Mn2+ were not as great as differences observed for Fe2+ in other streams because of the slower rate of Mn2+ oxidation and continued dark reduction of MnOx by sorbed humic substances. Laboratory experiments indicate that sorption of humic substances by oxides and interaction of Mn2+with DOC photoproducts influence Mn photoreduction in freshwaters. Finally, a reactive tracer experiment has been conducted, and the preliminary results show that an increase in H2O2 from the DOC addition enhanced MnOx photoreduction.