2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 10
Presentation Time: 1:30 PM-5:30 PM


CHAPIN, Thomas P. and WANTY, Richard B., U.S. Geological Survey, PO Box 25046, Denver Federal Center, Mailstop 973, Denver, CO 80225, tchapin@usgs.gov

Historically, most investigations of biogeochemical cycling in acid rock drainage systems have involved synoptic sampling over longer time periods (one sample per week or month) or intensive time series studies over a few hours or days. However, many important processes, such as diurnal and seasonal meltwater events, storm events, and photochemical cycling occur on timescales that are rarely observed by these traditional sampling approaches. Monitoring these transient processes, which may contribute the bulk of the annual metal loading, is essential for accurate assessment in acid rock drainage areas. To characterize a system that may respond to diurnal, seasonal, and episodic events, a monitoring program must sample at a frequency that captures the details of these major forcing events.

To examine low and high frequency temporal changes in acid rock drainage areas, we have developed a novel instrument, the Zn-DigiScanner, for hourly in-situ monitoring of Zn. The Zn-DigiScanner contains programmable micro pumps that preconcentrate and separate Zn from interfering metals with a strong anion exchange column, add spectrophotometric reagents, and propel the colored reaction product to a photometric detector. A battery-powered logger controls the Zn-DigiScanner and records the data. Reagent consumption currently limits the Zn-DigiScanner deployment duration to 15 days with hourly sampling.

The DigiSampler was developed for the long-term daily sampling of biogeochemically active elements. The DigiSampler can be programmed to take 0.1-10 mL sample volumes; either discrete samples every hour, or one sample integrated over 24 hours. The DigiSampler typically injects a 1 mL sample into an acid cleaned, 3.2 mm I.D. by 15 m Teflon coil which provides the capacity for taking over one hundred samples. A bubble of N2 gas is injected between water samples to prevent mixing between successive samples. Upon recovery, the 1 mL samples are diluted and run by IC and ICPMS for anions, cations, nutrients, and metals. Physical parameters (temperature, conductivity, dissolved oxygen, pH, turbidity, etc) are measured every 15 minutes with a YSI Sonde 6600. Details of the analytical design of the Zn-DigiScanner and DigiSampler, along with initial results from the Snake River near Montezuma, CO, will be presented.