U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Enhanced Removal of Dissolved Manganese in Hyporheic Zones: Centimeter-Scale Causes and Kilometer-Scale Consequences

By Judson W. Harvey, Christopher C. Fuller, and Martha H. Conklin

This report is available in pdf format: Harvey.pdf 63KB

ABSTRACT

Characterizing both the causes and consequences of enhanced oxidation of dissolved manganese (Mn) in the hyporheic zone required measurements with spatial resolution varying across five orders of magnitude. Our measurements at Pinal Creek basin, AZ ranged in scale from that of the fundamental interactions between surface and ground water (centimeters) to the scale of the perennial stream that receives ground-water discharge from the entire drainage basin (kilometers). Mean rate constants for the removal of dissolved manganese agreed closely between three scales of resolution in the field, ranging from centimeter-scale field measurements acquired in situ in hyporheic zones to kilometer-scale estimates determined using stream tracers. The laboratory estimate of the Mn removal-rate constant was approximately 30% lower than field estimates. In situ and laboratory rate constants had relatively large coefficients of variation (107% and 84%, respectively), which may be too large to be used reliably in transport simulations. Stream-tracer experiments provided estimates of the rate constant with lower uncertainties; 56% when averaged at the reach-scale (approximately 500 meters) and 26% when averaged at the basin-scale (3 kilometers). Because of the lower uncertainties the stream-tracer approach appeared to provide the most reliable basin-scale simulation of the effects of enhanced Mn-removal in hyporheic zones. The stream-tracer characterization alone, however, could not determine that removal of manganese was pH-dependent, or even that the reaction occurred in hyporheic zones (as opposed to slow-moving zones in surface water). Laboratory and in situ measurements within hyporheic zones provided the crucial evidence to support interpretations about the causal processes. Our experience at Pinal Creek basin leads us to conclude that a multi-scale approach is a necessity for characterizing enhanced biogeochemical reactions in hyporheic zones.