U.S. Department of Energy

Pacific Northwest National Laboratory

Where Riverbed and Groundwater Mix, a Cocktail for Ecological Changes

Map of hydraulic head in the floodplain aquifer across three sampling seasons. Samples for this study were collected at the location marked by a white circle. The black dots indicate groundwater monitoring well locations used for hydrologic calculations.

The Science                      

Researchers at The Ohio State University, Lawrence Berkeley National Laboratory, and Pacific Northwest National Laboratory assessed how seasonal changes affect microbial populations in the Colorado River, a snowmelt-dominated fluvial system. They did so by sampling vertical sediments and pore water profiles at three time points—Jan, May, and August—in the hyporheic zone.

The Impact

Their discoveries will help improve our understanding of how changes in watershed hydrology may lead to dynamic shifts in the ecology and microbiology of riverbed environments as a result of mixing variations in the hyporheic zone.

Summary

It’s an exerpt from the circle of life: snow falls from the sky, collects in the mountains, and runs into the riverbeds as it melts. The riverbed water mixes with the groundwater, creating a hyperhoic zone. The microbial communities within the riverbed and groundwater also mix, altering chemical reactions and the zone’s ecology. Then, as seasons change, so does the flow and volume of a river’s waters and its hyporheic mixing zone—further altering the ecology. 

In the case of the Upper Colorado River Basin (UCRB), scientists want to understand the links between microbial assemblages and seasons in order to develop more accurate models to predict ecological changes for the next 50 years. 

Riverbed microbial communities play a key role in watershed ecosystem functions. They process organic carbon, cycle nitrogen, and alter the mobility of metals. These functions impact water quality, soil fertility, plant growth, and more. Changes in the structure and activities of the microbial communities are affected by hyporheic exchange—the flux of river and groundwater through subsurface sediments. That hyporheic exchange is driven, in part, by changes in river elevation.

According to earlier studies, warming temperatures and decreasing snowpack in the UCRB will likely contribute to earlier snowmelt and decreases in Colorado River discharge. The loss of strong seasonal behavior may lead to longer base flow periods. Also, given the iron reduction identified during river base-flow in this study, longer periods of anoxic riverbed conditions will likely drive more metal mobilization and flux to the river channel – which has implications for water quality. It’s still unclear how microbial communities might respond to these hydrologic changes; scientists propose that greater focus is needed to perform depth-resolved, long-term temporal hyporheic zone studies to improve our understanding of how hydrologic change may alter the ecology and microbiology of riverbed environments.

The results presented in this study indicate that snowmelt-driven Colorado River discharge likely causes significant fluctuations in hyporheic transport, redox behavior, and microbial community structure throughout the hyporheic zone. As river stage declines from spring runoff throughout the summer and fall, increasing contributions from groundwater discharge reduce the depth of hyporheic mixing and allow reducing chemical conditions to develop at progressively shallower depths within the riverbed. As a result of these mixing dynamics, a large interval within the riverbed (up to 70 cm deep) undergoes annual redox cycling between oxic river water-dominated conditions and anoxic groundwater-dominated conditions. Researchers posit that this redox cycling creates a highly reactive zone within the riverbed and significantly contributes to the oversize role that hyporheic processes play in the function of riverine ecosystems.

Funding

This work was supported as part of the Genomes to Watershed Scientific Focus Area at Lawrence Berkeley National Laboratory. JCS was supported by BER as part of Subsurface Biogeochemical Research Program’s Scientific Focus Area (SFA) at the Pacific Northwest National Laboratory

Publications

Robert E. Danczak, et al. “Seasonal hyporheic dynamics control coupled microbiology and geochemistry in Colorado River sediments.” Journal of Geophysical Research35, 221 (2016). [DOI: 10.1002/2016JG003527]

Date: 
November 2016
| Pacific Northwest National Laboratory