Fate and Transport of Contaminants in a Stream-Aquifer System
USGS State Water Resources Research Institute Program
03/01/06 – 02/28/07
Streams and rivers transport sediments, natural organic matter, and, frequently, land-applied chemicals.
Many drinking-water wells located on streambanks and riverbanks induce a portion of the stream or river water to flow through the aquifer to well screens. The pumped well water is then a mixture of groundwater and induced infiltration water and is of a better quality than the stream or river water. This process is known as “riverbank filtration.”
Stream- and river-water contaminants are removed through straining, colloidal filtration, chemical precipitation, sorption, and microbial degradation. Also dilution is possible if the respective contaminants in the surface water are lower in concentration than in the groundwater. Riverbank filtration is a viable and low-cost water-treatment technology for water utilities.
As this natural filtration process works somewhat differently than engineered filtration systems, knowledge of the dynamic behavior of the system for various flow regimes of the stream or river is important for safe and sustainable operation. Advance understanding of the expected changes in water quality generated by scouring or bed clogging enables water utilities to better address filtrate quality during periods of flooding or heavy sedimentation.
Scouring and clogging of the stream or river bottom affect the rate of infiltration and the fate of the soil-resident or percolating contaminants. Scouring during floods can destroy the clogging layer and introduce oxygen-rich water into the aquifer, disturbing the previously established redox conditions. Conversely, a clogged streambed or riverbed would have a slower infiltration rate and a reduced flow compared to a normal streambed or riverbed.
It is not easy to study these processes in field settings because of high velocity in streams or rivers during high-flow events and our inability to accurately measure clogging and redox processes.
The research objectives were as follows:
(a) To retrofit a recirculating flume to serve as a model stream or river channel and attach a column to the channel bottom to simulate conditions in an aquifer under a streambed or riverbed;
(b) To study the impact of velocity profile on the scouring and deposition processes of particles;
(c) To study the redox conditions in the column as a function of stream velocity, particledeposition rate, natural-organic-matter content of the recirculating water, and the travel distance;
(d) To examine the effect of channel-bed scouring on the change in the redox conditions of the upper portions of the column and its impact on water quality.
Dr. Chittaranjan Ray
Water Resources Research Center
University of Hawaii at Manoa