USGS hydrologic researchers have found that the movement of nitrate through groundwater to streams can take decades to occur. This long lag time means that changes in the use of nitrogen-based fertilizer (the typical source of nitrate) — whether the change is initiation, adjustment, or cessation — may take decades to be fully observed in streams, according to a recent study published in the journal Environmental Science and Technology.
Water quality experts have been noting in recent years that nitrate trends in streams and rivers do not match their expectations based on reduced regional use of nitrogen-based fertilizer. The long travel times of groundwater discharge, like those documented in this study, have previously been suggested as the likely factor responsible for these observations.
"This study provides direct evidence that nitrate can take decades to travel from recharge at the land surface to discharge in streams," said Jerad Bales, acting USGS Associate Director for Water. "This is an important finding because long travel times will delay direct observation of the full effect of nutrient management strategies on stream quality."
Rivers and streams are fed by both groundwater held in underground aquifers and surface water from precipitation runoff. In low streamflow conditions, groundwater sources take a larger role.
In this study, USGS scientists closely examined surface and ground waters at seven study sites from across the nation to determine the portion of stream nitrate derived from groundwater. They found that most of the nitrate observed in streams located in groundwater-dominated watersheds was derived from groundwater sources. To determine the time it takes groundwater to reach a stream in a groundwater-dominated watershed, an age dating tracer study was conducted in the Tomorrow River in central Wisconsin. The findings indicated that decades-old nitrate-laden water was currently discharging to this stream. Consequently, base flow nitrate concentrations in this stream may be sustained for decades to come, regardless of current and future practices.
The slow release of groundwater nitrate to streams may also affect the water quality of large rivers. For example, increases in nitrate concentrations during low and moderate flows in large rivers in the Mississippi River Basin have been observed to be greater than or comparable to increases in nitrate concentrations during high flows. (See USGS website, Nitrate in the Mississippi River and its tributaries, 1980 to 2008.) These findings also suggest that increasing nitrate concentrations in groundwater are having a substantial effect on nitrate concentrations in rivers and nitrate transport to the Gulf of Mexico. Because nitrate moves slowly through groundwater to rivers, the full effect of management strategies designed to reduce nitrate movement to these rivers may not be seen for many years.
"Vulnerability of Streams to Legacy Nitrate Sources"
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