Ground Water Flow Rate
From: Andy Soos, ENN
Ground water flow rates can be a slow process. USGS hydrologic researchers, for
example, 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 their effect on 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, is the likely cause.
"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."
The nitrogen (N) and phosphorus (P) applied to agricultural land (via synthetic fertilizers, composts, manures, biosolids, etc.) can provide valuable plant nutrients. However, if not managed correctly, excess N and P can have negative environmental consequences. Excess N supplied by both synthetic fertilizers (as highly soluble nitrate) and organic sources such as manures (whose organic N is mineralized to nitrate by soil microorganisms) can lead to groundwater contamination of nitrate.
The nitrogen (N) and phosphorus (P) applied to agricultural land (via synthetic fertilizers, composts, manures, biosolids, etc.) can provide valuable plant nutrients. However, if not managed correctly, excess N and P can have negative environmental consequences. Excess N supplied by both synthetic fertilizers (as highly soluble nitrate) and organic sources such as manures (whose organic N is mineralized to nitrate by soil microorganisms) can lead to groundwater contamination of nitrate.
Nitrate-contaminated
drinking water can cause blue baby syndrome. Together with excess P from these
same fertilizer sources, eutrophication can occur downstream due to excess
nutrient supply, leading to anoxic areas called dead zones.
Rivers and streams are fed by both groundwater held in underground aquifers and surface water from precipitation runoff. In low stream flow 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.
Rivers and streams are fed by both groundwater held in underground aquifers and surface water from precipitation runoff. In low stream flow 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.)
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.
For further information see Ground Water Flows.
For further information see Ground Water Flows.