There was no change in concentrations of chloride, dissolved solids, or
nitrate in groundwater for more than 50 percent of well networks sampled
in a new analysis by the USGS that compared samples from 1988-2000 to
samples from 2001-2010. For those networks that did have a change, seven
times more networks saw increases as opposed to decreases.
The analysis was done by the USGS National Water Quality Assessment
Program (NAWQA) to determine if concentrations of these constituents
have increased or decreased significantly from the 1990's to the early
2000's nationwide.
"By providing a nation-wide, long-term, uniformly consistent analysis
of trends in groundwater quality, communities can see whether they
belong in the group of more than 50 percent which are maintaining their
water quality, or within the group of more than 40 percent for which
water quality is back sliding," said USGS Director Marcia McNutt.
"Communities in the latter group can decide whether and what action may
be warranted to address quality issues so they do not cause concern to
human health."
Though chloride, nitrate, and dissolved solids occur naturally in the
environment, human activities can cause concentrations to exceed levels
that would be found naturally. At high concentrations, these chemicals
can have adverse effects on human and environmental health.
High levels of chloride and dissolved solids in water don't present a
risk to human health, but are considered nuisance chemicals that can
cause the water to become unusable without treatment because of taste or
hardness. Additionally, these chemicals can have adverse effects on
ecosystems in streams and rivers when they discharge from the
groundwater to these water bodies.
Excessive nitrate concentrations in groundwater have the potential to
affect its suitability for drinking water. Also, when nitrate-laden
water is discharged from groundwater to streams, the nitrate can end up
in downstream water bodies, such as the Gulf of Mexico, and cause algal
blooms. These algal blooms lead to low oxygen zones, which can be deadly
to aquatic life.
Chloride, dissolved solids, and nitrate have many sources, including
agricultural fertilizers, wastewater disposal, and runoff from salt used
for deicing or other chemicals.
Understanding changes in groundwater
quality may help assess the effectiveness of management practices that
have been implemented to control these sources.
"This type of long-term trend analysis is crucial for assessing
whether the nation's groundwater is adequately protected from excessive
concentrations of these potential contaminants," said Bruce Lindsey,
lead scientist on the report. "USGS is uniquely positioned to provide
this type of nationally consistent, scientific information to managers
at the federal, state, and local level, so that they can make decisions
that protect people and the environment."
Though a majority of the well networks tested saw no change, chloride
concentrations increased in 43 percent of the well networks from the
first decade to the second decade of study. Dissolved solids
concentrations increased in 41 percent, and nitrate concentrations in 23
percent of well networks.
Although concentrations of these three constituents generally meet
their respective EPA drinking water standards or guidelines, the
proportion of samples exceeding the limits for nitrate and dissolved
solids increased significantly over the decadal period at the national
level.
Other important findings include:
- The largest increases in chloride concentrations were in urban areas in the Northeastern and Upper Midwestern United States, including suburban Boston, Chicago, Detroit and Milwaukee.
- Dissolved solids concentrations increased throughout the nation, including areas of Florida, Illinois, and the Rio Grande region.
- The largest increases in nitrate concentrations were in key agricultural areas, including the Great Plains, areas east of Lake Michigan, and in California.
- The magnitudes of increases in concentrations in deeper groundwater used as a source of drinking-water supply were generally less than in shallow groundwater. However, the proportions of networks with increases for both deep and shallow groundwater were similar.
The analysis consists of samples from 1,236 wells in 56 well
networks, representing major aquifers and urban and agricultural
land-use areas. Samples for chloride, dissolved solids, and nitrate
collected from 1988-2000 were compared to corresponding samples taken
from the same well between 2001 and 2010.
The NAWQA program continues to conduct studies on long-term
groundwater trends. This analysis, which provides an overview of current
water quality conditions and trends over time, is an important
foundation for future NAWQA studies that examine the causes of changing
concentrations and generate water-quality forecasts.
This report, "Methods
for Evaluating Temporal Groundwater Quality Data and Results of
Decadal-Scale Changes in Chloride, Dissolved Solids, and Nitrate
Concentrations in Groundwater in the United States, 1988-2010" as well as links to a series of interactive maps showing long-term groundwater trends, can be found online.
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