Nitrates in Groundwater
Michael R. Rosen – Nevada Water Science
Center, USGS and Christian Kropf – Dept. of
Water Resources, Washoe County, Nevada
O ne of the most common
contaminants found in
groundwater worldwide is nitrate
(NO3-), the oxidized form of dissolved
nitrogen. Accumulating from both natural
and anthropogenic sources, nitrate in low
concentrations is a necessary nutrient
of nitrate contamination to
for plants, but at high concentrations
groundwater where human activities
can impact human health and wreak
changed the hydrologic system by either
havoc on ecosystems (see sidebar, top To assess the extent of high nitrate
raising the water table or moving more
right). Nitrate is highly soluble, thus it is concentrations in the United States, 33
water through the vadose zone, such as
readily leached from soils and is mobile regional aquifers used for water supply were
from infiltration basins, thereby releasing
in groundwater. In the Southwest, nitrate evaluated by Nolan and Stoner (2000). On
the nitrate. Other, relatively minor natural
contamination issues are compounded by average, more than 15 percent of the wells
nitrate sources in the Southwest include
low precipitation, high evapotranspiration, contained water with nitrate concentrations
igneous rocks, deep geothermal fluids, and
and resultant low recharge that would above the U.S. Environmental Protection
dissolution of some evaporite minerals.
otherwise dilute subsurface nitrate. Agency maximum contaminant level
Anthropogenic sources: The three primary (MCL) of 10 milligrams of nitrogen per
liter (mg/l nitrate-N). Although relatively
Sources anthropogenic sources of nitrogen in rural
areas are farm animals, fertilizers and few regional studies of nitrate occurrence
Natural sources: The atmosphere is
manure applied to crops and landscapes, have been undertaken in the Southwest,
78 percent nitrogen gas. The natural process
and human waste from septic tanks and most states have water-quality databases
of fixation transforms inert atmospheric
small land treatment systems. Animal containing some information on statewide
nitrogen into bioavailable compounds,
wastes from confined animal feeding nitrate occurrence. In most southwestern
including nitrate and ammonia, which
operations are a major source of nitrate states that have surveyed nitrate-N
can be used by living organisms. Fixation concentrations, about 5 to 15 percent of
contamination of shallow groundwater
occurs in the atmosphere by lightning wells have concentrations above 10 mg/l,
(particularly if lagoons are unlined), acting
strikes, and terrestrially by certain bacteria with some reaching over 100 mg/l.
almost as point sources because of the
that often live with plants, particularly
large concentration of animals in small
legumes. After the plants die and decay, A recent U.S. Geological Survey study of
areas. Fertilizers are significant nitrogen
the stored nitrogen is released to the soil nutrient and pesticide concentrations in
sources in high-production pastures and
where it may be converted to nitrate and alluvial aquifers in Arizona, California,
vegetable farms. Soil tilling can mineralize
mobilized into the aquifer by precipitation, Nevada, New Mexico, south-central
natural organic nitrogen stored in the
irrigation, or other sources of recharge. Colorado, and Utah reported that
soil to nitrate, which can then be leached
nitrate exceeds the MCL in more than
The largest natural source of nitrogen from soils and mobilized during recharge
25 percent of the agricultural wells
in groundwater occurs from incomplete events. In urban areas, elevated nitrogen
sampled and 10 percent of urban wells
utilization of nitrate by sparse vegetation concentrations result from leaking sewer
(Paul and others, 2007). In agricultural
in arid regions. This nitrate accumulates pipelines and high-density septic systems
areas, the probability of exceeding the
where rapid urbanization has outgrown
in the unsaturated zone of alluvial aquifers MCL for nitrate is mostly influenced by
the sewer infrastructure (see page 24).
below the root zone. Walvoord and others three factors: fertilizer use, irrigation,
(2003) estimated that up to 2,000 pounds Localized high nitrate concentrations also and aquifer oxidation-reduction (redox)
of bioavailable nitrogen per acre is are associated with treated wastewater conditions. At smaller scales, differences in
available in arid regions of the Southwest, used for irrigation, fertilizer applied to nitrate concentrations between agricultural
where recharge is insufficient to move it turf, landfills and other types of industrial and urban land use are influenced more
away. Such deposits have become locally waste sites, and even some mine tailings. by groundwater redox conditions.
20 | JULY/AUGUST 2009 | Southwest Hydrology
In Arizona, high nitrate-N concentrations
(15 to 40 mg/l) associated with Why Should We Care?
agricultural practices were noted near
Phoenix in the West Salt River Valley. Nitrate in groundwater today causes health concerns. In 1993 the World
Confined animal feeding operations concern for humans and ecosystems. Health Organization established
and septic systems also are a major More than 50 years ago it was a similar MCL (11.3 mg/l NO3-N),
source of nitrate here, producing recognized that high concentrations now adopted by many countries
concentrations as high as 20 mg/l. of nitrate (greater than 20 mg/l as worldwide.
Concentrations as high as 150 mg/l in nitrogen) in drinking water supply
Although nitrogen is an essential
wells could cause health problems
groundwater beneath confined feeding nutrient for plant growth, high
such as methemoglobinemia (“blue
operations in New Mexico also have concentrations in groundwater
baby disease”). Infants under
been reported. Dissolved minerals from can deteriorate water quality
the age of 6 months are more
evaporite deposits present in the West in receiving lakes and streams.
susceptible to this disease because
Nitrate concentrations of less than
Salt River aquifer may also elevate they lack the enzyme that converts
1 mg/l NO3-N in groundwater can
groundwater nitrate-N concentrations methemoglobin back to hemoglobin.
Photo: Keith Weller, USDA-ARS
nevertheless cause serious effects
by about 5 mg/l. In California, high High nitrate concentrations have also
in plant communities where growth
nitrate-N concentrations occur mainly been linked to hypertension, central
is limited by the amount of nitrogen
in agricultural areas, particularly in nervous system birth defects, certain
available, such as in Lake Tahoe.
cancers, non-Hodgkin’s lymphoma,
the San Joaquin Basin (up to 75 mg/l), Additional nitrogen can cause
and diabetes (see Rosen and others,
but high densities of septic systems are eutrophication of the water body,
2006 for references). In 1975,
sources in urban-fringe areas, where wherein excessive plant growth
the U.S Environmental Protection
concentrations have reached 35 mg/l. (favoring certain species over others)
Agency established a maximum
High nitrate-N concentrations in Nevada and decay disrupts the normal
contaminant level (MCL) for nitrate-
balance of the ecosystem, ultimately
(as much as 100 mg/l) also are generally nitrogen (NO3-N) of 10 mg/l to
leading to a lack of oxygen in the
associated with high-density septic- regulate drinking water in the
water for fish and other species.
system usage. In areas of Nevada where United States based on these human
septic systems are used heavily, such as
Carson Valley and Washoe County in the
north and Pahrump in the south, some storage or dispose of treated wastewater Reno, Nevada. Nitrate concentrations in
domestic wells have groundwater nitrate can leach nitrogen out of the vadose groundwater from downgradient wells
concentrations exceeding the MCL. zone, raising the groundwater nitrate within 100 feet of the basin remained
concentrations. For example, wastewater relatively stable for seven years, abruptly
The rising use of infiltration basins treated to about 2 mg/l nitrate-N was increased to more than three times
in the Southwest to increase aquifer released into an infiltration basin near see SW Groundwater, page 34
The Many Forms of Nitrogen
Nitrogen makes up most of the (usually confined) aquifers; a product nitrogen equivalent: 44 mg/l NO3- =
atmosphere and its compounds are of ammonia and water under certain 10 mg/l NO3-N.
found everywhere on the surface of pH and temperature conditions.
nitrite (NO2-): intermediate compound
the planet. The gas is inert, but the
denitrification: microbial reduction of in the transformation of ammonium
compounds are among the most
nitrate to nitrogen gas under oxygen- to nitrate or nitrate to nitrogen
reactive. Nitrogen compounds can
poor conditions; the most significant gas; generally unstable but high
make things grow (fertlilizer, amino
natural degradation process of nitrate concentrations may occur near organic
acids) as well as kill them (cyanides);
in groundwater. waste disposal sites.
they can blow things up (TNT,
ammonium nitrate) or produce inert nitrate (NO3-): a highly soluble, nitrification: the oxidation of
atmospheres in laboratories (gas). relatively stable form of nitrogen in ammonium to nitrate.
The abundance of intentionally and oxygen-rich soils and aquifers; the nitrogen: a stable, inert gas that makes
unintentionally produced compounds most common form of groundwater up 78 percent of Earth’s atmosphere;
means much ends up in groundwater. contamination. the end product of denitrification.
ammonia (NH3): A common chemical nitrate as nitrogen (NO3-N or nitrosamine: a suspected carcinogen
both manufactured and produced in nitrate-N): standard means of that can be produced when ion-
human and animal waste; a nutrient in reporting measured nitrogen in a exchange technology is used to
low doses, toxic in high doses; used as nitrate analysis of water; assumes remove nitrogen from drinking water.
fertilizer. In groundwater, significant nitrate is the dominant form of
only where pH is greater than 9. nitrogen present and excludes organic nitrogen: can occur in either
oxygen-rich or oxygen-poor water
ammonium (NH4+): the most stable any other forms. Divide nitrate
but is rarely significant in
form of nitrogen in oxygen-depleted concentration by 4.4 to obtain the
JULY/AUGUST 2009 | Southwest Hydrology | 21
SW Groundwater, continued from page 21 contaminant transport models and Paul, A.P., R.L. Seiler, T.G. Rowe, and M.R. Rosen,
2007. Effects of agriculture and urbanization
the average wastewater concentration groundwater age dating will be on quality of shallow ground water in the arid
for two years, and then returned to important tools for assessing and to semiarid Western United States, USGS Sci.
predicting trends and groundwater Invest. Rept., 2007-5179.
earlier levels (see figure, below). Cores Rosen, M.R., 2003. Trends in nitrate and dissolved-
from the unsaturated zone of the wells nitrate budgets. Relatively few Southwest
solids concentrations in ground water, Carson
showed nitrate-N concentrations up basins have used groundwater models Valley, Douglas County, Nevada, 1985-2001,
to 50 mg/l in the upper 10 feet of for assessing nitrate budgets, but an USGS Water Res. Invest. Rept., 03-4152.
soil, suggesting that the infiltrating ongoing study in Carson Valley, Nevada Rosen, M.R., C. Kropf, and K.A. Thomas, 2006.
may provide an example of the utility Quantification of the contribution of nitrogen
wastewater mobilized the buried nitrate from septic tanks to ground water in Spanish
and carried it into the aquifer as a pulse. of determining nitrogen sources and Springs Valley, Nevada. USGS Sci. Invest.
amounts for management decisions. ■ Rept., 2006-5206.
Rupert, M.G., 2008. Decadal-scale changes of
Trends Contact Michael Rosen at USGS.gov nitrate in ground water of the United
Statistically significant, regional- States, 1988–2004, J. Environ. Qual., 37:
scale trends in groundwater nitrate References S-240–S-248.
Nolan, B.T., and J.D. Stoner, 2000. Nutrients in Walvoord, M.A., F.M. Phillips, D.A. Stonestrom,
concentrations in the Southwest are and others, 2003. A reservoir of nitrate
groundwaters of the conterminous United
difficult to determine because long-term States, 1992-1995, Environ. Sci. Technol., beneath desert soils, Science, 302: 1021-1024.
nitrate records do not exist at suitable 34(7): 1156-1165.
scales. However, nitrate is
known to be increasing locally
monitoring wells (MW) cores
(see, for example, Rosen,
2003). A recent national 12 MW 2S
trend analysis of nitrate from MW 3S
starts operation 1
wells in agricultural and
Depth below land surface (m)
Nitrate as nitrogen (mg/l)
urban lands found increasing 2
trends in the San Joaquin 3
Basin of California, but no
significant trends in Carson 4
City/Reno alluvial aquifers 6 Core 3S
5 Core 4S
or in the Rio Grande Basin
of New Mexico from 1988 4 6
to 2004 (Rupert, 2008).
Next Steps average nitrogen
Far more consistent and concentration of effluent
long-term monitoring over Jan 1 Jan 1 Jan 1 Jan 1 Jan 1 Jan 1 Jan 1 0 5 10 15 20 25 30 35 40 45 50
large areas is needed in the 1997 1999 2001 2003 2005 2007 2009 Nitrate as Nitrogen (mg/l)
Southwest to assess regional
or aquifer-wide nitrate (left) Eight years after an infiltration pond began operating, a pulse of high-nitrate groundwater was observed in monitoring
trends. Groundwater wells. (right) Nitrogen concentrations in sediment samples taken from the same wells are highest in the upper 10 feet.
34 | JULY/AUGUST 2009 | Southwest Hydrology