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Nitrates in Southwest Groundwater

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					Nitrates in Groundwater

 Nitrates in
 Southwest
 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
                                                  significant sources
 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
                                                                                                  Occurrence
 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
                                                                                                                       uncontaminated aquifers.


                                                                                                                        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,
                                                                                                                                                                         0
2003). A recent national               12                                                         MW 2S
                                                                      Infiltration pond
trend analysis of nitrate from                                                                    MW 3S
                                                                      starts operation                                                                                   1
                                                                                                  MW 4S
wells in agricultural and




                                                                                                                                          Depth below land surface (m)
                                       10
                                     Nitrate as nitrogen (mg/l)




urban lands found increasing                                                                                                                                             2

trends in the San Joaquin                                                                                                                                                3
                                         8
Basin of California, but no
significant trends in Carson                                                                                                                                             4
                                                                                                                                                                                               Core 2S
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).
                                                                                                                                                                         7
                                                                  2
Next Steps                                                            average nitrogen
                                                                                                                                (A)
                                                                                                                                                                         8
                                                                                                                                                                                                         (B)
Far more consistent and                                               concentration of effluent
                                                                  0                                                                                                      9
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

				
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