Nitrates in Groundwater

Larry W. Canter

• Publisher: Routledge
• Year: 2019
• Print ISBN:
  9780367448455
  9780873715690
• eBook ISBN:
  9781351428651
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Environmental Science

This time-saving book provides extensive coverage of all important aspects of nitrates in groundwater, ranging from prevention to problem assessment to remediation. It begins by highlighting the nitrogen cycle and related health concerns, providing both background information and a unique perspective on health issues. It then analyzes subsurface pr

1	FUNDAMENTAL ASPECTS OF NITRATES IN GROUNDWATER

INTRODUCTION

Groundwater pollution is a growing concern everywhere in the world. The basic problems are twofold: (1) extensive usage of groundwater leading to overdrafts and declining groundwater levels, and (2) pollution of fresh groundwater leading to undesirable effects on users or curtailment of groundwater usage. Groundwater is the source of drinking water to about 50% of the overall population in the U.S., and to over 90% of the rural population (Office of Technology Assessment, 1990). Groundwater is also essential to agricultural operations in many regions of the country.

Nitrates are one of the most problematic and widespread of the vast number of potential groundwater contaminants (Keeney, 1986). Nitrate test results can be expressed as either nitrate-nitrogen (NO₃–N) or as nitrate (NO₃). The drinking water quality standard in the U.S. for NO₃–N is 10 mg/l, whereas, for NO₃ it is 45 mg/l (Chandler, 1989).

To systematically address fundamental aspects of nitrates in groundwater, it is necessary to: (1) understand the nitrogen cycle from a chemical/microbiological perspective; (2) recognize the nitrogen cycle as related to the soil and groundwater environment; (3) delineate natural and man-related sources of nitrate to groundwater; and (4) identify health-related and other effects of excessive concentrations of nitrate in groundwater. This chapter highlights these four topics in addition to the results of a recent nationwide study that addressed, among other topics, nitrates in groundwater.

THE NITROGEN CYCLE

Nitrogen can exist in many forms in the environment. The movement and transformation of these nitrogen compounds through the biosphere can be characterized by the nitrogen cycle; this cycle is depicted in Figure 1.1 (U.S. Environmental Protection Agency, 1994). The atmosphere serves as a reservoir of nitrogen in the form of nitrogen gas (the atmosphere is 79% nitrogen). Atmospheric nitrogen must be combined with hydrogen or oxygen before it can be assimilated by higher plants; the plants, in turn, are consumed by animals. Human intervention through industrial nitrogen fixation processes and the large-scale cultivation of nitrogen-fixing legumes has played a significant role in altering the historical nitrogen cycle. The amount of nitrogen fixed annually by these two mechanisms now exceeds by as much as 10% the amount of nitrogen fixed by terrestrial ecosystems before the advent of agriculture (U.S. Environmental Protection Agency, 1994).

Figure 1.1 The nitrogen cycle. (From U.S. Environmental Protection Agency, in Nitrogen Control, Technomic Publishing, Lancaster, PA, 1994, pp. 1–22.)

Nitrogen can form a variety of compounds due to its different oxidation states. In the biosphere, most changes from one oxidation state to another are biologically induced. The following nitrogen forms are of interest in relation to the soil/water environment (U.S. Environmental Protection Agency, 1994).

Nitrogen compound	Formula	Oxidation state
Ammonia	NH3	–3
Ammonium ion	NH^+4	–3
Nitrogen gas	N2	0
Nitrite ion	NO^−2	+3
Nitrate ion	NO^−3	+5

The un-ionized, molecular ammonia exists in equilibrium with the ammonium ion, the distribution of which depends upon the pH and temperature of the biospheric system; in fact, very little ammonia exists at pH levels less than neutral. Transformation of nitrogen compounds can occur through several mechanisms, including fixation, ammonification, synthesis, nitrification, and denitrification. Each can be promulgated by particular microorganisms with either a net gain or loss of energy; energy considerations are basic to determining which reactions occur (U.S. Environmental Protection Agency, 1994).

Fixation of nitrogen refers to the incorporation of inert, gaseous nitrogen into a chemical compound such that it can be used by plants and animals. Fixation of nitrogen from N₂ gas to organic nitrogen is predominantly accomplished by specialized microorganisms and the associations between such microorganisms and plants (U.S. Environmental Protection Agency, 1994). Atmospheric fixation by lightning and industrial fixation processes (fertilizer and other chemicals) plays a smaller, but significant, role as a fixation method. The following processes and products are relevant.

Fixation process		Product
	→ Biological	→ Organic nitrogen compounds
N2 gas	→ Lightning	→ Nitrate
	→ Industrial	→ Ammonium, nitrate

Ammonification refers to the change from organic nitrogen to the ammonium form. An important hydrolysis reaction involves urea, a nitrogen compound found in urine:

$\begin{array}{l}{\text{H}}_{\text{2}}{\text{NCONH}}_{\text{2}}{\text{+2H}}_{\text{2}}\text{O}\underset{\text{urease}}{\overset{\text{enzyme}}{\to }}{\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{CO}}_{\text{3}}\hfill \\ \text{Urea}\text{Ammonium carbonate}\hfill \end{array}$

In general, ammonification occurs during decomposition of animal and plant tissue and animal fecal matter and can be expressed as follows:

$\underset{\text{(Protein, amino acids, etc}\text{.)}}{\text{Organic nitrogen}+\text{Microorganisms}\to {\text{NH}}_{\text{3}}{\text{/NH}}_{{\text{4}}^{\text{+}}}}$

Synthesis, or assimilation, refers to biochemical mechanisms that use ammonium or nitrate compounds to form plant protein and other nitrogen-containing compounds as follows:

$\begin{array}{c}{\text{NO}}_{3^{-}}+{\text{CO}}_{\text{2}}+\text{Green plants}+\text{Sunlight}\to \text{Protein}\\ {\text{NH}}_3/{\text{NH}}_{4^{+}}+{\text{CO}}_{\text{2}}+\text{Green plants}+\text{Sunlight}\to \text{Portein}\end{array}$

Animals require protein from plants and other animals. With certain exceptions, they are not capable of transforming inorganic nitrogen into an organic nitrogen form.

Nitrification refers to the biological oxidation of ammonium ions (U.S. Environmental Protection Agency, 1994). This is accomplished in two steps: first to the nitrite form, then to the nitrate form. Two specific chemoautotrophic bacterial genera are involved, using inorganic carbon as their source of cellular carbon as follows:

${\text{NH}}_4^{+}+{\text{O}}_2\underset{\text{bacteria}}{\overset{\text{Nitrosomonas}}{\to }}{\text{NO}}_2^{-}+{\text{O}}_2\underset{\text{bacteria}}{\overset{\text{Nitrosomonas}}{\to }}{\text{NO}}_3^{-}$

The transformation reactions are generally coupled and proceed rapidly to the nitrate form; nitrite levels at any given time are relatively low. The nitrate formed may be used in synthesis to promote plant growth, or it may be subsequently reduced by denitrification, as suggested by Figure 1.1 (U.S. Environmental Protection Agency, 1994).

Denitrification refers to the biological reduction of nitrate to nitrogen gas. The reduction can proceed through several steps in a biochemical pathway. A fairly broad range of heterotrophic bacteria are involved in the process, requiring an organic carbon source for energy as follows (U.S. Environmental Protection Agency, 1994):

${\text{NO}}_3^{-}+\text{Organic carbon}\to {\text{NO}}_2^{-}+\text{Organic carbon}\to {\text{N}}_2+{\text{CO}}_2+{\text{H}}_{\text{2}}\text{O}$

It should be noted that if both oxygen and nitrate are present the bacteria typically will preferentially us...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• 1. FUNDAMENTAL ASPECTS OF NITRATES IN GROUNDWATER
• 2. INFLUENCE OF SUBSURFACE PROCESSES
• 3. ILLUSTRATIONS OF NITRATE POLLUTION OF GROUNDWATER
• 4. VULNERABILITY MAPPING OF GROUNDWATER RESOURCES
• 5. DEVELOPMENT OF A NITRATE POLLUTION INDEX
• 6. MODELS FOR NITRATES IN THE SUBSURFACE ENVIRONMENT
• 7. MANAGEMENT MEASURES FOR NITRATE POLLUTION PREVENTION
• 8. TREATMENT MEASURES FOR NITRATES IN GROUNDWATER
• INDEX