An Introduction to Environmental Chemistry

It is difficult to estimate the natural pre-human NO 3 - concentration in rivers, since

most have now been affected by human activities to some extent. Nitrate con-

centrations in the Amazon are about 20mmol l-^1 and are probably close to natural

levels. Multiplying this NO 3 - concentration by the global river flux gives an esti-

mate of the natural nitrate input to the oceans:

eqn. 6.21

20 10¥ --^6 ()mol NO l 3 -^11 ¥¥3 6 10 1..61 1() yr-=¥0 72 10^2 mol NO yr 3 - -^1

236 Chapter Six

Low oxygen area

Gulf

of

Mexico

A

Year

NO

• – N (mg l 3

–1

)

4

19550 1965 1980 1985 1995

3

1960 1975 1990

1

12

Nitrogen fertilizer use (tonnes

×^10

6 per year)

2

2

1970

4

10

8

6

Nitrogen fertilizer use

Mean NO 3 – – N in

Mississippi River

A

Fig. 6.31Estimated nitrogen fertilizer use in the USA and mean nitrate-nitrogen

concentrations* in the Mississippi River at St Francisville near Baton Rouge, Louisiana (A on

inset map) between 1955 and 1995. Inset map shows the Mississippi drainage basin and the

region of low oxygen that forms offshore of the Mississippi delta in the Gulf of Mexico. Data

courtesy of United States Geological Survey. * Note on units: although we mainly use molar

units in this book, in some applications results are reported in mass units. Nitrate is a case in

point. Much nitrate data are reported in milligrams of nitrate per litre (mg NO 3 - l-^1 ). However,

another common unit is milligrams of nitrogen from the nitrate per litre (mg NO 3 - -N l-^1 ) as

used in this figure. Conversion of the former to the latter requires division by the molar mass

of NO 3 - (62) and multiplication by the atomic mass of nitrogen (14.01).