G6 



DATA OF GEOCHEMISTRY 



tions to explain the general uniformity of the water of 

 large rivers. 



This does not mean that sorption reactions can be 

 neglected in river chemistry. For some elements, 

 particularly the heavy metals, they are of the greatest 

 importance, and they have some effect on the contribu- 

 tion of river salt to sea water, but they do not seem to 

 be of great importance in controlling the general 

 composition of river waters. 



Because of the great temporal and spatial variability 

 of river water, a single sample from a river can give 

 only a very inadequate measure of its chemistry. 

 Particularly in lands with a very great seasonal varia- 

 tion in rainfall, such an estimate may be in error by 

 several orders of magnitude. This is the quality of the 

 data available, however, and the analysts have to rely 

 frequently on a single sample to characterize major 

 rivers, particularly those in tropical and arctic regions. 

 It is very gratifying to have a few rivers to which it is 

 possible to given annual weighted means of water 

 composition at a number of points. 



Chemical investigations of river water are usually 

 made to provide background information concerning 

 the utility of the water for industrial and agricultural 

 purposes. The ions that reduce the potability of the 

 water, or that produce objectionable hardness, are the 

 principal ones measured. A few minor elements 

 essential to plant or animal nutrition, such as boron, 

 fluorine, and fixed nitrogen are often included. Occa- 

 sionally heavy metals that may be of interest in measur- 

 ing industrial pollution, such as lead, arsenic, chromium, 

 zinc, and copper, are included in routine analyses of 

 this sort. 



A substantial number of analyses have been made 

 by limnologists. These are, in general, of httle value 

 for geochemical purposes because the elements de- 

 termined are usually those that have biological im- 

 portance, such as oxygen, phosphorus, and nitrogen in 

 its various combined forms. In a few cases, particularly 

 for lakes in remote parts of the world, the major con- 

 stituents of the dissolved mineral matter of lake and 

 river water have been determined. Although these 

 analyses have a special value, as they often come from 

 lands devoid of industrial development and hence of 

 other analytical data, they are to be found in a very 

 small percentage of limnological papers, even of those 

 whose titles suggest a chemical emphasis. 



Data that have been collected specifically for geo- 

 chemical purposes are extremely scarce. For the most 

 part they are restricted to a single element or a small 

 group of related elements, although some geochemists 

 working with trace elements are careful to present 

 data for the principal mineral constituents of the 



waters under analysis. These are the most valuable 

 sources of data, but they are also the most scarce. 



The most common geologic purpose for which the 

 chemical data dealing with river waters are used is the 

 calculation of the amount and nature of the substance 

 that is removed from the land by river waters and 

 deposited in the sea. It should be understood that 

 the data available for this purpose are scarce, incom- 

 plete, and not always accurate. By accepting them 

 at face value and ignoring the uncertainties involved 

 a spurious appearance of reliability can be given to 

 the calculated results, but they will, in fact, be less 

 reliable than ones incorporating a certain number of 

 reasonable assumptions. 



The first source of error lies in the incompleteness 

 of the data. In general, it is only the highly industrial- 

 ized nations of the temperate zones that make routine 

 chemical analyses of river water. As a result many 

 great river systems of the world, particularly in the 

 tropics and the Arctic, have been analyzed, if at all, 

 only by an occasional interested traveler. The samples 

 collected in this way are usually transported elsewhere 

 for analysis, and in the meantime they are stored in 

 glass bottles that exchange a variable quantity of 

 soluble material with the water. 



Even in the countries where water analyses are 

 routinely made, there is rather inadequate coverage. 

 As was demonstrated above, the concentration of 

 river water bears an inverse relation to the discharge. 

 If the discharge shows a great seasonal variation, a 

 single sample will not suffice for a calculation of the 

 quantity of dissolved material carried to the sea, even 

 if the annual variation of discharge is accurately 

 known. It is necessary to carry a systematic program 

 of sampling over a period of at least 12 months in 

 order to determine the chemical load of a stream. 

 Even this will not, of course, take into account varia- 

 tions due to wetter and dryer years. 



The U.S. Geological Survey has made long-term 

 investigations of a number of streams, so it is possible 

 to avoid the errors due to discharge variations in a 

 number of American rivers. The procedure is to take 

 a series of daily samples and to combine them into 

 composite samples, usually every 10 days, for analysis. 

 This, in effect, yields a series of simple 10-day averages 

 for the chemical nature of the water. Such simple 

 averages will suggest that the river water of the 

 sampling period is somewhat less dilute than it actually 

 is, and a closer approach to the true mean concentration 

 is sometimes achieved by combining the daily water 

 samples in quantities proportional to the discharge 

 on the days they were collected. 



