STREAM POLLUTION 
383 
determined by substances in solution — particularly carbonates, carbon dioxide, vari- 
ous salts, and a few organic substances — which collectively constitute a poor to fair 
buffer system; so that water more acid than pH 6.7 or more alkaline than pH 8.6 is 
not found generally in our inland streams, unless there be some unusual factor in the 
complex. Pond water, bog water, and lake water vary over a wider range, but the 
combination of stream flow, aeration and buffer substances holds the hydrogen-ion 
concentration of the larger rivers, the smaller streams, and even many brooks within 
the limits described. In pollution studies, therefore, it has been found advisable to 
view with suspicion any stream water having an hydrogen-ion concentration outside 
of the limits pH 6.7 to pH 8.6, until it could be definitely shown that such extra limital 
pH values were due to natural causes rather than to human agencies, as even badly 
polluted streams were usually within these limits. If the water of flowing streams 
were more acid than pH 6.7 or more alkaline than pH 8.6 as the result of the addition 
of municipal or industrial effluents the buffer and carbonate systems were usually so 
disturbed that conditions harmful to fishes were generally found. The determina- 
tion of pH, therefore, is an important aid in the study of polluted water in spite of the 
range of tolerance of fishes to pH changes in unpolluted waters, because excessive 
variation in hydrogen-ion concentrations is indicative of harmful changes in the com- 
plex of dissolved substances, both solids and gases, normally found in river water. 
Among the effluents which change the pH of stream water and break down the 
buffer systems are the wastes from wire-nail mills, tin-plate mills, and other metal 
works where acid washes are used; wastes from chemical works, particularly dye mor- 
dant and soda compounds; spent liquors from chrome tanning processes; whey- 
containing fluids from dairy products concerns; laundry waters; and some battery 
factory wastes. The waters from unsealed coal mines also add large quantities of acid 
wastes. The specific effects of high and low hydrogen-ion concentrations are dis- 
cussed in detail in the section on acid pollutants. (See p. 409.) 
IONIZABLE SALTS 
Specific Conductance 
Measurements of specific conductance were made with standard glass cells con- 
taining coated platinum electrodes in telephone circuit with a microlmmmer and 
standard variable resistance units. This apparatus was found to be very sturdy and 
was regularly used both in the laboratory and in the field. For convenience in pres- 
entation, the data are expressed as specific conductance in mhoXUU 6 at 25° C. 
Unpolluted natural waters contain in solution small quantities of carbonates, 
chlorides, phosphates, and sulphates, usually some nitrates and nitrites if organic 
matter be present, and traces of many other salts which vary with the region through 
which the stream flows. The metallic ions represented are largely calcium, magne- 
sium, sodium, potassium, iron, and manganese, with traces of various other elements. 
Owing to the fact that carbon dioxide is supplied to stream water from so many 
sources, carbonates are the dominant salts; but because of the low solubility of most 
carbonates and also of most phosphates, the mineral content of river water never 
rises very high unless some particular substance is added to the water which will 
raise the solubility of these compounds or transform them into other more soluble 
compounds. All of the substances in solution in river water collectively exert 
