APPLICATIONS OF RESISTIYITY 



MEASUREMENTS 



Relationship of resistivity to dissolved solids 



Measurements of electrical resistivity 

 have been utilized by industry for many years 

 as rough empirical indices of the total dissolved 

 solids content of waters (Gustafson and Behrman, 

 1939). The principal solids in non-turbid fresh 

 waters are Ca, Mg, Na, K, CI, SO4, HCO3, 

 and Si02' ^11 ^""^ ^^ ^^^2 are major electro- 

 lytes. Rodhe (1949) noted that the combined 

 total concentration of electrolytes may vary 

 greatly fron one body of freshwater to another 

 but that the proportions of specific electrolytes 

 in each body tended to be remarkably similar. 

 Welch (1948) recognized that a measure of total 

 electrolytes can be obtained by measuring the 

 electrical resistance of a sample of water. He 

 added that any measurement of the electrolyte 

 content of a body of water is of considerable im- 

 portance in limnological practice since, other 

 things being equal, the richer the water in elec- 

 trolytes the greater the biological productivity. 



Refinements in the resistivity method of 

 estimating dissolved solids in samples of waters 

 have been reported by Rossum (1949) and Syl- 

 vester (1958). Sylvester demonstrated the 

 straight line relationship between conductivity 

 and total solids in non -turbid waters of the Col- 

 umbia River Basin and presented a formula for 

 basin streams whereby any single conductivity 

 value (micromhos at 25° C.) minus 50 can be 

 multiplied by 0.74 to give the approximate value 

 of the total solids (mg./l). 



The motivation to improve the resistiv- 

 ity method for determining the concentration of 

 solids resulted from the fact that the old method 

 required the evaporation of 1 liter samples of 

 water, heating and then weighing the residue - 

 a method which is very slow and subject to great 

 errors. In contrast, the resistivity of a water 

 sample can be rapidly and precisely metered. 

 The developments cited above warrant considera- 

 tion in fishery investigations since field men 

 engaged in extensive surveys can convert meas- 

 urements of resistivity to estimates of total 

 solids or electrolytes in lakes and streams. The 

 results can serve as tentative indices of biologic- 



al productivity and permit classification, rank- 

 ing, or other comparisons of the waters. 

 Applications of resistivity measurements can 

 be made in more intensive surveys to obtain 

 qualitative and quantitative estimates not only of 

 total solids and total electrolytes but total hard- 

 ness, non -carbonate hardness, and concentrations 

 of specific electrolytes. 



Rodhe (1949) observed that among bodies 

 of freshwaters there may be local peculiarities, 

 particularly in geochemical and climatic con- 

 ditions, which might give certain waters a differ- 

 ent electrolyte composition from the general, 

 world wide "standard composition". It is advis- 

 able that fishery investigators test representative 

 waters of their regions to determine if the ac- 

 cepted relationship between resistivity and total 

 dissolved solids exists in them. 



The waters studied by Rossum, Rodhe, 

 and Sylvester ranged from soft to brine. Sylvester 

 indicated, however, that his curve and formula for 

 the conductivity -solids relationship was of little 

 value where conductivity was less than 150 

 micromhos. The linear relationship was there- 

 fore tested with data collected on 133 clear, soft 

 water streams in western North Carolina in which 

 conductivities were less than 150 micromhos. 

 Complete chemical analyses for these streams 

 were included in a study of mountain watersheds 

 made by the North Carolina Department of Con- 

 servation and Ctevelopment (1950, 1951, and 1953). 

 The measurements of total dissolved solids and 

 conductivity for the streams were grouped accord- 

 ing to toal hardness determinations (3-7, 8-12, 



13-17 38-42 ppm). Conductivities and total 



dissolved solids in each group were then averaged 

 and the products used in the line equation y = a -t-bx, 

 where y = TDsV and x = conductivity at 77° F . 

 The resulting equation was: TDS = 7.02 + 0.72 

 (conductivity in micromhos), with a standard 

 error of + 2.0 ppm. 



A comparison of the measured, grouped 

 values of conductivity and TDS with the values 

 calculated for same from the equation shows close 

 agreement (table 1). The small disparities in 

 measurements might be attributable to errors made 

 in the evaporation technique. The line drawn from 



1/ TDS hereafter used to denote total dissolved 

 solids 



