2— THE TRACING OF UNDERGROUND WATERS. 

 By J. McCrae, Ph.D. 



There are many instances in which it is advisable to ascertain 

 if a water connection exists underground between two points, where 

 water is found on the surface. It frequently happens, in limestone 

 and other geological formations, that a river sinks into the ground or 

 passes into a subterranean passage, and for the riparian owner at 

 some less elevated point, on whose ground water appears on the 

 surface, it is of the greatest importance to know if there is direct 

 connection between two such waters, in order that he may take 

 steps to protect his water-rights. Chemical analysis of samples from 

 the two spots may in some cases permit of the conclusion that there 

 is no connection, but it would be rash to deduce from similarity in 

 chemical composition that there is a connection : a geological survey 

 of the intervening ground will usually permit of the expression of an 

 opinion as to the probability of a direct connection between the two 

 waters, but this will be at best an opinion. If, however, some 

 characteristic substance be added to the water at the higher point, 

 and if this substance appear in the water at the lower point, then a 

 connection between the two points will have been established : a 

 negative result would require to be treated with circumspection, but 

 the more acccurate and delicate the process employed the greater will 

 be the value attached to a negative result. 



Furthermore, the tracing of a w-ater connection between two points 

 is often essential in ascertaining if a water (particularly a water-supply) 

 is being contaminated from some adjacent polluted river, ditch or 

 pond. 



Methods. — If the channel connecting the two waters is such 

 that the water flows freely through it without impediment, floats 

 of cork or other material may be used. The cases where floats can 

 be used are, however, extremely few. More hopeful is the method 

 of adding something characteristic which dissolves in the water. 



The properties required of such a substance are : — 



(i). It must be soluble in water; 



(2) It must be unacted upon by the material through w^hich it 



passes ; 



(3) It must be of such a nature that it will not be confused with 



anything naturally present in the waters ; 



(4) It must be easily recognised. 



Sodium chloride has been used, and it fulfils the requirements 

 in a large measure. But there are considerable objections to its use. 

 As it is not fer se easily recognised, it is taken as sufficient to 

 examine if the content of chlorine of the water at the lower point 

 increases after the addition of the sodium chloride at the higher 

 point. A consequence of this is that a very considerable amount of 

 sodium chloride, in proportion to the amount of water, is required 

 to be dissolved, and this may entail the damming back of the stream 

 in order to give the salt time to dissolve, for it is a sine qua non of 

 the method, that a large increase in the proportion of chlorine shall 

 be made. The method further suffers from the fact, that if the 



