Chapter 4
Part 2: Special cones: geo-environmental cones
Electric conductivity cone
ELECTRIC CONDUCTIVITY CONE 4.9 Subsoil conductivity In addition to the measured cone tip and sleeve resistances, the electric conductivity cone (Figure 28) also measures the soil conductivity with depth. As with the conventional CPT, the basic penetration parameters, qc and fs, enable the classification of soil. Contamination The measurement of conductivity in the subsoil facilitates separation of zones with differentiated water content including determining the water table depth and the thickness of the zone of capillary ascent. The most important application of the conductivity cone, however, is related to evaluating the degree of contamination of a soil medium containing electrolytes (the compounds dissociating into ions in water). Inorganic acids Since hydrogen and hydroxide ions have the highest conductivity, the easiest way to detect strong inorganic acids, hydroxides and their salts in soil is to use the electric conductivity cone. This method may be applied to identify zones with increased concentrations of nitrites and nitrates, sulphates, calcium and magnesium compounds and concentrations of heavy metals. If aromatic amines are found in the subsoil, compounds dissociating in the water solution which lead to an increase in conductivity may indicate an excess of N-nitrogen fertilizers. | |
Hydrocarbons Oil derivatives do not dissociate into ions, therefore rendering direct detection of them with the conductivity method impossible. In certain cases, however, it is possible to estimate indirectly the presence of hydrocarbons when there are phenols in the subsoil – phenols are products of the reaction of aromatic compounds which dissociate in water. Levels of pH The conductivity method also yields information on the aggressiveness of groundwater towards concrete. If the pH value of the soil changes from neutral, the conductivity increases. The presence of carbon dioxide in the water results in formation of carbonic acid – the factor that increases conductivity. Inorganic salts Water hardness related to the presence of magnesium, calcium and iron ions and the presence of sulphates (inorganic salts) is another factor which increases subsoil conductivity. Another application of the method can be the determination of the range of fresh and salt water in subsoil; the presence of sodium and chlorine ions in salt water increase conductivity, facilitating discrimination of zones where salt water occurs. Drawback From the perspective of analysis of pollutants, the universality of the application of this method is a significant drawback. The method measures the sum of all influences against the background of the soil’s natural conductivity based upon its structure, water content, mineralogical composition, and the physical, chemical and biological processes taking place in the subsoil. It is therefore best to use this method to monitor zones, range and intensity of contamination of an area under study. Execution Depending on the way in which the problem is formulated, execution of the studies can take two directions. Firstly, in cases where the contaminating agent is known but the range and intensity of contamination is unclear, penetration is carried out in a systematic grid to facilitate determining isolines (regions with similar concentrations of the agent). Secondly, where there are several pollutants or the pollutant is unknown, monitoring is carried out to select investigation points with the highest and the lowest levels of contamination from which water and soil samples are taken for special chemical investigation. | |