Development of an apparatus to measure groundwater qualities in situ and to sample groundwater using boreholes

Abstract

 To accurately measure the pH, Eh, EC and temperature of groundwater retrieved from boreholes, a deep groundwater sampling apparatus was developed which provided sensory measurements both in situ and in a flow-through cell at ground level. Under a pressure of 1×10⁶ Pa the in situ accuracy of the apparatus sensor was within the following limits: pH ±0.2, temperature ±0.1°C, Eh ±10 mV, and EC ±2.4%. The measuring and sampling of deep groundwater from a borehole of more than 1000 m in depth was performed continuously for 30 days. Values of pH were the same for the in situ sensor, the flow-through cell sensor and the laboratory measurements of the sampled water. At the beginning of the sampling period, Eh values of the in situ sensor indicated deep groundwater conditions. The apparatus is particularly useful for Eh measurement. Chemical composition and stable isotope ratios indicated that the groundwater sampled from more than 1000 m depth was a connate water with a chemical composition slightly different from seawater of the present time, and the groundwater retrieved from 800 m depth was a meteoric water. Natural radioactive elements are thought to be the origin of the tritium in the groundwater retrieved from the 1000 m depth.