Study on an in-situ dissolution experiment of gypsum boreholes

ZHOU Jianwei; HE Xiao; GAO Xiaofeng; PENG Tao; XU Ke; ZHAO Yong

doi:10.11932/karst20250508

Abstract：Gypsum is characterized by high solubility and a rapid dissolution rate,which can trigger geological disasters such as subsidence and collapse,especially under the influence of inappropriate human engineering activities like groundwater extraction and drainage.However,most research on gypsum has focused on itsn occurance in the marine sedimentary environments,with relatively few studies addressing gypsum in the lacustrine sedimentary settings.The latter is generally considered to exhibit rare karst phenomena and to occur on a smaller scale.The gypsum in the Cretaceous Guankou Formation(K2g)is a typical example of lacustrine sedimentary gypsum.Its dissolution has caused abnormal settlement of buildings constructed on it.However,most research on the dissolution of the gypsum has been limited to laboratory experiments,such as the static water and flowing water dissolution tests,which do not fully capture the actual dissolution conditions of gypsum.Some in-situ dissolution tests on gypsum have been carried out in Russia,Ukraine,Spain and Italy with the use of tablet or the MEM method in boreholes and cavities.But no in-situ dissolution research on gypsum has been conducted in China. In this study,three groups of standard gypsum(K2g)tablet specimen,packed in nylon bags,were suspended at depth of 15 m,25 m and 35 m within the borehole.Steel pipe were used to prevent the upper sandy and gravel aquifer from directly entering to the borehole.This setup simulated the dynamic dissolution process of gypsum under the in-situ hydrodynamic,hydrochemical and temperature conditions representative of the real stratigraphy environment.After 3 days,7 days,15 days and 30 days,the specimens were retrieved from the borehole for measurement and observation,and the chemistry of groundwater was analyzed simultaneously. The tests showed that the specimens at the depths of 15 m and 25 m exhibited intense dissolution phenomena,while the specimens at the depth of 35 m showed only slight dissolution.After 30 days of in-site testing,the average mass loss rates of the specimens at 15 m and 25 m reached 73.9％and 73.7％respectively,with average dissolution rates of 1.16×10-2 g·cm-2·d-1 and 1.13×10-2g·cm-2·d-1,respectively.The recession rates at 15 m and 25 m were 0.246 mm·d-1 and 0.245 mm·d-1,respectively.The average mass loss rate of the specimens at 35 m was only 18.4％,and the average dissolution rate was 0.39×10-2 g·cm-2·d-1,which was only a quarter of that of the specimens at 15 m and 25 m,and was comparable to the dissolution rate of the gypsum reported by other researchers in static water environments.This dissolution rate was much greater than that of gypsum specimens in boreholes measured by Calligaris C.The dissolution rates of the specimens at 15 m and 25 m initially increased and then decreased over time,while the dissolution rate of the specimens at 35m did not change significantly with time. Groundwater samples were collected from the borehole for chemical analysis both before and during the test.It was observed that the concentrations of SO42-and Ca2+in the groundwater at 15 m and 35 m were lower than those at 25 m.Nevertheless,neither the concentration of SO42-nor that of Ca2+reached saturation at any point during the experiment.The dissolution rate constants(K)at the site were approximately 0.030×10-5 to 0.114×10-5 m·s-1. Influenced by hydrogeological conditions at various depths,the rapid dissolution rate of gypsum was primarily controlled by mass transport driven by the hydraulic gradient,with surface reactions driven by ion concentration playing a secondary role.The dissolution processes of the specimens at 15 m and 25 m were affected by both mechanical erosion and chemical dissolution,progressing through four stages:adsorption and saturation,shallow pit formation,lateral pit merging,and deep disintegration.In contrast,the dissolution of the specimens at 35 m was mainly controlled by chemical dissolution and only underwent the first two stages:adsorption and saturation,and shallow pit formation. This in-situ dissolution experiment and evaluation method for gypsum supplements the existing dissolution data of gypsum in China,providing an important reference for karst risk assessment in gypsum-rich areas.