Evolution of groundwater chemical composition under human activity in an oilfield

 Analysis and hydrogeochemical modeling of hydrocarbonate groundwater, including its buffering geochemical properties, have demonstrated that this water has undergone a geochemical transformation almost throughout the whole of one industrial region. It is known that hydrocarbonate groundwater possesses a high protective natural geochemical potential, supporting neutralization of acid atmospheric precipitation. Natural hydrocarbonate water undergoes three stages of anthropogenic transformation caused by acid atmospheric precipitation over more than 50 years. The first stage is transformation of neutral hydrocarbonate water into alkaline water, accompanied by calcite precipitation and sodium carbonate dissolution from the unsaturated zone. The second stage is transformation of alkaline water into neutral hydrocarbonate water; the hydrocarbonate content, being less than the lower limit for background concentrations, showed reduced water buffering properties or protective potential relative to acid precipitation. The third stage is transformation of neutral hydrocarbonate water into acidic water, with a complete loss of protective geochemical potential. This unfavorable ecological situation with natural geochemically hydrocarbonate groundwater shows that natural groundwater less protected from the impact of acid atmospheric precipitation is in a worse ecological condition, which is confirmed by observations in northern and Western Europe and other regions of the world. Received: 30 October 1997 · Accepted: 23 March 1998