GROUNDWATER GEOCHEMISTRY EVOLUTION AND UNSATURATED ZONE ARCHIVES OF CLIMATIC CHANGE IN ARID NW CHINA

Isotopic and geochemical techniques were used to determine the groundwater age, understand the origins of salinity, and clarify the impact of climatic change on groundwater in the Minqin Basin. The radiocarbon age of the oldest groundwater is 40 ka. Minqin Basin groundwaters are generally negative, and the deepest water is notably depleted in heavy isotopes, signifying cooler recharge conditions during the late Pleistocene than at present. Shallower water, with values between ¨C7-¨C11°ë¦Ä18O, is mainly paleowater, mixed with limited modern recharge. Temperatures were 4-5°ã C lower than at present based on¦ÄD data. Trends in major elements in Minqin Basin groundwater, as well as in redox characteristics, follow a progressive sequence of geochemical evolution. Overall the groundwaters in the Minqin Basin evolved from alkali-sulphate types in the recharge area to alkali-sulphate-chloride types along the groundwater flow direction. A wide range of minor and trace elements indicate a reducing condition in deep groundwater and an oxidizing condition in shallow groundwater. Groundwater chemical accumulation and salinity increased slowly during the past 40 ka in the deep aquifer, while water quality deteriorated significantly in the upper shallow aquifer due to a combination of human activity and rapid climatic change during the last 2000 years. The history of groundwater recharge and climatic changes during the last 800 years has been estimated and reconstructed using environmental chloride from an unsaturated zone profile in the Badain Jaran Desert. The long-term recharge is estimated to be 1.3 mm yr-1. A dry episode occurred before 1290 AD. From 1500-1530 AD the climate underwent a rapid change from dry to wet. At the beginning of 1800s, important wet events occurred, but the climate deteriorated drastically during the last 200 yrs. These results correlate well with ice core records of Tibet Plateau from 1200 to 1900 AD, but differ during the most recent 100 yrs.