Controls on distributions of sulphate, fluoride, and salinity in aquitard porewater from the North China Plain: Long-term implications for groundwater quality

Groundwater interacts with porewater within clay-rich sequences. However, the effects of aquitard porewater on groundwater quality are generally poorly understood. This study explores potential links between porewater chemistry in a thick clay-rich sequence of Quaternary sediments, and groundwater quality in the North China Plain (NCP), one of China's most important water supply aquifers. Previous studies have hypothesised the release of salinity, fluoride, and sulphate from aquitards in response to intensive pumping, without explicitly analysing the chemistry of aquitard porewaters. This study further investigated this hypothesis through targeted analysis of porewater chemistry profiles through the NCP sedimentary sequence. Two cores were analysed - one in the central plain (Hengshui, HS) and the other in the coastal plain (Cangzhou, CZ). Both areas have experienced significant groundwater extraction over recent decades from confined aquifers inter-layered with clays. Aqueous extraction (AE) and squeezing (SE) methods were both used to assess porewater chemistry. The hydrochemical characteristics of groundwater are closer to those of the squeezed porewater, which can be regarded as more closely corresponding to in-situ porewater compositions than AE profiles. Chloride was generally consistent between the two methods, while sulphate and fluoride concentrations were greater using AE, ranging from 340 to 48,700 mg/L and 8.8 to 144 mg/L, compared to 350 to 8860 mg/L and 0.2 to 4.3 mg/L using SE, respectively. The much greater concentrations of fluoride derived from the AE method indicates a high susceptibility to release during changes to the ambient geochemical environment. While the primary fluoride sources in groundwater and porewater are likely minerals from weathering of volcanic rocks in the sedimentary sequence, release into groundwater appears linked to common geochemical influences such as high Na/Ca ratio (increased during cation exchange) and the presence of iron and aluminium oxides (providing sorption sites). The primary source of solutes is interpreted to be different in each profile, with evapotranspiration important in the central plain and marine solutes important in the coastal profile, indicating significant storage of salinity, fluoride, and sulphate within low permeability sediments. Our data supports the hypothesis that release of these ions is driving degradation of groundwater quality, in response to decades of intensive groundwater extraction, which is an ongoing threat to the quality of groundwater in the NCP.