Occurrence of ammonium in the acidic-circumneutral coastal groundwater of Beihai, Southern China: δ15N, δ13C, and hydrochemical constraints

Abstract

High levels of ammonium (NH4+) in groundwater can threaten the health of surface ecosystems and the safety of water supplies. To ascertain the occurrence of NH4+ in the coastal acidic groundwater of Beihai, Southern China, a combined hydrochemical and isotopic (δ15N in nitrate (NO3−) and NH4+; δ13C in dissolved inorganic carbon (DIC)) study was performed. In addition to a chloride concentration-based fresh water-seawater mixing model, δ15NNO3 vs δ15NNH4, Δδ13CDIC,react (changes in δ13CDIC values caused by reaction), and ΔCSO4,react (changes in sulfate concentrations caused by reaction) were applied to interpret the genesis of NH4+. The results show groundwater NH4+ loadings range from < 0.04 to 6.2 mg/L, with typically higher loadings along the coastline than in the plains. Elevated NH4+ loadings (>0.2 mg/L) are commonly accompanied by enriched δ15NNO3 and depleted δ15NNH4 values in relation to the anthropogenic endmember, depleted or enriched Δδ13CDIC,react values, and ΔCSO4,react > 0.00 or < 0.00 mmol/L. Such isotopic fingerprints and hydrochemical features indicate dissimilatory nitrate reduction to ammonium (DNRA), glucose fermentation, methanogenesis (acetate fermentation), and sulfide oxidation or sulfate reduction. The findings suggest acidic rainwater, sulfide oxidation, and NO3− reduction by ferrous iron (Fe2+) could jointly account for the formation of acidic groundwater (pH as low as 4.0) that is of meteoric origin. Fermentative and sulfide-driven DNRA with anthropogenic NO3− is notably responsible for the enrichment of groundwater NH4+ concentrations, with a minor contribution from anthropogenic NH4+ infiltration and algae decomposition. As a result of silicate and carbonate weathering, seawater inputs, and fermentation, the addition of bicarbonate (HCO3−) results in circumneutral pH values. These results indicate DNRA plays a critical role in the enrichment of NH4+ and, hence, improve understanding of its genesis in the groundwater of Beihai and other similar areas.