Occurrence of ammonium in the acidic-circumneutral coastal groundwater of Beihai, Southern China: delta N-15, delta C-13, and hydrochemical constraints

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 (delta N-15 in nitrate (NO3-) and NH4+; delta C-13 in dissolved inorganic carbon (DIC)) study was performed. In addition to a chloride concentration-based fresh water-seawater mixing model, delta N-15(NO3) vs delta N-15(NH4), Delta delta C-13(DIC),(react) (changes in delta C-13(DIC) values caused by reaction), and Delta C-SO4,C-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 delta N-15(NO3) and depleted Delta N-15(NH4) values in relation to the anthropogenic endmember, depleted or enriched Delta delta C-13(DIC,react) values, and Delta C-SO4,C-react > 0.00 or < 0.00 mmol/L. Such isotopic fingerprints and hydrochemical features indicate dissimi-latory 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.