Abnormally High Ammonium of Natural Origin in a Coastal Aquifer-Aquitard System in the Pearl River Delta, China

Abstract

High-nitrogen loadings of rivers and aquifers systems are a major concern because of potential effects on human health and water quality impacts such as eutrophication of lakes and coastal zones. This nitrogen enrichment is commonly attributed to anthropogenic sources such as sewage and agricultural and industrial wastes. The aims of this study were to delineate spatial distribution of groundwater ammonium in the coastal aquifer system in Pearl River Delta (PRD), China and to identify the origin of the abnormally high ammonium. A total of 40 boreholes were drilled to collect core samples of the aquitard and groundwater samples in the basal aquifer. The core samples were used for extraction of pore water for centrifugation and bulk chemical analyses in laboratory. Unlike previous studies which focused mainly on the aquifer, this study treated the aquifer-aquitard system as a hydrogeochemical continuum. The results show that the aquifer-aquitard system contains an exceptionally large total ammonium mass. Ammonium occurred at concentrations up to 390 mg/L in the basal sand Pleistocene aquifer 20-50 m deep, the largest concentration reported for groundwater globally. This ammonium was natural, areally extensive (1600 km(2)) and originated in the overlying Holocene-Pleistocene aquitard and entered the aquifer by groundwater transport and diffusion. Total ammonium in the aquifer (190 × 10(6) kg) was exceeded by total ammonium in the aquitard (8600 × 10(6) kg) by a factor of 45. Much organic nitrogen remained in the aquitard available for conversion to ammonium. This natural ammonium in the aquifer was slowly transported into the PRD river channels and the estuary of the South China Sea. The rate of this contribution will likely be greatly increased by sand dredging in the river channels and estuary. Although the ammonium in PRD groundwater occurred in the largest concentrations and mass reported globally, the literature shows no reports of other delta aquitards having been examined for ammonium occurrence and therefore abundant ammonium formed in aquitards rich in organic matter may not be uncommon and this "geologic" source of ammonium may present a large and hitherto unappreciated source of nitrogen discharging to surface waters.

Geochemical signature of pore water from core samples and its implications on the origin of saline pore water in Cangzhou, North China Plain

“…deposits (Jiao et al, 2010;Liu et al, 2014). An attempt was made to measure the TOC contents in the sediments, but they are below the detection limit.…”

Assessment of major ions and heavy metals in groundwater: a case study from Guangzhou and Zhuhai of the Pearl River Delta, China

“…SO 2 -4 is associated with the abundant authigenic pyrite identified in Quaternary sediments from the PRD (Lan, 1991;Wang and Jiao, 2012). Without significant nitrification, the concentrations of NH þ 4 in groundwater should primarily be influenced by sorption, which is primarily controlled by cation-exchange processes in porous media (Buss et al, 2004;Jiao et al, 2010). PC1 could therefore be said to reflect the influence of natural sources.…”

Effect of autotrophic denitrification on nitrate migration in sulfide-rich marine sediments

“…The ORP values of the sandy silt and clayey silt sediments were −236.9±4.5 and −175.2±3.9 mV, respectively. The watersoluble SO 4 2− and NO 3 − in sediment were extracted using deionized water (18.2 MΩ) at a 1:10w/v ratio (Jiao et al 2010). The concentrations of SO 4 2− and NO 3 − were then measured using HIC-20A SUPER ion chromatography (IC) (Shimadzu, Kyoto, Japan) (APHA 2012).…”