Coexistence and controlling factors of ammonium and phosphorus in groundwater along the middle reaches of the Yangtze River

Abstract

The plain area of the middle reaches of the Yangtze River is faced with a series of serious geological environment problems, among which the groundwater ammonium and phosphorus are the most abnormal, but the understanding of their coexistence is still very weak.In this paper, the hydrogeochemical analysis was carried out on the groundwater samples collected from the oxbows along the middle reaches of the Yangtze River, and the occurrence rules of ammonium and phosphorus were discussed by means of factor analysis and random forest model.The results showed that: Groundwater is in reductive environment as a whole, the mass concentration of NH4-N is 0.03-71.0 mg/L(the mean is 9.92 mg/L), and the mass concentration of P is 0.02-3.38 mg/L(the mean is 0.51 mg/L).High concentration of ammonium and phosphorus in groundwater are mainly from natural causes.However, the migration and enrichment processes of ammonium and phosphorus are different.The migration and enrichment of ammonium is closely related to the mineralization process of organic matter.The migration and enrichment of phosphorus is closely related to the reductive dissolution of iron oxide/hydroxide, and the mineralization of organic matter containing phosphorus is the secondary process of phosphorus enrichment.A low Eh groundwater environment is prone to produce high ammonium groundwater, while a relatively medium reduction environment will produce high phosphorus but usually not associated with high ammonium nitrogen.When S2-, DOC and I are in relatively high concentrations, a large amount of ammonium is associated, while the concentration of phosphorus is largely controlled by the concentration of Fe2+.When DOC, I, and Fe2+ are all high, high concentrations of ammonium and phosphorus are usually present.

Keywords

• groundwater
• ammonium
• phosphorus
• organic matter
• iron oxide