The distribution of nitrogen speciation and sources of nitrate in the north of Taihu Lake

Meiliang Bay and Gonghu Bay, in the north of Taihu Lake, are important water sources for the city of Wuxi, and increased eutrophication now threatens the safety of drinking water. The distribution of nitrogen (N) speciation and source of N in the surface waters in the north of Taihu Lake is studied, which was an important first step in controlling N pollution. The result shows that the average concentration of ammonia (NH₄⁺) and nitrate (NO₃⁻) of surface water in Meiliang Bay was 0.32 and 0.35 mg/L, while 0.21 and 0.74 mg/L of Gonghu Bay, in which both bays had serious nitrate pollution. The concentrations of NH₄⁺ and NO₃⁻ in the surface water of the two bays had a trend of gradual decrease from north to south. The maximum concentrations of NH₄⁺ and NO₃⁻ of two bays were observed near the inflowing rivers, and the maximum concentrations of NH₄⁺ in surface water of two bays were 0.49 and 0.61, and 0.77 and 1.38 mg/L of NO₃⁻. The concentration of NH₄⁺ in the interstitial water of the two bays had a trend of gradual decrease from west to east, but NO₃⁻ had the opposite tendency. The maximum concentrations of NH₄⁺ in the interstitial water of the two bays were 5.88 and 4.64, and 3.58 and 7.18 mg/L of NO₃⁻. The exchangeable NH₄⁺ content in the sediment of Meiliang Bay had a trend of gradual decrease from north to south, but Gonghu Bay showed the reverse. The exchangeable NO₃⁻ content in the sediment of Meiliang Bay had a trend of gradual decrease from east to west, but a decreasing trend from north to south was observed in Gonghu Bay. The maximum concentrations of exchangeable NH₄⁺ were determined, and the values were 96.25 and 74.90 mg/kg, as well as NO₃⁻ with the values of 12.06 and 7.08 mg/kg. Chemical fertilizer and domestic sewage were the major sources of nitrate in surface water of Gonghu Bay, contributing 39.16 and 47.79%, respectively. Domestic sewage was the major source of nitrate in Meiliang Bay, contributing 84.79%. The denitrification process in Gonghu Bay was more apparent than in Meiliang Bay. Mixing and dilution processes had important effects on changing the concentration of nitrate transportation in the two bays.