To understand the nitrogen sources and fate in the surface water of Tianjin, the concentrations of nitrogen and δ¹⁵N-NO₃⁻ were analyzed in different types of waters. Mostly, NO₃⁻ was the dominant species of DIN (Dissolved Inorganic Nitrogen), although NH₄+ was the main species in certain samples, such as sewage. The δ¹⁵N-NO₃⁻ values ranged from -5.5 to +28.6‰. The water chemical and isotopic results suggested that domestic sewage and agricultural activities were the two main sources of nitrate in surface waters. In addition, the nitrogen isotopic compositions were significantly influenced by nitrification, ammonia volatilization and denitrification.

speciation by sequential chemical extraction is important and necessary for REE research in surface waters (Yan et al., 1999).Many geochemical studies of REEs in rivers, estuaries, and oceans have been conducted and there is a wide range of REE data within the aquatic chemistry literature. However, the fundamental REE geochemistry of lakes is poorly documented Sholkovitz, 1993). Lakes are complex and dynamic biogeochemical, chemical, and physical systems that differ from rivers and oceans. Due to seasonal stratification and redox condition shifts, material exchange processes frequently occur between water, SPM, and bottom sediments in lake systems. These processes can markedly affect the chemical processes involving REEs in lake waters. Our knowledge of REE concentration and their fractionation behavior in terrestrial alkaline lakes is particularly limited, with few alkaline lakes having been investigated in this context (Johannesson and Lyons, 1994). One reason for this is the difficulties involving in analyzing the very low concentrations of dissolved REEs present in alkaline lakes. However, these lakes provide a useful opportunity to investigate and improve understandings of the mechanism of REE fractionation and behaviors in the aquatic systems. Many alkaline lakes Rare earth element geochemistry of waters and suspended particles in alkaline lakes using extraction and sequential chemical methods The concentrations of rare earth elements in the waters and suspended particles from two alkaline lakes in a carbonate drainage basin of the South China Karst region were determined via ICP-MS. Sequential chemical extraction experiments were conducted on the suspended matter to investigate water-particle interaction processes. Due to their high pH, the alkaline lakes have lower concentrations of dissolved rare earth elements than those reported for most other terrestrial surface waters of the world. Linear regressions between pH and the rare earth element (REE) concentrations over a wide pH range show a "three stage model," implying the geochemical behavior of dissolved REEs in surface waters is mainly controlled by pH. Shale-normalized patterns of dissolved REEs mainly show a marked enrichment in heavy REEs (HREE), while acid-extracted and organic REEs mainly show convex patterns and the residual fractions have flat patterns. Distinct positive La anomalies were observed in the Lake Aha and its river waters, which also exhibit a very high Mn concentration, indicating severe redox and scavenging conditions. Both negative and positive Ce anomalies were clearly observed in the dissolved phase and extracted fractions of suspended particulate matter. The positive correlation of Ce anomalies with Mn indicates that redox reactions control these Ce anomalies.

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