Source identification of groundwater phosphorus under different geological settings in the central Yangtze River basin

Abstract

• Diverse hydrochemical and geostatistical methods were used to identify P sources. • Groundwater P was mainly geogenic and little affected by anthropogenic activities. • Ca 2+ , Fe 2+ and NH 4 -N were key factors for groundwater P enrichment. • Ca-P minerals, OP and Fe-P minerals controlled P enrichment in different areas. • Determinants of geogenic P sources in groundwater were linked to regional geology. The contribution of geogenic phosphorus (P) in groundwater to surface water eutrophication has become a major concern for aquatic environment protection, but the differences in groundwater P sources in different geological settings are less concerned. In this study, three areas in the central Yangtze River basin under various geological conditions were selected for the identification of groundwater P sources and key factors affecting P enrichment, using hydrogeochemical and geostatistical methods. It was found that P in groundwater of middle and upper Pleistocene aquifers is mainly geogenic and is little affected by anthropogenic activities. In the lower reaches of the Han River (an area with phosphate mining upstream), the dissolution of phosphate minerals such as apatite from P-rich strata is the main source of groundwater P (feature importance of Ca 2+ 39%). Along the Jingjiang River (an area with widespread peat deposition in paleo-channels and ancient lakes), the mineralization of organic P (OP) is the dominant groundwater P source (feature importance of NH 4 -N 46%), with the reductive dissolution of iron (oxyhydr)oxides as a secondary source (feature importance of Fe 2+ 26%). In the western part of the Dongting Plain (an area with iron-rich sedimentary rocks and granites in the surrounding mountains), the reductive dissolution of iron (oxyhydr)oxides is the main source of P in groundwater (feature importance of Fe 2+ 34%), with mineralization of organic P as a secondary but important source (feature importance of NH 4 -N 25%). The differences in groundwater P sources in different subareas are clearly linked to specific geological conditions. This study provides new insights into the dependence of P source and enrichment in groundwater systems on regional geology and highlights the importance of using a wide range of research methods for understanding the extremely complex processes in the subsurface environment.