Assessment of P retention dynamics in tile drainage upon exfiltration of nutrient-rich groundwater in a marine clay polder

Abstract

The majority of the studies on phosphorus (P) immobilization in lowland catchments focus on the final mechanisms and pathway of phosphorus retention in surface water bodies, with little or no attention given to tile drainage and, mostly, regardless the effects of the landscape heterogeneity and the geochemical processes associated to it. Based on that, the aim of this research was to assess the P retention dynamics in the drains of a marine clay polder dominated by agricultural practices, by taking into account the heterogeneity of the surroundings. To do so, several components of the soil and water system of the study area were collected and analyzed for the main elements, including sequential chemical extraction procedures for iron (Fe) and P speciation of the sediment samples. The second part of this research consisted of geochemical modeling based on the fieldwork data in order to get a better insight into the potential degree of P immobilization in the drains. The results of the field work and laboratory analysis showed that Fe and phosphate concentrations were not only detectable but sometimes high in the drains, which can be attributed to the unfinished Fe oxidation as a result of the short residence time of drain water. Iron and phosphate concentrations also seemed to follow a large scale spatial pattern, mainly governed by the exfiltrating groundwater, which is rich in P and Fe. Nevertheless, the concentrations in the drains were sometimes larger than those found in groundwater and were highly variable in space. Such discrepancies may be ascribed to the heterogeneity of the parcel area and the subsequent geochemical transformations occurring in the shallow subsurface (i.e. pyrite oxidation), which are likely to be responsible for additional mobilization of Fe and phosphate to the drains. Additionally, phosphate and iron concentrations were inversely correlated with temperature and discharge, suggesting a possible effect of seasonality. However, these correlations were weak, indicating that temporal trends in drain water composition may be overshadowed by the large spatial variability. The sediment analysis pointed out that the ditch and drains particulate material holds nearly 20 to 80 times more phosphorus than in the geological sediment throughout the soil domain. From this particulate P (PP) content, nearly 81% was P bound to ferric iron particles (Fe-P) in the ditch, and more than 95% was Fe-P in the drains sediments. The calcium-bound P (Ca-P) fraction was of minor importance for the P retained in both drains and ditch sediments. Last but not least, the outcome of the geochemical modeling reinforced the findings that geochemical processes occurring in the shallow subsurface exert major influence in drain water composition and, thereafter, in P immobilization. The modelling simulations also revealed that P could be completely immobilized in nearly all the drains analyzed, for the two most extreme conditions, when the highest and the lowest P and Fe concentrations were found in solution. This outcome highlights the importance of subsurface drainage networks as a major sink of P in lowland catchments.