Building towards a conceptual model for phosphorus transport in lowland catchments

The release of P to surface water following P leaching from heavily fertilized agricultural fields to groundwater and the extent of P retention at the redox interphase are of major importance for surface water quality. We studied the role of biogeochemical and hydrological processes during exfiltration of groundwater and their impact on phosphorus retention in lowland catchments in the Netherlands. Our study showed that the mobility and ecological impact of P in surface waters in lowland catchments or polders like in the Netherlands is strongly controlled by the exfiltration of anoxic groundwater containing ferrous iron. Chemical precipitates derived from groundwater-associated Fe(II) seeping into the overlying surface water contribute to immobilization of dissolved phosphate and, therefore, reduces its bioavailability. Aeration experiments with Fe(II) and phosphate-containing synthetic solutions and natural groundwater showed that Fe(II) oxidation in presence of phosphate leads initially to formation of Fe(III) hydroxyphosphates precipitates until phosphate is near-depleted from solution. Continuing Fe(II) oxidation after depleting in dissolved phosphate results in the formation of Fe(III) oxyhydroxides. A field campaign on P specation in surface waters draining agricultural land showed, additionally, that in a large majority of surface water sampling locations the total-P concentration is strongly dominated by particle bound P. Sequential chemical extractions on suspended sediments samples revealed subsequently that iron-bound P was the most important particulate P fraction. Between 75 and 95% of the total-P concentration in the water samples was iron-bound particulate P. After the turnover of dissolved P to iron-bound particulate P, the P transport in catchments or polders is controlled by sedimentation and erosion of suspended sediments in the water body. Shear flow-induced surface erosion of sediment beds upon natural discharge events or generated by pumping stations is an important mechanism for P transport in downstream parts of catchments or polder outlets where flow velocities are high enough to exceed the critical shear stress for surface erosion. The flow velocities in headwaters like drainage ditches are generally much lower and not high enough to cause a bed shear stress that exceed the critical shear stress. We, therefore, hypothesize that floc erosion, which is defined as erosion at condition where the bed shear stress is less than the critical shear stress, and advective flow of exfiltrating groundwater through the bed sediment are major controls on the suspended sediment and particulate P dynamics in these headwaters.