Understanding Phosphorus Mobility and Bioavailability in the Hyporheic Zone of a Chalk Stream

Abstract

This paper investigates the changes in bioavailable phosphorus (P) within the hyporheic zone of a groundwater-dominated chalk stream. In this study, tangential flow fractionation is used to investigate P associations with different size fractions in the hyporheic zone, groundwater and surface water. P speciation is similar for the river and the chalk aquifer beneath the hyporheic zone, with 'dissolved' P (<10 kDa) accounting for~90% of the P in the river and >90% in the deep groundwaters. Within the hyporheic zone, the proportion of 'colloidal' (<0.45 μm and >10 kDa) and 'particulate' (>0.45 μm) P is higher than in either the groundwater or the surface water, accounting for~30% of total P. Our results suggest that zones of interaction within the sand and gravel deposits directly beneath and adjacent to river systems generate colloidal and particulate forms of fulvic-like organic material and regulate bioavailable forms of P, perhaps through coprecipitation with CaCO 3 . While chalk aquifers provide some degree of protection to surface water ecosystems through physiochemical processes of P removal, where flow is maintained by groundwater, ecologically significant P concentrations (20-30 μg/L) are still present in the groundwater and are an important source of bioavailable P during baseflow conditions. The nutrient storage capacity of the hyporheic zone and the water residence times of this dynamic system are largely unknown and warrant further investigation.

Groundwater conceptual models: implications for evaluating diffuse pollution mitigation measures

“…Similarly the complexities of phosphate sources (Holman et al, 2010) and mobility (Lapworth et al, 2011;Mellander et al, 2013) have been outside the scope of this study.…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…A number of studies have found that natural attenuation at the groundwater-surface water interface is a significant control on observed river nitrate concentrations (Allen et al, 2010;Lapworth et al, 2011;Pretty et al, 2006;Smith et al, 2009). Lapworth et al (2011) showed that ecologically significant P concentrations (20-30 μg/L) are present in the hyporheic zone in dissolved and colloidal forms.…”

“…Lapworth et al (2011) showed that ecologically significant P concentrations (20-30 μg/L) are present in the hyporheic zone in dissolved and colloidal forms.…”

“…humic and fulvic acids) dominate bulk groundwater DOC composition, and correlate with DOC (Baker and Curry, 2004;Baker and Lamont-BIack, 2001;Lapworth et al, 2009;Lapworth et al, 2008a;Lapworth et al, 2011;Rhymes et al, 2015). However, more labile protein-like dissolved organic matter (DOM) may also be present in detectable concentrations, particularly where rapid flow pathways in the subsurface are present, from surface sources such as landfills, rivers, urban waste water and agricultural inputs (e.g.…”

“…Additionally transport by the particulate phase is less important in many aquifers compared to surface water, however colloidal P can be a significant component of total P in the Chalk hyporheic zone and springs (e.g. Lapworth et al, 2011;Lapworth et al, 2013) The implementation of the Water Framework Directive (2000/60/EC) has changed the way many groundwater components are regarded as the importance of groundwater contribution to surface water must be taken into account. This has been shown to be important for both N and P (Bowes et al, 2009;Smith et al, 2009) and also has important controls on the temporal changes in DOC quality and bioavailability (Lapworth et al, 2009).…”