Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

Abstract

A 1D reactive transport model (RTM) is used to obtain a mechanistic understanding of the fate of phosphorus (P) in the saturated zone of two contrasting aquifer systems. We use the field data from two oxic, electron donor-poor, wastewater-impacted, sandy Canadian aquifers, (Cambridge and Muskoka sites) as an example of a calcareous and non-calcareous groundwater system, respectively, to validate our reaction network. After approximately 10 years of wastewater infiltration, P is effectively attenuated within the first 10 m downgradient of the source mainly through fast sorption onto calcite and Fe oxides. Slow, kinetic sorption contributes further to P removal, while precipitation of phosphate minerals (strengite, hydroxyapatite) is quantitatively unimportant in the saturated zone. Nitrogen (N) dynamics are also considered, but nitrate behaves essentially as a conservative tracer in both systems. The model-predicted advancement of the P plume upon continued wastewater discharge at the calcareous site is in line with field observations. Model results suggest that, upon removal of the wastewater source, the P plume at both sites will persist for at least 20 years, owing to desorption of P from aquifer solids and the slow rate of P mineral precipitation. Sensitivity analyses for the noncalcareous scenario (Muskoka) illustrate the importance of the sorption capacity of the aquifer solids for P in modulating groundwater N:P ratios in oxic groundwater. The model simulations predict the breakthrough of groundwater with high P concentrations and low N:P ratios after 17 years at 20 m from the source for an aquifer with low sorption capacity (b 0.02% w/w Fe(OH) 3 ). In this type of system, denitrification plays a minor role in lowering the N:P ratios because it is limited by the availability of labile dissolved organic matter.

rise and fall of dissolved phosphate in South African rivers

“…Phosphate is a highly charged and reactive ion that is strongly sorbed by most sediments and which is capable of binding with a number of metal cations, all of which limit its transport in groundwater. 44,45 In contrast, nitrates are more labile but can be removed by denitrification. 46 Nevertheless, phosphates can move through soil, albeit slowly [44][45] , and can be mobilised depending on soil and groundwater chemistry 47 .…”

“…44,45 In contrast, nitrates are more labile but can be removed by denitrification. 46 Nevertheless, phosphates can move through soil, albeit slowly [44][45] , and can be mobilised depending on soil and groundwater chemistry 47 . The implication is that while nitrogen can leach from soil where agricultural lands are fertilised or nutrient contaminated, phosphate leaching is likely to be more limited, but can occur, particularly when nutrient application has been prolonged.…”

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…High groundwater P concentrations (above 4 mg L À1 ) were also found in a residential area in the Lake Arendsee watershed reached associated leakages from on-site septic tank systems, the municipal sewage system, and house connection sewers . Leached wastewater and manure solution usually create anoxic conditions and have high organic matter, which are favor of inhibiting phosphate adsorption onto mineral surfaces, e.g., Al, Mn-and Fe-containing oxides and hydroxides, and calcite (Ptacek, 1998;Spiteri et al, 2007;Zanini et al, 1998) or onto solid organic carbon (Harman et al, 1996). Superimposed on these conditions is a long-lasting continuous supply of P, resulting in considerable P load transport to the vadose zone and consequent the groundwater for accumulating legacy P pools.…”

Geochemistry as an aid in archaeological prospection and site interpretation: current issues and research directions

“…Robinson et al, 1995;Gale et al, 2000;Hooda et al, 2001). In addition, due to the reactivity of P towards the soil exchange complex, soils that have received large quantities of P often display a decrease in sorption capacities compared with non-affected soils (Bö rling et al, 2004;Spiteri et al, 2007). Thus, modern P inputs may be inadequately retained in archaeological soil horizons.…”