Desorption of phosphate on sandy sediments by silicate in groundwater

Abstract

Abstract Recent studies have shown that the global input of dissolved inorganic phosphorus (P) to the oceans through submarine groundwater discharge (SGD) is twice as high as the input through rivers. This estimation is based on a relatively high level of particle-reactive P (∼2.0 μM) observed globally in coastal saline aquifers, although the mechanism for the presence of such high P concentrations is not yet understood. In this study, we examined the desorption mechanism of P due to the elevated level of inorganic silicon (Si) by ion exchange, which could potentially increase the level of P in seeping groundwater in addition to biological remineralization. Since SGD occurs generally in sandy aquifers, sandy sediments were used in our laboratory experiments. The beaker experiments showed that the amount of P desorption increased from 0.3 to 2.5 μM as Si concentrations increased from 10 to 400 μM. However, nitrogen (N) desorption was not affected by Si levels under the same conditions. In addition, we performed column experiments simulating natural groundwater seepage by percolating seawater through sediment columns over 250 h. The results showed that P concentrations in seeping water increased up to 0.7–3.8 µM as Si (∼200 µM) in water was rapidly removed by sediments (total 55–80% in the initial 24 h). The removal of Si was found to be related to the presence of Mn (r2 = 0.97) and Fe oxides (r2 = 0.67) in sediments. In general, the concentrations of Si in coastal groundwater range from 30 to 400 μM. Thus, our laboratory results revealed that relatively high levels of Si in coastal saline groundwater could potentially be a hidden process of P release through SGD to the coastal oceans.