5:15 PM - 6:30 PM
[AHW22-P11] Identification of enriched phosphate in groundwater: insights from distribution of phosphate oxygen isotope ratio in aquifer sediments
Keywords:Phosphorus, Groundwater
1. Introduction
Phosphorus (P) input through groundwater discharge plays a significant role in nutrient cycling and primary productivity in the coastal area (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems.
Recently, phosphate oxygen isotope ratio (δ18OPO4) has been used as a promising tool to elucidate the P cycling (Paytan and McLaughlin, 2012). Previous studies showed the possibility to evaluate P sources, metabolism by organism in some ecosystems. However, it is not clear whether δ18OPO4 is useful for evaluating the P cycling of in underground environment, because few research has applied the δ18OPO4 analysis for underground P cycling (Neidhardt et al., 2018). Determination of δ18OPO4 values in groundwater and lithological sediments would allow us to clarify the P biogeochemistry in underground environment and to predict δ18OPO4 values in groundwater.
In the present study, we aimed to clarify the biogeochemical P cycling in underground environment by comparing δ18OPO4 values in groundwater and lithological sediment from a boring core.
2. Material and method
Our study was conducted on the eastern shore of Lake Biwa in central Japan. The study area is covered by forest in upstream and rice field and built-up in downstream. To monitor shallow (1.8~10.0 m) and deep (24.7~27.5 m) aquifers, two monitoring wells were excavated by rotary drilling at The University of Shiga Prefecture.
The recovered 28 m-long sediment core was divided by 1 m and air-dried. The top 10 cm of each core were ground to powder using a multi-bead shocker (Yasui Kikai) with tungsten carbide beads to homogenize the samples and to enhance the reaction between extraction solution and the sample. The powdered sediment samples were stored at room temperature until analysis. The shallow and deep groundwater samples were collected in May 2019 by siphon technic with a silicon tube. Samples of the lake water and river water were collected near the monitoring wells.
To extract labile P, iron-bound P, authigenic P and detrital P in the sediment sample, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). To extract inorganic P in the PP of groundwater, the filter samples were immersed in 1 M HCl for 16 h. The P concentrations in the extractions and soluble reactive P (SRP) in the water samples were measured using the molybdenum-blue method.
The δ18OPO4 analysis was conducted on the groundwater, PP and sediment samples corresponding to shallow and deep aquifers using SEDEX extractions. DIP in the samples were converted to Ag3PO4 according to Tamburini et al. (2010). The δ18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature.
3. Result and discussion
The groundwater, especially in the deep aquifer, was characterized by high SRP concentration (0.74~6.78 μmol L-1) and low ORP value (-80~66 mV). The SRP concentration in groundwater was higher than those in the river water (0.33~0.73 μmol L-1) and in the lake water (<0.45 μmol L-1), indicating groundwater discharge is additional P source to the oligotrophic lake.
The δ18OPO4 values of DIP in shallow (15.1‰) was close to biological equilibrium value based on latest formula (Chang and Blake, 2015). Therefore, DIP in shallow groundwater may be heavily recycled by organisms. Also, PP in shallow groundwater, which had the similar δ18OPO4 value to DIP (15.9‰), was likely affected by biological recycling. In contrast, for deep groundwater, the δ18OPO4 values of DIP (17.4‰) and PP (11.8‰) differed from biological equilibrium value, which suggests the influence of other biological processes and/or P sources. In our poster, we will show the result of δ18OPO4 in sediment samples and discuss the P cycling in groundwater.
Reference
Chang, S. J. and Blake, R. E. (2015), Geochimica et Cosmochimica Acta. 150, pp. 314–329. doi: 10.1016/j.gca.2014.10.030.
Neidhardt, H. et al. (2018), Science of the Total Environment. 644, pp. 1357–1370. doi: 10.1016/j.scitotenv.2018.07.056.
Paytan, A. and McLaughlin, K. (2012), Handbook of Environmental Isotope Geochemistry. pp. 419–436. doi: 10.1007/978-3-642-10637-8.
Slomp, C. P. and Van Cappellen, P. (2004), Journal of Hydrology, 295(1–4), pp. 64–86. doi: 10.1016/j.jhydrol.2004.02.018.
Phosphorus (P) input through groundwater discharge plays a significant role in nutrient cycling and primary productivity in the coastal area (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems.
Recently, phosphate oxygen isotope ratio (δ18OPO4) has been used as a promising tool to elucidate the P cycling (Paytan and McLaughlin, 2012). Previous studies showed the possibility to evaluate P sources, metabolism by organism in some ecosystems. However, it is not clear whether δ18OPO4 is useful for evaluating the P cycling of in underground environment, because few research has applied the δ18OPO4 analysis for underground P cycling (Neidhardt et al., 2018). Determination of δ18OPO4 values in groundwater and lithological sediments would allow us to clarify the P biogeochemistry in underground environment and to predict δ18OPO4 values in groundwater.
In the present study, we aimed to clarify the biogeochemical P cycling in underground environment by comparing δ18OPO4 values in groundwater and lithological sediment from a boring core.
2. Material and method
Our study was conducted on the eastern shore of Lake Biwa in central Japan. The study area is covered by forest in upstream and rice field and built-up in downstream. To monitor shallow (1.8~10.0 m) and deep (24.7~27.5 m) aquifers, two monitoring wells were excavated by rotary drilling at The University of Shiga Prefecture.
The recovered 28 m-long sediment core was divided by 1 m and air-dried. The top 10 cm of each core were ground to powder using a multi-bead shocker (Yasui Kikai) with tungsten carbide beads to homogenize the samples and to enhance the reaction between extraction solution and the sample. The powdered sediment samples were stored at room temperature until analysis. The shallow and deep groundwater samples were collected in May 2019 by siphon technic with a silicon tube. Samples of the lake water and river water were collected near the monitoring wells.
To extract labile P, iron-bound P, authigenic P and detrital P in the sediment sample, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). To extract inorganic P in the PP of groundwater, the filter samples were immersed in 1 M HCl for 16 h. The P concentrations in the extractions and soluble reactive P (SRP) in the water samples were measured using the molybdenum-blue method.
The δ18OPO4 analysis was conducted on the groundwater, PP and sediment samples corresponding to shallow and deep aquifers using SEDEX extractions. DIP in the samples were converted to Ag3PO4 according to Tamburini et al. (2010). The δ18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature.
3. Result and discussion
The groundwater, especially in the deep aquifer, was characterized by high SRP concentration (0.74~6.78 μmol L-1) and low ORP value (-80~66 mV). The SRP concentration in groundwater was higher than those in the river water (0.33~0.73 μmol L-1) and in the lake water (<0.45 μmol L-1), indicating groundwater discharge is additional P source to the oligotrophic lake.
The δ18OPO4 values of DIP in shallow (15.1‰) was close to biological equilibrium value based on latest formula (Chang and Blake, 2015). Therefore, DIP in shallow groundwater may be heavily recycled by organisms. Also, PP in shallow groundwater, which had the similar δ18OPO4 value to DIP (15.9‰), was likely affected by biological recycling. In contrast, for deep groundwater, the δ18OPO4 values of DIP (17.4‰) and PP (11.8‰) differed from biological equilibrium value, which suggests the influence of other biological processes and/or P sources. In our poster, we will show the result of δ18OPO4 in sediment samples and discuss the P cycling in groundwater.
Reference
Chang, S. J. and Blake, R. E. (2015), Geochimica et Cosmochimica Acta. 150, pp. 314–329. doi: 10.1016/j.gca.2014.10.030.
Neidhardt, H. et al. (2018), Science of the Total Environment. 644, pp. 1357–1370. doi: 10.1016/j.scitotenv.2018.07.056.
Paytan, A. and McLaughlin, K. (2012), Handbook of Environmental Isotope Geochemistry. pp. 419–436. doi: 10.1007/978-3-642-10637-8.
Slomp, C. P. and Van Cappellen, P. (2004), Journal of Hydrology, 295(1–4), pp. 64–86. doi: 10.1016/j.jhydrol.2004.02.018.