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[AHW24-P12] Comparison and Origin Estimation of Groundwater Dissolved Phosphorus in Coastal Agricultural Waters
Keywords:phosphorus, groundwater, stable isotope, redox conditions
Introduction
Phosphorus (P) is an essential element for all living organisms and can be a limiting factor for primary production. Previous studies have shown P input through groundwater discharge plays a significant role in nutrient cycling and primary productivity in the coastal area a (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems. However, the mechanisms of P formation in the groundwater system and the effects of human activities on it have not been fully clarified. In this study, we aimed to clarify the effects of lithology and human activity, especially agriculture, on the P dynamics in groundwater.
Material and method
Investigation was conducted in Osakishimo-jima (OSA) and Ikuchi-Jima (IKU), the Seto Inland Sea, Hiroshima Prefecture. The main industry on the islands is citrus production. The bedrock in OSA and IKU are rhyolite and granite, respectively. In OSA, we collected groundwater (20 sites), river water (2 sites) forest soil (3 sites), and orchard soil (4 sites). In addition, a 30 m long lithological sediment core was excavated by rotary drilling. The sediment core was divided by 1 m and powdered. In IKU, groundwater samples were collected from three monitoring wells by depths.
We analyzed soluble reactive phosphorus (SRP), ion concentrations, oxidation reduction potential (ORP), and phosphate oxygen isotope ratio (d18OPO4) in the groundwater samples. The d18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature. The d18OPO4 has been used as a promising tool to evaluate P sources and metabolism by organism in some ecosystems. (Paytan and McLaughlin, 2012). To extract labile P, iron-bound P, authigenic P and detrital P in the powdered sediment sample, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). The SRP concentration and d18OPO4 in the extracts were analyzed.
Result and discussion
The SRP concentration of groundwater in OSA was high in the shallow aquifer and low in the deep aquifer. In IKU, high SRP concentrations were observed in both shallow and deep aquifer. The ORP values in groundwater was lower on IKU than on OSA. The total inorganic P content (sum of each extraction fraction) in the lithological sediments in OSA were low in 0-20 m depths and high in 20-30 m depths and the main P fraction changed from iron-bound P to authigenic P. The spatial distribution of d18OPO4 in groundwater of OSA was lower in the upstream and higher in the downstream of the watershed. This result suggests that P with high d18OPO4 may be dissolved in the groundwater from upstream to downstream. In our poster, we will show the result of d18OPO4 values in sediment samples and discuss the P cycling in groundwater.
Reference
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.
Paytan, A. and McLaughlin, K. (2012), Handbook of Environmental Isotope Geochemistry. pp. 419–436. doi: 10.1007/978-3-642-10637-8.
Ruttenberg, K. C. (1992), Limnology and Oceanography, 37 (7), pp. 1460-1482. doi: 10.4319/lo.1992.37.7.1460.
Phosphorus (P) is an essential element for all living organisms and can be a limiting factor for primary production. Previous studies have shown P input through groundwater discharge plays a significant role in nutrient cycling and primary productivity in the coastal area a (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems. However, the mechanisms of P formation in the groundwater system and the effects of human activities on it have not been fully clarified. In this study, we aimed to clarify the effects of lithology and human activity, especially agriculture, on the P dynamics in groundwater.
Material and method
Investigation was conducted in Osakishimo-jima (OSA) and Ikuchi-Jima (IKU), the Seto Inland Sea, Hiroshima Prefecture. The main industry on the islands is citrus production. The bedrock in OSA and IKU are rhyolite and granite, respectively. In OSA, we collected groundwater (20 sites), river water (2 sites) forest soil (3 sites), and orchard soil (4 sites). In addition, a 30 m long lithological sediment core was excavated by rotary drilling. The sediment core was divided by 1 m and powdered. In IKU, groundwater samples were collected from three monitoring wells by depths.
We analyzed soluble reactive phosphorus (SRP), ion concentrations, oxidation reduction potential (ORP), and phosphate oxygen isotope ratio (d18OPO4) in the groundwater samples. The d18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature. The d18OPO4 has been used as a promising tool to evaluate P sources and metabolism by organism in some ecosystems. (Paytan and McLaughlin, 2012). To extract labile P, iron-bound P, authigenic P and detrital P in the powdered sediment sample, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). The SRP concentration and d18OPO4 in the extracts were analyzed.
Result and discussion
The SRP concentration of groundwater in OSA was high in the shallow aquifer and low in the deep aquifer. In IKU, high SRP concentrations were observed in both shallow and deep aquifer. The ORP values in groundwater was lower on IKU than on OSA. The total inorganic P content (sum of each extraction fraction) in the lithological sediments in OSA were low in 0-20 m depths and high in 20-30 m depths and the main P fraction changed from iron-bound P to authigenic P. The spatial distribution of d18OPO4 in groundwater of OSA was lower in the upstream and higher in the downstream of the watershed. This result suggests that P with high d18OPO4 may be dissolved in the groundwater from upstream to downstream. In our poster, we will show the result of d18OPO4 values in sediment samples and discuss the P cycling in groundwater.
Reference
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.
Paytan, A. and McLaughlin, K. (2012), Handbook of Environmental Isotope Geochemistry. pp. 419–436. doi: 10.1007/978-3-642-10637-8.
Ruttenberg, K. C. (1992), Limnology and Oceanography, 37 (7), pp. 1460-1482. doi: 10.4319/lo.1992.37.7.1460.