Hydrochemical behaviour of an underground dammed limestone aquifer in the subtropics

Abstract

In an island area with limited surface water resources, the conservation of fresh groundwater resources is important for the maintenance of the water supply and irrigation system to be used in agricultural activity. In a limestone aquifer used for irrigation on a subtropical island, the progress of the dissolution behaviour of the aquifer and the effects of CO2 storage and chemical weathering of the aquifer must be understood to ensure sustainable groundwater use. In this study, hydrochemical behaviour of the Ryukyu limestone aquifer was investigated using the residence time of slowly flowing groundwater, which is contained by a 2.3‐km‐long underground man‐made dam constructed in 2005 located at the southern edge of Okinawa Island, Japan. Groundwater dating and horizontal flow velocity measurements indicate that the groundwater horizontal flow is slow in the underground dam area, whereas the saturation index (SI) for calcite indicates that the majority of the limestone exists in a supersaturated condition. The range of SI for gypsum was indicative of subsaturation, and the SI increased with the groundwater residence time, suggesting that the dissolution of CaSO4 tends to proceed slowly through the common ion effect. The dissolution of CaCO3, however, tends to be in suspension within about a residence time of 20 years. The apparent dissolution rate of Ca in the Ryukyu limestone aquifer in this area was estimated to be approximately 2.70–4.06 nmol cm−2 year−1, a value much lower than that of the early stages of the limestone dissolution. The estimated groundwater CO2 (gas) concentrations are two order high (about 0.3–7%) relative to atmospheric CO2 (about 0.04%), and the groundwater pH shows a tendency to decrease from 7.4 to 6.9 over the last approximately 20 years, indicating that the hydration reaction of CO2 occurs in the Ryukyu limestone aquifer. These findings imply that high CO2 produced in the subtropical zone is consumed slower than CO2 production during limestone dissolution, with the exclusion of the early stage of the chemical weathering process, and causes groundwater acidification in the underground dammed limestone aquifer.

Shotgun metagenomic analysis of the microbial communities and associated network of nitrogen metabolism genes in the Ryukyu limestone aquifer

“…The groundwater at site 2 showed a markedly high concentration of NO 3 -N (not less than 20 mg/L, Table 1) and thus was thought to be unsuitable for drinking water purposes because the drinking water standard in Japan is set to < 10 mg/L. In contrast, site 3 tended to have the lowest DO values throughout the year among the three sampling sites 54 . DO is considered an indicator of a denitri cation response and suggested that site 2 had a mild reducing environment 25 .…”

“…The topography of the southern part of Okinawa Island is characterized by the Pleistocene Ryukyu limestone terrace, which unconformably overlies the basement mudstone of the Miocene-to-Pliocene Shimajiri Group as the surface geology 54 . The Shimajiri mudstone layer has an impermeable basement, and the Ryukyu limestone layer, which is 20 to 80 m thick, serves as an aquifer 54 . These layers are transected by faults, and the Ryukyu limestone aquifer is separated into several permeable blocks by a conjugate fault system with strike directions 54 .…”

“…The Shimajiri mudstone layer has an impermeable basement, and the Ryukyu limestone layer, which is 20 to 80 m thick, serves as an aquifer 54 . These layers are transected by faults, and the Ryukyu limestone aquifer is separated into several permeable blocks by a conjugate fault system with strike directions 54 . This southern part of Okinawa Island has agricultural elds mainly in Itoman City, Yaese Town, and Nanjo City near Naha City, and sugarcane elds cover this area (Fig.…”

Temporal and Spatial Variation of Radon Concentrations in Environmental Water from Okinawa Island, Southwestern Part of Japan

“…Based on the radon attenuation between the measured radon concentrations in the water samples and the estimated radon concentrations in the pore water, the residence times in the Ryukyu limestone at Ukinju spring and Komesu spring were estimated to be 12 to 21 d and 12 to 19 d, respectively. Nakaya et al (2018) reported an average residence time in the Ryukyu limestone aquifer of 14-34 years based on the results of SF 6 concentrations in the spring and well water around the Komesu spring site [38]. The residence time estimated in this study is very limited because it was calculated based on observations at only one point in the catchment.…”