Groundwater flow and transport and potential sources of groundwater nitrates in the Ryukyu Limestone as a mixed flow aquifer in Okinawa Island, Japan

Abstract

We investigated the groundwater flow and the transport and potential source of groundwater nitrates in the typical karst setting of the Ryukyu Limestone aquifer in the southern part of Okinawa Island, Japan. Analysis of groundwater hydrographs indicated that this is a ''mixed flow'' aquifer with the coexistence of slow diffuse flow in the matrices and rapid conduit flow in the caves and caverns. This relationship is indicated by the travel time of groundwater flow: 70 days in the matrices of the aquifer and 6 days through the caves and caverns. The conduit flow system was also confirmed by the distribution of relatively low concentrations of 222 Rn near caverns. The sampling sites were categorized into upland field (UF) type and residential area (RA) type according to the land-use ratio on the upstream side with a 600-m influential radius, and cave and cavern (CC) type according to the hydrogeologic setting near two large caverns, even though the CC type should be categorized as the UF type from the viewpoint of land use. Cross plots of NO 3 -N versus SO 4 2-showed that the predominant source of UF groundwater nitrates was chemical fertilizer. A difference was observed in average d 15 N values between UF (8.9%) and RA (10.0%). On the other hand, the average d 15 N value for CC (10.5%) was similar to that for RA, indicating that CC nitrates were not related to the surrounding land use. This phenomenon is considered as evidence that CC groundwater nitrates were carried by rapid groundwater flow through caves and caverns from residential areas located higher upstream compared to the influential areas. According to previous studies, animal and human waste was considered the predominant sources of RA and CC groundwater nitrogen. The contribution ratio of chemical fertilizer (R CF ) was calculated using mass balance equations under assumed predictability. There was a relatively high correlation between the rate of upland areas and of residential areas and R CF . Average R CF for UF, RA and CC was 41, 27, and 25%, respectively.

Hydrochemical behaviour of an underground dammed limestone aquifer in the subtropics

“…The anthropogenic source of NO 3 − and SO 4 2 − in the groundwater in the study area has been reported to be chemical fertilizer, (NH 4 ) 2 SO 4 , used in the catchment (e.g., Yoshimoto et al, ; Yoshimoto et al, , ; Nakano et al, ; Yasumoto, Hijikawa, Nakaema, Towata, & Nakano, ). NO 3 − and SO 4 2 − are formed by the dissolution and nitrification of (NH 4 ) 2 SO 4 , according to Equation (e.g., Kondo, Tase, & Hirata, ): Because H + ions form according to Equation , limestone can dissolve.…”

“…The Shimajiri mudstone layer has an impermeable basement, and the Ryukyu limestone layer, which is 20 to 80 m thick, serves as an aquifer. These layers are transected by faults, and the Ryukyu limestone aquifer can be separated into several permeable blocks by a conjugate fault system with strike directions showing predominantly NE-SW and NW-SE orientations; the stepwise geological structure is shown in Figure 1 Imaizumi, Okushima, Shiono, Takeuchi, & Komae, 2002;Sagae et al, 2012;Yoshimoto et al, 2007;Yoshimoto, Tsuchihara, Ishida, & Imaizumi, 2011a, 2011b. The blocks are characterized by their double porosity, a matrix-and-fracture system, and caves.…”

“…Figure 9a,b shows the relationships between SO 4 and Cl content of the groundwater in 2011 and in 2015 with those of Ryukyu limestone (Tokuyama, 1986), air deposits (Tohyama, 1983), and general seawater (Nozaki, 1997 do not include seawater, as shown by Figure 9a,b. The anthropogenic source of NO 3 − and SO 4 2− in the groundwater in the study area has been reported to be chemical fertilizer, (NH 4 ) 2 SO 4 , used in the catchment (e.g., Yoshimoto et al, 2007;Yoshimoto et al, 2011aYoshimoto et al, , 2011bNakano et al, 2013;Yasumoto, Hijikawa, Nakaema, Towata, & Nakano, 2013). NO 3 − and SO 4 2− are formed by the dissolution and nitrification of (NH 4 ) 2 SO 4 , according to Equation 10 (e.g., Kondo, Tase, & Hirata, 1997): Figure 9c,d); groundwater nitrate nitrogen (NO 3 -N) pollution was first reported in 1990 (Tokuyama et al, 1990).…”

Examination of the Effectiveness of Controlled Release Fertilizer to Balance Sugarcane Yield and Reduce Nitrate Leaching to Groundwater

“…When surface water is scarce, the water requirements for domestic and agricultural purposes are fulfilled primarily from GW [21]. For instance, on Miyako Island in Okinawa, drinking water availability depends on the rainwater accumulated underground [22,23].…”

Digital Filters to Eliminate or Separate Tidal Components in Groundwater Observation Time-Series Data

“…In such areas, strategies for the sustainability and development of agriculture, along with the incorporation of appropriate groundwater resource management are needed (Van der Velde et al 2007, Nawa & Miyazaki 2009, Ishida et al 2011, Yoshimoto et al 2011, Duncan 2012, Kobayashi & Koda 2012, Baharuddin et al 2013, Koda et al 2014. Shirahata et al (2014) estimated hydraulic properties of an unconfined aquifer on an island where groundwater development was desired for agricultural use.…”