9:50 AM - 10:05 AM
[AHW30-03] Groundwater Contamination by Bird Droppings at Takashima Island in Kojima Bay, Okayama Prefecture
Keywords:tiny island, granite, bird droppings, shallow groundwater pollution, nitrogen isotope, groundwater residence time
Takashima Island in Kojima Bay, Okayama Prefecture, is a small island with a circumference of about 1.7 km and a maximum elevation of about 31 m, consisting of granite and its weathered materials. The island has been uninhabited for more than 40 years. Until the early 2010s, a large colony of river cormorants existed on the southern part of the island, and the large amount of bird droppings from the cormorants caused the trees to die and the vegetation and forest floor to turn white over a wide area. The cormorants were subsequently removed from the island, but some of them have recently returned to form a colony again. However, the colony is much smaller than before.
There are three wells on Takashima Island: Jinja Well (3.5 m deep), Koya Well (3.2 m deep), and Yoshida Well (2.2 m deep), as well as several springs. Hydrological surveys were conducted in 2011 and 2012 to determine the effect of bird droppings on shallow groundwater quality in the island. An additional survey was conducted in 2024 to understand the changes in shallow groundwater quality after the release of river cormorants. It was found that NO3- concentrations were 957 mg/L (pH 3.4) in Jinja Well and 620 mg/L (pH 4.2) in Koya Well in November 2011, but decreased to 573 mg/L (pH 3.7) in the former well and 43 mg/L (pH 5.3) in the latter well in November 2024. In this presentation, we will introduce the changes in shallow groundwater quality on the island over the past decade or so. The hydrogeological structure and groundwater chemistry evolution of Takashima Island are discussed based on the results of geological investigations, soil tests, pumping tests, and isotope (oxygen, hydrogen, nitrogen, and sulfur) measurements.
There are three wells on Takashima Island: Jinja Well (3.5 m deep), Koya Well (3.2 m deep), and Yoshida Well (2.2 m deep), as well as several springs. Hydrological surveys were conducted in 2011 and 2012 to determine the effect of bird droppings on shallow groundwater quality in the island. An additional survey was conducted in 2024 to understand the changes in shallow groundwater quality after the release of river cormorants. It was found that NO3- concentrations were 957 mg/L (pH 3.4) in Jinja Well and 620 mg/L (pH 4.2) in Koya Well in November 2011, but decreased to 573 mg/L (pH 3.7) in the former well and 43 mg/L (pH 5.3) in the latter well in November 2024. In this presentation, we will introduce the changes in shallow groundwater quality on the island over the past decade or so. The hydrogeological structure and groundwater chemistry evolution of Takashima Island are discussed based on the results of geological investigations, soil tests, pumping tests, and isotope (oxygen, hydrogen, nitrogen, and sulfur) measurements.