An Analytical Method for Assessing Recharge Using Groundwater Travel Time in Dupuit-Forchheimer Aquifers

Estimation of Groundwater and Spring Water Residence Times near the Coast of Fukushima, Japan

The massive Tohoku earthquake, colloquially known as "The 2011 Great East Japan Earthquake," occurred off the Pacific coast of Japan on March 11, 2011. The coastal area of the Tohoku region was severely affected by the tsunami, and the tsunami also caused severe damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP) releasing a large amount of radioactive material into the atmosphere and environment. Determining the residence time of groundwater is important for evaluating how long radioactive materials are present after nuclear accidents such as FDNPP and multiple methods are needed to account for mixing between old/young water that can occur. The apparent residence times of spring water, groundwater, and artesian well water samples obtained during 2014 to 2018 from the northern coastal area of Fukushima Prefecture were estimated using tritium (³ H), chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF₆ ) concentrations. The lowest ³ H concentrations were within the background range (1-5 TU) and were observed in artesian wells in Shinch, Soma and Minamisoma. The highest ³ H concentrations (∼8-15 TU) were observed near the FDNPP, and were probably affected by the accident following the 2011 earthquake. The average apparent residence time of spring water/shallow groundwater was 29 years based on the SF₆ concentration and exponential mixing model, whereas that of artesian well water was 62 years based on the CFC-12 concentration and piston flow model. Considering the relatively short apparent residence time of spring water/shallow groundwater, it is important to conduct continuous observations to understand the influence of the FDNPP accident.

Combinations of geoenvironmental data underline coastal aquifer anthropogenic nitrate legacy through groundwater vulnerability mapping methods

Groundwater quality is strongly dependent on land use. Past and current anthropogenic activities can lead to the diffusion of contaminants in aquifers. This diffusion can threaten the resource exploitation for decades, thereby endangering the ecological health of groundwater dependent ecosystems. Thus, groundwater stakeholders need methods for long-term management which integrate groundwater vulnerability. This study was conducted on the shallow alluvial aquifer of the groundwater-dependent Biguglia lagoon on Corsica Island, France. The aquifer is exposed to anthropogenic contamination for many decades with nitrate contamination legacy linked to agricultural activities, uncontrolled urbanization and sewage leakages. In most cases, vulnerability mapping is done in the objective of comparing groundwater situation regarding an on-going contamination process. But the question is still pending for aquifers where contamination is inherited from past practices or contaminations and where anthropogenic influences have changed through time. To propose an effective and innovative method for territorial management in Mediterranean alluvial aquifers, four index-based groundwater vulnerability mapping methods were tested and compared: two intrinsic vulnerability mapping methods (DRASTIC and SINTACS) and two specific vulnerability mapping methods (Modified-DRASTIC and SI), the latter integrating land use in the accuracy of groundwater vulnerability. Novelty is coming from the comparison between vulnerability maps and their application and validation in a hydrosystem affected by nitrate legacy-type contamination. The specific vulnerability mapping methods are more likely to represent the current pressures to which groundwater are subject. Thus, specific vulnerability methods such as the SI one revealed here very relevant to assess groundwater quality and to react retrospectively. The comparison between groundwater nitrate legacy and intrinsic groundwater vulnerability methods appeared also useful to define priority protection areas in long-term territorial management planning (EU Water Framework Direction). In this sense, the SINTACS method seems to be the more appropriate in the Mediterranean and alluvial context of this study.