Temporal characteristics of groundwater chemistry affected by the 2016 Kumamoto earthquake using self-organizing maps

Possibilities to perform pre- and post-seismic groundwater chemical comparisons on regional groundwater flow systems are rare due to lack of data and observations. The Kumamoto earthquake provides an unusual opportunity to improve the knowledge on earthquake hydrology and earthquake effects on hydrochemistry of groundwater due to a wealth of pre- and post-quake observations. We analyzed 12 physiochemical parameters (SiO2, (NO3-+ NO2-)-N, Fe-total, Mn-total, pH, F-, Cl, SO42-, Na+, K+, Ca2+, and Mg2+) using self-organizing maps (SOM) combined with hydrological and geological characteristics to improve the understanding of changes in groundwater chemistry after a major earthquake. The results indicate that the earthquake induced hydrological and environmental change via fault forming (Suizenji fault systems), liquefaction, rock fracturing, and ground shaking. These geological processes created rock fresh reactive surfaces, rock loosening, and enhancement of hydraulic conductivity. In turn, this lead to secondary processes in groundwater chemistry by advection, dilution, and chemical reaction. The most obvious indicator of hydrological and environmental change was from the increased dissolved silica content stemming from fracturing and Si-O bond cleavage in silicate rocks. Besides this, decreasing concentration of common ions (Cl-, F-, Na+, K+,Ca2+) was found due to dilution from mountain-side water release. Increase in (NO3-+ NO2-)-N, SO42-, and Mg2+ concentration occurred locally due to soil leaching of contaminants or agricultural fertilizers through surface ruptures in recharge areas. Increase of SO42- content also originated from leaching of marine clay in coastal areas and possibly sporadic deep crustal fluid upwelling. Increase in (NO3-+ NO2-)-N and Cl- content occurred from sewage water pipe breaks in the Suizenji fault formation in urban areas. Decrease of pH occurred in a few wells due to mixing of river water and different types of aquifer groundwater. Increase of Fe-total and Mn-total concentration possibly originated from leaching of marine clay by liquefaction in coastal areas. However, in most cases the water chemistry changes were subtle, thus not resulting in any groundwater quality deterioration of water supplies.