Possi
bilities to perform pre- and post-seismic groundwater chemical comparisons on regional groundwater flow sy
stems are rare due to lack of data and o
bservations. 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 o
bservations. We analyzed 12 physiochemical parameters (SiO2, (NO3– + NO2– )-N, Fetotal, Mntotal, pH, F−, Cl−, SO4 2−, Na+, K+, Ca2+, and Mg2+) using self-organizing maps (SOM) com
bined 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 sy
stems), 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 o
bvious 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, SO4 2−, and Mg2+ concentration occurred locally due to soil leaching of contaminants or agricultural fertilizers through surface ruptures in recharge areas. Increase of SO4 2− content also originated from leaching of marine clay in coastal areas and possi
bly 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 ur
ban areas. Decrease of pH occurred in a few wells due to mixing of river water and different types of aquifer groundwater. Increase of Fetotal and Mntotal concentration possi
bly originated from leaching of marine clay
by liquefaction in coastal areas. However, in most cases the water chemistry changes
b(0,0,0); FONT-SIZE: 7.173pt; mso-spacerun: 'yes'">were subtle, thus not resulting in any groundwater quality deterioration of water supp