Po
ssibilitie
s to perform pre- and po
st-
sei
smic groundwater chemical compari
son
s on regional groundwater flow
sy
stem
s are rare due to lack of data and ob
servation
s. The Kumamoto earthquake provide
s an unu
sual opportunity to improve the knowledge on earthquake hydrology and earthquake effect
s on hydrochemi
stry of groundwater due to a wealth of pre- and po
st-quake ob
servation
s. We analyzed 12 phy
siochemical parameter
s (SiO2, (NO3– + NO2– )-N, Fetotal, Mntotal, pH, F−, Cl−, SO4 2−, Na+, K+, Ca2+, and Mg2+) u
sing
self-organizing map
s (SOM) combined with hydrological and geological characteri
stic
s to improve the under
standing of change
s in groundwater chemi
stry after a major earthquake. The re
sult
s indicate that the earthquake induced hydrological and environmental change via fault forming (Suizenji fault
sy
stem
s), liquefaction, rock fracturing, and ground
shaking. The
se geological proce
sse
s created rock fre
sh reactive
surface
s, rock loo
sening, and enhancement of hydraulic conductivity. In turn, thi
s lead to
secondary proce
sse
s in groundwater chemi
stry by advection, dilution, and chemical reaction. The mo
st obviou
s indicator of hydrological and environmental change wa
s from the increa
sed di
ssolved
silica content
stemming from fracturing and Si-O bond cleavage in
silicate rock
s. Be
side
s thi
s, decrea
sing concentration of common ion
s (Cl−, F−, Na+, K+, Ca2+) wa
s found due to dilution from mountain-
side water relea
se. Increa
se in (NO3– + NO2– )-N, SO4 2−, and Mg2+ concentration occurred locally due to
soil leaching of contaminant
s or agricultural fertilizer
s through
surface rupture
s in recharge area
s. Increa
se of SO4 2− content al
so originated from leaching of marine clay in coa
stal area
s and po
ssibly
sporadic deep cru
stal fluid upwelling. Increa
se in (NO3– + NO2– )-N and Cl− content occurred from
sewage water pipe break
s in the Suizenji fault formation in urban area
s. Decrea
se of pH occurred in a few well
s due to mixing of river water and different type
s of aquifer groundwater. Increa
se of Fetotal and Mntotal concentration po
ssibly originated from leaching of marine clay by liquefaction in coa
stal area
s. However, in mo
st ca
se
s the water chemi
stry change
s
style="FONT-FAMILY: CharisSIL; COLOR: rgb(0,0,0); FONT-SIZE: 7.173pt; mso-spacerun: 'yes'">were subtle, thus not resulting in any groundwater quality deterioration of water supp