Geoscience Reference
In-Depth Information
The study area is divided uniformly into 12,000 spaced grids. Each grid
cell represents 1,127
1,228 m
2
. Lateral exchange of groundwater with the
region immediately adjacent to the study area is adjusted by the general
head boundary of the MODFLOW according to the water table difference
between inside and outside of the boundary.
Simulating stage is from 1980 to 1989, and the water heads in 1990
are used for subsequent prognoses. By reason of records of groundwater
pumping and injection go by months, the simulating stage is divided into
12 stress periods. Each stress period consists of two time steps.
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3. Hydrochemical Evolution of Groundwater
Long-term groundwater regime reveals that water table in phreatic aquifer
is controlled mainly by micro-geomorphology and rainfall, and rainwater
is the most important recharge source. The composition of hydrogen iso-
topes supports this view that tritium concentration of groundwater in 1980
(21.3-35.6 tritium units (TU)) is similar to that of rainwater (30 TU).
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Furthermore, three facts can be found from Fig. 1: (a) saltwater bodies
distribute inside transgression scope of early Holocene, (b) groundwater
samples with the highest TDS distribute in east coast, and (c) the ground-
water ridge is approximately accordant with a high belt of geomorphology
in south of estuary. These indicate that the saltwater in phreatic aquifer was
primitively formed from seawater in Holocene and was diluted by rainwater
subsequently.
From Fig. 1(b), the distribution of water quality in confined aquifers
is characterized obviously by: (a) increase of saltwater area and gradual
decrease of freshwater area from lower to upper; (b) expansion of saltwater
area from west to east region of the study area, and (c) decrease land-
ward of TDS values. Sedimentologic, paleontologic and geologic investiga-
tion demonstrated that there were 4-7 times transgression events during
Quaternary in the estuarine region and the intensity of these events tended
to increase from early Pleistocene to Holocene.
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,
7
It is thus to believe that
the temporal and spatial distribution of water quality was generally con-
trolled by the Quaternary sea-level fluctuation.
By analyzing the chemical composition of water samples, cation
exchange is discovered. It is characterized that the cation exchange capacity
(CEC), expressed by the value of Na/(Cl+Na),
8
increases along with TDS
decreasing, and the CEC of IV- and V-aquifer (average 0.65) is greater than
that of I-III aquifers (average 0.45).
