The Choushui River alluvial fan is located in southwestern Taiwan (Fig. 4a). The
groundwater catchment of this region is surrounded by the Taiwan Strait (to the west) and
the Central Mountain Range (to the east), and it is broadly partitioned into the proximal
fan, mid-fan, and distal fan areas. Two major rivers flow through the alluvial fan: the
Choushui River to the north and the Peikang River to the south.
On the basis of accelerator mass spectrometry 14C (radiocarbon isotope) dating of
mollusk shells in core samples of the Choushui River alluvial fan (Central Geological
Survey, 1999), the geologic ages of core samples in the distal fan to the mid-fan could be
grouped as follows: 2,931 to 5,364 yr, 7,090 to 9,230 yr, and older than 36,400 yr.
Sedimentary formation was in the late Quaternary period and extended to a depth of
approximately 300 m (Central Geological Survey, 1999). The shallow aquitard with
depths of 0 to −55 m was deposited 3–9 ka ago during the Holocene transgression, the
middle aquitard with depths of −100 to −155 m was deposited 35–50 ka ago, and the deep
aquitard was deposited 80–120 ka ago. On the basis of subsurface hydrogeological
analysis up to a depth of approximately 300 m, the hydrogeological environment is
divided into four types of aquifers (Fig. 5): aquifer 1 with depths of 0–103 m, aquifer 2
with depths of 35–217 m (divided into 2-1 and 2-2 with depths of 35–155 m and 100–
217 m, respectively), aquifer 3 with depths of 140–275 m, and aquifer 4 with depths
exceeding 271 m (Central Geological Survey, available from
http://hydro.moeacgs.gov.tw/). Fig. 5 shows that mud layer and gravel or sand bed cross
from the top layer to the bottom layer; rock strata and mountain layer distribute from the
inland area to the coastal area.
The watershed area of the Choushui River is 3,156.9 km2 (Water Resources Agency,
available from http://www.wra.gov.tw/). The annual average water quality of the
Choushui River in 2015 is as follows: pH = 8.33, electrical conductivity (EC) = 506
μmho/cm, dissolved oxygen (DO) = 8.58 mg/L, NH4+ = 0.13 mg/L, total organic carbon
(TOC) = 1.4 mg/L, NO3− = 3.89 mg/L, Mn = 0.482 mg/L, and As = 0.0031 mg/L
(Environmental Protection Administration, available from http://www.epa.gov.tw/). The
annual precipitation in the Choushui River alluvial fan is 1,972 mm in 2015, mostly
concentrated in April to October (rainy season), and the historical yearly rainfall averages
2,366 mm (Water Resources Agency, available from http://gweb.wra.gov.tw/wrhygis/).
The amount of irrigation area of the Choushui River alluvial fan is 47,680 ha, and the
yearly irrigation water is 782.31 million ton (Water Resources Agency, available from
http://www.wracb.gov.tw/). The annual average temperature monitored by near climate
station is 24.3°C (Central Weather Bureau, Taiwan, available from
http://www.cwb.gov.tw/). The main land use of the Choushui River alluvial fan is for
agriculture, including rice cropping and upland farming, accounting for 60%, whereas the
main land use in the coastal area is for aquaculture (Fig. 6; Environmental Protection
Administration, 2014). The amount of fertilizer application in Taiwan is 347,039 ton,
including the N-containing fertilizers of 182,412 ton (Table 1; Council of Agriculture,
available from http://www.afa.gov.tw/). The average percentage of sewage permeating to
the Choushui River alluvial fan is 26.11% (Construction and Planning Agency, available
from http://www.cpami.gov.tw/). Hsu et. al (2013) reported that pumpage for
non-irrigation or non-irrigation purposes was regarded as known but illegal pumping was not
accounted for in the Choushui River alluvial fan. Recharge sources including rainfall,
rivers, boundary inflow, and groundwater irrigation have not been individually accounted
for in previous studies. However, Hsu et. al (2015) used groundwater storage hydrograph
and isotope analysis to estimate the pumpage and recharge of groundwater in the
Choushui River alluvial fan, and the result showed that the amount of yearly pumpage
for irrigation averaged 1.49 billion ton in 2012 to 2014, whereas that for non-irrigation
was 0.867 billion ton. The amounts of yearly average recharge from rainfall, rivers,
boundary inflow, and groundwater irrigation were 0.796, 0.682, 0.879, and 0.377 billion
ton, respectively. The yearly groundwater loss averaged 0.485 billion ton.
As, NO3−
, and NH4+ are the target contaminants in the groundwater of the Choushui
River alluvial fan. To frame a sound policy for remediation of groundwater contamination,
it is crucial to determine the sources of contaminants and understand their biogeochemical
cycling. The highest As concentration in sediments of the shallow aquifer of the Choushui
River alluvial fan was found at a depth of approximately 50 m, where the deposits of the
Holocene transgression are located (Liu et al., 2006). In 35% of the monitoring wells in
the Choushui River alluvial fan, the As concentrations exceeded the World Health
Organization guideline of 0.01 mg/L (Agricultural Engineering Research Center, 2012),
with the highest As concentration being 0.96 mg/L (Agricultural Engineering Research
Center, 2010). Lu et al. (2010) reported that the major As sinks and sources are As-bearing
iron minerals and As-bearing sulfides, and authigenic framboidal pyrite commonly
occurred in sediment of the Choushui River alluvial fan. Furthermore, in 76% of the
monitoring wells, NH4+ concentrations exceeded the quality standard (0.1 mg/L)
specified by the Taiwan Environmental Protection Administration for drinking water
sources (Agricultural Engineering Research Center, 2012). The concentrations of NO3−
and NH4+ in the groundwater of the Choushui River alluvial fan ranged from not detected
(ND) to 9.08 mg/L and from 0.02 to 15.6 mg/L, respectively (Kao et al., 2011). High
concentrations of NO3−
were found in the proximal fan, whereas high concentrations of
NH4+ were mostly detected in the distal fan (Agricultural Engineering Research Center,
2012). Notably, NH4+ concentrations of shallow wells were greater than those of deep
wells (Liu et al., 2003; Wang et al., 2007). The heterogeneous vertical distribution of
NH4+ concentrations may be attributed to the frequent local agricultural use of N
fertilizers or manure. Because groundwater pumping for crop irrigation is ubiquitous in
the local region, overpumping of groundwater and anthropogenic activities have led to
land subsidence and other adverse effects on the local environment.
The spatial distribution of NO3−, NH4+ contamination, and high As concentrations
which occurred in the Choushui River alluvial fan may not be affected by a single process.
They may be governed by multiple geochemical processes, including either the
co-precipitation or the adsorption of the reduction products, or both, that control the
mobilization of As into the reductive groundwater.
Table 1. Amount of fertilizer consumptions in Taiwan (modified from Council of Agriculture, available from http://www.afa.gov.tw/)
Fertilizer types Consumption amount (tons)
Consumption rate of elements in fertilizers Total
Chemical fertilizers 1,010,722 347,039 182,412 65,039 99,588
ammonium sulfate 144,802 30,408 30,408 - -
compound fertilizer 679,091 251,264 117,483 53,648 80,133
others fertilizers 14,839 - - - -
Fig. 4. (a) Study area and the division of the Choushui River alluvial fan into different fan regions. (b) Sampling locations of wells in the study area.
Fig. 5. Conceptual hydrogeological profile of the aquifer system in the Choushui River alluvial fan (modified from Central Geological Survey, 1986).
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mud layer (aquitard) groundwater flow line Mountain layer
rock strata
Sea Level
stream outlet
(a) (b)
Fig. 6. Schematic for land use in the Choushui River alluvial fan (modified from
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2590000