The distribution of ratings of index D is shown in Fig. 7. The depth to groundwater is shallower, i.e. 10 to 20 m, in west Kinmen such that most cells are rated 3 or 5; while the depth of groundwater table is considerable deeper, i.e. 20 to 30 m, for the hilly east Kinmen, the corresponding ratings are 1 and 2 for most cells.
As determined by using Eq. (2), the net recharge near the coastal area which is mostly sandy farmland or undeveloped zone ranges 80 – 100 cm per year and the index R was rated 8 to 9 (Fig. 8). The net recharge of agricultural zones with loamy soils that occupy the most area of central parts of east and west Kinmen is estimated to be around 50 – 70 cm. The rating of R of these area ranges from 6 – 7. Furthermore, for the townships where the a large portion of the ground surface is covered by concrete buildings or asphalt concrete, the net recharge is less than 40 cm so that the R rating is 4.
N
The boring logs collected in this study show the porous media of the west Kinmen aquifer are alluvial sands deposited by the streams that changed their courses numerous times in the past. The aquifer media ranges from medium to coarse sands and silty sands with rating of index A of 8 and 5, respectively (Fig. 9).
The aquifer of east Kinmen near the Taiwu Mountain consists of weathered sandstone and granite with a low A-rating of 1. The aquifer materials of the other part of east Kinmen are often sandy silt and sandy clay which correspond to a rating of 3.
N
The soil within less than 1 m below the ground surface of Kinmen is largely sands and silty loam. In the central part and coastal zones of east Kinmen the top soil is sand. The relatively high potential to allow
pollution to infiltrate into the aquifer leads to a high rating of index S of 9 (Fig. 10). The central-east of Kinmen are farmlands and protected natural reserve areas with sandy top soils of an S-rating of 8. The center of Kinmen with clay top soils was rated 1 for index S.
For west Kinmen with a rather flat terrain, the rating of index T is either 9 or 10 for most cells (Fig. 11).
For east Kinmen, the steeper slopes around Taiwu Mountain result in lower ratings of T.
Approximately 70% of the vadose zone of Kinmen consists of medium to coarse sands and weathered sandstone with considerable ability to allow vertical migration of pollutants with little retardation. These areas are assigned with a rating of index I of 6 (Fig. 12). The areas near the central part of Kinmen with silty or clayey soils in the vadose zone are assigned a low I-rating of 1 and 2.
N
The majority of the aquifer media in northern west Kinmen is sandy or silty soils of medium hydraulic conductivity ranging from 10-6 to 10-4 cm/s. As a result, the cells in these areas were given rating of index C from 4 to 6 (Fig. 13). For southern part of west Kinmen, the aquifer material is mostly medium to coarse sand, the I-rating is significantly higher and range from 8 to 10. For Taiwu Mountain region in east Kinmen whose aquifer consists of porous media of lower hydraulic conductivity, such as clayey soils or weather rock, the C-rating ranges from 1 to 2.
The total DRASTIC score of the 96 cells are shown in Fig. 14. There are 10 cells with DRASTIC score less than 90 all located in the region of Taiwu Mountain which extends towards the center of the island.
There are 13 cells given DRASTIC scores higher than 140, among which 5 higher than 160. Among the cells of top 15 DRASTIC score, only 2 are in east Kinmen. The frequency distribution of the DRASTIC score is shown in Fig. 15.
The southwestern part of the island obtained the highest scores. It happens to be the most developed area of Kinmen where the county government and business dictrict are located. The pumping wells of the water treatment plant are all located in this region. The Kinmen Liquor Company near the southwestern tip of Kinmen also draws groundwater from this aquifer. On the other hand, since the ground surface is steeper, aquifer material is less permeable, and the groundwater table is deeper, the vulnerability of aquifer to pollution of east Kinmen is significantly lower.
In addition to the vulnerability of aquifer to pollution due to hydrogeological settings, various types of human activities also give rise to potential of groundwater pollution of different degrees. The density of population and economic activities in southwestern Kinmen further increases the risk of groundwater pollution.
0 2 4 6 8 10 12 14
West Kinmen East Kinmen
Numberof 1 km x 1 km cell
DRASTIC Score
Fig. 15 Frequency Distribution of DRASTIC Score
Table 9 Cells of High DRASTIC Scores and Other Critical Concerns Cell
Number
DRASTIC Score
Land Use Groundwater Related
Characteristics
Priority for Monitoring Well
1 165 Harbor Groundwater downstream zone,
Coastal zone
2 138 Harbor Groundwater downstream zone,
Coastal zone
9 145 Protected Coastal Zone Groundwater downstream zone, Coastal zone
11 73 Liquor Factory, Residential Area Groundwater upstream zone, Coastal zone
16 108 Business District Major groundwater pumping zone
20 170 National Park Groundwater middle-stream zone, Coastal zone
21 133 Agriculture, Residential Area Major groundwater pumping zone
23 140 Agriculture, Residential Area Major groundwater pumping zone 24 142 Business and Residential Area Major groundwater pumping zone
25 152 Business and Residential Area Major groundwater pumping zone 26 172 Agriculture, Residential Area Major groundwater pumping zone 27 172 Agriculture, Residential Area Groundwater upstream zone,
Coastal zone
28 164 National Park Groundwater middle-stream zone,
Coastal zone
29 138 Agriculture, Residential Area Groundwater downstream zone, Coastal zone
32 147 Liquor Factory, Agriculture, Government Agencies
Groundwater downstream zone 33 150 Cemetery, Agriculture,
Residential Area
Groundwater downstream zone, Major groundwater pumping zone
34 138 Agriculture, Residential Area Groundwater downstream zone,
Coastal zone
35 147 Agriculture Groundwater upstream zone,
Coastal zone
37 101 Liquor Factory, National Park No groundwater use
40 81 Air Port, Scenic Area No groundwater use
54 145 Agriculture, Public Pumping Wells
Groundwater upstream zone, Coastal zone
60 152 Residential Area, Public
Pumping Wells
Groundwater upstream zone 64 85 Agriculture, Residential Area No groundwater use 66 120 Agriculture, Public Pumping
Wells
Groundwater middle-stream zone
70 67 Business District Groundwater upstream zone
78 110 Industry, National Park Groundwater middle-stream zone
86 107 Industry Groundwater downstream zone
93 135 Harbor, Agriculture Groundwater downstream zone, Coastal zone
96 134 Agriculture Coastal zone
Based on the result of DRASTIC analysis, regions of high vulnerability of groundwater to human impact
could be identified. However, when selecting locations to install groundwater monitoring wells, the direction of groundwater flow and the nature and intensity human activities must also be taken into account. Among the cells with highest DRASTIC scores, the cells in which pumping wells of water supply companies are located, the cells with potential source of pollution or important groundwater use, and the cells of major recharge area are listed in Table 9. The cells determined to have priority for installing groundwater
monitoring wells are also indicated. A total of 15 groundwater monitoring wells are proposed to be installed to establish the groundwater quality monitoring network (Fig. 16).
Out of the 15 proposed monitoring wells, 10 are located in west Kinmen where groundwater is the major source of water supply, to safeguard the quality of water for public and private use. On the other hand, the other 5 monitoring wells are proposed to be installed in east Kinmen to establish long-term groundwater quality database. The cells where groundwater is extracted in a large amount, such as 16, 21, 22, 23, 24, 25, 26, 33, 54, 60, and 66, groundwater monitoring wells are proposed to be installed in cells upstream, such as 16, 23, 32, 33, 54, and 66.
Fig. 16 Locations of Fifteen Proposed Groundwater Monitoring Wells
5. Results and Discussion
In this study, hydrogeologic data of the aquifer of Kinmen were collected and mapped to obtain ratings corresponding to the seven DRASTIC indices. The resultant total DRASTIC scores reveal that the aquifer of west Kinmen, where groundwater is the major source of water supply, is also more vulnerable to pollution induced by human activity than east Kinmen. Since Kinmen is very limited in size and its terrain is rather flat there is no clearly defined upstream recharge zone of groundwater. In other words, the entire west Kinmen is simply the recharge zone. It is thus even more critical to regulate land use and human activities in order to prevent pollution infiltrating into the aquifer.
A total of 15 groundwater monitoring wells are proposed as an initial effort to establish a groundwater quality monitoring network. The locations are selected not only based on DRASTIC score but also taking into account the direction of groundwater flow, the land use pattern, and the potential of pollution by human activities.
Furthermore, the study also shows that DRASTIC is a very simple and straight forward tool for evaluating vulnerability of aquifer. The rating of indices based on relative magnitude of the hydrogelogic parameters is a very powerful tool. Even without accurate experimental data on properties such as hydraulic conductivity, their values can still be obtained by using general values for the basic soil types.
Ratings based on these data can still yield discrete ratings ready to be used to compute the total score. If coupled with more detailed data on the potential of discharge of pollutants into the aquifer, a risk assessment and management system for aquifer can be developed based on DRASTIC.
References
[1] Kinmen Water Supply Company, 1998, Water Resources Planning of Kinmen, 256 p. (in Chinese) [2] Kinmen Water Supply Company, 1995, Proposal for Groundwater Monitoring Network of Kinmen, 135 p.
(in Chinese)
[3] U.S. Environmental Protection Agency, 1987, DRASTIC:A Standardized System for Evaluating Ground Water Pollution Using Hydrogeologic Settings, EPA Number: 600287035, 643 p.
[4] Kinmen County Government, 1987, Project Report on Planning of Kinmen Water Right Registration Management System, 344 p. (in Chinese)
[5] ASCE, 1993, Design and Construction of Urban Stormwater Management Systems (Manual of Practice No. 77), 724 p.
[6]Bear, J., 1988, Dynamics of Fluids in Porous Media, Dover Publications, 784 p.