| Nonpoint source pollution (NPSP) generally occurs: over a large area, over an intermittent period of time, from diverse sources, sporadically, and is often correlated with climatic events. Geographic Information System (GIS) is an advanced spatial-tool, commonly applied to the study of NPSP with high research significance and practical value. Integration of Blackland GRASS GIS and NPSP models on West Lake watershed was the main focus of this research from which the following relevant conclusions were obtained. The research of shallow groundwater in West Lake watershed had found: 1. All the water samples collected have cation abundances of Ca>Na>Mg>K>NH4 on an equivalent basis. Anion abundances are more variable. The ranking C1>N03>HCO3>S04 is the most abundant (27%), although there are 6 other rankings above 5%: Cl>N03>S04>HCO3 (14%), HCO3>Cl>N03>S04 (13%), HCO3>Cl>S04>N03 (10%), HCO3>N03>CI>S04 (9%), HCO3>N03>S04>CI (6%), ~f1 HCO3>S04>Cl>N03 (5%) 2. The groundwater district can be divided into two groups in the West Lake watershed. Group I locates at the northwest part of the watershed, is dominated with calcium-sulfate rich waters. The groundwater of Group 2 is dominantly calcium-bicarbonate waters. Unlike group 1, group 2 had a high concentration of bicarbonate (over 4 times), calcium and chloride (approximately 2 times), which reflected the average of EC-value nearly 2 times higher. 3. Concentrations of nitrate inside shallow groundwater ranged from <0.01 to 25.3 mg N/liter with a median of 8 mgN/liter. Nitrate contamination was discovered in a watershed-wide area, about 24% of the samples exceeded the MCL of 10 mg N/liter, and 35% of the sampling sites exceeded the MCL in at least one sampling procedures. 4. The spatial variance of nitrate in the shallow groundwater had showed a relatively high level of nitrate at the south of watershed (mainly near the Kuoshiban residential area), and at the north of watershed (mainly near the Xixialin residential area). The forest area had been found a relatively low concentration of nitrate in the shallow groundwater. 5. Among the three land uses, shallow groundwater in forested areas commonly had a low concentration of all the analyzed ions, especially for sodium, magnesium and chloride. In residential areas, a relatively high concentration of sodium and chloride were detected, which may originate mainly from household sewage effluent. Agricultural areas show a higher nitrate level than other two land use areas, relatively high concentrations of magnesium and sulfate were also found in the area, which need the further investigation to learn the reason. 6. Most of the groundwater samples had the low concentration of nitrite blow the detection limitation, but there were about 10% water samples exceeded the MCL of 0.02 mg N/liter, which had indicated a serious nitrite contamination in the watershed. Well N-38 had detected a high nitrite level of 1.50 mg N/liter with the land use type of paddy fields surrounded, which had a moderate nitrate concentration of 2.61-5.02 mgN/liter. It reflected an intensive denitrification on the site, which suggested more 103 consideration should be given to nitrite contamination together with nitrate contamination and methane emission from rice agriculture. 7. Among the three types of land use, agricultural area showed the highest concentration of nitrate in the...
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