Regional Mapping of Vertical Hydraulic Gradient Using Uncertain Well Data: A Case Study of the Toyohira River Alluvial Fan, Japan
- 1 1Division of Earth and Planetary Dynamics, Faculty of Science, Hokkaido University, Sapporo, Japan 2Geotechnical Department, Division of Environmental Engineering, Docon, Sapporo, Japan
- 2 Division of Earth and Planetary Dynamics, Faculty of Science, Hokkaido University, Sapporo, Japan
Abstract
Vertical hydraulic gradient (VHG) provides detailed information on 3D groundwater flows in alluvial fans, but its regional mapping is complicated by a lack of piezometer nests and uncertainty in conventional well data. Especially, determining representative depth of well screen in each well is problematic. Here, a VHG map of the Toyohira River alluvial fan, Sapporo, Japan, is constructed based on groundwater table elevation (GTE), using available well-data of various screen lengths and depths. The water-level data after 1988, when subway constructions are mostly completed in the city, are divided into those of shallow wells (≤20 m deep), and those of deep wells (>20 m deep). First, the GTE map is generated by kriging interpolation of shallow well data with topographic drift. Next, the individual VHG value of each deep well is calculated using its top, middle, and bottom elevations of the screen depths, respectively. The VHG maps of three cases are then obtained using neighborhood kriging. The VHG map of the bottom screen depths has proven most valid by cross-validation. The VHG map better visualizes that downward flows of groundwater are predominant over the fan. Positive area of VHG is mostly vanished around the fan-toe, indicating urbanization effect such as artificial withdrawals. A negative peak of VHG corresponds to recharge area, and is seen along the distinct losing section in the river. The negative peak also expands upstream to the fan-apex where a basement is suddenly depressed.
- C. F. Tolman, “Ground Water,” McGraw-Hill, New York, 1937.
- B. R. Rust, “Facies Models 2 Coarse Alluvial Deposits,” In: R. G. Warker, Ed., Facies Models, Geological Association of Canada, Toronto, 1979.
- G. Einsele, “Sedimentary Basins: Evolution, Facies, and Sediment Budget,” Springer, Heidelberg, 2000. doi:10.1007/978-3-662-04029-4
- C. W. Fetter, “Applied Hydrogeology,” Prentice-Hall, Upper Saddle River, 2001.
- S. G. Hu, T. Kobayashi, E. Okuda, A. Oishi, M. Saito, T. Kayaki and S. Miyazaki, “Alluvial Fans-Importance and Relevance: A Review of Studies by Research Group on Hydro-Environments around Alluvial Fans in Japan,” In: M. Taniguchi and I. P. Holman, Eds., Groundwater Response to Changing Climate, CRC Press, AK Leiden, 2010, pp. 131-139. doi:10.1201/b10530-12
- R. A. Freeze and J. A. Cherry, “Groundwater,” Prentice-Hall, Englewood Cliffs, 1979.
- C. J. Taylor and W. M. Alley, “Ground-Water-Level Monitoring and the Importance of Long-Term Water-Level Data,” US Geological Survey Circular 1217, US Geological Survey, Denver, 2001.
- W. A. McIlvride and B. M. Rector, “Comparison of Short- and Long-Screen Monitoring Wells in Alluvial Sediments,” Proceedings of the Second National Outdoor Action Conference on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods, Dublin, 1988, pp. 375-390.
- K. R. Rushton, “Groundwater Hydrology Conceptual and Computational Models,” John Wiley & Sons, West Sussex, 2003.
- E. Pardo-Igúzquiza and M. Chica-Olmo, “Estimation of Gradients from Sparse Data by Universal Kriging,” Water Resources Research, Vol. 40, No. 12, 2004. doi:10.1029/2004WR003081
- H. Daimaru, “Holocene Evolution of the Toyohira River Alluvial Fan and Distal Floodplain, Hokkaido, Japan,” Geophysical Review of Japan, Vol. 62A, 1989, pp. 589-603.
- Y. Sakata, “Geostatistical Reservoir Modeling of Trending Heterogeneity Specified in Focused Recharge Zone: A Case Study of Toyohira River Alluvial Fan, Sapporo, Japan,” PhD Dissertation, Hokkaido University, Sapporo, 2013.
- S. G. Hu, S. Miyajima, D. Nagaoka, K. Koizumi and K. Mukai, “Study on the Relation between Groundwater and Surface Water in Toyohira-Gawa Alluvial Fan, Hokkaido, Japan,” In: M. Taniguchi and I. P. Holman, Eds., Groundwater Response to Changing Climate, CRC Press, AK Leiden, 2010, pp. 141-158. doi:10.1201/b10530-13