The achievable reliability in modeling fluxes and transport in groundwater basins would be greatly enhanced by a practical method for reliable basin-scale aquifer characterization. Where it is not practical to conduct pumping tests to induce stresses throughout a basin stretched over hundreds of square kilometers, the downstream passage of river hydrographs create aquifer stresses that can be used. The large area of influence, frequency of occurrence, and spatial mitigation of the excitation make river stage variation an ideal energy source for use in basin-scale hydraulic tomographic surveys to map aquifer heterogeneity where aquifer responses are monitored. Previous work has demonstrated the viability of a 2D depth-average coupled river and groundwater model for groundwater basin characterization and this new project is now engaged in expanding on this past work to develop a 3D model for aquifer characterization. The approach is being tested and validated through application to the intensely instrumented Cho-Shui River alluvial fan in central western Taiwan. The method starts from an immense amount of data to determine the stress induced by river-stage variation at various locations sequentially and aquifer responses recorded in numerous wells. It then employs a stochastic parameter estimating procedure using geo-statistically-based methodology to estimate parameters for representing a heterogeneous aquifer. The approach imports the tomography paradigm by recognizing that the recurrent nature of temporally and spatially variable precipitation-driven hydrologic processes provides non-fully overlapping information for estimating aquifer hydrogeologic parameters. The work is targeted to have broad impacts through greater understanding of the interactions between rivers and groundwater basins that would support water resources management and greater environmental protection through conjunctive water use. Practitioners will also greatly benefit by having a non-invasive technology for characterizing basin hydrologic heterogeneity and the uncertainty associated with the characterization. They will also gain a valuable tool for use in evaluating the effects of climate change on groundwater resources. An effort will be made to publicize and explain the methodology through communications that would facilitate applying these contributions.
Normal 0 false false false EN-US ZH-CN AR-SA We hypothesize that river stage fluctuation induces pressure fluctuations in aquifers adjacent to the river, these fluctuations then propagate throughout the entire aquifers, and they are modified by aquifers heterogeneity. The groundwater level fluctuations recorded in wells distributed over the entire aquifers therefore are tantamount to a snapshot of the aquifer heterogeneity with a light source at a location along the river. As the river stage fluctuation migrates downstream, continuous records of groundwater level fluctuations are analogous to continuous snapshots of heterogeneity with the light source moving downstream. Synthesizing all these snapshots taken at different locations and perspectives leads to a detailed spatial distribution of aquifer heterogeneity. This is the river stage tomography. To test this hypothesis, we first analyzed the cross correlation between the groundwater level and river stage data collected in an alluvial fan. A physically based 3-D surface-subsurface hydrologic model was also developed to numerically simulate responses of heterogeneous aquifers in a groundwater basin due to migrating river stage fluctuations. Results of the cross correlation analysis of both field and numerically simulated data demonstrated that the groundwater level fluctuations in a large-scale groundwater basin indeed are related to the river stage fluctuation, and the relation between them is affected by the aquifer heterogeneity. The correlation patterns between river stage recorded at different gaging stations and groundwater level fluctuations are similar, reflecting the general pattern of the aquifer heterogeneity in the basin. But they are different in details because they represent snapshots taken at different angles and perspectives. These results corroborate our river stage tomography concept. In addition, we found that the unsaturated zone of the aquifers filters out pressure variations of high frequencies, and retains the low frequency trend of the fluctuations. On the other hand, once a part of the unsaturated zone is fully saturated and the river become connected with the saturated region of the aquifer, the variations of high and low frequencies of the river stage can propagate over long distances. Based on these findings, we are currently developing data fusion algorithms, which use these cross-correlations, and groundwater level and river stage data, to estimate the spatial distribution of aquifer properties. Normal 0 false false false EN-US ZH-TW X-NONE Normal 0 false false false EN-US ZH-TW X-NONE
