Which flanked the west and south granitic hard rock formations

The study area under consideration constitutes a river valley, which is flanked to the west and south by granitic hard rock formations. The hydraulic connection between the fractured hard rock terrain and the river valley is debated, however, any potential fluxes of groundwater across these boundaries are only expected within the upper weathered and more fractured approximately 15 metres of the granitic rock. The model area is bound to the north by a river, which is regulated by a weir. The river and upper alluvial aquifer are hydraulically connected, but a culmination layer within the flow channel restricts the exchange of water to some degree. Besides recharge through the river system, the river valley aquifers receive rainfall recharge. Discharge from the aquifers is mainly due to evapotranspiration, groundwater extraction and outflow through the eastern boundary. The model area itself is of 4.17 km extend in the W-E direction and 2.7 km in the north to south direction.

The valley aquifer is a sedimentary aquifer, made up of river valley deposits: i.e. of sand, gravel and silt. The different sedimentary deposits in the valley are a result of the river meandering with time. However, weathering of the granitic basement rocks have had some effect on the river valley deposits towards the west: here, the alluvial sequence is gradually becoming less permeable due to weathering products of the granitic rock formation. At the base of the valley deposits a thick layer of mudstone restricts flow at depth. Observed groundwater heads in the area describe a flow field, in which groundwater enters the river valley in the west through leakage from the upper weathered zone within the hard rock formation and then groundwater flow is directed towards the east, where the river valley aquifer narrows (Figure 1). The full thickness of the valley deposits varies between about 26 metes in the east and 35 metres in the west towards the granitic outcrop area. The upper alluvium constitutes an unconfined aquifer.

Spot height measurements are available for the area and are provided to you in a text file format: “spot_heights.txt”. This information will be useful for creating the top of the model (the land surface). You can use ModelMuse interpolation of the Z values of the points, once you have the data points loaded.

Borehole Logs

Table 1. Slug Test Results

It is noticeable that OW-4 exhibits a much lower hydraulic conductivity compared to OW-1, even though both bores penetrate the upper alluvial aquifer. However, the geologists from the geological survey speculate, that OW-4 is representative for the finer sand encountered close to the granitic outcrop and which is believed to have its origin in weathering products of the basement rocks.

Additionally, there is data available from three bores drilled into the granitic hard rock formation just to the west of the study area. Water levels of three bores are available and show relatively stable water level measurements in all three bores, just outside of the study area, varying between 30masl in the north, to 32 masl further towards the south and 31.8 metres in the south-west (Figure 1). No data is available from the south-eastern granitic outcrop areas, however, it is assumed, that there is no hydraulic connection to the alluvial aquifer.

In the east, where the alluvial valley narrows, water levels in the alluvial plain deposits are approximately 23 m a.s.l and hardly fluctuate throughout the seasons.

Scope of work

With the data you have and the skills you’ve developed during the assignments, you’re in a position to start putting together a groundwater model of the study area. Before launching into model construction using ModelMuse, develop a conceptual model first. Make some sketches to help to understand the spatial characteristics of how to conceptualize the hydrogeological system, the parameters, boundary conditions, etc.:

5. How will you represent the river?

Next, develop your model in the following steps:

4. Document the water balance of the model. How much water is being taken out by ET? Which areas are affected by evaporation?

5. Which parameters effect the model results the most? You need to undertake a sensitivity analysis, where you show sensitivities of the model results to changes in parameters (the calibration and sensitivity can be done manually, if you are able to do it by PEST it is fantastic, but there is no need for it). Only change parameters in realistic ranges and show sensitivities in form of graphs (e.g. change in SRMS for the different model runs).

10. What is the range of groundwater ages of water arriving at the well (i.e. min , max in years)?

11. Write down all your findings in a report. Model reporting should encompass your conceptual model, model design, construction, its performance (calibration, water balance), the sensitivity analysis and outputs from predictions. Your findings should include the discussions of model results obtained in steps 3 to 10. Your text should be accompanied by figures (e.g. water levels in the model domain, graphs showing the capture zone of the well, scatter plots, etc). The model, the data and information created through the modelling process need to be archived in the report, so the results presented can be reproduced and the model can be used in future studies. Add a chapter, where you discuss uncertainties of the model and any shortcomings e.g. in terms of data availability. How do model uncertainties effect your model results? Don’t forget referencing any literature sources you may use. If you need help with structuring your groundwater modelling report, have a look through the Australian Modelling guidelines. However, be aware, that the reporting described in the guidelines refers to more comprehensive modelling studies, compared to what we do in this course. There is no min-max number of pages for the report, most important is that you document clearly what you have done and what your results are.