A numerical groundwater flow model was developed to estimate compaction in the Jasper Aquifer in the study area from a hypothetical brackish groundwater development project. The numerical model was developed using the United States Geological Survey code MODFLOW (Harbaugh et al., 2000). This model simulated subsidence with the MODFLOW SUB package (Hoffman et al., 2003) which is the same package utilized in the most recent regional MODFLOW model published in the region (Kasmarek, 2012). The model developed for this study is called the Jasper Compaction Model (JCM).

The JCM accounts for the variability in clay properties and parameters controlling compaction which are correlated to aquifer depth of burial across the study area. Clay bed location and properties were determined by the nearest analysed geophysical log. Because the analysis desires to estimate risk within the brackish Jasper Aquifer, the model must simulate compaction over a large study area. The model is composed of one-mile square cells. To make the analysis practicable, brackish projects were simulated at the central cell of regions approximately 23 km² in area. This resulted in 117 modelled brackish projects which provided adequate coverage both spatially and with depth. Each hypothetical project was represented in the model as a head-controlled boundary assumed to have an equal drawdown of approximately 150 m. The use of a constant drawdown versus a constant rate boundary normalized the compaction results.

Table 1Summary of direction of parameter variation for Jasper Compaction Model sensitivity analysis.

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Due to the uncertainty in the model parameters used in the JCM, a sensitivity analysis was performed to better understand the influence of model parameters on the results Three assemblages of model parameters were used grouped in a low impact, base (average), and high impact scenarios (Table 1).

Figure 2 plots compaction versus depth for the three sensitivity cases. The analysis shows that generally, for all scenarios, as depth exceeds about 1200 m, there is little change in model results as depth increases. Above about 600 m of depth, compaction rates for the base scenario are similar to typical observed rates of subsidence in the Houston region. While not the focus of this study, it is clear from a review of Fig. 2, that areas where the developed resources is within about 600 m of land surface in the Jasper aquifer (typically fresh water in the Houston Area), the risk of compaction in the aquifer materials is relatively high.

Figure 3Total subsidence normalized risk score in the Jasper aquifer, Gulf Coast Aquifer system, near Houston, TX, USA.

The approach used to develop a relative risk map for subsidence from development of the brackish Jasper Aquifer is based upon Multi-Attribute Utility Theory. Utility theory is a tool widely used by decision analysts for converting their preferences, expressed in monetary terms or other relevant performance measures, into a normalized scale to facilitate comparison of options (Clemen, 1986). In this case, an option is the relative risk of subsidence from pumping the brackish Jasper Aquifer at a given location in the study area. Performance measures are selected that inform risk of subsidence. One obvious performance measure is the amount of compaction in the Jasper Aquifer predicted by the JCM (Fig. 3). Other performance measures include: land subsidence, and the consequence from subsidence.

The land subsidence risk category intends to account for differences between the total compaction at depth and the total amount of subsidence observed at land surface. Research by Geertsma (1973) and Du and Olsen (2001) show that, for a given radius of compaction, the percentage of total compaction at depth that equals subsidence at land surface decreases as the depth of the compacting radius decreases. This performance measure assumes there is a correlation between the risk of subsidence occurring at land surface and the depth at which groundwater production (compaction) occurs. Consistent with the literature, the deeper the depth of burial, the lower the risk of subsidence at land surface for a given radius of compaction at depth.

A third performance measure was considered based upon the fact that the consequence of subsidence can vary by location. For the consequence performance measure, we used flood risk. The 100-year flood plain as determined by the U.S. Federal Emergency Management Agency was used to determine those areas most at risk from subsidence contributing to flooding. In our analysis, this performance measure was binary based upon being within or outside the flood plain. The risk grid was coincident with the MODFLOW grid and all three performance measures were normalized and either upscaled or downscaled to the risk grid.

A combined Total Subsidence Normalized Risk Score (TSNRS) was calculated based on the performance measures throughout the model on a one-mile risk grid.

Each normalized performance measure was weighted based on its overall contribution to risk. The Jasper Compaction Normalized Risk score accounts for 50 % of the TSNRS with the depth of development Normalized Score accounting for 40 % of the TNRS. The consequence parameter accounts for 10 % of the TSNRS due to the generally flat topography in the region and the assumption that any amount of appreciable subsidence over a 50-year period will have a consequence.

The TSNRS ranges from zero to 1.0 with 1.0 being the maximum relative risk of subsidence and zero being the minimum relative risk of subsidence. Figure 3 plots the TSNRS across the entire brackish Jasper Aquifer study area. Results of the assessment generally indicate that development of groundwater in the shallower areas of the Jasper aquifer is at a higher risk of causing subsidence. Areas of high risk include southern Waller County, Northern Harris County, and Southern Montgomery County.

The data, model, and analysis presented in this paper were completed by Intera, Inc., published by the Harris-Galveston Subsidence District, archived locally, and are available from the District upon request.

ND and VK completed the hydrogeologic survey, model development, and subsidence risk analysis. MT provided additional contributions on the subsidence mechanics of the Coastal Lowlands aquifer and contemporary development of brackish resources within the study area. MT, CP, and SB contributed knowledge of the historical subsidence and water use in the brackish zones of the aquifer.

The authors declare that they have no conflict of interest.

This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.

This research was supported by the Harris-Galveston Subsidence District.