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Regional groundwater and subsidence analysis

Since the mid-19^th century the aquifer system below New Orleans has been exploited for its groundwater resources. The extraction volumes increased rapidly after the 1940’s and reached a maximum around 1970. By then the industrial exploitation of groundwater had caused a hydraulic head decline exceeding 50 meters in the Gonzales-New Orleans aquifer below the City Centre. Several studies relate a significant component of observed subsidence to these extractions (Dokka, 2011; Jones et al., 2016).

To increase our understanding about the contribution of groundwater extractions to subsidence in New Orleans, a model study was carried out to reproduce the historical subsidence pattern. Hydraulic head dynamics were simulated using a MODFLOW-based groundwater model and showed reasonable agreement with observed heads. Land subsidence calculations were made with the NEN-Bjerrum model. However, the lack of fundamental base for the geotechnical parameterization and high-quality land subsidence observations complicated reliable model computations. An ensemble of model runs with a range of plausible geotechnical parameters suggest a mean cumulative subsidence in the order of 10 to 20 cm in the greater New Orleans area by 1970. Hereafter, the model shows significant rebound of the land surface. The magnitude of subsidence varies over the greater New Orleans area and subsidence hot-spots are shown at the industrial facilities at Michoud and Norco.  Although those locations correlate well with the locations as indicated by the observations, generally the model results poorly match the few point observations.

Although it is hard to validate the model results in an absolute sense, the model shows that in response to the industrial exploration of the New Orleans groundwater resources land subsidence has very likely developed. The model also supports indications of InSAR-studies that a significant part of the land subsidence has been recovered after 1970’s.

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Modelled cumulative land subsidence between 1900 and 1970. The model results show that in the greater New Orleans until 1970 between 100 and 200 mm of subsidence developed. According to the model results around 1970 the cumulative subsidence was worst. Hereafter, the model shows significant rebound of the land surface. In comparison to the observed subsidence trends (red points), the model results show relatively little subsidence. However, it is difficult to make a direct comparison between the model results and observations, because they might account for different processes.