Groundwater

Introduction

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 center. 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.

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.

Groundwater drainage in New Orleans

During a study investigating groundwater drainage in New Orleans, the interactions between different water infrastructure systems and the current groundwater drainage were analyzed. The three infrastructure systems looked at were: the sanitary sewer system, the storm water drainage system, and the drinking water system.

Due to the age of these networks, damage caused during natural disasters like Hurricane Katrina, the ground movement sensitivity of the region and the existing land subsidence, these networks are in poor shape. To quantify the groundwater drainage component of each system, an existing QGIS wastewater model was used along with different time series, including hourly and daily data for precipitation and sewer station influents, and a storm drainage analysis based on pumping station data. The following conclusions were made: 1) 50% of the total water treated in the wastewater treatment plant is groundwater, adding a large amount of unnecessary stress to the treatment process, 2) the storm water drainage system is the largest groundwater drainage component contributing to 58% of the total groundwater drainage and 3) 55% of the produced drinking water is lost during distribution, meaning the drinking water losses are a larger groundwater recharge than annual precipitation surplus.

Urban groundwater model of New Orleans

Little is known about shallow and deep groundwater flow in New Orleans. Therefore, two groundwater studies have been conducted: (1) a regional study focusing on deep groundwater and its relation to subsidence, reported in 'Assessment of Land Subsidence in New Orleans' (Kooi et al., 2023), and (2) a more detailed urban groundwater flow study focusing on the shallow subsurface (approximately 40 meters). This model study aims to enhance our understanding of shallow (phreatic) groundwater levels, both presently and in the future. The deliverables for this study is a well-described time-dependent groundwater model for New Orleans, supported by current and potential users. The model is capable of analyzing and calculating the effects of future changes in groundwater management on the groundwater system and surface elevation. An additional deliverable will be a protocol for the continuous improvement of the model by incorporating results from the integral monitoring network.

Fresh - salt groundwater in New Orleans

Fresh-salt groundwater interaction is studied at three scales: (1) at a regional scale (Broxkx, 2019), (2) at a city-wide scale. This study analyzes the potential impact of brackish-salt groundwater flow from Lake Pontchartrain (and connected canals) towards the urban area using an urban groundwater model based on groundwater flow paths analysis. (3) In addition, fresh-salt interaction in the shallow soil and root zone is analyzed by a detailed time-dependent 1D vertical soil moisture-groundwater model, considering the interaction between infiltrating rainwater (fresh) and groundwater discharge (salt).

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