An official website of the United States government
Abstract. In groundwater pumping optimization (GPO), offline-trained data-driven surrogates can be used to replace numerical-intensive simulators in order to save computing time. The traditional offline training approach involves building surrogates prior to optimization, fitting training datasets that cover the input space uniformly or randomly, which can prove inefficient due to the potential oversampling of low-gradient areas and under-sampling of high-gradient areas. This study proposes an offline machine-learning (ML) algorithm that ranks candidate training points by scoring them based on their distance to the closest training point and on the local gradient of the surrogate estimate and then choosing the highest-rank point. This method is applied to develop surrogates for solving a two-objective GPO problem formulated on a three-dimensional (3D) island aquifer, using hydrogeological conditions representative of San Salvador Island, Bahamas. The objectives are to minimise the supply cost (fOC) resulting from groundwater pumping and desalination and maximise fresh groundwater supply (Qp), subject to constraints on seawater intrusion (SWI) control expressed in terms of aquifer drawdown Δs at pumping locations and aquifer salt mass increase ΔSM. Gaussian Process (GP) is the technique applied to construct surrogates of objectives and constraints, alongside the estimation of uncertainties. Using GP models, it is possible to estimate the probability of “Pareto optimality” for each pumping scheme by Monte Carlo simulation. Pareto optimal pumping schemes (POPS) are then characterized by a probability of occurrence, which can be verified by numerical simulation. The GP training strategy's effectiveness in generating POPS is compared to traditional training approaches, showing that such a strategy can efficiently identify reliable POPS.
more »
« less
Investigating the Impact of Seawater Intrusion on the Operation Cost of Groundwater Supply in Island Aquifers
Abstract Managing fragile island freshwater resources requires identifying pumping strategies that trade off the financial cost of groundwater supply against controlling the seawater intrusion (SWI) associated with aquifer pumping. In this work, these tradeoffs are investigated through a sensitivity analysis conducted in the context of an optimization formulation of the groundwater management problem, which aims at minimizing the groundwater supply operation cost associated with groundwater pumping and desalination treatment, subject to constraints on SWI control, as quantified by the water table drawdown over the well (∆s), the reduction in freshwater volume (∆FV) in the aquifer, or the salt mass increase (∆SM) in the aquifer. This study focuses on a simplified two‐dimensional model of the San Salvador Island aquifer (Bahamas). Pumping strategies are characterized by the distance of the pumping system from the shoreline (WL), the abstraction screen depth (D) and overall pumping rate (Q), constituting the decision variables of the optimization problem. We investigate the impacts of pumping strategies on the operation cost, ∆s, ∆FVand ∆SM. Findings indicate increasingDor decreasingWLreduces ∆s, ∆FVand ∆SM, thus preserving the aquifer hydrogeologic stability, but also leads to extracting saltier groundwater, thus increasing the water treatment requirements, which have a strong impact on the overall groundwater supply cost. From a financial perspective, groundwater abstraction near the island center and at shallow depths seems the most convenient strategy. However, the analysis of the optimization constraints reveals that strategies where the pumping system approaches the island center tend to cause more severe SWI, highlighting the need to trade off groundwater supply cost against SWI control.
more »
« less
- Award ID(s):
- 2022278
- PAR ID:
- 10477522
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 59
- Issue:
- 10
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Groundwater management policy around the world increasingly seeks to protect groundwater-dependent ecosystems and associated human uses and values. This includes uses of ecosystems and agricultural systems linked to natural spring discharge. Yet, there are few examples of practical tools to balance human groundwater use with ecological water demand related to spring discharge. Using a simulation optimization framework, we directly incorporate a spring discharge constraint into the analysis of sustainable yield for operationalizing groundwater policy in the state of Hawai‘i. Our application on the island of O‘ahu is a spring discharge-dependent watercress farm with historical, cultural, and ecological significance. This research provides decision-makers in Hawai‘i with information regarding the trade-off between groundwater pumping and spring discharge, which is connected to multiple benefits, including historical and cultural values in line with codified state beneficial use protections. Because this trade-off provides an important step in operationalizing sustainable yield policy in Hawai‘i, we conclude by discussing further conceptual and technical developments necessary to move groundwater policy in Hawai‘i closer to full incorporation of the public trust principles of the state water code.more » « less
-
Abstract Many irrigated agricultural areas seek to prolong the lifetime of their groundwater resources by reducing pumping. However, it is unclear how lagged responses, such as reduced groundwater recharge caused by more efficient irrigation, may impact the long‐term effectiveness of conservation initiatives. Here, we use a variably saturated, simplified surrogate groundwater model to: (a) analyze aquifer responses to pumping reductions, (b) quantify time lags between reductions and groundwater level responses, and (c) identify the physical controls on lagged responses. We explore a range of plausible model parameters for an area of the High Plains aquifer (USA) where stakeholder‐driven conservation has slowed groundwater depletion. We identify two types of lagged responses that reduce the long‐term effectiveness of groundwater conservation, recharge‐dominated and lateral‐flow‐dominated, with vertical hydraulic conductivity (KZ) the major controlling variable. When highKZallows percolation to reach the aquifer, more efficient irrigation reduces groundwater recharge. By contrast, when lowKZimpedes vertical flow, short term changes in recharge are negligible, but pumping reductions alter the lateral flow between the groundwater conservation area and the surrounding regions (lateral‐flow‐dominated response). For the modeled area, we found that a pumping reduction of 30% resulted in median usable lifetime extensions of 20 or 25 years, depending on the dominant lagged response mechanism (recharge‐ vs. lateral‐flow‐dominated). These estimates are far shorter than estimates that do not account for lagged responses. Results indicate that conservation‐based pumping reductions can extend aquifer lifetimes, but lagged responses can create a sizable difference between the initially perceived and actual long‐term effectiveness.more » « less
-
Abstract Low‐elevation coastal areas are increasingly vulnerable to seawater flooding as sea levels rise and the frequency and intensity of large storms increase with climate change. Seawater flooding can lead to the salinization of fresh coastal aquifers by vertical saltwater intrusion (SWI). Vertical SWI is often overlooked in coastal zone threat assessments despite the risk it poses to critical freshwater resources and salt‐intolerant ecosystems that sustain coastal populations. This review synthesizes field and modeling approaches for investigating vertical SWI and the practical and theoretical understanding of salinization and flushing processes obtained from prior studies. The synthesis explores complex vertical SWI dynamics that are influenced by density‐dependent flow and oceanic, hydrologic, geologic, climatic, and anthropogenic forcings acting on coastal aquifers across spatial and temporal scales. Key knowledge gaps, management challenges, and research opportunities are identified to help advance our understanding of the vulnerability of fresh coastal groundwater. Past modeling studies often focus on idealized aquifer systems, and thus future work could consider more diverse geologic, climatic, and topographic environments. Concurrent field and modeling programs should be sustained over time to capture interactions between physical processes, repeated salinization and flushing events, and delayed aquifer responses. Finally, this review highlights the need for improved coordination and knowledge translation across disciplines (e.g., coastal engineering, hydrogeology, oceanography, social science) to gain a more holistic understanding of vertical SWI. There also needs to be more education of communities, policy makers, and managers to motivate societal action to address coastal groundwater vulnerability in a changing climate.more » « less
-
Abstract Coastal deltaic aquifers are vulnerable to degradation from seawater intrusion, geogenic and anthropogenic contamination, and groundwater abstraction. The distribution and transport of contaminants are highly dependent on the subsurface sedimentary architecture, such as the presence of channelized features that preferentially conduct flow. Surface deposition changes in response to sea‐level rise (SLR) and sediment supply, but it remains unclear how these surface changes affect the distribution and transport of groundwater solutes in aquifers. Here, we explore the influence of SLR and sediment supply on aquifer heterogeneity and resulting effects on contaminant transport. We use realizations of subsurface heterogeneity generated by a process‐based numerical model, DeltaRCM, which simulates the evolution of a deltaic aquifer with different input sand fractions and rates of SLR. We simulate groundwater flow and solute transport through these deposits in three contamination scenarios: (a) vertical transport from widespread contamination at the land surface, (b) vertical transport from river water infiltration, and (c) lateral seawater intrusion. The simulations show that the vulnerability of deltaic aquifers to seawater intrusion correlates to sand fraction, while vertical transport of contaminants, such as widespread shallow contamination and river water infiltration, is influenced by channel stacking patterns. This analysis provides new insights into the connection between the depositional system properties and vulnerability to different modes of groundwater contamination. It also illustrates how vulnerability may vary locally within a delta due to depositional differences. Results suggest that groundwater management strategies may be improved by considering surface features, location within the delta, and the external forcings during aquifer deposition.more » « less
