Groundwater Throughflow and Seawater Intrusion in High Quality Coastal Aquifers

Abstract

High quality coastal aquifer systems provide vast quantities of potable groundwater for millions of people worldwide. Managing this setting has economic and environmental consequences. Specific knowledge of the dynamic relationship between fresh terrestrial groundwater discharging to the ocean and seawater intrusion is necessary. We present multi- disciplinary research that assesses the relationships between groundwater throughflow and seawater intrusion. This combines numerical simulation, geophysics, and analysis of more than 30 years of data from a seawater intrusion monitoring site. The monitoring wells are set in a shallow karstic aquifer system located along the southwest coast of Western Australia, where hundreds of gigalitres of fresh groundwater flow into the ocean annually. There is clear evidence for seawater intrusion along this coastal margin. We demonstrate how hydraulic anisotropy will impact on the landward extent of seawater for a given groundwater throughflow. Our examples show how the distance between the ocean and the seawater interface toe can shrink by over 100% after increasing the rotation angle of hydraulic conductivity anisotropy when compared to a homogeneous aquifer. We observe extreme variability in the properties of the shallow aquifer from ground penetrating radar, hand samples, and hydraulic parameters estimated from field measurements. This motived us to complete numerical experiments with sets of spatially correlated random hydraulic conductivity fields, representative of karstic aquifers. The hydraulic conductivity proximal to the zone of submarine groundwater discharge is shown to be significant in determining the overall geometry and landward extent of the seawater interface. Electrical resistivity imaging (ERI) data was acquired and assessed for its ability to recover the seawater interface. Imaging outcomes from field ERI data are compared with simulated ERI outcomes derived from transport modelling with a range of hydraulic conductivity distributions. This process allows for interpretation of the approximate geometry of the seawater interface, however recovery of an accurate resistivity distribution across the wedge and mixing zone remains challenging. We reveal extremes in groundwater velocity, particularly where fresh terrestrial groundwater discharges to the ocean, and across the seawater recirculation cell. An overarching conclusion is that conventional seawater intrusion monitoring wells may not be suitable to constrain numerical simulation of the seawater intrusion. Based on these lessons, we present future options for groundwater monitoring that are specifically designed to quantify the distribution of; (i) high vertical and horizontal pressure gradients, (ii) sharp variations in subsurface flow velocity, (iii) extremes in hydraulic properties, and (iv) rapid changes in groundwater chemistry. These extremes in parameter distribution are common in karstic aquifer systems at the transition from land to ocean. Our research provides new insights into the behaviour of groundwater in dynamic, densely populated, and ecologically sensitive coastal environments found worldwide.

Aquifer Unit Management Plan (AUMP) For Groundwater Resources Management of Multi-layered Coastal Aquifer System

“…The change in hydraulic head dynamics (from positive hydraulic pressure to negative hydraulic pressure)facilitates the salt water wedge (Holland, 1998) to move inland. To maintain hydraulic equilibrium, equivalent freshwater head that represents the column of fresh groundwater is required to balance the hydraulic pressure at a particular depth and groundwater density (Costall, 2020). The present hydraulic heads of the aquifer units III and IV in the coastal zone ranged between − 3 and − 5 and msl to -2 m respectively.…”

Groundwater Hydrograph Decomposition With the HydroSight Model

“…In the coastal areas, the climate, land cover and pumping exert limited impacts on the groundwater level changes, and other factors such as seawater intrusion, may impose major impact on the groundwater level changes. The limited changes in the groundwater level along the coast also occur due to the extremely high hydraulic conductivity of the Tamala limestone and proximity to the discharge zone (Costall et al, 2020). Other possibility of limited groundwater level changes may be from the boundary conditions (Morgan et al, 2012).…”

The Role of Hydrogeological Investigation in Geotechnical Engineering

“…Therefore, it is necessary to continuously realize data collation through specific investigation work. Therefore, when carrying out investigation operations, relevant staff needs to use specific requirements as the basic basis to make a correct distinction between upper stagnant water and diving (7). Accurate identification and scientific evaluation of groundwater types can be carried out on the conditions of pore water, rock fissure water, and upper stagnant water, so as to achieve a comprehensive understanding and grasp of the specific impact of different types of groundwater on the project.…”