Characterization of Flow Dynamics and Vulnerability in a Coastal Aquifer System

Abstract

Traditional aquifer vulnerability techniques primarily rely on spatial property data for a region and are limited by their ability to directly or indirectly assess flow and transport processes occurring from the surface to depth within an aquifer system. The main objective of this study was to investigate groundwater vulnerability in terms of aquifer interconnectivity and flow dynamics. A combination of stable isotopes, groundwater age-dating (radiocarbon), and geomorphic/geogenic spatial analyses was applied to a regional, highly developed coastal aquifer to explain the presence of nitrate at depth. The average δ(13) C value (-17.3 ± 2‰ VPDB, n = 27) is characteristic of groundwater originating from locally infiltrated precipitation through extensively cultivated soils. The average δ(18) O and δD values (-4.0 ± 0.1‰ VSMOW, n = 27; δD: -19.3 ± 1‰ VSMOW, n = 27, respectively) are similar to precipitation water derived from maritime sources feeding the region's surface water and groundwater. Stable and radioactive isotopes reveal significant mixing between shallow and deep aquifers due to high velocities, hydraulic connection, and input of local recharge water to depths. Groundwater overdevelopment enhances deeper and faster modern water downward flux, amplifying aquifer vulnerability. Therefore, aquifer vulnerability is a variable, dependent on the type and degree of stress conditions experienced by a groundwater system as well as the geospatial properties at the near surface.

Effect of variable-density groundwater flow on nitrate flux to coastal waters

“…Simulated residence time increases as water flows locally through semi‐confining and/or more heterogeneous lenses and layers. In the deeper parts of the aquifer system, residence times exceed 1000 years (the maximum simulation time) and closely match the radiocarbon ages reported by Murgulet and Tick () (Figure ; Table ).…”

“…The input salinity values at the IW and the canal were set to 18 450 and 29 750 mg∙L −1 , respectively, using average observed salinity data collected from each water body. Constant nitrate concentrations were assigned at the top boundary cells, along identified nitrate and recharge source zones (Murgulet and Tick, , ) (Figure ). Concentrations varied based upon the type of nitrate source and land use identified in the region (e.g.…”

“…The thickness of these sediments regionally decreases towards the GOM where they are divided into three distinct units (Figure ): (1) the upper, unconfined aquifer A1, which extends a few kilometres (km) north from the shoreline (at which point A1 and A2 become the same overall unit not separated by the clay aquitard) and consists of white fine‐grained to coarse‐grained quartzose sands, silt and locally sandy clay; (2) the middle unit, aquifer A2, which generally exhibits confined conditions (southern portion of the study area) to semi‐confined and unconfined conditions (throughout most of the study area), consists of coarse siliciclastic sands and gravel, and sporadic clay lenses; and (3) the lower, predominantly confined aquifer A3, similar in lithology to A2, which consists of well‐defined sand and clay layers (Walter and Kidd, ; Chandler et al ., ). A more detailed description of the stratigraphy and hydrostratigraphy of this area can be found in previous studies (Walter and Kidd, ; Chandler et al ., ; Murgulet and Tick, , ).…”

“…The primary source of groundwater recharge is through local infiltration of rainfall in the outcrop areas (Chandler et al ., ; Murgulet and Tick, ). Under natural conditions, groundwater moves southward, from the higher sandy plateau regions [the inland area with elevations around 50 m above mean sea level (m.s.l.)]…”

“…A water balance estimated for the model area (46 850 m × 2000 m), encompassing the wells represented in the model, shows that the observed pumping discharge rate from high capacity irrigation and city wells developed in aquifers A2 and A3 (i.e. 3.6 × 10 13 cm 3 ∙year −1 for 2008) accounts for the volume of water removed from the system and is equivalent to that produced by reducing the recharge rate in the simulation – that is, 2.2 × 10 13 cm 3 ∙year −1 for a recharge rate of 24 cm∙year −1 (Murgulet and Tick, ) and 3.7 × 10 13 cm 3 ∙year −1 for a recharge rate of 40 cm∙year −1 (Dowling et al ., ). The annual pumping volumes for the large wells that account for most of the pumping within the simulated model domain were compared with the difference in recharge rates.…”