groundwater

Dual porosity flow is an important mechanism for groundwater transport in fractured rock aquifers. However, quantification and characterization of fracture flow systems remains challenging, as it often involves complex procedures such as the injection of tracers. In this study we conducted single-well pumping tests in 11 uncased wells in a coastal fractured rock aquifer while monitoring in-well salinity and temperature gradients through downhole casts using a Conductivity-Temperature-Depth (CTD) logger. In this way, we aimed to observe how naturally occurring salinity gradients in the well become disturbed by induced groundwater flow to the well, and if these gradients may serve as natural tracers for fracture flow. Since natural temperature gradients in the wells are minimal, we applied point electrical heating at the bottom of the well to create a plume of slightly warmer water to migrate up the wellbore during pumping from the top. During the pumping tests in this set-up, repeated CTD casts suggest that groundwater flow to these wells is strongly focused along narrow zones and is occurring at various rates over a range of salinities and temperatures. Hence, the observed patterns in both salinity and temperature presumably reflect the presence of fracture zones, which could indeed be confirmed by downhole camera observations for some wells. Further data analysis resulted in detailed hydrogeological characterization of the 11 wells, comprising an assessment of the fracture density and hydraulic conductivity of the aquifers, as well as the origin of the inflowing water being meteoric mostly fresh water or deeper saline groundwater.

Abstract

For this thesis a hydrochemical groundwater survey was carried out on the Caribbean island of Curaçao in 2020 as part of the NWO SEALINK project. In order to be well-adapted against anthropogenic and natural pressures, Curaçao needs representative data, yet thorough hydrochemical datasets were only sparsely collected in the wet seasons of 1977 and 1992. The aim of this thesis was to determine the current chemical state of the groundwater and analyze for long-term pollution trends with an extended database spanning four decades, also including data that was collected in another fieldwork campaign in 2021.

Curaçao was divided into a western and eastern section to facilitate data interpretation. Willemstad is located in the east, with a higher population density and most drinking/wastewater treatment plants and pipelines. The west is less populated, more rural and mostly agricultural. Four geological formations occur: Diabaas, Knip Group, Limestones, and Mid Curaçao Formation, of which the Diabaas is distinctly present as both Diabaas East (DE) and Diabaas West (DW). During the 2020 fieldwork campaign, wells that were sampled in 1992 were revisited (n=96), of which 20 could be remeasured, but 76 were inaccessible. To acquire more data, an additional 71 wells were sampled, bringing the total to 91 (neast=52; nwest=39). To obtain representativity, wells were as homogeneously distributed across the island as fieldwork conditions allowed for. 27 water quality parameters were analyzed in the field and lab (EC, DO, pH, turbidity, T, alkalinity, NO3, NO2, NH4, Al, B, Br, Ca, Cl, Fe, F, K, Mg, Na, Ni, P, PO4, S, Si, SO4, V and Zn). Datasets were assessed with a variety of boxplots, diagrams, descriptive and multivariate statistics (e.g. cluster analysis).

Of the groundwater wells measured in 2020, 29% were fresh, 53% slightly brackish (EC = 1.5 – 5 mS/cm), 17% brackish (5 – 15 mS/cm) and 1% saline. The majority was used for private irrigation (51%) or commercial agriculture (11%). The expectation was that the 2020 and a later obtained 2021 (n=72) dataset could be combined, but the results showed that this was not possible; reasons are further discussed within this thesis. To determine the influence of field methodology, wells measured in 2020 and 2021 (n=8) were compared for EC and sampling strategy, and wells situated closely together (n2020=7; 21-74 m) were assessed for heterogeneity (EC, pH, NO3, Ca, Cl), showing the effects of different sampling techniques (3 mg/L’92,’20,’21=81%, 87%, 65%; NO3>50 mg/L’92,’20,’21=38%, 39%, 47%), likely linked to wastewater and fertilizer inputs. Throughout the years, groundwater acidified, but the rural west did so with a higher onset pH (pH1977=7.83, DW) and steeper decline (ΔpH=0.7) than the urbanized east (pH1977=7.49; ΔpH=0.25, DE). An upward alkalinity trend is observed for the east, but not the west. Eastern acidification is likely caused by leaking wastewater. This is substantiated by higher eastern NO3 concentrations (east: 74±63 mg/L, west: 27±44 mg/L; p<0.001), more exposure to wastewater due to the presence of Willemstad, and a negative correlation between alkalinity and pH only found in the east, probably caused by the conversion of wastewater-related organics to HCO3/H+ . For the rural west, acidification is also attributed to wastewater pollution, but to a much lesser extent than the east. Instead, exploratory assessments point towards a plausible influence of atmospheric pollution spreading westward from the petrochemical facility “Isla Refinería” during its operating years. More vegetation, increasing the degradation of subsurface organic matter, is speculated to have an acidifying effect in the west, but no detailed analyses was done. Overall, further research into the causations and influences on long-term groundwater trends is recommended.

As was shown in this thesis, multi-annual datasets are valuable tools to disclose long-term groundwater quality trends regarding contamination, freshening, salinization and acidification. Such knowledge can greatly support the management and protection of groundwater resources and interlinked systems, such as marine aquatic life. This underlines the value of extended research, and also stresses the importance of an accessible network of groundwater wells, allowing for more representative data, further improving the advisory potential of such fieldwork campaigns.

Abstract

Worldwide, coral reef health is declining rapidly due to both global stressors (climate change) and local stressors (pollution). Reef maintenance on Curaçao focusses on reducing local stressors including terrestrial pollution. One f low path for terrestrial pollution is via submarine groundwater discharge (SGD). Yet, on the island of Curaçao these hotspots of submarine groundwater discharge have not yet been located, let stand quantified. With this research we aimed to increase our understanding of groundwater flow on the island. This will serve as a foothold for future SGD research. To do so, we have conducted 9 electrical resistivity tomography (ERT) measurements to understand (1) the heterogeneity of the lava formation, the main aquifer, (2) the saltwater-fresh water interface at the coast, and (3) the groundwater flow at geological interfaces. To assist in the interpretation of the data we measured groundwater levels and the discharge of Hato spring, in addition to observing local geological outcrops. We discovered that the lava formation is a very heterogenous aquifer due to variety in degree of weathering of the pillow basalt. As a result of the heterogenous permeability, the extent of seawater intrusion in the coastal lava formation is strongly variable. Yet, the lava formation does form a better barrier for sea-fresh water mixing than the limestone terraces. The midden formation (sedimentary rock) forms a thin aquifer near the surface. And, at the interface of lava and midden formation, the groundwater flow is hampered. As a result, the groundwater is confined within the lava formation until the lava formations reservoir “overflows”. The same occurs at the interface of the lava formation and the diorite intrusion. The next step in SGD research is quantifying the seaward groundwater flux where the lava formation is in direct contact with the sea or limestone formation. Here seaward groundwater flow is not hampered by the midden formation or the intrusion and thus forms hotspots of Submarine (polluted) Groundwater Discharge.

Abstract

The research aimed to recollect specimens from Curaçao of the genus Halosbaena belonging to the rare crustacean order Thermosbaenacea, a genus with a relictual Tethyan distribution. It resulted in recording the apparent extinction of the entire, species-rich, subterranean fauna on the island that had occurred within a period of 43 years up to 2015. The composition of the groundwater fauna on Curaçao was sampled in 2015 for comparison with sampling undertake 43 years previously. Despite using the same sampling methods previously used and comprehensive coverage of the available sampling sites, no stygofauna specimen was collected from sampling in 2015 in contrast to more than 50 species collected from the initial sampling in 1973. It is hypothesised that this faunal extinction was associated with the oil industry, but it is unclear whether it resulted from petroleum pollution of groundwater drawdown or recharge. This record of the extinction of an entire ecosystem, rather than merely some members of it, is perhaps unique, but is concerning amongst the more diffuse extinction events happening globally.

EXECUTIVE SUMMARY

The southern wetlands of Bonaire represent a unique environment for the island. Consisting of a wide variety of habitat types including caves, karsts, dry tropical forests, coastal areas, salt pans and mangroves. The Ramsar site Pekelmeer lies completely in this area, as well as a small portion of the buffer zone of the Ramsar site Lac Bay.

Culturally, a number of Bonaire’s historic monuments and tributes to its past can be found as you drive around the perimeter, from ruins of old salt pans to the remains of slave huts and gravestones. Maintaining and respecting these sober reminders of Bonaire’s history is vital to ensuring the sacrifices of the enslaved populations are not forgotten. It would be impossible to separate the historic and cultural identity of Bonaire from this area.

Economically the southern wetlands represent commercial opportunities for salt extraction by Cargill Salt Works as well as a significant driver of tourism, whether it is history enthusiasts, cyclists, kiteboarders, recreational fishers, scuba divers or bird watchers.

The cultural and economic value of this area is only surpassed by its environmental value. The southern wetlands are recognized internationally as an Important Bird Area (IBA), as a site of regional importance by the Western Hemisphere Shorebird Reserve Network, as an area important for sea turtle nesting and as a Ramsar site. The Ramsar site Pekelmeer, which encompasses most of the southern portion of the wetlands, is critical to a number of threatened, endangered or keystone species. Pekelmeer offers a much-needed rest stop for a number of migratory bird species while also serving as an important breeding ground for the Caribbean Flamingo and five different tern species. Furthermore, the southern wetlands constitute most of the natural habitat of the rare and endemic Bonaire Sabal Palm.

This management plan offers a description of the southern wetlands (chapter 1), a legal and legislative overview (chapter 2), a description of resources and utilities (chapter 3), an explanation of the spatial development plan (chapter 4), an overview of conservation target habitats (chapter 5), an analysis of threats and issues (chapter 6), an outline of management actions and strategies (chapter 7), and provides recommendations for the management plan evaluation and review (chapter 8). Conserving this unique wetland will be a major challenge. A critical first step is to designate Pekelmeer as a protected area under island and national legislation, and appoint a management authority.