Groundwater Quality in Curaçao: A hydrochemical multi-annual assessment (1977-2021) of a Caribbean island

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.