## Doctoral Dissertations

Spring 1987

Dissertation

Earth Sciences

#### Degree Name

Doctor of Philosophy

#### Abstract

Groundwaters near two Bermuda marshes were sampled and studied for variations in major and minor ion geochemistry. Factors which influence groundwater geochemistry include Ghyben-Herzberg/freshwater-seawater mixing, carbonate dissolution and recrystallization, redox and the hydrology. The major ions calcium, magnesium and strontium have been previously studied by Plummer et al. (1976) near these two marshes and across the Devonshire lens. This study focuses on two wells and the chemical variations which occur with depth down the groundwater column.

Samples were collected at specific depths from two wells, one near the Pembroke Rest Home (PRH) adjacent to the Pembroke marsh, and the other along Jubilee Road (JR) near the Devonshire marsh. Collections were performed by bailing water from the borehole with a thief-type sampler and also from the aquifer using an inflatible packer. Analyses were performed to determine sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), strontium (Sr), chloride (Cl), sulfate $({\rm SO}\sb4),$ fluoride (F), bromide (Br), pH, alkalinity, nitrate $({\rm NO}\sb3\sp-),$ nitrite $({\rm NO}\sb2\sp-),$ ammonium $({\rm NH}\sb4\sp+),$ phosphate $({\rm PO}\sb4\sp{3-}),$ iron (Fe), manganese (Mn) and silicate $({\rm H\sb4 SiO\sb4).}$ With these data, the groundwater could be traced from its rainfall origin through carbonate reactions, biogeochemical reactions and mixing to its discharge to the inshore waters.

The mixing of fresh water and sea water shows conservative mixing for sodium and bromide, while the other major ions are all influenced by non-conservative input and removal mechanisms. An alkalinity model has been developed, and the contributions to the alkalinity from carbonate dissolution and redox have been quantified. Results suggest that carbon dioxide generation from aerobic respiration and nitrate reduction are probably the dominant reactions influencing the carbonate geochemistry, while iron, manganese and sulfate reduction, although clearly detectible, play only a minor role. The geochemistry of phosphate and fixed nitrogen has also been considered. Phosphate uptake from the groundwater does not appear to result from the formation of the mineral apatite. However, evidence does point toward ${\rm PO}\sb4\sp{3-}$ removal by calcium-carbonate phosphate surface adsorption. Flux calculations were performed to determine groundwater fixed nitrogen and phosphorus discharge to the Bermuda inshore waters. The results indicate that groundwaters do contribute significantly to the inshore water nutrient supply.

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