The BAPs

Now there comes the science...

North Andros is a perfect natural laboratory for the study of carbonate island hydrogeology, geochemistry, biogeochemistry and geology.  Not only does it hosts the Bahamas' largest freshwater lens  but there is an extensive degree of shallow karstification which is exposed.  The research is split into a number of different projects, detailed below, each with a specific scientific aim and/or set of questions we want answers to (and we are not leaving until we get them!). A simplified explanation of what we want to find out can be found in blue after the heavy science part! 

Quantifying shallow karstification

Surveys of karst development in young limestones are limited largely to core-scale descriptions and cave surveys. However, understanding karst features across a range of scales is critical for assessing aquifer behaviour. Extended sections of the upper Pleistocene Lucayan Limestone exposed in shallow water supply trenches provide an unparalleled opportunity to quantify lateral variability of vadose dissolutional features, contrasting different lithologies and elevations.  Limestone is easily dissolved and these dissolution features are referred to as karst and the process is called karstification.  The limestone is not uniform in its characteristics and can often have areas that are preferentially areas which are karstified.  There are few studies of surveys of these features and as part of the unsaturated zone (the area above the water table) is exposed on North Andros it allows us to map these karst features.

Understanding groundwater recharge

North Andros has a strongly seasonal climate, with most rain falling during storm events. The rapidity with which water recharges the aquifer will be strongly influenced by the degree of karstification, vadose zone thickness and antecedent moisture.  We will monitor the rainfall rate, specific electrical conductivity (SEC) and isotopic composition, and the response of the water table elevation and SEC to individual and sequential rainfall events. The expedition targets the early wet season to evaluate recharge behaviour over this key period, but the monitoring network will operate for 18 months to capture behaviour over two (potentially variable) wet seasons and the intervening dry season.  Rainfall is key to recharging the freshwater on the island and the speed at which it reaches the water table is influenced by how heavy the rain is, how holey (or karstified) the limestone is and by the distance the water has to travel (how much rock is between the surface and the water table).  Most of the recharge on Andros occurs during the rain/hurricane season which is May til November.  We want to understand how long the water takes to reach the water table and what effects this time.

Evaluating pollutant transmission through the karstified vadose zone

Central Andros has a well-established and extensive network of arable farms. Given the paucity of the soil, there is widespread use of chemical fertilisers, pesticides and herbicides. We will study transmission of these contaminants to the shallow aquifer, which also supplies domestic consumption, comparing groundwaters from farmed and pristine areas of freshwater lens. Field measurements will quantify rates of biological activity (soil gas CO₂, and bacterial numbers).  North Andros has very little developed soil so the farming is very dependent on chemicals.  These chemicals can get into the groundwater and pollute the area.  We want to understand if they do get into the water, how far they could travel and if they could affect peoples drinking water.

 Analysing and predicting the hydrological impact of human activity

For the last 40 years c.14x10⁶ m³/d were extracted from the North Andros lens and barged to Nassau. The aquifer response to variations in abstraction rate, natural fluctuations in recharge and tidal head, offer insights into the large-scale hydrological functioning. SEC profiling in boreholes and karst lakes will characterize the thickness of the lens and underlying mixing zone, for comparison with historical data. This data will be used to develop a 3D groundwater flow model of the northern Andros freshwater system, to evaluate the impact on freshwater resources of a) the proposed development of a large (60 km²) quarry in the abandoned wellfield and b) long-term global sea-level rise and storm surge events.  Using information gathered from the fieldwork and also from historical data we want to create a 3D computer model of the island so that we can understand the impacts of quarrying, sea level rise and storm surge events.

Characterisation of rocks samples

Sediment and rock from representative depositional and diagenetic settings will be sampled to understand the evolution of the porosity/permeability network and changes in reactive surface area. Microfacies and diagenesis will be characterised through routine petrography, SEM, and trace element analysis. We will measure porosity and permeability in core-plugs, and determine reactive surface area by BET.  Basically, we want to understand the rocks on the island better!

Mapping processes driving carbonate dissolution and cementation

Current understanding of the distribution of processes driving dissolution and cementation within individual hydrological zones (vadose, phreatic lens, mixing-zone) is limited. We will characterise rainfall, throughflow, stemflow and runoff, to vadose percolation and phreatic waters with depth below the water table, over individual storms and intervening periods to capture the temporal and spatial complexity of the geochemical evolution of meteoric waters. pH and alkalinity will be determined in the field, and major ions and isotopes analysed in the UK.  We want to understand how the water changes chemically from source (rainfall) to sink (groundwater) over a range of different timescales.

Understanding the role of biogeochemical processes in diagenesis

Our previous studies focussed largely on understanding dissolution from cave water chemistry, but also suggested biological processes likely play an important role in diagenesis. We will determine distribution of microbes, metabolic rates and pathways of microbial activity, and their influence on diagenesis. We will focus on careful evaluation of the biogeochemical and diagenetic response to temporal variations in recharge and tides, and interactions between with the continuum of scales of voids from pores to caverns. O₂ and CO₂ flux and biological oxygen demand will be measured in the field, and samples preserved for nutrient, DOC, fluorescence, bacterial counts and isotopic analysis.  Microbes are important in carbonate environments and we want to understand more about what is living in the water at a microscopic scale and they effect the water and the karstification.

 Comparing rates of water-rock interaction

We will also install a series of pre-weighed, carefully characterised rock tablets of differing lithologies in environments where the hydrological and biogeochemical conditions have also been studied. These will provide insitu measurements of long-term (12-18 month) relative rates of diagenesis in different environments, and an opportunity to study biological colonisation, inorganic precipitates and/or changes to surface reactivity.  We want to understand more about how the rock is actually dissolving so we are leaving out pieces of rock to dissolve, simple!