FIELD WORKSHOP - ISLAND HYDROGEOLOGY
This 2-credit-hour workshop is offered within the framework of VARIABLE TITLE WORKSHOP IN GEOLOGY: GEOL-40093/60093/70093
Objective
The objective of this field workshop is to provide the student field experience in hydrogeology, involving sampling and analyzing water samples, collecting other field data and observations, and synthesizing the data into a comprehensive report of investigations.
Timing and site
Field work is conducted at Andros Island, Bahamas, and is scheduled for seven days during winter break, starting with the last Saturday of the year.
Description
Andros is the largest of the Bahama islands, lying on the western side of the U-shaped Great Bahama Bank. Its surface rock to a depth of approximately 40 feet below mean sea level is composed of oolithic and ooliclastic limestones, and below that, to a depth of approximately 500 feet, of fossiliferous reef limestones. The two formations, and particularly the lower one, are honeycombed by and extensive system of caves, solution channels and sinkholes. Development of caves appears to have two major controlling factors: (a) the position of the fresh/sea water interface, and (b) slump faulting and associated fractures running parallel to the deep oceanic trough that borders the eastern coast of the island, the "Tongue of the Ocean".
Fresh ground water occurs on Andros Island in form of a thin lens floating on top of denser sea water underneath. The shape and the thickness of the fresh ground water lens on Andros Island is largely controlled by the three factors: (1) the spatial and temporal distribution of fresh water recharge from atmospheric precipitation, (2) hydraulic properties of the karstified limestone formations, and (3) fresh/sea water density contrast.
As ground water flows toward discharge areas along the coast, it mixes with sea water that has encroached into the aquifer. This mixing generates brackish water, less dense and therefore more buoyant than sea water, that moves upward to discharge near the shore. The numerous submarine and near-shore brackish water springs represent discharge from this mixing zone. This discharge requires a replacement of sea water, thereby setting up a cyclic flow system for water from the sea, through the mixing zone, and back to the sea. The mixing zone associated with the fresh/sea water interface is hydrochemically reactive area that has been a major influence in the development and distribution of porosity and permeability, particularly in carbonate rocks.
One of the most spectacular limestone solution features on Andros Island are the deep, steepsided sinkholes called "blue holes". They are as much as 320 feet in diameter and occur all over Andros, as well as in the surrounding shallow submarine Pleistocene platform. The blue holes provide a unique opportunity to probe the chemical stratification of the ground water column in various parts of the Island. Workshop participants will be divided into teams of 3-5 per one blue hole. Each team will sample and analyze water profiles in a number of blue holes, to investigate the physical and chemical nature of the fresh/sea water interface, as well as the spatial distribution of this interface throughout the Island. Students will also investigate the hydrochemical environments at different depths in blue holes. Water profiles in most of the blue holes contain a layer of dark, turbid, rich in suspended organic particles water, with highly reductive environment. Chemical analyses will be conducted on the site and in the lab using portable kits, including spectrophotometer, titrators, specific ion electrodes, electric conductivity/pH-meter, etc. Graduate students, in addition, will use computer software, to investigate the hydrochemical processes occurring at different depths in blue holes.
The base of operations for the workshop will be at Forfar Station of International Field Studies, Inc. The Station is situated in a beach-front grove of coconut palms on the northeastern coast of Andros Island. It consists of a number of stone or wooden cabins with bunk beds, a lodge with a kitchen, dining room, lounge, classroom, laboratory and library, and a maintenance building. The Station is well equipped to provide the necessary transportation on the Island, as well as on the coastal waters.
Following return home, students will have to prepare project completion report. Preparation of the report will involve literature search and compiling all the information on the geology and hydrogeology of the Island. Following completion of the chapters on general geology and hydrogeology, the students will have to prepare interpretive materials, as follows:
1. Interpretation of the results of the chemical analyses, consisting of:
2. Spatial interpretation and presentation of the results, consisting of:
3. In addition, graduate students will have to use hydrochemical software (e.g. WATEQ) to describe the chemical processes taking place at various levels along the vertical profiles in each blue hole.